Thyristors are semiconductors that function as bistable switches (switches equipped with dual stable states that can be used for switching all types of electric loads). This 4-layer device has a gate, cathode, and anode, which are known as terminals. The current that flows between the cathode and anode are controlled by the gate. The 4 layers consist of alternating P & N type materials.
Thyristors are unidirectional. They may be lightweight and small, but they are capable of protecting both the current and voltages. Thyristors are actually similar to diodes, at least in terms of characteristics.
Different States of Thyristors
Thyristors have three states: forward conducting mode, forward blocking mode, and reverse blocking mode.
- Forward conducting mode is when voltage is applied and the thyristor is triggered. As a result, the voltage is conducted until it dips low – low enough to reach the holding current point.
- In the forward blocking mode, the voltage is directed to a particular direction. This will lead to diodes conducting a current.
- The reverse blocking mode is a bit similar to forward blocking mode except that after voltage is applied, the diodes will block the currents.
Types of Thyristors
There are five primary types of Thyristors:
- Inverter thyristors
- Phase control thyristors
- Asymmetrical thyristors
- Light-triggered thyristors
- Gate turn-off thyristors
Inverter thyristors are commonly used for high-speed switching applications. These DC power supply-operated thyristors has inverse functions capable of producing different voltages. Turn-off and turn-on times are fast.
Phase control thyristors are ideal for some power transmission applications, resistance welding, DC drives, and other power frequency applications. They are not fast-switching.
Asymmetrical thyristors, also known as ASCR, are made for applications with low reverse voltage and forward voltage that falls anywhere between 400 Volts and 2000 Volts.
Light-triggered thyristors are known as LTTs or photothyristors. They react to optically-produced excess carriers.
Gate turn-off thyristors or GTOs are ideal for an over 400 Ampere current or more than 2500 Volt voltage applications.