Home > Circuits > Drill Speed Controller
Modified:21:44, 22 October 2013

Designed by Phil Townshend 04

This circuit uses Pulse Width Modulation (PWM) to control the amount of power to a load. It can be used to power a variety of medium current devices, including halogen lamps and motors.

The heart of the circuit is a triangle wave astable whose frequency is controlled by VR1 and is set to about 2.5KHz at halfway. The triangle output is compared against a reference voltage provide by R5/VR2/R6. When the reference voltage is higher than the triangle wave, the output to the comparator switches on and activates the Darlington pair in turn driving the output device. By varying the reference voltage, an adjustable mark space ratio can be generated.

For motors a rule of thumb is that the frequency of the pulses should be at least 5 times the RPM of the motor. In most cases this should be in the KHz range.

You should check that when the speed potentiometer is at its lowest setting, the transistors are fully off. Check this with an oscilloscope. If this does not happen, try reducing the value of R6. Equally check the output is fully on when turned to the maximum position. If notm and low spikes are still present, then decrease the value of R5 until it does.

Q1 and Q2 can be replaced with a high current Darlington transistor such as TIP122 but must be mounted on a heatsink..

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An output of 5%

An output of 50%

An output of 80%

The principle is fairly simple. It works on the mark/space ratio (on and off time) of the output. For example if the output is like a square wave with an even mark/space ratio, then the output is on for 50% of the time. To increase or decrease the average output power, increase or decrease the "on" time or the mark time. At 5% mark time the output will be very low. At 80% it will be quite high. This happens repeatedly and fast enough for the motor to see a constant DC voltage.

Designed and Written by Phil Townshend 2008
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