A diode is a two-terminal electronic component that conducts current in one direction. A diode vacuum tube or thermionic diode is a vacuum tube with two electrodes, a heated cathode and a plate, in which electrons can flow in only one direction, from cathode to plate. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. Semiconductor diodes were the first semiconductor electronic devices; the discovery of asymmetric electrical conduction across the contact between a crystalline mineral and a metal was made by German physicist Ferdinand Braun in 1874. Today, most diodes are made of silicon, but other materials such as gallium arsenide and germanium are used; the most common function of a diode is to allow an electric current to pass in one direction, while blocking it in the opposite direction. As such, the diode can be viewed as an electronic version of a check valve; this unidirectional behavior is called rectification, is used to convert alternating current to direct current.
Forms of rectifiers, diodes can be used for such tasks as extracting modulation from radio signals in radio receivers. However, diodes can have more complicated behavior than this simple on–off action, because of their nonlinear current-voltage characteristics. Semiconductor diodes begin conducting electricity only if a certain threshold voltage or cut-in voltage is present in the forward direction; the voltage drop across a forward-biased diode varies only a little with the current, is a function of temperature. Diodes' high resistance to current flowing in the reverse direction drops to a low resistance when the reverse voltage across the diode reaches a value called the breakdown voltage. A semiconductor diode's current–voltage characteristic can be tailored by selecting the semiconductor materials and the doping impurities introduced into the materials during manufacture; these techniques are used to create special-purpose diodes. For example, diodes are used to regulate voltage, to protect circuits from high voltage surges, to electronically tune radio and TV receivers, to generate radio-frequency oscillations, to produce light.
Tunnel, Gunn and IMPATT diodes exhibit negative resistance, useful in microwave and switching circuits. Diodes, both vacuum and semiconductor, can be used as shot-noise generators. Thermionic diodes and solid-state diodes were developed separately, at the same time, in the early 1900s, as radio receiver detectors; until the 1950s, vacuum diodes were used more in radios because the early point-contact semiconductor diodes were less stable. In addition, most receiving sets had vacuum tubes for amplification that could have the thermionic diodes included in the tube, vacuum-tube rectifiers and gas-filled rectifiers were capable of handling some high-voltage/high-current rectification tasks better than the semiconductor diodes that were available at that time. In 1873, Frederick Guthrie observed that a grounded, white hot metal ball brought in close proximity to an electroscope would discharge a positively charged electroscope, but not a negatively charged electroscope. In 1880, Thomas Edison observed unidirectional current between heated and unheated elements in a bulb called Edison effect, was granted a patent on application of the phenomenon for use in a dc voltmeter.
About 20 years John Ambrose Fleming realized that the Edison effect could be used as a radio detector. Fleming patented the first true thermionic diode, the Fleming valve, in Britain on November 16, 1904. Throughout the vacuum tube era, valve diodes were used in all electronics such as radios, sound systems and instrumentation, they lost market share beginning in the late 1940s due to selenium rectifier technology and to semiconductor diodes during the 1960s. Today they are still used in a few high power applications where their ability to withstand transient voltages and their robustness gives them an advantage over semiconductor devices, in musical instrument and audiophile applications. In 1874, German scientist Karl Ferdinand Braun discovered the "unilateral conduction" across a contact between a metal and a mineral. Indian scientist Jagadish Chandra Bose was the first to use a crystal for detecting radio waves in 1894; the crystal detector was developed into a practical device for wireless telegraphy by Greenleaf Whittier Pickard, who invented a silicon crystal detector in 1903 and received a patent for it on November 20, 1906.
Other experimenters tried a variety of other minerals as detectors. Semiconductor principles were unknown to the developers of these early rectifiers. During the 1930s understanding of physics advanced and in the mid 1930s researchers at Bell Telephone Laboratories recognized the potential of the crystal detector for application in microwave technology. Researchers at Bell Labs, Western Electric, MIT, Purdue and in the UK intensively developed point-contact diodes during World War II for application in ra
Silicon controlled rectifier
A silicon controlled rectifier or semiconductor controlled rectifier is a four-layer solid-state current-controlling device. The principle of four-layer p–n–p–n switching was developed by Moll, Tanenbaum and Holonyak of Bell Laboratories in 1956; the practical demonstration of silicon controlled switching and detailed theoretical behavior of a device in agreement with the experimental results was presented by Dr Ian M. Mackintosh of Bell Laboratories in January 1958; the name "silicon controlled rectifier" is General Electric's trade name for a type of thyristor. The SCR was developed by a team of power engineers led by Gordon Hall and commercialized by Frank W. "Bill" Gutzwiller in 1957. Some sources define silicon-controlled rectifiers and thyristors as synonymous, other sources define silicon-controlled rectifiers as a proper subset of the set of thyristors, those being devices with at least four layers of alternating n- and p-type material. According to Bill Gutzwiller, the terms "SCR" and "controlled rectifier" were earlier, "thyristor" was applied as usage of the device spread internationally.
SCRs are unidirectional devices as opposed to TRIACs. SCRs can be triggered only by currents going into the gate as opposed to TRIACs, which can be triggered by either a positive or a negative current applied to its gate electrode. There are three modes of operation for an SCR depending upon the biasing given to it: Forward blocking mode Forward conduction mode Reverse blocking mode In this mode of operation, the anode is given a positive voltage while the cathode is given a negative voltage, keeping the gate at zero potential i.e. disconnected. In this case junction J1 and J3 are forward-biased, while J2 is reverse-biased, due to which only a small leakage current exists from the anode to the cathode until the applied voltage reaches its breakover value, at which J2 undergoes avalanche breakdown, at this breakover voltage it starts conducting, but below breakover voltage it offers high resistance to the current and is said to be in the off state. An SCR can be brought from blocking mode to conduction mode in two ways: Either by increasing the voltage between anode and cathode beyond the breakover voltage, or by applying a positive pulse at the gate.
Once the SCR starts conducting, no more gate voltage is required to maintain it in the ON state. There are two ways to turn it off: Reduce the current through it below a minimum value called the holding current, or With the gate turned off, short-circuit the anode and cathode momentarily with a push-button switch or transistor across the junction; when a negative voltage is applied to the anode and a positive voltage to the cathode, the SCR is in reverse blocking mode, making J1 and J3 reverse biased and J2 forward biased. The device behaves as two reverse-biassed diodes connected in series. A small leakage current flows; this is the reverse blocking mode. If the reverse voltage is increased at critical breakdown level, called the reverse breakdown voltage, an avalanche occurs at J1 and J3 and the reverse current increases rapidly. SCRs are available with reverse blocking capability, which adds to the forward voltage drop because of the need to have a long, low-doped P1 region; the reverse blocking voltage rating and forward blocking voltage rating are the same.
The typical application for a reverse blocking SCR is in current-source inverters. An SCR incapable of blocking reverse voltage is known as an asymmetrical SCR, abbreviated ASCR, it has a reverse breakdown rating in the tens of volts. ASCRs are used where either a reverse conducting diode is applied in parallel or where reverse voltage would never occur. Asymmetrical SCRs can be fabricated with a reverse conducting diode in the same package; these are known for reverse conducting thyristors. Forward-voltage triggering gate triggering dv/dt triggering temperature triggering light triggeringForward-voltage triggering occurs when the anode–cathode forward voltage is increased with the gate circuit opened; this is known as avalanche breakdown. At sufficient voltages, the thyristor changes to its on state with low voltage drop and large forward current. In this case, J1 and J3 are forward-biased. SCRs are used in devices where the control of high power coupled with high voltage, is demanded, their operation makes them suitable for use in medium- to high-voltage AC power control applications, such as lamp dimming, power regulators and motor control.
SCRs and similar devices are used for rectification of high-power AC in high-voltage dc power transmission. They are used in the control of welding machines GTAW processes similar, it is used as switch in various devices. A silicon-controlled switch behaves nearly the same way as an SCR. Unlike an SCR, an SCS can be triggered into conduction when a negative voltage/output current is applied to that same lead. SCSs are useful in all circuits that need a switch that turns on/off through two distinct control pulses; this includes power-switching circuits, logic circuits, lamp drivers, etc. A TRIAC resembles an SCR. Unlike an SCR, a TRIAC can pass curre
A power supply is an electrical device that supplies electric power to an electrical load. The primary function of a power supply is to convert electric current from a source to the correct voltage and frequency to power the load; as a result, power supplies are sometimes referred to as electric power converters. Some power supplies are separate standalone pieces of equipment, while others are built into the load appliances that they power. Examples of the latter include power supplies found in desktop computers and consumer electronics devices. Other functions that power supplies may perform include limiting the current drawn by the load to safe levels, shutting off the current in the event of an electrical fault, power conditioning to prevent electronic noise or voltage surges on the input from reaching the load, power-factor correction, storing energy so it can continue to power the load in the event of a temporary interruption in the source power. All power supplies have a power input connection, which receives energy in the form of electric current from a source, one or more power output connections that deliver current to the load.
The source power may come from the electric power grid, such as an electrical outlet, energy storage devices such as batteries or fuel cells, generators or alternators, solar power converters, or another power supply. The input and output are hardwired circuit connections, though some power supplies employ wireless energy transfer to power their loads without wired connections; some power supplies have other types of inputs and outputs as well, for functions such as external monitoring and control. Power supplies are categorized including by functional features. For example, a regulated power supply is one that maintains constant output voltage or current despite variations in load current or input voltage. Conversely, the output of an unregulated power supply can change when its input voltage or load current changes. Adjustable power supplies allow the output voltage or current to be programmed by mechanical controls, or by means of a control input, or both. An adjustable regulated power supply is one, both adjustable and regulated.
An isolated power supply has a power output, electrically independent of its power input. Power supplies are classified accordingly. A bench power supply is a stand-alone desktop unit used in applications such as circuit test and development. Open frame power supplies have only a partial mechanical enclosure, sometimes consisting of only a mounting base. Rack mount. An integrated power supply is one. An external power supply, AC adapter or power brick, is a power supply located in the load's AC power cord that plugs into a wall outlet; these are popular in consumer electronics because of their safety. Power supplies can be broadly divided into linear and switching types. Linear power converters process the input power directly, with all active power conversion components operating in their linear operating regions. In switching power converters, the input power is converted to AC or to DC pulses before processing, by components that operate predominantly in non-linear modes. Power is "lost" when components operate in their linear regions and switching converters are more efficient than linear converters because their components spend less time in linear operating regions.
A DC power supply is one. Depending on its design, a DC power supply may be powered from a DC source or from an AC source such as the power mains. DC power supplies use AC mains electricity as an energy source; such power supplies will employ a transformer to convert the input voltage to a higher or lower AC voltage. A rectifier is used to convert the transformer output voltage to a varying DC voltage, which in turn is passed through an electronic filter to convert it to an unregulated DC voltage; the filter removes most, but not all of the AC voltage variations. The electric load's tolerance of ripple dictates the minimum amount of filtering that must be provided by a power supply. In some applications, high ripple is tolerated and therefore no filtering is required. For example, in some battery charging applications it is possible to implement a mains-powered DC power supply with nothing more than a transformer and a single rectifier diode, with a resistor in series with the output to limit charging current.
In a switched-mode power supply, the AC mains input is directly rectified and filtered to obtain a DC voltage. The resulting DC voltage is switched on and off at a high frequency by electronic switching circuitry, thus producing an AC current that will pass through a high-frequency transformer or inductor. Switching occurs at a high frequency, thereby enabling the use of transformers and filter capacitors that are much smaller and less expensive than those found in linear power supplies operating at mains frequency. After the inductor or transformer secondary, the high frequency AC is rectified and filtered to pr
Switched-mode power supply
A switched-mode power supply is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently. Like other power supplies, an SMPS transfers power from a DC or AC source to DC loads, such as a personal computer, while converting voltage and current characteristics. Unlike a linear power supply, the pass transistor of a switching-mode supply continually switches between low-dissipation, full-on and full-off states, spends little time in the high dissipation transitions, which minimizes wasted energy. Ideally, a switched-mode power supply dissipates no power. Voltage regulation is achieved by varying the ratio of on-to-off time. In contrast, a linear power supply regulates the output voltage by continually dissipating power in the pass transistor; this higher power conversion efficiency is an important advantage of a switched-mode power supply. Switched-mode power supplies may be smaller and lighter than a linear supply due to the smaller transformer size and weight.
Switching regulators are used as replacements for linear regulators when higher efficiency, smaller size or lighter weight are required. They are, more complicated. 1836 Induction coils use switches to generate high voltages. 1910 An inductive discharge ignition system invented by Charles F. Kettering and his company Dayton Engineering Laboratories Company goes into production for Cadillac; the Kettering ignition system is a mechanically-switched version of a flyback boost converter. Variations of this ignition system were used in all non-diesel internal combustion engines until the 1960s when it began to be replaced first by solid-state electronically-switched versions capacitive discharge ignition systems. 1926 On 23 June, British inventor Philip Ray Coursey applies for a patent in his country and United States, for his "Electrical Condenser". The patent mentions furnaces, among other uses. C. 1932 Electromechanical relays are used to stabilize the voltage output of generators. See Voltage regulator#Electromechanical regulators.
C. 1936 Car radios used electromechanical vibrators to transform the 6 V battery supply to a suitable B+ voltage for the vacuum tubes. 1959 Transistor oscillation and rectifying converter power supply system U. S. Patent 3,040,271 is filed by Joseph E. Murphy and Francis J. Starzec, from General Motors Company c. 1967 Bob Widlar of Fairchild Semiconductor designs the µA723 IC voltage regulator. One of its applications is as a switched mode regulator. 1970 Tektronix starts using High-Efficiency Power Supply in its 7000-series oscilloscopes produced from about 1970 to 1995. 1972 HP-35, Hewlett-Packard's first pocket calculator, is introduced with transistor switching power supply for light-emitting diodes, timing, ROM, registers. 1973 Xerox uses switching power supplies in the Alto minicomputer 1977 Apple II is designed with a switching mode power supply. "Rod Holt was brought in as product engineer and there were several flaws in Apple II that were never publicized. One thing Holt has to his credit is that he created the switching power supply that allowed us to do a lightweight computer".
1980 The HP8662A 10 kHz – 1.28 GHz synthesized signal generator went with a switched mode power supply. A linear regulator provides the desired output voltage by dissipating excess power in ohmic losses. A linear regulator regulates either output voltage or current by dissipating the excess electric power in the form of heat, hence its maximum power efficiency is voltage-out/voltage-in since the volt difference is wasted. In contrast, a switched-mode power supply changes output voltage and current by switching ideally lossless storage elements, such as inductors and capacitors, between different electrical configurations. Ideal switching elements have no resistance when "on" and carry no current when "off", so converters with ideal components would operate with 100% efficiency. For example, if a DC source, an inductor, a switch, the corresponding electrical ground are placed in series and the switch is driven by a square wave, the peak-to-peak voltage of the waveform measured across the switch can exceed the input voltage from the DC source.
This is because the inductor responds to changes in current by inducing its own voltage to counter the change in current, this voltage adds to the source voltage while the switch is open. If a diode-and-capacitor combination is placed in parallel to the switch, the peak voltage can be stored in the capacitor, the capacitor can be used as a DC source with an output voltage greater than the DC voltage driving the circuit; this boost converter acts like a step-up transformer for DC signals. A buck–boost converter works in a similar manner, but yields an output voltage, opposite in polarity to the input voltage. Other buck circuits exist to boost the average output current with a reduction of voltage. In an SMPS, the output current flow depends on the input power signal, the storage elements and circuit topologies used, on the pattern used to drive the switching elements; the spectral density of these switching waveforms has energy concentrated at high frequencies. As such, switching transients and rip
A mercury-arc valve or mercury-vapor rectifier or mercury-arc rectifier is a type of electrical rectifier used for converting high-voltage or high-current alternating current into direct current. It is a type of cold cathode gas-filled tube, but is unusual in that the cathode, instead of being solid, is made from a pool of liquid mercury and is therefore self-restoring; as a result, mercury-arc valves were much more rugged and long-lasting, could carry much higher currents than most other types of gas discharge tube. Invented in 1902 by Peter Cooper Hewitt, mercury-arc rectifiers were used to provide power for industrial motors, electric railways and electric locomotives, as well as for radio transmitters and for high-voltage direct current power transmission, they were the primary method of high power rectification before the advent of semiconductor rectifiers, such as diodes and gate turn-off thyristors in the 1970s. These solid state rectifiers have since replaced mercury-arc rectifiers thanks to their higher reliability, lower cost and maintenance and lower environmental risk.
In 1882 Jemin and Meneuvrier observed the rectifying properties of a mercury arc. The mercury arc rectifier was invented by Peter Cooper Hewitt in 1902 and further developed throughout the 1920s and 1930s by researchers in both Europe and North America. Before its invention, the only way to convert AC current provided by utilities to DC was by using expensive and high-maintenance rotary converters or motor-generator sets. Mercury-arc rectifiers or "converters" were used for charging storage batteries, arc lighting systems, the DC traction motors for trolleybuses and subways, electroplating equipment; the mercury rectifier was used well into the 1970s, when it was replaced by semiconductor rectifiers. Operation of the rectifier relies on an electrical arc discharge between electrodes in a sealed envelope containing mercury vapor at low pressure. A pool of liquid mercury acts as a self-renewing cathode; the mercury emits electrons whereas the carbon anodes emit few electrons when heated, so the current of electrons can only pass through the tube in one direction, from cathode to anode, which allows the tube to rectify alternating current.
When an arc is formed, electrons are emitted from the surface of the pool, causing ionization of mercury vapor along the path towards the anodes. The mercury ions are attracted towards the cathode, the resulting ionic bombardment of the pool maintains the temperature of the emission spot, so long as a current of a few amperes continues. While the current is carried by electrons, the positive ions returning to the cathode allow the conduction path to be unaffected by the space charge effects which limit the performance of vacuum tubes; the valve can carry high currents at low arc voltages and so is an efficient rectifier. Hot-cathode, gas discharge tubes such as the thyratron may achieve similar levels of efficiency but heated cathode filaments are delicate and have a short operating life when used at high current; the temperature of the envelope must be controlled, since the behaviour of the arc is determined by the vapor pressure of the mercury, which in turn is set by the coolest spot on the enclosure wall.
A typical design maintains a mercury vapor pressure of 7 millipascals. The mercury ions emit light at characteristic wavelengths, the relative intensities of which are determined by the pressure of the vapor. At the low pressure within a rectifier, the light appears pale blue-violet and contains much ultraviolet light; the construction of a mercury arc valve takes one of two basic forms — the glass-bulb type and the steel-tank type. Steel-tank valves were used for higher current ratings above 500 A; the earliest type of mercury vapor electric rectifier consists of an evacuated glass bulb with a pool of liquid mercury sitting in the bottom as the cathode. Over it curves the glass bulb, which condenses the mercury, evaporated as the device operates; the glass envelope has one or more arms with graphite rods as anodes. Their number depends on the application, with one anode provided per phase; the shape of the anode arms ensures that any mercury that condenses on the glass walls drains back into the main pool to avoid providing a conductive path between the cathode and respective anode.
Glass envelope rectifiers can handle hundreds of kilowatts of direct-current power in a single unit. A six-phase rectifier rated 150 amperes has a glass envelope 600 mm high by 300 mm outside diameter; these rectifiers will contain several kilograms of liquid mercury. The large size of the envelope is required due to the low thermal conductivity of glass. Mercury vapor in the upper part of the envelope must dissipate heat through the glass envelope in order to condense and return to the cathode pool; some glass tubes were immersed in an oil bath to better control the temperature. The current-carrying capacity of a glass-bulb rectifier is limited by the fragility of the glass envelope and by the size of the wires fused into the glass envelope for connection of the anodes and cathode. Development of high-current rectifiers required leadwire materials and glass with similar coefficients of thermal expansion in order to prevent leakage of air into the envelope. Current ratings of up to 500 A had been achieved by the mid-1930s, but most rectifiers for current ratings above this were realised using the more robust steel-tank design.
For larger valves, a steel tank with ceramic insulators for
A thyratron is a type of gas-filled tube used as a high-power electrical switch and controlled rectifier. Thyratrons can handle much greater currents than similar hard-vacuum tubes. Electron multiplication occurs when the gas becomes ionized, producing a phenomenon known as Townsend discharge. Gases used include mercury vapor, xenon and hydrogen. Unlike a vacuum tube, a thyratron cannot be used to amplify signals linearly. In the 1920s, thyratrons were derived from early vacuum tubes such as the UV-200, which contained a small amount of argon gas to increase its sensitivity as a radio signal detector, the German LRS relay tube, which contained argon gas. Gas rectifiers, which predated vacuum tubes, such as the argon-filled General Electric "Tungar bulb" and the Cooper-Hewitt mercury-pool rectifier provided an influence. Irving Langmuir and G. S. Meikle of GE are cited as the first investigators to study controlled rectification in gas tubes, about 1914; the first commercial thyratrons appeared circa 1928.
The term "thyristor" was derived from a combination of "thyratron" and "transistor". Since the 1960s thyristors have replaced thyratrons in most low- and medium-power applications. Thyratrons resemble vacuum tubes both in appearance and construction but differ in behavior and operating principle. In a vacuum tube, conduction is dominated by free electrons because the distance between anode and cathode is small compared to the mean free path of electrons. A thyratron, on the other hand, is intentionally filled with gas so that the distance between anode and cathode is comparable with the mean free path of electrons; this means. Due to the high conductivity of plasma, a thyratron is capable of switching higher currents than vacuum tubes which are limited by space charge. A vacuum tube has the advantage that conductivity may be modulated at any time whereas a thyratron becomes filled with plasma and continues to conduct as long as a voltage exists between the anode and cathode. A pseudospark switch operates in a similar regime of the Paschen curve as a thyratron and is sometimes called a cold cathode thyratron.
A thyratron consists of a hot cathode, an anode, one or more control grids between the anode and cathode in an airtight glass or ceramic envelope, filled with gas. The gas is hydrogen or deuterium at a pressure of 300 to 500 mTorr. Commercial thyratrons contain a titanium hydride reservoir and a reservoir heater that together maintain gas pressure over long periods regardless of gas loss. Conductivity of a thyratron remains low as long as the control grid is negative relative to the cathode because the grid repels electrons emitted by the cathode. Space charge limited electron current flows from the cathode through the control grid toward the anode if the grid is made positive relative to the cathode. Sufficiently high space charge limited current initiates Townsend discharge between anode and cathode; the resulting plasma provides high conductivity between anode and cathode and is not limited by space charge. Conductivity remains high until the current between anode and cathode drops to a small value for a sufficiently long time that the gas ceases to be ionized.
This recovery process takes 25 to 75 μs and limits thyratron repetition rates to a few kHz. Low-power thyratrons were manufactured for controlling incandescent lamps, electromechanical relays or solenoids, for bidirectional counters, to perform various functions in Dekatron calculators, for voltage threshold detectors in RC timers, etc. Glow thyratrons were optimized for high gas-discharge light output or phosphorized and used as self-displaying shift registers in large-format, crawling-text dot-matrix displays. Another use of the thyratron was in relaxation oscillators. Since the plate turn-on voltage is much higher than the turn-off voltage, the tube exhibits hysteresis and, with a capacitor across it, it can function as a sawtooth oscillator; the voltage on the grid controls the breakdown voltage and thus the period of oscillation. Thyratron relaxation oscillators were used in power inverters and oscilloscope sweep circuits. One miniature thyratron, the triode 6D4, found an additional use as a potent noise source, when operated as a diode in a transverse magnetic field.
Sufficiently filtered for "flatness" in a band of interest, such noise was used for testing radio receivers, servo systems and in analog computing as a random value source. The miniature RK61/2 thyratron marketed in 1938 was designed to operate like a vacuum triode below its ignition voltage, allowing it to amplify analog signals as a self-quenching superregenerative detector in radio control receivers, was the major technical development which led to the wartime development of radio-controlled weapons and the parallel development of radio controlled modelling as a hobby; some early television sets British models, used thyratrons for vertical and horizontal oscillators. Medium-power thyratrons found applications in machine tool motor controllers, where thyratrons, operating as phase-controlled rectifiers, are utilized in the tool's armature regulator and in the tool's field regulator. Examples include Monarch Machine Tool 10EE lathe, which used thyratrons from 1949 until solid-state devices replaced them in 1984.
High-power thyratrons are still manufactured, are capable of operation up to tens of kiloamperes and tens of kilovolts. Modern applications include p