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 resonant–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.