DC motors with permanent magnets are widely used and popular among model builders. A particular characteristic of these motors is a large discrepancy between the startup torque and the nominal torque. If a permanent magnet motor is to be powered from a DC supply and a constant, high torque is required at a variable operating speed, a pulsewidth modulator (PWM) of the type described here is needed. We construct an astable multivibrator with the help of a couple of gates from a 40106 hex Schmitt trigger IC.
The multivibrator has a mark-space ratio (‘duty factor’) that can be adjusted over a very wide range, independent of its operating frequency. Adjusting the potentiometer changes the ratio between Ra and Rb, which together make up the total resistance of the potentiometer. Capacitor C1 is charged via Rb and is discharged via Ra. The corresponding mark-space ratio is present at the output of the oscillator on pin 4 of the 40106. The output high time is determined by Ra, while the output low time is determined by Rb. The oscillator frequency is constant at approximately 115 Hz. Transistor T1 provides current gain: when pin 4 of the 40106 is low, T1 turns on, and when the output is high, T1 is turned off. Sufficient current is available at the collector of this transistor to drive the base of the 2N3055 power transistor. R4 and C2 provide decoupling for the oscillator from the large currents switched by the power output stage.
The moving-coil meter connected via R7 serves to monitor the state of the battery, which is useful when rechargeable cells are used. The circuit has been used by the author to drive a motor salvaged from an old cassette tape recorder. In this case the 2N3055 did not require a heatsink. Interested readers will find that a search of the Internet turns up plenty of information on the theory and practice of driving DC motors using pulsewidth modulation.
