After the SCR turns on, CI charges up to almost the full supply voltage via R3 and the anode of the SCR. Whe
n S2 is subsequently closed, it clamps the positive end of CI to ground, and the charge on CI forces the anode of the SCR to swing negative momentarily, thereby reverse-biasing the SCR and causing it to turn off. The capacitor"s charge bleeds away rapidly, but it has to hold the SCR"s anode negative for only a few /is to ensure turn-off. CI must be a nonpolarized type.