Modulated triac zero-point switch eliminates electromagnetic interference generated by phase control while providing proportional ON/ OFF switching for accurate temperature regulation of heater load. Circuit at right of dashed line is basic zero-point switch that tums triac on almost immediately after each zero crossing between half-cycles. R7 is set so bridge in control circuit is balanced at desired temperature.When temperature overshoots, thermistor RT decreases in resistance and Q2 tums on to providegate drivefor SCR Q3. Q3 then tums on and shunts gate signal away from triac Q4, to remove power from load. When temperature drops, Q2 and Q3 tum off and full-wave power is applied to load. Modulation is achieved by supelimposing sawtooth voltage from Q1 on one arm of bridge through R3, With sawtooth period equal to 12 cycles ol line frequency. From 1 to 12 of these cycles can be applied to load for modulating power in 8% steps from 0% to 100% duty cycle.-"Circuit Applications for the Triac," Motorola, Phoenix, AZ, 1971, AN-466, p 13.
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