While we have published a number of DC speed control circuits over the years, none has had the ability to control speed over the full range in both forward and reverse (with the exception of the Bi-directional Train Speed Control in the April 1997 issue of SILICON CHIP).

We have published a very popular 12V/24V 20A switchmode speed control (June 1997) but it works in only one direction. If you want it to change direction, you need a double-pole changeover switch or relay to change the polarity of the applied voltage and the motor spins the other way! However, this has the disadvantage that you then have two things to control the motor – a direction switch as well as the speed control.

Also, it is not a good idea to suddenly reverse the voltage on a DC motor while it is spinning. It can cause a big current surge that could burn out the speed controller, as well as causing big electrical and mechanical stresses on the motor itself.

This circuit overcomes both these problems. The direction and speed is controlled using a single potentiometer. Turning the pot in one direction causes the motor to start spinning. Turning the pot in the other direction causes the motor to spin in the opposite direction.

The centre position of the pot is the "off" position, forcing the motor to slow and stop before changing direction.