This page shows how to construct a low powerd variable speed motor controller that can handle around 40A, Ideal for powering Car windscreen wiper motors, Car fan motors and possibly some wheelchair motors. It consists of the same PWM circuit that the full bridge controller uses and a direction circuit using 2 40A relays. The output stage can be described as pretty primative but it does work well on lower powered applications such as this but it does lose reliability if you up the power too much through burning out the Relay contacts and even blowing the MOSFET. The coils of both Relays are wired in parallel, this is so both sets of contacts change at the same time, and as you can see from the circuit the polarity on the motor will be reversed when the Relay coils are energised. The motor speed is produced by turning ON/OFF the MOSFET in it’s GND return. The switching of the MOSFET is controlled by the PWM circuit, this will give a speed range from 0% (STOP) to 99% (virtually full speed). It is important that the direction of the motor is not changed while it is being powered as this can result in burnt relay contacts and a blown MOSFET, so we must only change direction when the PWM is at 0%. This can be done in the servo interface by using a Microswitch to change the direction signal, and a variable resistor to generate the speed voltage for the PWM circuit, both of which are connected to your R/C servo by a custom made cam. It must be aligned so that a direction change only occurs when the servo is in (or near) the center position