A Thyristor is a semiconductor component. They are similar in performance to other solid-state silicon components such as diodes and transistors. Thyristors are also switching devices like transistors.


A thyristor is a four-layer three-junction semiconductor switching device. It has three terminals, anode, cathode, and gate. It is a unidirectional device, like a diode, which currently only flows in one direction. Although it consists of three PN junctions connected in series and consists of four layers. The Gate terminal is used to trigger the SCR by applying a small current to this terminal, which we also called the gate trigger method to turn on the SCR.

The first thyristor equipment was used commercially in 1956. Because thyristors can control relatively large power and voltage with a small device, they have wide applications in electrical control, ranging from dimmers and motor speed control to high voltage—DC power transmission.

Fast switching thyristor is a fast disconnected semiconductor device called “inverter thyristor”, “fast thyristor”. Fast switching thyristor is an important component of the thyristor in the furnace power supply. It is a four-layer, three-terminal semiconductor device, each of which is alternately N-type or P-type material.

The thyristors are used in power-switching circuits, relay replacement circuits, inverter circuits, generator circuits, level sensor circuits, light-dimming circuits, low-cost timer circuits, speed-control circuits, phase circuits. Control schemes, etc.

Working Principle of Thyristor

The thyristor acts like a diode. It consists of two layers of semiconductors, namely p-type and n-type, sandwiched together to form a junction. The anode is connected to the p-layer, the cathode is connected to the n-layer, and the gate is connected to the player. At the same time, it has three junctions, which are J1, J2, and J3.

It is used in AC circuits. In an Alternating Current circuit, the forward current drops to zero during each cycle, so there will always be a trip function. However, this means that the gate must be started every cycle to re-enable it. In the relative synchronization of these two functions, the thyristor plays the most important role, that is, power regulation.

A thyristor is a two-position switch for controlling the power output of an electrical circuit by turning the load circuit on and off at regular intervals. While P (cathode) and N (anode) connected in series, so we get three contact contacts: anode, gate, and cathode.

When we move forward, the anode and cathode, that is, the anode and cathode connected to the positive and negative terminals of the battery, the first PN junction, and the last PN junction (j1 and j3), become forward biased due to depletion layer destruction. Junction j2 remains biased in the opposite direction because no current applied to the gate.

When we apply current to the gate, the j2 layer begins to break, and the current begins to flow in the circuit. When enough positive signal current or pulse applied to the gate pin, it causes the thyristor to conduct. It can only be fully turned on and off. This makes the thyristor unsuitable as an analog amplifier but can use as a switching device.

Types of Thyristors

1: SCR or Silicon controlled Rectifier

2: GTO or Gate turn off thyristor

3: ETO or Emitter turn off thyristor

4: RCT or Reverse conducting thyristor

5: LASCR or Light-activated silicon-controlled rectifier

1: SCR or Silicon controlled Rectifier

SCR or Silicon controlled rectifier is also called thyristor rectifier. It is normally OFF, but it goes ON when a small current applied to its G gate. SCRs can only conduct current in one direction of unidirectional devices.

SCRs can normally be triggered by a current applied to the gate terminal. If the gate current removed, the thyristor remains in the ON state, and to turn it off, it is necessary to remove the current between the anode and the cathode or set a negative voltage at the anode concerning the cathode.

The current flows in only one direction from the anode to the cathode. SCRs commonly used in switching circuits, phase control circuits, inverting circuits, VFD, etc.

2: GTO or Gate turn off thyristor

GTO or Gate Turn off is a special type of high-power semiconductor device. The shutter controls the limit switch for on and off. The GTO turns on like any normal thyristor by applying a positive gate voltage. However, it can be closed by applying a negative gate voltage.

It is a non-leaching device; At least 1% plus is required to maintain delivery status. A thyristor gate or GTO is a three-pole bipolar (currently controlled minority carrier) semiconductor switching device. As with a conventional thyristor, the leads are the anode, cathode. As the name suggests, it can be disabled.

This gate can switch on the main current and switch it off through the excitation circuit. A small positive gate activates an existing GTO in delivery mode and can also close it with a negative gate pulse.

If a positive pulse applied between the cathode and the gate leads, the device would turn on. The cathode and gate lead behave like a PN junction, and there is relatively little voltage between the leads. It is as unreliable as SCR. If a negative voltage pulse applied between the gate and cathode leads, the device would shut down.

Some forward currents stole to stimulate the gate cathode voltage, which can cause the forward current to drop, and the GTO will automatically switch to a locked state.

3: ETO or Emitter turn off thyristor

ETO the Emitter Turn Off Thyristor is a type of thyristor that uses a MOSFET to turn on and off. ETO is the first in a new family of high power devices suitable for high-performance power conversion systems (PCS), an important part of energy storage systems.

.It combines the advantages of GTO and MOSFET. It has two gates – one normal for on and one with a serial MOSFET for off. ETO devices can have high switching frequencies and low damping requirements, which can increase the system’s dynamic response and reduce its cost and size.

4: RCT or Reverse conducting thyristor

RCT Reverse conduction thyristor is also called feedback thyristor. RCT – a reverse conduction thyristor differs from a conventional high-power thyristor by a built-in reverse conduction diode. Due to the flyback diode, this thyristor cannot reverse blocking.

RCT is beneficial where a freewheel or free-wheeling diode must be used. Since the SCR and diode never conduct simultaneously, they do not generate heat at the same time and can be easily combined and cooled together.

The diode is connected back in parallel between the anode and cathode of the thyristor so that the transmitting junction of the anode and cathode is short-circuited. Due to this, a special circuit structure has high voltage resistance, high-temperature resistance, short turn-off time, low switching voltage, and other good characteristics.

5: LASCR or Light-activated silicon-controlled rectifier

LASCR Light-Activated Silicon Controlled Rectifier is a silicon rectifier that conducts current when the gate is exposed to light. LASCR a Photo SCR is just a regular SCR, except that it can also be light-triggered.

The gate still functions as a regular gate in an SCR but remains disabled in many cases. LASCR is also a unidirectional device that conducts current in one direction only. Most LASCRs also has a gate terminal for triggering an electrical pulse, just like a regular SCR. Positive feedback starts and the LASCR turns on, and the LASCR will continue to conduct even if the light source is removed.

LASCRs find many applications, including optical light control, relays, phase control, motor control, and a wide variety of computer applications.

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