FAQ00615 of Temperature Controllers FAQ

ON/OFF operation is shown in the following illustration.

The output turns ON to provide current to the heater when the present temperature is lower than the set point.

The output turns OFF to shut OFF the heater when the present value is above the set point.
Thus, ON and OFF are repeated near the set point to maintain a constant temperature. This contort method is called ON/OFF operation.

Because the manipulated variable is either 0% or 100% with the set point serving as the boundary, it is also called 2-position operation.