1361098 Control systems for consuming devices GENERAL ELECTRIC CO 31 Jan 1972 [12 Feb 1971] 4397/72 Heading H2H [Also in Divisions H1 and H3] In a circuit wherein the current flowing in a load 6 is controlled by means of a triac 8, the trigger pulse for the gate 12 of the triac being produced by a capacitor 14, charged by way of a variable resistor 16 and a breakdown device 10 has the device 10 formed as a monolithic structure which comprises two pairs of complementary transistors Q 1 , Q 2 and Q 3 , Q 4 , a plurality of Zener diodes, say D 1 , D 2 , D 3 and a pair of resistors R 1 , R 2 . In operation, as the capacitor 14 charges the device 10 is initially non-conductive and thus presents a high impedance between terminals 15 and 12. When the capacitor acquires a voltage at terminal 15 equal to the breakdown voltage of Zener diode D 1 and the forward bias voltage of the base-emitter junction of transistor Q 2 , the pair of transistors Q 1 , Q 2 become regenerative in the PNPN mode thus going into a high conduction state whereupon the device 10 presents a low impedance between terminals 15, 12. Capacitor 14 then discharges through the device 10 into the gate of the thyristor to cause turn-on thereof. In opposite half cycles of the applied A.C. when the capacitor 14 chargeswith the opposite polarity the transistors Q 3 , Q 4 function as the regenerative switching pair. With a trigger control device 10 of the kind shown in Fig. 6 the Zener diode D 3 ensures that the capacitor 14 only discharges in alternate half cycles to the breakdown voltage of the diode, thus ensuring that the triac 8 fires at the identical point in every half cycle of the A.C. supply.