The thyristor is also known as the silicon controlled rectifier (S.C.R.).
It has the same charactaristics as the diode, current flowing from cathode to anode, when the anode is positive with respect to the cathode. However, it will only do this when the gate is also positive with respect to the cathode.
In the circuit, with the switch open as shown, no current flows. When the switch is closed, the diode begins to conduct and current flows from cathode to anode. There is a problem. If the switch is now opened, current continues to flow.
Conduction can be stopped by removing the the cathode/anode voltage.
Another method of stopping current flow is to reverse the polarity of the cathode/anode voltage.
If the thyristor is used with an ac supply then it will conduct on the positive half cycles and automatically switch off during the negative half cycles.
The resistor in series with the gate connection limits the gate current to a safe value.
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