GB1361677A - Device for delivering a firing voltage for semiconductor switches - Google Patents

Abstract

1361677 Triggering circuits for controlled rectifiers HOLZER PATENT AG 31 Aug 1971 [17 Sept 1970] 40598/71 Heading H2F A circuit for producing a train of trigger pulses in synchronism with an A.C. supply (e.g. for triggering a triac 3) at a phase relative to the A.C. supply, adjustable over substantially a full cycle in dependence on an input signal, has means for producing an additional trigger pulse after any trigger pulse which is produced at more than 180 degrees before the end of a cycle of the A.C. supply, the additional pulse being timed to coincide with the passage through zero of the A.C. supply voltage following that pulse. The triac 3 can thus provide cycles of current the duration of which may be adjustable from a full wave to a small part of a half wave. Alternatively, the triac 3 may be replaced by a rectifying arrangement. As described, each positive half cycle of the A.C. supply is limited by a Zener diode 6 to provide a trapezoidal waveform which is rectified at 7 and smoothed at 8 to form a D.C. supply used to charge capacitor 19 via resistor 18 so as to provide the sawtooth wave (Fig. 3d). Transistor 9 of a reciprocallycoupled complementary pair of transistors 9, 10 is turned on at a point in each sawtooth determined by a D.C. control voltage at terminal 2, so as to produce a pulse to trigger the triac 3. The trapezoidal wave (Fig. 3a) is differentiated by capacitors such as 32, to provide positive and negative pulses (Figs. 3e and 3b) at the bases of transistors 26, 17, respectively. The negative pulses turn on transistor 17 which allows diode 25 to discharge capacitor 19 at the end of each positive half-cycle thus terminating each cycle of the sawtooth wave. If transistors 9, 10 have generated a pulse during the negative half-cycle, the positive pulse will turn on transistor 26 so that diode 25 will trigger transistors 9, 10 to generate a further triggering pulse for triac 3 thus ensuring that it is conductive during the whole of the positive half-cycle. Transistors 9, 10 may be replaced by a four-electrode siliconcontrolled switch.