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20A12/24V DC Motor Speed Controller Mk.2

This gutsy little speed controller has a wide range of applications and is simple to build and use. There are no software settings to fiddle with but it does have some really useful features such as low-battery protection, soft start and adjustable pulse frequency. It can run from 12V or 24V batteries at currents up to 20A.

By John Clarke

Main Features

• 20A current rating

• 12V or 24V operation

• On-board trimpot or external potentiometer for speed adjustment

• Optional soft start

• 0-100% speed control range

• Efficient PWM control

• PWM frequency adjustment

• Low battery protection


Supply Voltage........12-30VDC

Supply Current........20A maximum

Output Current........20A maximum

Standby Current........20mA

Control Range........0-100%

Low Voltage Cut-out........typically set for 11.5V (for 12V battery)
or 23V for a 24V battery

Pulse Frequency Adjustment........~100Hz to 1.1kHz (129Hz to 1.28kHz
measured on prototype)

Soft Start........from 0-100% (or to set speed) over about 1s

Mosfet gate rise and fall times........1.5μs & 1.6μs respectively­

The motor speed Controller described in the June 1997 issue of SILICON CHIP has to be one of the most popular projects we have presented; many thousands have been built. But as popular as it’s been, readers have often requested simple modifications to suit myriad applications. So we have come up with a revised design which should cope with virtually every possible variation that readers are likely to envisage. That’s a brave statement but it is based on literally hundreds of emails and letters we have answered on the original project in the 14 years since it was published.

The original design is still OK but we strongly recommend this new version since it has more capabilities and is easier to build and connect. In fact, if you have an unassembled version of the old kit, we suggest you toss the old PCB and buy the new PCB plus a few extra bits to make up the new design; it will be worth it.

Click for larger image
Fig.1: the circuit is based on a TL494 Switchmode Pulse Width Modulation (PWM) Control IC. External timing components RT & CT on pins 5 & 6 set the PWM frequency, while output transistors Q1 & Q2 can be configured for either push-pull or single-ended operation.

New features

First, the new PCB has provision for an on-board speed control trimpot (VR1) or as most builders seem to want, an off-board potentiometer.

For ease of wiring, we have added heavy-duty screw terminals to the PCB for the power supply and motor connections. As well, the in-line fuse and fast recovery power diode are now mounted on the PCB and the power Mosfets and power diode have small heatsinks fitted.

The circuit now provides full range speed control from zero to full power; the older design did not allow full speed. Apart from letting the motor operate at full power it also eliminates switching noise caused by the controller (at the full speed setting).

Variable pulse frequency is another new feature. Because the speed controller works by pulse width modulation, with the pulse width varying the voltage fed to the motor, this can produce more noise from the motor. This is due to magnetostriction of the core laminations and rattling of the armature windings. You can often reduce this lamination noise by changing the pulse frequency and there is a trimpot (VR3) on the PCB to provide this feature.

As mentioned above, we have also provided low-battery protection. This is mainly to prevent damage when the circuit is powered from 12V sealed lead-acid (SLA) batteries which will fail completely if they are discharged below 11V.

Soft start is also included. This will bring the motor smoothly up to speed each time power is connected to the circuit, regardless of the speed setting. If soft start is not required, it can be disabled by removing a jumper link.

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