Understanding On-Off Temperature Controllers

Posted by Ian Parnell in on 22 January 2014

How Does An On Off Temperature Controller Work?

An on-off temperature controller, also known as a signaller or "bang-bang" controller, will set its output so that the process value increases whenever it is below the desired temperature setpoint. As the process temperature rises above the setpoint, the output is changed to make it fall.


In a typical heating application, we use reverse acting control action. This involves turning on heaters when it is too cold and turning them off when it is too hot. Most people are familiar with this method because this is how home central heating thermostats work.

A cooling application such as a refrigerator would normally use direct acting control action, where the output turns on when the process is too hot.

Why Use An On-Off Temperature Controller? 

On-Off control is chosen for simplicity. The only parameters that need to be set are the switching differential and setpoint.

On-off control is suitable if the response delay is short when the output is switched and the maximum rate of rise is small. Alternatively if a very stable process isn’t needed then on off controllers are ideal.

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