2.4 Electronic Control with Thermal Switch

A basic electronic control can be made using a thermal switch as the load handling device. Since the resistor heater on the thermal switch will operate with any waveform, the electronics can supply half wave DC or any phase angle fired DC. This simplifies design. Also, since the long bilateral time delay of the thermal switch effectively filters out short ON or OFF cycles, the electronics can be made with no hysterysis. In other words, the control's OFF/ON ratio can be 1:1 since the thermal switch is slower than the sunset or sunrise.

The first electronic control I made was in the early 1970's. It used a modified thermal switch and a GE light activated silicon controlled rectifier (LASCR). It worked well as long as you weren't too concerned about change in switch points with line voltage change or temperature change. It also made the control a RF noise generator in the AM radio band. Finally, it was expensive.

A simplified circuit is shown.

Electronic thermal controls had advantages over previous conventional controls. Historically the much higher prices and long delay kept them away from most utilities. During the early 1990's these controls came down in price from the $12-14 range to $6.00-10.00. This is in part because of reduced electronics costs. Even so, most utilities don't like the thermal control's 30-60 second time delay for ON & OFF and have found that there is a better method: electronic controls with DC relays.