Electrical Revolution: Resistance welding using Triac

The power circuit for the resistance welding using TRIAC – DIAC is shown in the figure A.
The current passes through transformer primary winding when TRIAC is turned on.
As soon as the alternating supply is on, the charging of capacitor is done through path AC supply – R – C.
When the voltage across capacitor becomes equal to DIAC break over voltage, the DIAC turns on.
Now the gate of the TRIAC gets pulse and resulting TRIAC turns on.
The current starts to flow through primary winding of the transformer.
The control of current is done by variable resistor R.
The firing angle of the TRIAC increases when the setting of variable resistor is made at higher value.
The current passes through transformer primary winding decreases as the firing angle of the TRIAC increases.
The transient voltage generated due to switching on the TRIAC and this voltage may damage the transformer primary winding.
The thyride resistor is connected across transformer primary winding in order to suppress transient voltage.
The resistor of the thyride resistor decreases as the voltage increases and this will protect the transformer primary.

Electrical Revolution