Chapter-6a-1 Page 36
A Motor Speed Control
This is a simple motor control circuit. It has good starting characteristics and it will run at a slow speed. It could be used for a loco motor, turntable drive motor, accessory equipment such as cranes, winches, etc. Use it for a single direction motor or with a change over switch, bi- direction control. It can be powered by an AC plugpack (Wall Wart) of 14 to 16 volts AC or a transformer. With the pulsing DC developed from the AC input it has good start and slow running characteristics and it is gentle enough to be used with all types of motors. There is some feed-back from the motor that will increase the drive voltage if a heavier load is encountered. The power source selected should have enough voltage and amperage to match the motor. The circuit components can be altered to suit the motor. It is not a precise controller and the speed will vary slightly as the transistor heats up, so a heat sink is essential. With the speed control at minimum (to the right hand side on the diagram) there is still a trickle of voltage being supplied to the motor which, if it is in a loco, will 'tickle' a block detector or if driving a turntable, it can be used to bias the motor and hold it onto the stop at the selected track position. The transistor used is a Darlington type which gives a high power output for low base drive current. As it has to absorb the surplus voltage not used by the motor it can get warm and a heatsink is needed. The bridge rectifier gives a full wave rectification and the voltage out of it goes from zero volts to about 19 volts and back to zero at twice the -
Components mains frequency (120 times per second in the USA) This gives an average voltage of about 16 volts. The zener diode limits this to a zero to 12 volt pulsing wave. With the potentiometer 'off' there is a small current through the resistors and diode to the motor. Not enough to run it, but it could be used to hold the motor against a stop. The potentiometer resistance can be changed to adjust this bias power, 500 ohms gives about 1 volt, 100 ohms about 3 volts. When the speed control is moved off 'Low' the transistor starts conducting and it supplies a higher voltage to the motor and starts it. The voltage to the motor can go as high as the zener diode regulated voltage value.
One resistor of 100 ohms at 1 watt, Potentiometer with PCB mounting of 500 ohms, ďž˝ to 1 watt, a 2 to 4 amp bridge rectifier, W06 a 12 volt 1 watt zener diode (depending on the motor running voltage needed), a 1 amp diode type 1N4004, Darlington transistor BD681 for motors to 1.5 amp and TIP141 for up to 3 amp power. A Heat sink to suit the transistor that has a screw or solder mount to secure it to the 18 by 13 hole veroboard (resize if necessary to suit the heatsink)
