Over the years, motor speed controls have always been popular and this one is a beauty. Its Mos-fet bridge circuit can be used for speed control in an R/C system using standard 1-2ms pulse control or you can simply connect a 10kΩ (linear)
potentiometer or joystick to give single-handed forward/reverse control. As such, it would be suitable for a golf buggy, electric wheelchair, go-kart or whatever motor control application you have in mind.

The bridge driver circuit employs 80A N-channel Mosfets that have an on-resistance of just five milliohms (5mW) and are suitable for 10-30V operation. In practice, that will mean operation from 12V or 24V batteries.

When tested with a loaded 24V motor at a continuous 10A the MOSFETs became just slightly warm. No additional heatsinking would be required for operation at 20A. This test was conducted with four MOSFETs in the output bridge but there is provision for another four MOSFETs to be paralleled with the existing ones in the output bridge driver. This would result in each of the paralleled MOSFETs having one quarter of the power dissipation when compared to the original single devices! In a 24V system, there would be no problem powering motors with a power rating of up to 1kΩ.

This view shows the top side of the assembled PC board. Be careful not to get the two ICs mixed up and take care to ensure that all polarised parts (ICs, diodes, zener diodes & electrolytic capacitors) go in the right way around. The power Mosfets are mounted on the underside of the board (see below).

The surface-mount Mosfets are soldered to the underside of the PC board while the external connections are run via crimped eyelet assemblies which are fastened in place using M3 machine screws and nuts.

The complete circuit of the Speed Control For DC Motors is shown in Fig.1. With a total of four op amps, four comparators and four Mosfets, it may look fairly complicated but we can break it down into two sections in order to understand how it works.