A thyristor is a four-layer semiconductor device with alternating P-type and N-type (PNPN) materials. A thyristor also consists of three terminals: anode, cathode, and gate (also known as the control terminal).
The most common type of thyristor is the Silicon Controlled Rectifier (SCR). If the cathode is negatively charged with respect to the anode, no current will flow until the gate is pulsed. The SCR then conducts current until the voltage between the cathode and anode reverses or falls below a certain threshold or hold value. This type of thyristor allows a small trigger current or small trigger voltage to switch or control a large output.
Working of Thyristor
A thyristor works like a diode. A junction is formed by sandwiching two semiconductor layers of p-type and n-type. The anode is connected to the outer p-layer, the cathode is connected to the outer n-layer, and the gate is connected to the inner p-layer. It has three junctions namely J1, J2, and J3 as shown in the figure below.
When the anode is at a positive potential with respect to the cathode, no voltage is applied to the gate. Junctions J1 and J3 are forward-biased and J2 is reverse-biased. Hence, no conduction occurs.
Now, when the positive potential increases beyond the breakdown voltage, junction J2 breaks down and begins to conduct. Once breakdown occurs, it continues to conduct regardless of the gate voltage until the potential at the anode is removed or the current through the device is less than the holding current. Junction J2 breaks down when a positive potential is applied to the gate connection compared to the cathode. To turn on the thyristor quickly, a suitable potential must be chosen.
The gate acts as a control electrode. A small voltage, called a gate pulse, is applied to its gate to trigger the device into a conducting state. This continues until the voltage across the device is reversed or removed.
The gate trigger current varies inversely with the gate voltage and requires a minimum gate charge to trigger it. Therefore, thyristor switching can be controlled via gate pulses.
Symbol of Thyristor
Modes of Thyristor
Thyristor has three operating modes which are given below
- Forward Blocking
- Reverse Blocking
- Forward Conducting
Forward Blocking
In this operating mode, the forward current conduction is blocked. The top and bottom diodes are forward-biased and the middle junction is reverse-biased. Therefore the thyristor will not turn on. This is because the gate will not fire and no current will flow.
Reverse Blocking
In this operating mode, the anode and cathode connections are reversed and no current still flows. A thyristor can only conduct current in one direction and block it in the opposite direction, thus blocking current flow.
Forward Conducting
When current is applied to the gate, the thyristor is triggered, and conducting will start. It remains on until the forward current drops below the threshold. This can be achieved by turning off the circuit.
Types of Thyristor
Three main types of thyristors are given below:
- Silicon Controlled Rectifier (SCR)
- Gate Turn Off Thyristor (GTO)
- MOS-controlled thyristor (MCT)
Applications of Thyristor
- Variable Speed Motor Drives
- Camera Flashes
- Timer Circuits
- Burglar Alarms
- Electric Fan Speed Control
- Car Ignition Switches.