Silicon controlled rectifier

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Silicon controlled rectifier
SCR de potencia.jpg
Silicon controlled rectifier
Type Passive
Working principleIan M. Mackintosh (Bell Laboratories)
InventedGordon Hall and Frank W. "Bill" Gutzwiller
First production General Electric, 1957
Pin configuration Anode, gate and cathode
Electronic symbol
Thyristor circuit symbol.svg
SCR 4-layer (p-n-p-n) diagram Thyristor layers.svg
SCR 4-layer (p-n-p-n) diagram

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, Goldey and Holonyak of Bell Laboratories in 1956. [1] 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. [2] [3] 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 [4] and commercialized by Frank W. "Bill" Gutzwiller in 1957.

Solid-state electronics circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material

Solid-state electronics means semiconductor electronics; electronic equipment using semiconductor devices such as semiconductor diodes, transistors, and integrated circuits (ICs). The term is also used for devices in which semiconductor electronics which have no moving parts replace devices with moving parts, such as the solid-state relay in which transistor switches are used in place of a moving-arm electromechanical relay, or the solid-state drive (SSD) a type of semiconductor memory used in computers to replace hard disk drives, which store data on a rotating disk.

Electric current flow of electric charge

An electric current is the rate of flow of electric charge past a point or region. An electric current is said to exist when there is a net flow of electric charge through a region. In electric circuits this charge is often carried by electrons moving through a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in an ionized gas (plasma).

General Electric American industrial company

General Electric Company (GE) is an American multinational conglomerate incorporated in New York City and headquartered in Boston. As of 2018, the company operates through the following segments: aviation, healthcare, power, renewable energy, digital industry, additive manufacturing, venture capital and finance, lighting, and oil and gas. GE has a subsidiary in Bermuda.


Some sources define silicon-controlled rectifiers and thyristors as synonymous, [5] 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. [6] [7] According to Bill Gutzwiller, the terms "SCR" and "controlled rectifier" were earlier, and "thyristor" was applied later, as usage of the device spread internationally. [8]

SCRs are unidirectional devices (i.e. can conduct current only in one direction) as opposed to TRIACs, which are bidirectional (i.e. charge carriers can flow through them in either direction). SCRs can be triggered normally only by currents going into the gate as opposed to TRIACs, which can be triggered normally by either a positive or a negative current applied to its gate electrode.

TRIAC generic trademark for a three-terminal thyristor that conducts current in either direction when triggered

TRIAC, from triode for alternating current, is a generic trademark for a three terminal electronic component that conducts current in either direction when triggered. Its formal name is bidirectional triode thyristor or bilateral triode thyristor. A thyristor is analogous to a relay in that a small voltage induced current can control a much larger voltage and current. The illustration on the right shows the circuit symbol for a TRIAC where A1 is Anode 1, A2 is Anode 2, and G is Gate. Anode 1 and Anode 2 are normally termed Main Terminal 1 (MT1) and Main Terminal 2 (MT2) respectively.

Modes of operation

Characteristic curve of a silicon-controlled rectifier Scr curve.jpg
Characteristic curve of a silicon-controlled rectifier

There are three modes of operation for an SCR depending upon the biasing given to it:

  1. Forward blocking mode (off state)
  2. Forward conduction mode (on state)
  3. Reverse blocking mode (off state)

Forward 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 (0) potential i.e. disconnected. In this case junction J1 and J3 are forward-biased, while J2 is reverse-biased, allowing only a small leakage current from the anode to the cathode. When the applied voltage reaches the breakover value for J2, J2 undergoes avalanche breakdown. At this breakover voltage J2 starts conducting, but below breakover voltage J2 offers very high resistance to the current and the SCR is said to be in the off state.

Forward conduction mode

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:

  1. Reduce the current through it below a minimum value called the holding current, or
  2. With the gate turned off, short-circuit the anode and cathode momentarily with a push-button switch or transistor across the junction.

Reverse blocking mode

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-biased diodes connected in series. A small leakage current flows. This is the reverse blocking mode. If the reverse voltage is increased, then at critical breakdown level, called the reverse breakdown voltage (VBR), 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. Usually, 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 typically has a reverse breakdown rating in the tens of volts. ASCRs are used where either a reverse conducting diode is applied in parallel (for example, in voltage-source inverters) or where reverse voltage would never occur (for example, in switching power supplies or DC traction choppers).

Asymmetrical SCRs can be fabricated with a reverse conducting diode in the same package. These are known as RCTs, for reverse conducting thyristors.

Thyristor turn-on methods

  1. forward-voltage triggering
  2. gate triggering
  3. dv/dt triggering
  4. temperature triggering
  5. light triggering

Forward-voltage triggering occurs when the anode–cathode forward voltage is increased with the gate circuit opened. This is known as avalanche breakdown, during which junction J2 will break down. 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 already forward-biased.

Biasing Predetermined voltages or currents establishing proper operating conditions in electronic components

Biasing in electronics means establishing predetermined voltages or currents at various points of an electronic circuit for the purpose of establishing proper operating conditions in electronic components.

In order for gate triggering to occur, the thyristor should be in the forward blocking state where the applied voltage is less than the breakdown voltage, otherwise forward-voltage triggering may occur. A single small positive voltage pulse can then be applied between the gate and the cathode. This supplies a single gate current pulse that turns the thyristor onto its on state. In practice, this is the most common method used to trigger a thyristor. [9] [10]

Simple SCR Circuit

A simple SCR circuit with a resistive load Simple scr circuit.png
A simple SCR circuit with a resistive load

A simple SCR circuit can be illustrated using an AC voltage source connected to a SCR with a resistive load. Without an applied current pulse to the gate of the SCR, the SCR is left in its forward blocking state. This makes the start of conduction of the SCR controllable. The delay angle α, which is the instant the gate current pulse is applied with respect to the instant of natural conduction (ωt = 0), controls the start of conduction. Once the SCR conducts, the SCR does not turn off until the current through the SCR, is, becomes negative. is stays zero until another gate current pulse is applied and SCR once again begins conducting. [11]


SCRs are mainly used in devices where the control of high power, possibly 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 also used in the control of welding machines, mainly GTAW (gas tungsten arc welding) processes similar. It is used as switch in various devices.

Comparison with SCS

A silicon-controlled switch (SCS) behaves nearly the same way as an SCR; but there are a few differences: Unlike an SCR, an SCS switches off when a positive voltage/input current is applied to another anode gate lead. Unlike an SCR, an SCS can also be triggered into conduction when a negative voltage/output current is applied to that same lead.

SCSs are useful in practically all circuits that need a switch that turns on/off through two distinct control pulses. This includes power-switching circuits, logic circuits, lamp drivers, counters, etc.

Compared to TRIACs

A TRIAC resembles an SCR in that both act as electrically controlled switches. Unlike an SCR, a TRIAC can pass current in either direction. Thus, TRIACs are particularly useful for AC applications. TRIACs have three leads: a gate lead and two conducting leads, referred to as MT1 and MT2. If no current/voltage is applied to the gate lead, the TRIAC switches off. On the other hand, if the trigger voltage is applied to the gate lead, the TRIAC switches on.

TRIACs are suitable for light-dimming circuits, phase-control circuits, AC power-switching circuits, AC motor control circuits, etc.

See also

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Rectifier AC-DC conversion device; electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction.

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Insulated-gate bipolar transistor three-terminal power semiconductor device

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Thyristor semiconductor device with three or more p-n junctions, having two steady states: off (non-conducting) and on (conducting)

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Thyratron Gas filled tube, electrical switch, rectifier

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DIAC diode that conducts current only after its breakover voltage has been reached momentarily

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Integrated gate-commutated thyristor

The integrated gate-commutated thyristor (IGCT) is a power semiconductor electronic device, used for switching electric current in industrial equipment. It is related to the gate turn-off (GTO) thyristor.

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Mercury-arc valve electrical equipment for converting high-voltage or -current alternating current into direct current

A mercury-arc valve or mercury-vapor rectifier or (UK) mercury-arc rectifier is a type of electrical rectifier used for converting high-voltage or high-current alternating current (AC) into direct current (DC). 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, and could carry much higher currents than most other types of gas discharge tube.

Crowbar (circuit) Type of electrical circuit

A crowbar circuit is an electrical circuit used for preventing an overvoltage condition of a power supply unit from damaging the circuits attached to the power supply. It operates by putting a short circuit or low resistance path across the voltage output (Vo), quite like were one to drop a crowbar across the output terminals of the power supply. Crowbar circuits are frequently implemented using a thyristor, TRIAC, trisil or thyratron as the shorting device. Once triggered, they depend on the current-limiting circuitry of the power supply or, if that fails, the blowing of the line fuse or tripping the circuit breaker.

Phase-fired controller

Phase-fired control (PFC), also called phase cutting or "phase angle control", is a method for power limiting, applied to AC voltages. It works by modulating a thyristor, SCR, triac, thyratron, or other such gated diode-like devices into and out of conduction at a predetermined phase of the applied waveform.

Solid-state relay

A solid-state relay (SSR) is an electronic switching device that switches on or off when a small external voltage is applied across its control terminals. SSRs consist of a sensor which responds to an appropriate input, a solid-state electronic switching device which switches power to the load circuitry, and a coupling mechanism to enable the control signal to activate this switch without mechanical parts. The relay may be designed to switch either AC or DC to the load. It serves the same function as an electromechanical relay, but has no moving parts.

Gate turn-off thyristor

A gate turn-off thyristor (GTO) is a special type of thyristor, which is a high-power semiconductor device. It was invented by General Electric. GTOs, as opposed to normal thyristors, are fully controllable switches which can be turned on and off by their third lead, the gate lead.

Quadracs are a special type of thyristor which combines a "diac" and a "triac" in a single package. The diac is the triggering device for the triac. Thyristors are four-layer (PNPN) semiconductor devices that act as switches, rectifiers or voltage regulators in a variety of applications. When triggered, thyristors turn on and become low-resistance current paths. They remain so even after the trigger is removed, and until the current is reduced to a certain level. Diacs are bi-directional diodes that switch AC voltages and trigger triacs or silicon-controlled rectifiers (SCRs). Except for a small leakage current, diacs do not conduct until the breakover voltage is reached. Triacs are three-terminal, silicon devices that function as two SCRs configured in an inverse, parallel arrangement. They provide load current during both halves of the AC supply voltage. By combining the functions of diacs and triacs, quadracs eliminate the need to buy and assemble discrete parts.

A Triggering device is an electronic circuit, such as a Schmitt trigger, which is used to control another electronic circuit.

Holding current (electronics)

The holding current (hypostatic) for electrical, electromagnetic and electronic devices is the minimum current which must pass through a circuit in order for it to remain in the 'ON' state. The term can be applied to a single switch or to an entire device. A simple example of holding current is in a Spark gap.


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  2. Vasseur, J. P. (2016-06-06). Properties and Applications of Transistors. Elsevier. ISBN   9781483138886.
  3. Twist, Jo (2005-04-18). "Law that has driven digital life". BBC News. Retrieved 2018-07-27.
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  5. Christiansen, Donald; Alexander, Charles; Jurgen, Ronald (2005). Standard Handbook of Electronic Engineering, 5th Edition. Mcgraw-hill. ISBN   9780071384216.
  6. International Electrotechnical Commission 60747-6 standard
  7. Dorf, Richard C. (1997-09-26). The Electrical Engineering Handbook,Second Edition. CRC Press. ISBN   9781420049763.
  8. Ward, Jack. "The Early History of the Silicon Controlled Rectifier". p. 7. Retrieved 12 April 2014.
  9. "Thyristor SCR Firing & Triggering |Circuit Design | Electronics Notes". Retrieved 2019-05-06.
  10. "The Silicon-Controlled Rectifier (SCR) | Thyristors | Electronics Textbook". Retrieved 2019-05-06.
  11. Mohan, Ned (2012). Power Electronics: A First Course. United States: Don Fowley. pp. 230–231. ISBN   978-1-118-07480-0.

Further reading