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An SCR can be turned off in a DC circuit using Forced Commutation techniques. This is the Class B commutation circuit, which uses an LC circuit to commutate the SCR. An LC circuit placed parallel to the SCR helps turn it off. Initially, when the SCR is off, the capacitor charges up to the supply voltage through the inductor and load. When the SCR is triggered, the capacitor first discharges through the SCR and then charges into the opposite polarity (due to the inductor which forms a resonant circuit with the capacitor). Due to this, a negative voltage appears across the SCR and it's current reduces to zero, turning it off. The capacitor charges back to the supply voltage and this continues.
Care must be taken that the LC current must be greater than the load current in order to turn the SCR off. And the pulses triggering the SCR must be far enough apart so that it isn't triggered on way too soon, or else it will never be able to turn off again.
Such communication circuits are needed when an SCR is used as a DC chopper.
SCR model http://everycircuit.com/circuit/5270816124829696
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