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Inverter (electrical)

Inverter (electrical)

An inverter refers to two distinct types of electric circuits. In analog electronics, an inverter is a circuit for converting direct current to alternating current. In digital electronics, an inverter is a circuit that converts a logic level 1 to a logic-level 0, and vice-versa.

Analog inverters

alternating current Analog inverters are used in a wide range of applications, from small power supplies for a computer to large industrial applications to transport bulk power. An inverter can have one or two switched-mode power supplies (SMPS). Simple inverters consist of an oscillator driving a transistor that is used to interrupt the incoming direct current to create a square wave. This is then fed through a transformer to produce the required output voltage. Advanced inverters have started using more advanced forms of transistors or similar devices such as thyristors. More efficient inverters use various tricks to try to get a reasonable sine wave at the transformer input, rather than relying on the transformer to smooth it. Capacitors and inductors can be used to smooth the flow of current into and out of the transistor. Also, it is possible to produce a more sinusoidal wave by having split-rail direct current inputs at two voltages, or positive and negative inputs with a central ground. By connecting the transformer input terminals in sequence between the positive rail and ground, the positive rail and the negative rail, the ground rail and the negative rail, then both to the ground rail, a stepped sinusoid is generated at the transformer input and the current drain on the direct current supply is less choppy. These methods result in an output that is called a "modified-sinewave". Modified-sine invertors may cause some loads, such as motors, to operate less efficiently. More expensive power inverters use Pulse Width Modulation (PWM) with a high frequency carrier to more closely approximate a sine function. The quality of an inverter is described by its pulse-rating: a 3-pulse is a very simple arrangement, utilising only 3 transistors, whereas a more complex 12-pulse system will give an almost exact sine wave. In remote areas where a utility generated power is subject to significant external, distorting influences such as inductive loads or semiconductor-rectifier loads, a 12-pulse inverter may even offer a better, "cleaner" output than the utility-supplied power grid.

Digital inverters

A Digital Inverter is a circuit which outputs a voltage representing the opposite logic-level as its input. Digital electronics are circuits that operate at fixed voltage levels corresponding to a logical 0 or 1 (see Binary). An inverter circuit serves as the basic logic gate to swap between those two voltage levels. Implementation determines the actual voltage, but common levels include (0, +5V) for TTL circuits. Common types include resistive-drain, using one transistor and one resistor; and CMOS, which uses two (opposite type) transistors per inverter circuit. Digital inverter quality is often measured using the Voltage Transfer Curve, which is a plot of input vs. output voltage. From such a graph, device parameters including noise tolerance, gain, and operating logic-levels can be obtained.

External links

- [http://www.4lots.com DC to AC Power Inverters (Sale and info)]
- [http://www.vesa.org/inverterpr.htm VESA Announces Formation of LCD Inverter Special Interest Group]
- [http://www.donrowe.com donrowe: a company that sells power inverters]
- [http://www.solarpanelinfo.com/solar-panels/inverters/ Solar Panel Inverters] explained in great detail.
- [http://www.bosstar.com CCFL Inverter] Category:Power supplies ja:インバータ

Direct current

Direct current (DC or "continuous current") is the constant flow of electric charge from high to low potential. This is typically in a conductor such as a wire, but can also be through semiconductors, insulators, or even through a vacuum as in electron or ion beams. In direct current, the electric charges flow always in the same direction, which distinguishes it from alternating current (AC). A term formerly used for direct current was Galvanic current. term formerly used Historically, the first commercial electric power transmission (developed by Thomas Edison in the late nineteenth century) used direct current. Because alternating current is more convenient than direct current for electric power distribution and transmission, today nearly all electric power transmission uses alternating current. See War of Currents

Various Definitions

Within Electrical Engineering, the term DC is also a synonym for constant. For example, the voltage across a DC voltage source is constant as is the current through a DC current source. The DC solution of an electric circuit is that solution where all voltages and currents are constant. In this context, a voltage (current) that is changing with time cannot be a DC voltage (current) even if the polarity (direction) does not change. However, it can be shown that such a changing voltage or current can be decomposed into the sum of a DC component and an AC component. The DC component is defined to be the average value of the voltage or current over all time. The average value of the AC component is exactly zero as with, for example, a sine wave. Although DC stands for "Direct Current", DC is generically used to refer to constant polarity voltages. Some forms of DC vary wildly in voltage, such as the raw output of a rectifier. Running them through an RC low-pass filter will produce more stable voltage. Other forms of DC (such as that produced by a voltage regulator) have almost no variations in voltage (but may still have wild variations in output electric power and current).

Applications

Direct current installations usually have different types of sockets, switches, and fixtures, mostly due to the very low voltages used, from those suitable for alternating current. It is usually extremely important with a direct current appliance to not reverse polarity unless the device has a diode bridge to correct for this. (Most battery-powered devices don't.) High voltage direct current is used for long-distance point-to-point power transmission and for submarine cables, with voltages from a few kilovolts to approximately one megavolt. DC is commonly found in many low-voltage applications, especially where these are powered by batteries, which can produce only DC, or solar power systems, since solar cells can produce only DC. Most automotive applications use DC, although the generator is an AC device which uses a rectifier to produce DC. Most electronic circuits require a DC power supply. Most telephones connect to a twisted pair of wires, and internally separate the AC component of the voltage between the two wires (the audio signal) from the DC component of the voltage between the two wires (used to power the phone).

External links

- "[http://www.pbs.org/wgbh/amex/edison/sfeature/acdc.html AC/DC: What's the Difference]?". Edison's Miracle of Light, [http://www.pbs.org/wgbh/amex/index.html American Experience]. (PBS) Category:Electricity Category:Electrical engineering ko:직류 ja:直流

Alternating current

An alternating current (AC) is an electrical current where the magnitude and direction of the current varies cyclically, as opposed to direct current, where the direction of the current stays constant. The usual waveform of an AC power circuit is a sine wave, as this results in the most efficient transmission of energy. However in certain applications different waveforms are used, such as triangular or square waves. Used generically, AC refers to the form in which electricity is delivered to businesses and residences. However, audio and radio signals carried on electrical wire are also examples of alternating current. In these applications, an important goal is often the recovery of information encoded (or modulated) onto the AC signal.

History

William Stanley Jr designed one of the first practical coils to produce alternating currents. His design was an early precursor of the modern transformer, called an induction coil. From 1881 to 1889, the system used today was devised by Nikola Tesla, George Westinghouse, Lucien Gaulard, John Gibbs, and Oliver Shallenger. These systems overcame the limitations imposed by using direct current, as found in the system that Thomas Edison first used to distribute electricity commercially. The first long-distance transmission of alternating current took place in 1891 near Telluride, Colorado, followed a few months later in Germany. Thomas Edison strongly advocated the use of direct current (DC), having many patents in that technology, but eventually alternating current came into general use (see War of Currents). Charles Proteus Steinmetz of General Electric solved many of the problems associated with electricity generation and transmission using alternating current.

Distribution and domestic power supply

AC voltage can be stepped up or down by a transformer to a different voltage. High-voltage, direct current electric power transmission systems contrast with the more common alternating-current systems as a means for the bulk transmission of electrical power. However, these tend to be more expensive and less efficient than transformers, or did not exist when Edison, Westinghouse and Tesla were designing their power systems. Use of a higher voltage leads to more efficient transmission of power. The power losses in a conductor are a product of the square of the current and the resistance of the conductor, described by the formula

P= I^2R

. This means that when transmitting a fixed power on a given wire, if the current is doubled, the power loss will be four times greater. Since the power transmitted is equal to the product of the current, the voltage and the cosine of the phase difference φ (

P = IV \cos \phi

), the same amount of power can be transmitted with a lower current by increasing the voltage. Therefore it is advantageous when transmitting large amounts of power to distribute the power with extremely high voltages (sometimes as high as hundreds of kilovolts). However, high voltages also have disadvantages, the main ones being the increased danger to anyone who comes into contact with them, the extra insulation required, and generally increased difficulty in their safe handling. In the power plant the voltage is generated on three phase low voltage, with a frequency of either 50 or 60 hertz, and stepped up to a high voltage for distribution and stepped down, with a neutral, to a relatively low level for the consumer, generally around 200 V to 500 V between phases and 100 V to 250 V between each phase and the neutral. Three-phase electrical generation is very common and is a more efficient use of commercial generators. Electrical energy is generated by rotating a coil inside a magnetic field, in large generators with a high capital cost. However, it is relatively simple and cost effective to include three separate coils in the generator stator (instead of one). These sets of coils are physically separated and at an angle of 120° to each other. Three current waveforms are produced that are 120° out of phase with each other, but of equal magnitude. Three-phase systems are designed so that they are balanced at the load; if a load is correctly balanced no current will flow in the neutral point. Also, even in the worst-case unbalanced (linear) load, the neutral current will not exceed the highest of the phase currents. For three-phase at low (normal mains) voltages a four-wire system like this is normally used, reducing the cable requirements by one third over using a separate neutral per phase. When stepping down three-phase, a transformer with a Delta primary and a Star secondary is often used so there is no need for a neutral on the supply side. For smaller customers (just how small varies by country and age of install) only a single phase and the neutral or two phases and the neutral are taken to the property. For larger installs all three phases and the neutral are taken to the main board. From a three-phase main board both single and three-phase circuits may lead off (and in some cases also circuits with two phases (not to be confused with two-phase) and a neutral are led off). Three-wire single phase systems, with a single centre-tapped transformer giving two live conductors, is a common distribution scheme for residential and small commercial buildings in North America. A similar method is used for a different reason on construction sites in the UK. Small power tools and lighting are supposed to be supplied by a local center-tapped transformer with a voltage of 55V between each power conductor and the earth. This significantly reduces the risk of electric shock in the event that one of the live conductors becomes exposed through an equipment fault whilst still allowing a reasonable voltage for running the tools. A third wire is usually (should be always but there are many older, non-compliant, or third-world installs where it is not) connected between the individual electrical appliances in the house and the main consumer unit or distribution board. The third wire is known in Britain and most other English-speaking countries as the earth wire, but in (English-speaking) North America it is the ground wire. Exactly what happens to the ground wire before the main board varies, but there are three main possibilities, which are listed here by their European names:
- TT (customer's earth not connected to neutral at all)
- TN-S (neutral and earth run back separately to the transformer star point)
- TN-C-S (neutral and earth are joined at the intake position). There is also TN-C where neutral and earth are joined right through the install, but this is much less common than the others and requires special procedures to make it safe. A system should be designed so that, in the event of a short to earth on any part of the system, some form of fuse or breaker will make the system safe. In a TT system the high earth loop impedance means that a Residual-Current Device (RCD) must be used. In other earthing systems this can be covered by the normal overcurrent protection devices. RCDs may still be used on such systems though as they can protect against small earth faults such as through a person.

AC power supply frequencies by country

Electrical equipment is made by the manufacturer to be used on a specific frequency, in general 50 or 60 hertz or for both frequencies. If specified for one frequency this equipment cannot and should not be used on the other frequency, because of burn out and therefore fire reasons. The frequency of the electrical system varies by country; most electric power is generated at either 50 or 60 Hz. The 60 hertz countries are: American Samoa, Antigua and Barbuda, Aruba, Bahamas, Belize, Bermuda, Canada, Cayman Islands, Colombia, Costa Rica, Cuba, Dominican Republic, El Salvador, French Polynesia, Guam, Guatemala, Guyana, Haiti, Honduras, South Korea, Marshall Islands, Mexico, Micronesia, Montserrat, Nicaragua, Northern Mariana Islands, Palau, Panama, Peru, Philippines, Puerto Rico, Saint Kitts and Nevis, Suriname, Taiwan, Trinidad and Tobago, Turks and Caicos Islands, United States, Venezuela, Virgin Islands (U.S.), Wake Island._{[http://www.philip.allen.org/voltages.htm]} The following countries have a mixture of 50 Hz and 60 Hz supplies: Bahrain, Brazil (mostly 60 Hz), Japan (60 Hz used in western prefectures), Liberia (now officially 50 Hz, formerly 60 Hz and many independent 60 Hz generating plants still exist). _{[http://www.50hz.com/pwchrt2.htm]} Also see List of countries with mains power plugs, voltages and frequencies. Very early AC generating schemes used arbitrary frequencies based on convenience for steam engine, water turbine and generator design, since frequency was not critical for incandescent lighting loads. Frequencies between 16 2/3 Hz and 133 Hz were used on different systems, with lower frequencies favoured where loads were primarily composed of motors, and higher frequencies preferred to reduce lighting flicker. For example, the city of Coventry, England, in 1895 had a unique 87 Hz single-phase distribution system that was in use until 1906. Once induction motors became common, it was important to standardize frequency for compatibility with the customer's equipment. Standardizing on one frequency also, later, allowed interconnection of generating plants on a grid for economy and security of operation. It is generally accepted that Nikola Tesla chose 60 hertz as the lowest frequency that would not cause street lighting to flicker visibly. The origin of the 50 hertz frequency used in other parts of the world is open to debate but seems likely to be a rounding off of 60 Hz to the 1-2-5-10 structure, called a set of preferred numbers, popular with metric standards. Other frequencies were somewhat common in the first half of the 20th century, and remain in use in isolated cases today, often tied to the 60 Hz system via a rotary converter or static inverter frequency changer. 25 Hz power was used in Ontario, Quebec, the northern USA, and for electrified railroads. In the 1950s, much of this electrical system, from the generators right through to household appliances, was converted and standardised to 60 Hz. Some 25 Hz generators still exist at the Beck 1 and Rankine generating stations near Niagara Falls to provide power for large industrial customers who did not want to replace existing equipment; and some 25 Hz motors in New Orleans' floodwater pumps [http://www.dotd.louisiana.gov/press/pressrelease.asp?nRelease=513]. A low frequency eases the design of low speed electric motors, particularly for hoisting, crushing and rolling applications, and commutator-type traction motors for applications such as railways, but also causes a noticeable flicker in incandescent lighting and objectionable flicker of fluorescent lamps. 16.67 Hz power (1/3 of the mains frequency) is still used in some European rail systems, such as in Sweden and Switzerland. Off-shore, textile industry, marine, computer mainframe, aircraft, and spacecraft applications sometimes use 400 Hz, for benefits of reduced weight of apparatus or higher motor speeds. AC-powered appliances can give off a characteristic hum at the multiples of the frequencies of AC power that they use. Most countries have chosen their television standard to match (or at least approximate) their mains supply frequency. This helps prevent unfiltered powerline hum and magnetic interference from causing visible beat frequencies in the displayed picture.

Mathematics of AC voltages

television Alternating currents are usually associated with alternating voltages. An AC voltage v can be described mathematically as a function of time by the following equation: :

v(t)=A \times\sin(\omega t),

where :A is the amplitude in volts (also called the peak voltage), :ω is the angular frequency in radians per second, and :t is the time in seconds. Since angular frequency is of more interest to mathematicians than to engineers, this is commonly rewritten as: :

v(t)=A \times\sin(2 \pi f t),

where :f is the frequency in hertz. The peak-to-peak value of an AC voltage is defined as the difference between its positive peak and its negative peak. Since the maximum value of sin(x) is +1 and the minimum value is −1, an AC voltage swings between +A and −A. The peak-to-peak voltage, written as V_P-P, is therefore (+A) − (−A) = 2 × A. In power distribution, the AC voltage is nearly always given as a root mean square (rms) value, written V_rms. For a sinusoidal voltage: :

V_\mbox=

V_rms is useful in calculating the power consumed by a load. If a DC voltage of V_DC delivers a certain power P into a given load, then an AC voltage of V_rms will deliver the same average power P into the same load if V_rms = V_DC. Because of this fact rms is the normal means of measuring voltage in mains (power) systems. To illustrate these concepts, consider the 240 V AC mains used in the UK (it should be noted that the UK is now officially 230 V +10% −6% but in reality voltages are still closer to 240 V than 230 V in most cases). It is so called because its rms value is (at least nominally) 240 V. This means that it has the same heating effect as 240 V DC. To work out its peak voltage (amplitude), we can modify the above equation to: :

A=V_\mbox \times \sqrt 2

For our 240 V AC, the peak voltage V_P or A is therefore 240 V × √2 = 339 V (approx.). The peak-to-peak value V_P-P of the 240 V AC mains is even higher: 2 × 240 V × √2 = 679 V (approx.) Note that non-sinusoidal waveforms have a different relationship between their peak magnitude and effective (RMS) value. This is of practical significance when working with non-linear circuit elements that produce harmonic currents, such as rectifiers. Also notice the balanced 3 phase system: the neutral is supposed to have zero current. This follows from: :

\cos + \cos + \cos = 0

(

2\pi/3

is an angle of 120°). The European Union (including the UK) have now officially harmonized on a supply of 230 V 50 Hz. However they made the tolerance bands very wide at ±10%. Some countries actually specify stricter standards than this for example the UK specifies 230 V +10% −6%. Most supplies to the old standards therefore conform to the new one and do not need to be changed.

External links

- "AC/DC: [http://www.pbs.org/wgbh/amex/edison/sfeature/acdc.html What's the Difference]?". Edison's Miracle of Light, [http://www.pbs.org/wgbh/amex/index.html American Experience]. (PBS)
- "AC-DC: [http://www.pbs.org/wgbh/amex/edison/sfeature/acdc_insideacgenerator.html Inside the AC Generator]". Edison's Miracle of Light, American Experience. (PBS)
- Kuphaldt, Tony R., "Lessons In Electric Circuits : [http://www.faqs.org/docs/electric/AC/index.html Volume II - AC]". March 8, 2003. (Design Science License)
- Nave, C. R., "[http://hyperphysics.phy-astr.gsu.edu/hbase/electric/accircon.html Alternating Current Circuits Concepts]". HyperPhysics.
- "[http://www.ndt.net/article/az/mpi/alternating_current.htm Alternating Current] (AC)". Magnetic Particle Inspection, Nondestructive Testing Encyclopedia.
- "[http://www.apcs.net.au/nav/article/fg40400.html Alternating current]". Analog Process Control Services.
- Hiob, Eric, "[http://www.math.bcit.ca/examples/elex/trig_vectors/ An Application of Trigonometry and Vectors to Alternating Current]". British Columbia Institute of Technology, 2004.
- "[http://www.tpub.com/neets/book2/index.htm Introduction to alternating current and transformers]". Integrated Publishing.
- "Wind Energy Reference Manual Part 4: [http://www.windpower.org/en/stat/unitsac.htm Electricity]". Danish Wind Industry Association, 2003.
- Chan. Keelin, "[http://www.jcphysics.com/toolbox_indiv.php?sub_id=17 Alternating current Tools]". [http://www.jcphysics.com/ JC Physics], 2002.
- "[http://www.apcs.net.au/nav/kn/ff4050901.html Measurement -> ac]". Analog Process Control Services.
- Williams, Trip "Kingpin", "[http://www.alpharubicon.com/altenergy/understandingAC.htm Understanding Alternating Current], Some more power concepts".
- "[http://www.salestores1.com/woreltab.html Table of Voltage, Frequency, TV Broadcasting system, Radio Broadcasting, by Country]".
- [http://www.technology.niagarac.on.ca/people/mcsele/Rankine.html Professor Mark Csele's tour of the 25 Hz Rankine generating station]
- [http://www.henkpasman.com 50/60 hertz information] Category:Electric power Category:Nikola Tesla ja:交流

Switched-mode power supply

A switched-mode power supply, or SMPS, is an electronic power supply unit (PSU) that incorporates a switching regulator — an internal control circuit that switches the load current rapidly on and off in order to stabilise the output voltage. Switching regulators are used as replacements for simpler linear regulators when higher efficiency, smaller size or lighter weight are required. They are, however, more complicated and more expensive, their switching currents can cause noise problems if not carefully suppressed, and simple designs can have a poor power factor. SMPS can also be classified into four types according to the input and output waveforms, as follows.
- AC in, DC out: rectifier, off-line converter
- DC in, DC out: voltage converter, or current converter, or DC to DC converter
- AC in, AC out: frequency changer, cycloconverter
- DC in, AC out: inverter AC and DC are abbreviations for alternating current and direct current. Switched-mode PSUs in domestic products such as personal computers often have universal inputs, meaning that they can accept power from most mains supplies throughout the world, with frequencies from 50 Hz to 60 Hz and voltages from 100 V to 240 V (although a manual voltage "range" switch may be required). In the case of TV sets, for example, one can test the excellent regulation of the power supply by using a variac. For example, in some models made by Philips, the power supply starts when the voltage reaches around 90 volts. From then, one can change the voltage with the variac, and go as low as 40 volts and as high as 260, and the image will show absolutely no alterations.

SMPS compared with linear PSUs

Two main types of regulated power supplies are available: SMPS and linear. The reasons for choosing one type or the other can be summarized as follows.
- Size and weight. Linear power supplies use a mains-transformer operating at the mains frequency of 50/60 Hz and line-frequency smoothing filters. These components are larger and heavier than the corresponding parts of a SMPS, which operate at higher frequencies.
- Efficiency. Linear power supplies regulate their output by generating a higher voltage than needed at the output, then reducing it by converting some of the electrical power to heat. This loss is a necessary part of the operation of the circuit, and cannot be eliminated by improving the design. SMPS generate no more voltage than they require, and only a small amount of energy is wasted.
- Heat output or power dissipation. This is determined by the efficiency, above. Linear PSUs produce much more heat than SMPS.
- Complexity. Linear PSUs are simple enough to be designed by beginners. SMPS are complicated, difficult to design well, and the higher number of components makes them more expensive to assemble and to repair.
- Noise. The switching currents in a SMPS contain much more energy at high frequencies than those in a linear PSU. This high-frequency energy can be transmitted by electromagnetic induction to other nearby equipment, or as radio waves over long distances, causing interference. Care is therefore needed to eliminate this energy at source, or to contain it by screening.
- Power factor. If the current drawn by a load such as a SMPS from an AC supply is nonsinusoidal and/or out of phase with the supply voltage waveform, the power factor will be less than unity and the efficiency, capacity and reliability of generating plants and the transmission grid can be substantially decreased. The simplest and most common SMPS designs have a power factor of about 0.6, and their use in increasingly popular personal computers and compact fluorescent lamps presents a growing problem. Power factor correction (PFC) circuits can reduce this problem, and are required in some countries by regulation. Notably, power factor correction is not yet widely required or used in North America.

How an SMPS works

Power factor correction

Rectifier stage

Power factor correction If the SMPS has an AC input, then its first job is to convert the input to DC. This is called rectification. The rectifier circuit is often the same as that in a linear power supply, and produces an unregulated DC voltage which is then smoothed by a filter capacitor. The current drawn from the mains supply by this rectifier circuit occurs in short pulses around the AC voltage peaks. These pulses have significant high frequency energy which reduces the power factor. Special control techniques can be employed by the following SMPS to force the average input current to follow the sinusoidal shape of the AC input voltage thus correcting the power factor. A SMPS with a DC input does not require this stage. A SMPS designed for AC input can often be run from a DC supply, as the DC passes through the rectifier stage unchanged. (The user should check the manual before trying this!) If an input range switch is used, the rectifier stage is usually configured to operate as a voltage doubler when operating on the low voltage (~120 VAC) range and as a straight rectifier when operating on the high voltage (~240 VAC) range. If an input range switch is not used, then a full-wave rectifier is usually used and the downstream inverter stage is simply designed to be flexible enough to accept the wide range of dc voltages that will be produced by the rectifier stage. In higher-power SMPSs, some form of automatic range switching may be used.

Inverter stage

The inverter stage converts DC, whether directly from the input or from the rectifier stage described above, to AC by switching it on and off ('chopping') at a frequency of tens or hundreds of kilohertz (kHz). The frequency is usually chosen to be above 20 kHz, to make it inaudible to humans. The switching is done by MOSFETs, which are a type of transistor with a low on-resistance and a high current-handling capacity. This section refers to the block marked "Chopper" in the block diagram.

Voltage converter

If the output is required to be isolated from the input, as is usually the case in mains power supplies, the inverted AC is used to drive the primary winding of a high-frequency transformer. This converts the voltage up or down to the required output level on its secondary winding. The output transformer in the block diagram serves this purpose. If a DC output is required, the AC output from the transformer is rectified and smoothed by a filter consisting of inductors and capacitors. The higher the switching frequency, the smaller these components can be made. Simpler, non-isolated power supplies contain an inductor instead of a transformer. This type includes boost converters, buck converters, and the so called "buck-boost converter". These belong to the simplest class of single input, single output converters which utilise one inductor and one active switch (MOSFET). The buck converter reduces the input voltage, in direct proportion, to the ratio of the active switch "on" time to the total switching period, called the Duty Ratio. For example an ideal buck converter with a 10V input operating at a duty ratio of 50% will produce an average output voltage of 5V. A feedback control loop is usually employed to maintain (regulate) the output voltage by varing the duty ratio to compensate for variations in input voltage. The output voltage of a boost converter is always greater than the input voltage and the buck-boost output voltage is inverted but can be greater than, equal to, or less than the magnitude of its input voltage. There are many variations and extensions to this class of converters but these three form the bases of almost all isolated and non-isolated DC to DC conveters. By adding a second inductor the Cuk and SEPIC converters can be implemented or by adding additional active switches various bridge converters can be realised. Other types of SMPS use a capacitor-diode voltage multiplier instead of inductors and transformers. These are mostly used for generating high voltages at low currents.

Regulation

A feedback circuit monitors the output voltage and compares it with a reference voltage, which is set manually or electronically to the desired output. If there is an error in the output voltage, the feedback circuit compensates by adjusting the timing with which the MOSFETs are switched on and off. This part of the power supply is called the switching regulator. The "Chopper controller" shown in the block diagram serves this purpose. Depending on design/safety requirements, the controller may or may not contain an isolation mechanism (such as opto-couplers) to isolate it from the DC output. Open-loop regulators do not have a feedback circuit. Instead, they rely on feeding a constant voltage to the input of the transformer or inductor, and assume that the output will be correct.

Power factor

Unlike most other appliances, switched mode power supplies tend to be constant power devices, drawing more current as the line voltage reduces. Also, in common with many static rectifiers, maximum current draw occurs at the peaks of the waveform cycle. This means that basic switched mode power supplies tend to produce more harmonics in the mains power line and have a worse power factor than other types of appliances. This may cause stability problems in some situations such as emergency generator systems or for very heavy loads on ordinary power mains (as it can lead to increased neutral current and increased heating of the utility transformers. However, higher-quality switched-mode power supplies with power factor correction (PFC) are available; these are designed to present a near resistive load to the mains. European regulatory standards are now beginning to require power factor correction and harmonic reduction.

Types

Switched-mode power supplies can classified according to the circuit topology.
- buck regulator (single inductor; output voltage < input voltage)
- boost regulator (single inductor; output voltage > input voltage)
- buck-boost regulator (single inductor; output voltage can be more or less than the input voltage)
- flyback regulator (uses output transformer; allows multiple outputs and input-to-output isolation)
- forward regulator (uses output transformer; allows multiple outputs and input-to-output isolation)
- Ćuk converter (uses a capacitor for energy storage; produces negative voltage for positive input)
- Inverting charge-pump (Modified Ćuk with single inductor; output voltage negative and higher-magnitude than positive input voltage)
- SEPIC converter (two inductors; output voltage can be higher or lower than input voltage)

External articles

- [http://www.smpstech.com/tutorial/t00con.htm Switching-Mode Power Supply Design]
- [http://www.smps.us/Unitrode.html Unitrode Power Supply Design Seminar Books Online]
- [http://www.hills2.u-net.com/electron/smps.htm Switched Mode Power Supplies]. A fairly detailed discussion of converter types and control schemes. Does not cover modern switcher ICs.
- Watkins, Steve, "[http://www.steve-w.dircon.co.uk/fleadh/mphil/history.htm History and development of switched-mode power supplies pre 1987]". 1998 (ed. the bibliography is [http://www.steve-w.dircon.co.uk/fleadh/mphil/references.htm here].)
- [http://www.powerdesigners.com/InfoWeb/design_center/articles/DC-DC/converter.shtm]. A general description of DC-DC converters.

Book References

- Ned Mohan, Tore M. Undeland, William P. Robbins (2002). Power Electronics : Converters, Applications, and Design. Wiley. ISBN 0-471-22693-9.
- Muhammad H. Rashid (2003). Power Power electronics : circuits, devices, and applications. Prentice Hall. ISBN 0-131-22815-3.
- Fang Lin Luo, Hong Ye (2004). Advanced DC/DC Converters. CRC Press. ISBN 0-849-31956-0.
- Mingliang Liu (2006). Demystifying Switched-Capacitor Circuits. Elsevier. ISBN 0-750-67907-7.
- Fang Lin Luo, Hong Ye, Muhammad H. Rashid (2005). Power Digital Power Electronics and Applications. Elsevier. ISBN 0-120-88757-6.
- Abraham I. Pressman (1997). Switching Power Supply Design. McGraw-Hill. ISBN 0-070-52236-7. Category:Power supplies

Electronic oscillator

An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. A low-frequency oscillator (or LFO) is an electronic oscillator that generates an AC waveform between 0.1 Hz and 10 Hz. This term is typically used in the field of audio synthesizers, to distinguish it from an audio frequency oscillator.

Types of electronic oscillator

There are two main types of electronic oscillator: the harmonic oscillator and the relaxation oscillator.

Harmonic oscillator

synthesizers The harmonic oscillator produces a sinusoidal output. The basic form of an harmonic oscillator is an electronic amplifier with the output attached to a narrow-band electronic filter, and the output of the filter attached to the input of the amplifier. When the power supply to the amplifier is first switched on, the amplifier's output consists only of noise. The noise travels around the loop, being filtered and re-amplified until it increasingly resembles the desired signal. A piezoelectric crystal (commonly quartz) may be coupled to the filter to stabilise the frequency of oscillation, resulting in a crystal oscillator. There are many ways to implement harmonic oscillators, because there are different ways to amplify and filter. For example:
- Hartley oscillator
- Colpitts oscillator
- Clapp oscillator
- Pierce crystal oscillator
- Phase-shift oscillator
- RC oscillator (Wien Bridge and "Twin-T")

Relaxation oscillator

The relaxation oscillator is often used to produce a non-sinusoidal output, such as a square wave or sawtooth. The oscillator contains a nonlinear component such as a transistor that periodically discharges the energy stored in a capacitor or inductor, causing abrupt changes in the output waveform. Square-wave relaxation oscillators can be used to provide the clock signal for sequential logic circuits such as timers and counters, although crystal oscillators are often preferred for their greater stability. Triangle-wave or sawtooth oscillators are used in the timebase circuits that generate the horizontal deflection signals for cathode ray tubes in analogue oscilloscopes and television sets. In function generators, this triangle wave may then be further shaped into a close approximation of a sine wave. The multivibrator is another type of relaxation oscillator.

Transformer

:This article is about electrical and electronic transformers. For other meanings, see Transformers A transformer is an electrical device that transfers energy from one electrical circuit to another by magnetic coupling without using any moving parts. It is often used to convert between high and low voltages and in impedance transformation.

Basic principles

|- align = "center" | Image:Xfmrair.png | Image:Xfmriron.png | Image:Xfmrwithtickler.png | Image:Xfmrstepdown.png |- align = "center" | Air core | Iron core | Coil with
tickler | Step down |- align = "center" | Image:Xfmrstepup.png | Image:Xfmrcentertap.png | Image:Autoxfmr.png |- align = "center" | Step up | Center tap
(iron core) | Autotransformer A simple single phase(1φ) transformer consists of two electrical conductors called the primary coil and the secondary coil. The primary is fed with a varying (alternating or pulsed continuous) electric current which creates a varying magnetic field around the conductor. According to the principle of mutual inductance, the secondary, which is placed in this varying magnetic field, will develop an electromotive force or EMF. If the ends of the secondary are connected together to form an electric circuit, this EMF will cause a current to flow in the secondary. Thus, some of the electrical power fed into the primary is delivered to the secondary. In practical transformers, the primary and secondary conductors are coils of conducting wire because a coil creates a denser magnetic field (higher magnetic flux) than a straight conductor. Transformers alone cannot do the following:
- Convert DC to AC or vice versa
- Change the voltage or current of DC
- Change the frequency (the "cycles") of AC. However, transformers are components of the systems that perform all these functions.

Electrical laws

Consider the following two laws: # According to the law of conservation of energy, the power delivered by a transformer cannot exceed the power fed into it. # The power dissipated in a load at any instant is equal to the product of the voltage across it and the (in phase) current passing through it (see also Ohm's law). It follows from the above two laws that a transformer is not an amplifier. If the transformer is used to change power from one voltage to another, the magnitudes of the currents in the two windings must also be different, inversely proportional to the voltages. If the voltage were to be stepped down by the transformer, the secondary current available to the load would be greater. For example, suppose a power of 50 watts is supplied to a resistive load from a transformer with a turns ratio of 25:2.
- P = E·I (power = electromotive force · current) 50 W = 2 V · 25 A in the primary circuit
- Now with transformer change: 50 W = 25 V · 2 A in the secondary circuit. The high-current, low-voltage windings have fewer turns of (usually) thicker wire. The high-voltage, low-current windings have more turns of (usually) thinner wire. amplifier The electromotive force (EMF) developed in the secondary is proportional to the ratio of the number of turns in the secondary coil to the number of turns in the primary coil. Neglecting all leakage flux, an ideal transformer follows the equation: :::

\frac=\frac.

Where

V_\mathrm

is the voltage in the primary coil,

V_\mathrm

is the voltage in the secondary coil,

N_\mathrm

is the number of turns of wire on the primary coil, and

N_\mathrm

is the number of turns of wire on the secondary coil. This leads to the most common use of the transformer: to convert power at one voltage to power at a different voltage. Neglecting leakage flux, the relationship between voltage, number of turns, magnetic flux intensity and core area is given by the universal emf equation: :::

E=4.44 \cdot f \cdot n \cdot a \cdot B

Where

E

is the sinusoidal root mean square (RMS) voltage of the winding,

f

is the frequency in hertz,

n

is the number of turns of wire,

a

is the area of the core (square units) and

B

is magnetic flux density in webers per square unit. The value 4.44 collects a number of constants required by the system of units. In normal operation, a transformer winding should never be energised from a constant DC voltage source, as this would cause a large direct current to flow. In such a situation, in an ideal transformer with an open circuit secondary, the current would rise indefinitely as a linear function of time. In practice, the series resistance of the winding limits the amount of current that can flow, until the transformer either reaches thermal equilibrium or is destroyed. DC is occasionally applied to large power transformers in order to "bake out" water prior to adding the cooling oil and commencing normal operation.

Practical transformers

weber weber weber The transformer was an important element in the development of high-voltage power transmission and central generating stations. Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to gigawatt units used to interconnect large portions of national power grids, all operating with the same basic principles and with many similarities in their parts. A rough classification of transformers by the power handled in the circuit, in watts (or, more accurately, VA (volt amperes)):
- Up to 1 watt: Signal transformers, interstage coupling
- 1 - 1000 watts: Small power transformers, filament transformers, audio output transformers
- 1 kilowatt - 1 megawatt: Power transformers; larger units in this range may be oil filled
- 1 megawatt and over: Large power transformers, used for substations, large electrical consumers, and for power plants and transmission. Transformers can be classified into various types according to the ratio of the numbers of turns in the coils, as well as whether or not the primary and secondary are isolated: :Step-up :
- the secondary has more turns than the primary :Step-down :
- the secondary has fewer turns than the primary :Isolating :
- intended to transform from one voltage to the same voltage. The two coils have approximately equal numbers of turns, although often there is a slight difference in the number of turns, in order to compensate for losses (otherwise the output voltage would be a little less than, rather than the same as, the input voltage). :Variable :
- the primary and secondary have an adjustable number of turns which can be selected without reconnecting the transformer. The transformer may be an autotransformer used for regulation or adjustability. For example, a typical Variac™ can transform 120 volts to an adjustable voltage that ranges from zero to 140 volts is an autotransformer with a sliding tap on the winding to allow adjustment. In all cases the primary winding, or the secondary winding, or both, may have taps that allow selection of one of several different ratios of primary to secondary turns. A transformer with a single winding where part serves as both primary and secondary is known as an autotransformer.

Losses

An ideal transformer would have no loss, and would therefore be 100% efficient. However, the coils of a real transformer have resistance. When modeling a real transformer the resistance can be considered as existing in series with the winding of an ideal transformer. Large power transformers are often more than 98% efficient, in terms of energy supplied to the primary winding of the transformer and coupled to the secondary. The remaining 2% (or less) of the input energy is lost to: :
- Winding resistance :: The current flowing through the windings causes resistive heating of the conductors. This is referred to as copper loss (to distinguish this from the rest of the losses below which are primarily attributable to the magnetic core and known as core losses, also called iron losses) :
- Eddy currents :: Induced currents circulating in the core causing resistive heating of the core. :
- Stray losses :: Not all the magnetic field produced by the primary is intercepted by the secondary. A portion of the leakage flux may induce eddy currents within nearby conductive objects, such as the transformer's support structure, and be converted to heat. :
- Hysteresis losses :: Each time the magnetic field is reversed, a small amount of energy is lost to hysteresis in the magnetic core. Differing core materials will have different levels of hysteresis loss. :
- Mechanical losses :: The alternating magnetic field causes fluctuating electromagnetic forces between the coils of wire, the core and any nearby metalwork, causing vibrations and noise which consume power. :
- Magnetostriction :: A minor effect that causes the core to physically expand and contract slightly with the alternating magnetic field. This in turn causes losses due to frictional heating in susceptible ferromagnetic cores. The familiar hum or buzzing noise heard near transformers is a result of stray fields causing components of the tank to vibrate, and is also due to magnetostriction vibration of the core itself. :
- Cooling system :: Large power transformers may be equipped with cooling fans, oil pumps or water-cooled heat exchangers designed to remove the heat caused by copper losses and core losses. The power used to operate the cooling system is typically considered part of the losses of the transformer. Small transformers, such as a plug-in "wall wart"/"power brick" used to power small consumer electronics, often have high losses and may be less than 85% efficient.

High frequency transformers

The universal transformer emf equation indicates that at higher frequency, the core flux density will be lower for a given voltage. This implies that a core can have a smaller cross-sectional area and thus be physically more compact without reaching saturation. It is for this reason that the aircraft manufacturers and the military use 400 hertz supplies. They are less concerned with efficiency, which is lower at higher frequencies (mostly due to increased hysteresis losses), but are more concerned with saving weight. Similarly, flyback transformers which supply high voltage to cathode ray tubes operate at the frequency of the horizontal oscillator, many times higher than 50 or 60 hertz, which allows for a more compact component.

Designs

Invention

Those credited with the invention of the transformer include:
- Michael Faraday, who invented an 'induction ring' on August 29, 1831. This was the first transformer, although Faraday used it only to demonstrate the principle of electromagnetic induction and did not foresee the use to which it would eventually be put.
- Lucien Gaulard and John Dixon Gibbs, who first exhibited a device called a 'secondary generator' in London in 1881 and then sold the idea to American company Westinghouse. This may have been the first practical power transformer, but was not the first transformer of any kind. They also exhibited the invention in Turin in 1884, where it was adopted for an electric lighting system. Their early devices used a linear iron core, which was later abandoned in favour of a more efficient circular core.
- William Stanley, an engineer for Westinghouse, who built the first practical device in 1885 after George Westinghouse bought Gaulard and Gibbs' patents. The core was made from interlocking E-shaped iron plates. This design was first used commercially in 1886.
- Hungarian engineers Ottó Bláthy, Miksa Déri and Károly Zipernowsky at the Ganz company in Budapest in 1885, who created the efficient "ZBD" model based on the design by Gaulard and Gibbs.
- Nikola Tesla in 1891 invented the Tesla coil, which is a high-voltage, air-core, dual-tuned resonant transformer for generating very high voltages at high frequency.

Circuit symbols

Standard symbols

circuit symbol	Transformer with two windings and iron core.
circuit symbol	Transformer with three windings. The dots show the adjacent ends of the windings.
circuit symbol	Step-down or step-up transformer. The symbol shows which winding has more turns, but does not usually show the exact ratio.
circuit symbol	Transformer with electrostatic screen, which prevents electrostatic coupling between the windings.

Construction

A transformer usually has:
- two or more insulated windings, to carry current
- a core, in which the mutual magnetic field couples the windings. In transformers designed to operate at low frequencies, the windings are usually formed around an iron or steel core. This helps to confine the magnetic field within the transformer and increase its efficiency, although the presence of the core causes energy losses. Transformers made to operate at high frequencies may use other lower loss materials, or may use an air core. Power transformers are further classified by the exact arrangement of the core and windings as "shell type", "core type" and also by the number of "limbs" that carry the flux (3, 4 or 5 for a 3-phase transformer). The differences in the performance of each of these types, while of continuing interest to specialists, is perhaps more detail than is appropriate for a general encyclopedia.

Steel cores

steel Transformers often have silicon steel cores to channel the magnetic field. This keeps the field more concentrated around the wires, so that the transformer is more compact. The core of a power transformer must be designed so that it does not reach magnetic saturation. Carefully designed gaps are sometimes placed in the magnetic path to help prevent saturation. Practical transformer cores are always made of many stamped pieces of thin steel. The high resistance between layers reduces eddy currents in the cores that waste power by heating the core. These are common in power and audio circuits. A typical laminated core is made from E-shaped and I-shaped pieces, leading to the name "EI transformer". One problem with a steel core is that due to the material's magnetic hysteresis it may retain a static magnetic field when power is removed. When power is then reapplied, the residual field may cause the core to temporarily saturate. This can be a significant problem in transformers of more than a few hundred watts output, since the higher inrush current can cause mains fuses to blow unless current-limiting circuitry is added. More seriously, inrush currents can physically deform and damage the primary windings of large power transformers.

Solid cores

In higher frequency circuits such as switch-mode power supplies, powdered iron cores are sometimes used. These materials combine a high magnetic permeability with a high material resistivity. At even higher frequencies (radio frequencies typically) other types of core made of non-conductive magnetic materials, such as various ceramic materials called ferrites are common. Some transformers in radio-frequency circuits have adjustable cores which allow tuning of the coupling circuit.

Air cores

High-frequency transformers may also use air cores. These eliminate the loss due to hysteresis in the core material. Such transformers maintain high coupling efficiency (low stray field loss) by overlapping the primary and secondary windings.

Toroidal cores

Toroidal transformers are built around a ring-shaped core, which is made from a long strip of silicon steel or permalloy wound into a coil, or from ferrite, depending on frequency. This construction ensures that all the grain boundaries are pointing in the optimum direction, making the transformer more efficient by reducing the core's reluctance, and eliminates the air gaps inherent in the construction of an EI core. The cross-section of the ring is usually square or rectangular, but more expensive cores with circular cross-sections are also available. The primary and secondary coils are wound concentrically to cover the entire surface of the core. This minimises the length of wire needed, and also provides screening to prevent the core's magnetic field from generating electromagnetic interference. Toroidal cores for use at frequencies up to a few tens of kilohertz may also be made of ferrite material to reduce losses. Such transformers are used in switch-mode power supplies. Toroidal transformers are more efficient (around 95%) than the cheaper laminated EI types. Other advantages, compared to EI types, include smaller size (about half), lower weight (about half), less mechanical hum (making them superior in audio amplifiers), lower exterior magnetic field (about one tenth), low off-load losses (making them more efficient in standby circuits), single-bolt mounting, and more choice of shapes. This last point means that, for a given power output, either a wide, flat toroid or a tall, narrow one with the same electrical properties can be chosen, depending on the space available. The main disadvantage is higher cost. When fitting a toroidal transformer, it is important to avoid making an unintentional short-circuit through the core (e.g. by carelessly fitting a steel mounting bolt through the middle and fastening it to metalwork at both ends). This would cause a large current to flow through the bolt, converting all of the mains input power into heat, and blowing the input fuse. To avoid this, only one end of the mounting bolt must be fixed to the surrounding metalwork.

Windings

The winding material depends on the application. Small power and signal transformers are wound with insulated solid copper wire, often enameled. Larger power transformers may be wound with wire, copper or aluminum rectangular conductors, or strip conductors for very heavy currents. High frequency transformers operating in the tens to hundreds of kilohertz will have windings made of Litz wire, to minimize the skin effect losses in the conductors. Windings on both primary and secondary of a power transformer may have taps to allow adjustment of the voltage ratio; taps may be connected to automatic on-load tap changer switchgear for voltage regulation of distribution circuits.

Insulation

The conductor material must have insulation to ensure the current travels around the core, and not through a turn-to-turn short-circuit. In power transformers, the voltage difference between parts of the primary and secondary windings can be quite large. Layers of insulation are inserted between layers of windings to prevent arcing, and the transformer is immersed in transformer oil that provides further insulation and acts as a cooling medium.

Shielding

Although an ideal transformer is purely magnetic in operation, the close proximity of the primary and secondary windings can create a mutual capacitance between the windings. Where transformers are intended for high electrical isolation between primary and secondary circuits, an electrostatic shield can be placed between windings to minimize this effect. Transformers may also be enclosed by magnetic shields, electrostatic shields, or both to prevent outside interference from affecting the operation of the transformer or to prevent the transformer from affecting the operation of other devices (such as CRTs in close proximity to the transformer). Transformers may also be enclosed for reasons of safety, both to prevent contact with the transformer during normal operation and to contain possible fires that occur as a result of abnormal operation. The enclosure may also be part of the transformer's cooling system.

Coolant

Small transformers up to a few kilowatts in size usually are adequately cooled by air circulation. Larger "dry" type transformers may have cooling fans. High-power or high-voltage transformers are bathed in transformer oil - a highly-refined mineral oil that is stable at high temperatures. Large transformers to be used indoors must use a non-flammable liquid. Formerly, polychlorinated biphenyl (PCB) was used as it was not a fire hazard in indoor power transformers. Due to the stability of PCB and its environmental accumulation, it is no longer permitted in new equipment. Today, nontoxic, stable silicone-based oils or fluorinated hydrocarbons may be used, where the expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Other less-flammable fluids such as canola oil may be used but all fire resistant fluids have some drawbacks in performance, cost, or toxicity compared with mineral oil. The oil cools the transformer, and provides part of the electrical insulation between internal live parts. It has to be stable at high temperatures so that a small short or arc will not cause a breakdown or fire. To improve cooling of large power transformers, the oil-filled tank may have radiators through which the oil circulates by natural convection. Very large or high-power transformers (with capacities of millions of watts) may have cooling fans, oil pumps and even oil to water heat exchangers. Large and high-voltage transformers undergo prolonged drying processes, using electrical self-heating, the application of a vacuum, or both to ensure that the transformer is completely free of water vapor before the cooling oil is introduced. This helps prevent electrical breakdown under load. Oil transformers tend to be equipped with Buchholz relays - safety devices sensing gas buildup inside the transformer (a side effect of an electric arc inside the windings) and switching off the transformer. Experimental power transformers in the 2 MVA range have been built with superconducting windings which eliminates the copper losses, but not the core steel loss. These are cooled by liquid nitrogen or helium.

Terminals

Very small transformers will have wire leads connected directly to the ends of the coils, and brought out to the base of the unit for circuit connections. Larger transformers may have heavy bolted terminals, bus bars or high-voltage insulated bushings made of polymers or porcelain. A large bushing can be a complex structure since it must both provide electrical insulation, and contain oil within the transformer tank. helium

Autotransformers

An autotransformer has only a single winding, which is tapped at some point along the winding. AC or pulsed DC power is applied across a portion of the winding, and a higher (or lower) voltage is produced across another portion of the same winding. Autotransformers are used to compensate for voltage drop in a distribution system or for matching two transmission voltages, for example 115 kV and 138 kV. For voltage ratios not exceeding about 3:1, an autotransformer is less costly, lighter, smaller and more efficient than a two-winding transformer of a similar rating. Variac is a trademark of General Radio (mid-20th century) for a variable autotransformer intended to conveniently vary the output voltage for a steady AC input voltage. The term is often used to describe similar variable autotransformers made by other makers. To provide very small increments of adjustment, the secondary connection is made through a brush that slides across the winding coils. A variable autotransformer is an efficient and quiet method for adjusting the voltage to incandescent lamps. While lightweight and compact semiconductor light dimmers have replaced variacs in many applications such as theatrical lighting, variable autotransformers are still used when an undistorted variable voltage sine wave is required.

Polyphase transformers

For three phase power, three separate transformers can be used, or all three phases can be connected to a single polyphase transformer. The three primary windings are connected together and the three secondary windings are connected together. The most common connections are Y-Δ, Δ-Y, Δ-Δ and Y-Y. If a winding is connected to earth (grounded) the earth connection point is usually the center point of a Y winding. There are many possible configurations that may involve more or fewer than six windings and various tap connections. three phase power three phase power

Resonant transformers

A resonant transformer is one that operates at the resonant frequency of one or more of its coils. The resonant coil, usually the secondary, acts as an inductor, and is connected in series with a capacitor. If the primary coil is driven by a periodic source of alternating current, such as a square or sawtooth wave, each pulse of current helps to build up an oscillation in the secondary coil. Due to resonance, a very high voltage can develop across the secondary, until it is limited by some process such as electrical breakdown. These devices are therefore used to generate high alternating voltages. The current available from this type of coil can be much larger than that from electrostatic machines such as the Van de Graaff generator and Wimshurst machine. Examples:-
- Tesla coil
- Oudin coil (or Oudin resonator; named after Paul Marie Oudin, 1851-1923)
- D'Arsonval apparatus
- ignition coil or induction coil used in the ignition system of a petrol engine
- Flyback transformer of a CRT television set or video monitor. Other applications of resonant transformers are as coupling between stages of a superheterodyne receiver, where a large measure of the selectivity of the receiver is provided by the tuned transformers of the intermediate-frequency amplifiers. A voltage regulating transformer uses a resonant winding and allows part of the core to go into saturation on each cycle of the alternating current. This effect stabilizes the output of the regulating transformer, which can be used for equipment that is sensitive to variations of the supply voltage. Saturating transformers provide a simple rugged method to stabilize an ac power supply. However, due to the hysteresis losses accompanying this type of operation, efficiency is low.

Instrument transformers

Current transformers

superheterodyne receiver 400 ampere electricity supply]]A current transformer is designed to provide a current in its secondary which is accurately proportional to the current flowing in its primary. Current transformers are commonly used in electricity meters to facilitate the measurement of large currents which would be difficult to measure more directly. Care must be taken that the secondary of a current transformer is not disconnected from its load while current is flowing in the primary as in this circumstance a very high voltage would be produced across the secondary. Current transformers are often constructed with a single primary turn either as an insulated cable passing through a toroidal core, or else as a bar to which circuit conductors are connected.

Voltage transformers

Voltage transformers (also known as potential transformers) are used in the electricity supply industry to measure accurately the voltage being supplied. They are designed to present negligible load to the voltage being measured.

Pulse transformers

A pulse transformer is a transformer that is optimised for transmitting rectangular electrical pulses (that is, pulses with fast rise and fall times and a constant amplitude). Small versions called signal types are used in digital logic and telecommunications circuits, often for matching logic drivers to transmission lines. Medium-sized power versions are used in power-control circuits such as camera flash controllers. Larger power versions are used in the electrical power distribution industry to interface low-voltage control circuitry to the high-voltage gates of power semiconductors such as TRIACs, IGBTs, thyristors and MOSFETs. Special high voltage pulse transformers are also used to generate high power pulses for radar, particle accelerators, or other pulsed power applications. To minimise distortion of the pulse shape, a pulse transformer needs to have low values of leakage inductance and distributed capacitance, and a high open-circuit inductance. In power-type pulse transformers, a low coupling capacitance (between the primary and secondary) is important to protect the circuitry on the primary side from high-powered transients created by the load. For the same reason, high insulation resistance and high breakdown voltage are required. A good transient response is necessary to maintain the rectangular pulse shape at the secondary, because a pulse with slow edges would create switching losses in the power semiconductors. The product of the peak pulse voltage and the duration of the pulse (or more accurately, the voltage-time integral) is often used to characterise pulse transformers. Generally speaking, the larger this product, the larger and more expensive the transformer.

RF transformers

For radio frequency use, transformers are sometimes made from configurations of transmission line wound around ferrite cores. This style of transformer can give an extremely wide bandwidth. The windings are sometimes bifilar and sometimes made from coaxial cable. Only a limited number of ratios (such as 1:9,1:4,1:2) can be achieved with this technique. The cores of such transformers help performance at the lower frequency end of the band. This style of transformer is frequently used as an impedance matching balun to convert from 300 ohm balanced to 75 ohm unbalanced in FM receivers. [http://upload.wikimedia.org/wikipedia/en/b/bb/Tvbalun.jpg]

Uses of transformers

- Electric power transmission over long distances. The simplicity, reliability, and economy of conversion of voltages by stationary transformers was the principal factor in the selection of alternating current power transmission (see War of Currents)
- High-voltage direct-current HVDC power transmission systems
- Large, specially constructed power transformers are used for electric arc furnaces used in steelmaking.
- Rotating transformers are designed so that one winding turns while the other remains stationary. These can pass power or radio signals from a stationary mounting to a rotating mechanism, or radar antenna.
- Sliding transformers can pass power or signals from a stationary mounting to a moving part such as a machine tool head. See linear variable differential transformer,
- Some rotary transformers are precisely constructed in order to measure distances or angles. Usually they have a single primary and two or more secondaries, and electronic circuits measure the different amplitudes of the currents in the secondaries. See synchro and resolver.
- Small transformers are often used to isolate and link different parts of radio receivers and audio amplifiers, converting high current low voltage circuits to low current high voltage, or vice versa. See electronics and impedance matching. See also isolation transformer and repeating coil.
- Balanced-to-unbalanced conversion. A special type of transformer called a balun is used in radio and audio circuits to convert between balanced circuits and unbalanced transmission lines such as antenna downleads. A balanced line is one in which the two conductors (signal and return) have the same impedance to ground: twisted pair and "balanced twin" are examples. Unbalanced lines include coaxial cables and strip-line traces on printed circuit boards. A similar use is for connecting the "single ended" input stages of an amplifier to the high-powered "push-pull" output stage.

Thyristors

The thyristor is a solid-state semiconductor device similar to a diode, with an extra terminal which is used to turn it on. Once turned on, the thyristor will remain on (conducting) as long as there is a significant current flowing through it. If the current falls to zero, the device switches off. Some resources define silicon controlled rectifiers and thyristors as synonymous¹, while others define SCRs as a subset of thyristors². The thyristor is a four-layer semiconducting device, with each layer consisting of an alternately N-type or P-type material, for example N-P-N-P. The main terminals, labeled anode and cathode, are across the full four layers, and the control terminal, called the gate, is attached to one of the middle layers. The operation of a thyristor can be understood in terms of a pair of tightly coupled transistors, arranged to cause the self-latching action. image:thyristor.png Thyristors are mainly used where high currents and voltages are involved, and are often used to control alternating currents, where the change of polarity of the current causes the device to automatically switch off; refered to as Zero Cross operation. The device can also be said to be in synchronous operation as, once the device is open, it conducts in phase with the voltage applied over its cathode to anode junction. This is not to be confused with symmetrical operation, as the output is unidirectional, flowing only from cathode to anode, and so is asymmetrical in nature. These properties are used control the desired load regulation by adjusting the frequency of the trigger signal at the gate. The load regulation possible is broad as semiconductor based devices are capable of switching at extremely high speeds over extremely large numbers of switching cycles; something difficult to achieve with mechanical switching elements. This is a frequency domain method of control. With phase angle control a thyristor is turned on at a specific and adjustable portion of the cycle of the controlling sinusoidal input. Moving the point at which the thyristor is turned on regulates power output. An example of this method of control is a dimmer switch for lights. The turn on point of a thyristor is controlled to occur at a particular point on the sine curve of the AC supply. The thyristor stays on for the remainder of that cycle and the longer the thyristor stays on, the brighter the light. Fine adjustment of the system's output is possible with this method. Phase angle control is particularly suitable for slow-responding loads such as tungsten filament lamps or temperature variable resistive loads, with the method often being used to control the voltage across the resistive elements found in electrical ovens and furnaces. Phase angle is also utilised in a large number of inductive load controllers. Phase angle triggered controllers, also known as phase fired controllers, can be used to control resistive and inductive loads directly from the AC mains, substantially reducing the cost and weight of systems previously requiring transformers. Thyristors are also often also found in power supplies for digital circuits, where they can be used as a sort of "circuit breaker" or "crowbar" to prevent a failure in the power supply from damaging downstream components. The Thyristor is used in conjunction with a zener diode attached to its gate, and when the output voltage of the supply rises above the zener voltage, the thyristor opens, shorting the power supply output to ground (and in general blowing an upstream fuse). The functional drawback of a thyristor is that, like a diode, it only conducts in one direction. A similar self-latching 5-layer device, called a triac, is able to work in both directions. This added capability, though, also can become a shortfall. Because the triac can conduct in both directions, reactive loads can cause the triac to fail to turn off during the zero-voltage instants of the ac power cycle. Because of this, use of triacs with (for example) heavily-inductive motor loads usually requires the use of a "snubber" circuit around the triac to assure that it will turn off with each half-cycle of mains power. Inverse-parallel SCRs can also be used in place of the triac; because each SCR in the pair has an entire half-cycle of reverse polarity applied to it, the SCRs, unlike triacs, are sure to turn off. snubber An earlier gas filled tube device called a Thyratron provided a similar electronic switching capability, where a small control voltage could switch a large current. It is from a combination of "thyratron" and "transistor" that the term "thyristor" is derived. Modern thyristors can switch large amounts of power (up to megawatts). In the realm of very high power applications, they are still the primary choice. However, in low and medium power (from few tens of watts to few tens of kilowatts) they have almost been replaced by other devices with superior switching characteristics like MOSFETs or IGBTs. One major problem associated with the thyristor is that it is not a fully controllable switch in the sense that triggering current direction needs to be reversed to switch it off. The GTO (Gate Turn-off Thyristor) is another related device which addresses this problem. In high-frequency applications, thyristors are poor candidates due to large switching times arising out of bipolar conduction. MOSFETs, on the other hand, have much faster switching capability because of their unipolar conduction (only majority carriers carry the current).

Types of Thyristors

- Silicon controlled rectifier
- Distributed Buffer - Gate Turn-off Thyristor (DB-GTO)
- Triac, a bidirectional switching device containing two thyristor structures
- Gate turn-off thyristor (GTO thyristor)
- Mosfet controlled thyristor (MCT), two additional FET structures for on and off control.
- Modified anode - gate turn-off thyristor (MA-GTO)
- Static induction thyristor (SITh) or Field controlled thyristor (FCTh), a gate structure can shut down anode current flow.

References

#Dorf, Richard C., editor (1997), Electrical Engineering Handbook (2nd ed.). CRC Press, IEEE Press, Ron Powers Publisher, ISBN 0-8492-8574-1 # Christiansen, Donald; Alexander, Charles K. (2005); Standard Handbook of Electrical Engineering (5th ed.). McGraw-Hill, ISBN 0-07-138421-9

Capacitor

:See Capacitor (component) for a discussion of specific types. A capacitor is a device that stores energy in the electric field created between a pair of conductors on which equal but opposite electric charges have been placed. A capacitor is occasionally referred to using the older term condenser. condenser condenser

History

In circa 600 BC, Thales of Miletus recorded that the Ancient Greeks could generate sparks by rubbing balls of amber on spindles. This is the triboelectric effect, the mechanical separation of charge in a dielectric. This effect is the basis of the capacitor. In October 1745, Ewald Georg von Kleist of Pomerania invented the first recorded capacitor: a glass jar coated inside and out with metal. The inner coating was connected to a rod that passed through the lid and ended in a metal sphere. By layering the insulator between two metal plates, von Kleist dramatically increased charge density. Before Kleist's discovery became widely known, a Dutch physicist Pieter van Musschenbroek independently invented a very similar capacitor in January 1746. It was named the Leyden jar, after the University of Leyden where van Musschenbroek worked. Benjamin Franklin investigated the Leyden jar, and proved that the charge was stored on the glass, not in the water as others had assumed. Originally, the units of capacitance were in 'jars'. A jar is equivalent to about 1 nF. Early capacitors were also known as condensers, a term that is still occasionally used today. It was coined by Volta in 1782 (derived from the Italian condensatore), with reference to the device's ability to store a higher density of electric charge than a normal isolated conductor. Most non-English languages still use a word derived from "condensatore", like the French condensateur or the German kondensator. 1782

Physics

Overview

A capacitor consists of two electrodes or plates, each of which stores an opposite charge. These two plates are conductive and are separated by an insulator or dielectric. The charge is stored at the surface of the plates, at the boundary with the dielectric. Because each plate stores an equal but opposite charge, the total charge in the capacitor is always zero. dielectric dielectric

Capacitance

The capacitor's capacitance (C) is a measure of the amount of charge (Q) stored on each plate for a given potential difference or voltage (V) which appears between the plates: :

C = \frac

In SI units, a capacitor has a capacitance of one farad when one coulomb of charge causes a potential difference of one volt across the plates. Since the farad is a very large unit, values of capacitors are usually expressed in microfarads (µF), nanofarads (nF) or picofarads (pF). The capacitance is proportional to the surface area of the conducting plate and inversely proportional to the distance between the plates. It is also proportional to the permittivity of the dielectric (that is, non-conducting) substance that separates the plates. The capacitance of a parallel-plate capacitor is given by: :

C \approx \frac; A >> d^2

[http://www.ttc-cmc.net/~fme/captance.html] where ε is the permittivity of the dielectric, A is the area of the plates and d is the spacing between them.

Stored energy

As opposite charges accumulate on the plates of a capacitor due to the separation of charge, a voltage develops across the capacitor owing to the electric field of these charges. Ever increasing work must be done against this ever increasing electric field as more charge is separated. The energy (measured in joules, in SI) stored in a capacitor is equal to the amount of work required to establish the voltage across the capacitor, and therefore the electric field. The energy stored is given by: :

E_\mathrm =  C V^2

where V is the voltage across the capacitor.

Hydraulic model

As electrical circuitry can be modeled by fluid flow, a capacitor can be modeled as a chamber with a flexible diaphragm separating the input from the output. As can be determined intuitively as well as mathematically, this provides the correct characteristics: the pressure across the unit is proportional to the integral of the current, a steady-state current cannot pass through it but a pulse or alternating current can be transmitted, the capacitance of units connected in parallel is equivalent to the sum of their individual capacitances; etc.

In electric circuits

Circuits with DC sources

Electrons cannot directly pass across the dielectric from one plate of the capacitor to the other. When there is a current through a capacitor, electrons accumulate on one plate and electrons are removed from the other plate. This process is commonly called 'charging' the capacitor even though the capacitor is at all times electrically neutral. In fact, the current through the capacitor results in the separation rather than the accumulation of electric charge. This separation of charge causes an electric field to develop between the plates of the capacitor giving rise to voltage across the plates. This voltage V is directly proportional to the amount of charge separated Q. But Q is just the time integral of the current I through the capacitor. This is expressed mathematically as: :

I = \frac = C\frac

where :I is the current flowing in the conventional direction, measured in amperes :dV/dt is the time derivative of voltage, measured in volts / second. :C is the capacitance in farads For circuits with a constant (DC) voltage source, the voltage across the capacitor cannot exceed the voltage of the source. Thus, an equilibrium is reached where the voltage across the capacitor is constant and the current through the capacitor is zero. For this reason, it is commonly said that capacitors block DC current.

Circuits with AC sources

The capacitor current due to an AC voltage or current source reverses direction periodically. That is, the AC current alternately charges the plates in one direction and then the other. With the exception of the instant that the current changes direction, the capacitor current is non-zero at all times during a cycle. For this reason, it is commonly said that capacitors 'pass' AC current. However, at no time do electrons actually cross between the plates. Since the voltage across a capacitor is the integral of the current, as shown above, with sine waves in AC or signal circuits this results in a phase difference of 90 degrees, the current leading the voltage phase angle. It can be shown that the AC voltage across the capacitor is in quadrature with the AC current through the capacitor. That is, the voltage and current are 'out-of-phase' by a quarter cycle. The amplitude of the voltage depends on the amplitude of the current divided by the product of the frequency of the current with the capacitance, C. The ratio of the voltage amplitude to the current amplitude is called the reactance of the capacitor. This capacitive reactance is given by: :

X_C = -\frac = -\frac

where : $\omega = 2 \pi f$ , the angular frequency measured in radians per second :X_C = capacitive reactance, measured in ohms :f = frequency of AC in hertz :C = capacitance in farads and is analogous to the resistance of a resistor. Clearly, the reactance is inversely proportional to the frequency. That is, for very high-frequency alternating currents the reactance approaches zero so that a capacitor is nearly a short circuit to a very high frequency AC source. Conversely, for very low frequency alternating currents, the reactance increases without bound so that a capacitor is nearly an open circuit to a very low frequency AC source. Reactance is so called because the capacitor doesn't dissipate power, but merely stores energy. In electrical circuits, as in mechanics, there are two types of load, resistive and reactive. Resistive loads (analogous to an object sliding on a rough surface) dissipate energy that enters them, ultimately by electromagnetic emission (see Black body radiation), while reactive loads (analogous to a spring or frictionless moving object) retain the energy. The impedance of a capacitor is given by: :

Z_C = \frac = \frac

Hence, capacitive reactance is the negative imaginary component of impedance. The negative sign indicates that the current leads the voltage by 90° for a sinusoidal signal, as opposed to the inductor, where the current lags the voltage by 90°. Also significant is that the impedance is inversely proportional to the capacitance, unlike resistors and inductors for which impedances are linearly proportional to resistance and inductance respectively. This is why the series and shunt impedance formulae (given below) are the inverse of the resistive case. In series, impedances sum. In shunt, conductances sum. In a tuned circuit such as a radio receiver, the frequency selected is a function of the inductance (L) and the capacitance (C) in series, and is given by: :

f = \frac

This is the frequency at which resonance occurs in an RLC series circuit. For an ideal capacitor, the capacitor current is proportional to the time rate of change of the voltage across the capacitor where the constant of proportionality is the capacitance, C: :

i(t) = C \frac

The impedance in the frequency domain can be written as :

Z = \frac = -