For ON/OFF control, the heating circuit is switched on when the temperature is below the set point, and off above it. There could also be a cooling circuit, switched on above the set point and off below it. For variable control, it is often carried out by a PID controller (3-term controller). This controller uses a refined algorithm on the error (the difference between the set point and the measured value) which produces an output designed to achieve and maintain the set point with minimum overshoot and hold it as stable as possible.