Class B: Self Commutated by an L-C Circuit
The Capacitor C charges up in the dot as positive before a gate pulse is applied to the SCR.
When SCR is triggered, the resulting current has two components. The constant load current Iload flows through R - L load. This is ensured by the large reactance in series with the load and the freewheeling diode clamping it. A sinusoidal current flows through the resonant L- C circuit to charge-up C with the dot as negative at the end of the half cycle. This current will then reverse and flow through the SCR in opposition to the load current for a small fraction of the negative swing till the total current through the SCR becomes zero. The SCR will turn off when the resonantâ€“circuit (reverse) current is just greater than the load current. The SCR is turned off if the SCR remains reverse biased for tq >toff, and the rate of rise of the reapplied voltage < the rated value.