Synchronous control of a static switch - overcomes parasitic effects at zero crossing by superposition of ramp and train of trapezoidal waveforms

摘要

The control circuitry for solid state switches, such as triacs initiates the switching operation on a change in value of some parameter such as temperature. The parameter is represented by the impedance of a thermistor. The circuit avoids parasitic effects at the change of state of the switches. Trigger pulses to the gate of the triac are derived from the superposition of a train of trapezoidal signals on a ramp waveform, the overall level rising to the trigger level. The amplitude of the trapezoidal waveform depends on the state of the thermistor which is connected between earth and the collector of an earthed-emitter transistor whose base receives a half-wave rectified supply. The output is fed in parallel with the ramp waveform to a trigger generator and the triac.