Motor Control Unit
| Introduction | Constant Voltage | Pulse-Width Modulation | Design Ideas |
The pulse-width modulation (PWM) version gives improved reliability at low speeds when compared with the Constant Voltage version since full power is always delivered, except that the time for which the voltage is reduced thus preventing the motor from reaching its maximum speed. However, this causes the motor to become quite noisy in operation since a square wave is being fed into it - this is especially the case if the frequency happens to be close to the resonant frequency of the model or container where the motor is located!
Another reason for the use of this system is that the output stage of the electronics is used as a switch, and is either fully on or fully off, thus reducing the voltage drop through the device. This reduces the need for a large heatsink, thus reducing the overall cost of the system.
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.
Design Description
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. This is ideal for most model trains, however if another frequency is desired the values of these can be components, according to the current formula:
where R is in ohms, C if Farads and f in Hertz. So, for example, say a frequency of 50Hz is required, R = 1 / 1.4 x 0.1 x 10-6 x 50 = 143kΩ, so a 150kΩ resistor should be used.
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 (up the maximum). This is because for the worst case, the device will have a voltage drop of 4 Volts at a current of 5 Amperes this is a power of 20W. Therefore the heatsink must be able to dissipate 20J of heat per second, with a suitable increase in temperature. An increase of 20K is satisfactory at a temperature near room temperature (293K) as this is well below the limit of 343K. Of course, this can be varied backwards accordingly for smaller loads as both the current and voltage drop through the device decrease. (In the worst case, a variable-voltage control unit will cause the output transistor to dissipate the entire output power of the system, i.e. 90Watts. This, of course, requires a 5W/K heatsink which is about ten times the size due to the way in which heat flows, but will cost more still.)
The diode in the circuit works by shorting out any voltage spikes produced by the motor in the reverse direction. Note that it is desirable to use a diode that can conduct a larger current with a much larger motor. For example if the motor nominally draws 2A of current, then a 1N5401 diode would be more suitable.
The circuit should be powered from a smoothed DC supply. Regulation isn't essential, indeed it isn't desirable since the voltage drop through a regulator is likely to be more than that through the output transistor.
Construction Details
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.
