temperature regulator with zero crossing switch

elektor july/august 1985 This temperature regulator can be built without special ICs and may be used with powers up to 3.5 kVA. The circuit is based on a two-point regulator with a thermistor as the tem- perature sensor. As the load current is switched only during zero crossing of the mains, no additional interference suppression is necessary. The series combination R1C1serves to lower the mains voltage to a level suitable as supply voltage for trigger T1. As Ri is small compared with the reactance of C1, the current leads the voltage by nearly 90°. If the ambient temperature is higher than a given value, determined by potentiometer P1, the resistance of Rth is low enough to cause T1to con- duct. Silicon controlled rectifier Th1is supplied with gate current and switches on during the negative half cycle of the mains, because the cur- rent through R1C1leads the voltage. When Th1is on, thyristors Th2and Th3remain in the blocked state, so that no current flows through heating elemen...