PWM Motor Control circuit

Posted on Feb 5, 2014

As this controller has been designed primarily for controlling model trains, it is designed to deliver around 12-15 Volts, although this system should work with as low a voltage as 3V (indeed it also works on a 6V supply to control some accessories I have). With regard to maximum current, the output stage should in theory be able to deliver around 5 Amperes, although I have never tested the unit beyond 2A. The above diagram describes the electronic configuration of the controller. At its heart is the 555 timer/multivibrator Integrated Circuit (I.C.). This generates the pulses of varying widths. The 36kΩ resistor and 0.1μF capacitor set the mid-range frequency at 200Hz. If two controllers are required, the 556 dual timer circuit can be used in order to reduce the package count and overall circuit size. This circuit has been built and tested using the standard NE555/6 and the CMOS TS555 parts, all of which have been successful. At the limits of the voltage range, strange effects occur at the output with the standard part; something which does not occur with the TS555.

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Note that the 10kΩ resistor just pulls up the discharge pin so it is not left "floating", and is particularly important if a CMOS version of the 555 is employed. The potentiometer connected to the Control Voltage pin varies the voltage set internally by a potential divder. This causes the voltage internally compared with the voltage on the capacitor to be altered, thus at a lower voltage on the Control Voltage pin a lower capacitor voltage is needed, hence the 555 switches earlier. However as the voltage falls at the approximately the same rate, the output pulse width is relativly shorter, hence less power per cycle is output. As a side effect, the frequency is higher at lower speed and lower at higher speeds as the rise time varies, and the fall time remains largely unaffected. This variation is of the order of about a quarter of the frequency, but since the frequency is not greatly important the resultant effect is neglegable (except for the varied audio pitch produced by the motor). At the output stage, the TIP122 acts as a darlington-pair transistor amplifier with a gain of approximately 5000. This means that in theory only 1mA needs to be sourced from the 555 at the maximum 5A load. In practice more current is often drawn, however. At low currents of around 200mA or less, a small heatsink (or even no heatsink at all) will suffice. However a medium-sized heatsink of about 1W/K will be needed for higher loads...

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