Controlling a small dc motor without speed control sounds like a trivial task; a switch or a relay should suffice. However, several problems accompany this approach. For one, the switch, because of the inductive load and the low starting resistance of the motor, tends to wear out prematurely (with all the related sparks and EMI problems). Second, when you cut the power, the motor continues to rotate for a certain time, depending on its initial speed and inertia.
The circuit in Figure 1 can be useful for designs that don't need precise control of speed and stopping position but can benefit from enhanced deceleration. The circuit comprises two parts. Q1 plays the role of the switch. D2 protects Q1 against inductive surges. Resistor R2 keeps Q1 off as long as switch S1 is open. R1 limits the base current of Q1 when S1 is closed. S1 can be a manual switch, a relay contact, an optocoupler, or a transistor. If you close S1, Q1 turns on, and the motor runs.
|Figure 1.||This circuit provides both motor-drive
and braking functions.
Q2, D1, and R3 constitute the braking circuit. This circuit is similar to the output circuit of TTL gates. D3 protects Q2 from inductive surges. When S1 closes, Q1 turns on, and the voltage at Point A goes high (near VCC). The voltage at the base of Q2 is higher than the voltage at the emitter, because of the voltage drop in D1. If you open S1 while the motor is running, Q1 turns off. The voltage at Point A is near zero. The self-induced, back-EMF voltage from the motor sees a short circuit in Q2, whose emitter is more positive than its base and thus conducts. Short-circuiting the motor results in braking it. The higher the speed of the motor, the stronger the braking effect.
You should mount the circuit of Q2 as near as possible to the motor to reduce the series resistance of the wiring. This parasitic resistance limits the braking current and, thus, the deceleration. The circuit of Q1 can be remote. The dividing line between the two circuits is at Point A. This design mounts the circuit on the tool-changer motors of small machine tools, and it has worked perfectly for years. The values of the components are not critical. The transistors should preferably be Darlington pairs and, like the diodes, should be types commensurate with the power-supply voltage and the motor current. (Also, don't forget the high inductance of the motor.) The components in Figure 1, for example, are suitable for a 24 V, 3.5 A motor.
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