In Vol. III of Semiconductors in the Thryistors section, it discusses the use of a rectifier diode to be placed before the gate of a silicon controlled rectifier when the SCR has a gate-cathode resistor. Other than preventing reverse flow of current through the noted gate-cathode resistor, what other purpose does this protection diode perform? Does it prevent damage to the gate junction itself via reverse voltage/current, or is it simply to stop reverse flow through the gate for other reasons not noted? I have used both sensitive gate SCR's and SCR's with a gate-cathode resistor, and I am having trouble finding information about my question noted above. Can anyone help me out on this one?

Thanks!

 

The usual reason for such a diode is to prevent excess reverse voltage on the SCR gate junction above its rate maximum which could damage the junction.

 

They also prevent the high anode voltage from feeding back down the gate driver circuit.