Temperature Controller


This circuit is a simple temperature controller using an NTC thermistor sensor, an op-amp Schmidt trigger, npn driver transistor and 12v fan which may be directed towards a heatsink, valve etc. Use a 12v relay with appropriately rated contacts in place of the fan to control an external high voltage fan, for example 120/240V AC.

The unit is powered from 12v DC which is derived from a half wave rectified 12.6v AC source and a 12V regulator IC.

The photo on the right shows the printed circuit board holding the controller components plus two larger electrolytic capacitors at the back that are not part of the controller and may be ignored.

The sensor head consists of the NTC thermistor held in a simple bent metal (tin plate) clip with an epoxy resin adhesive. The metal clip should be a snug but not tight fit with the thermistor.

This controller is used in two of my home built power supplies and has proved to be very reliable.

To reduce the hysterisis increase R2 to a maximum of 4M7. Some hysterisis is required to prevent the circuit oscillating at the control temperature threshold.