Circuit for producing turn-on signal for a silicon controlled rectifier (SCR), which signal abruptly and automatically returns to a lower level at which the SCR can commutate off upon reversal of its anode voltage. When a switch opens, a capacitor starts to charge and when the charge reaches a given level, a normally reverse biased transistor turns on. In response to the turn on of the transistor, a turn-on signal is applied via a second capacitor to the gate electrode of the SCR. After a short interval, a second transistor responsive to base current flow through the first transistor, clamps the gate electrode of the SCR to said lower level. After turn off of the SCR, the switch closes and the second capacitor discharges through a circuit including the gate-to-cathode path of the SCR, reverse biasing the gate of the SCR and holding the latter off.