Solid-State Proportional Temperature Controller

A new proportional temperature control circuit that uses a unique thermistor oscillator as the temperature sensing element is discussed. A series connected positive temperature coefficient (PTC) thermistor and negative temperature coefficient (NTC) thermistor form an electrothermal relaxation oscillator. The environmental temperatures which the thermistors experience determine the amplitude and frequency of the oscillator. Coupling the oscillator with a thyristor and using the temperature dependence of the oscillator, proportional temperature control can be achieved.