In thyristor converters high ac voltages exists between anode and cathode of the thyristor, while low voltage level pulses are placed between gate and cathode. Isolation is necessary between the gate-cathode circuit and the anode-cathode circuit. This isolation is accomplished with the use of:
- Optocouplers and
- Pulse transformers.
In the case of optocoupler isolation, the low voltage gate drive circuit is optically isolated from the high voltage anode-cathode circuit as shown in figure 4.22.
Figure 4.22 Optical Isolation Using Photo SCR
In the above circuit, the gate drive circuit is connected to the light emitting diode D1 via a current limiting resistor R1. Pulses sent to the light emitting diode D1 turns on the photo SCR T1 which in turn triggers the power thyristor TL. Hence the gate drive circuit is optically isolated from the output circuit. Instead of using an optocoupler, a pulse transformer could be used to magnetically isolate the gate drive circuit from the anode-cathode circuit.