A power thyristor is provided with a pair of antiseries back to back diode junctions forming an open base transistor thermally coupled to the thyristor and electrically connected between the thyristor gate and cathode. The diode junctions shunt gate current above a given sensed temperature of the power thyristor, for protecting the latter by preventing turn-on thereof. The diode junctions have nonsymmetrical leakage current characteristics to insure protective shunting of forward gate drive in one direction but blocking leakage in the other direction to prevent draining a reverse charged phasing capacitor. The latter is thus charged beginning from the same starting reference point in each cycle whereby to insure accurate phase control in an AC gating system.