A Thyristor: What Is It?

A thyristor, also known as a silicon-controlled rectifier (SCR), is referred to as a semi-controlled device since a signal may be used to regulate its conduction but not its turn-off. Although a basic form of the thyristors are often referred to by the name “thyristor,” the term also widely refers to a wide range of derivative devices, including the Tri-Electrode AC switch (TRIAC), Fast Switching Thyristor (FST), Reverse-Conducting Thyristor (RCT), and Light-Triggered Thyristor (LTT).

I. Composition

Bolt-type and plate-type high-power thyristors appearance structures are now most often employed. It is made up of four layers and three PN connections. The figure depicts its look, organizational structure, and visual symbols.

A PNPN four-layer semiconductor structure with the letters P1, N1, P2, and N2 is found within the thyristor. Anode A and cathode K are the two electrodes taken from the outermost P and N layers, respectively, while gate electrode G is the electrode derived from the middle P2 layer (also called the control electrode). The four areas combine to create J1, J2, and J3 PN junctions. A thyristor may thus be compared to three PN junctions connected in series. The thyristor is known by its scientific name, Thyristor, which is shortened to VT.

A radiator must be fitted since thyristors are power electrical devices that produce heat as a result of loss while operating. The anode is often the bolt in the bolt type package, which is designed to be simple to install and snugly linked to the radiator. For natural cooling, the anode (bolt) is attached to the aluminum radiator; the flat thyristor is made up of two Thyristors clamped by a pair of mutually insulated radiators that receive cold air cooling.

The plate type is used in thyristors with rated currents larger than 200A because it effectively dissipates heat on both sides of the device. Additionally, it may be cooled using cooling techniques like oil and water cooling.

2. The Operating Principle

The thyristor is a powerful electrical device with a semi-controlled operating environment.

1. No matter what voltage the gate carries, a thyristor is in the reverse blocking condition when it is exposed to the reverse anode voltage.
2. The thyristor will only switch on when the gate is exposed to a positive voltage when the thyristor is subjected to a positive anode voltage. The thyristors are now in the forward conduction condition, which is its controlled characteristic or thyristor characteristic.
3. Once the thyristor is switched on, the gate loses its purpose since the thyristor stays on as long as there is a certain positive anode voltage, independent of the gate voltage. Only the gate acts as a trigger.
4. After the thyristor is switched on, it shuts off when the voltage (or current) of the main circuit approaches zero.

To demonstrate the thyristor’s operation, do a simple experiment using the circuit shown in the image below. The power supplies, the incandescent light, and the thyristor’s anode and a cathode made constitute the device’s primary circuit. The components of a control circuit, also known as a trigger circuit, are the power supply Uc, the switch S, the thyristor’s gate, and the cathode.

1. Gate Turn-Off Thyristor – KG

When a regular thyristor is activated by the positive trigger signal and the control electrode G, the control electrode G ceases to be functional. Anode A and cathode K’s positive voltages must be zero or higher to switch off the standard thyristor. Put on a negative charge. By providing a suitably strong and broad negative trigger current to the control electrode G, the turn-off thyristor may be efficiently switched off.

Usage: Common applications include chopper speed regulation, variable frequency speed regulation, inverter power supplies, and DC control circuits that turn on and off DC loads such as relay coils, solenoids, electromagnetic clutches, DC motors, etc.

2. KS-based bidirectional controlled rectifier

Even if a positive trigger signal is provided to the control electrode G and a negative voltage is connected to the anode A and cathode K, an ordinary thyristor will not switch on. You may turn on the Tri-Electrode AC switch. The voltage between anode A and cathode K has to be zero to turn the bidirectional thyristor off.

Usage: Voltage regulators, inverters, AC/DC load switches, etc. are often used in AC/DC control electronic component distributors.