During the development of systems that include small motors, a simple, bidirectional motor controller with speed adjustment may be helpful. Figure 1 shows such a controller. The circuit uses everyday components whose tolerances and ratings are unimportant as long as they sustain the required voltage, current, and power. The circuit’s advantages are low cost, small size, flexibility, and ready availability. You can assemble it in less than an hour on a board measuring approximately 75 × 100 mm; its height is less than 12 mm. A transistor-based H-bridge allows two directions of rotation. A chopper controls the upper arms of the H-bridge, thereby enabling the speed adjustment. To start the rotation in one direction, you must connect one of the inputs (In CW or In CCW) to 0 V. You can do this through switches, transistors, or open-collector TTL circuits, for example. If both inputs are high (no command), transistors Q2 and Q4 do not conduct, and the motor stops. The motor receives a slight braking action from the pulsing Q1 and Q3 transistors.
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| Figure 1. | You can set a motor’s rotational direction and speed using this simple circuit. |
If one input is low (connected to 0 V) – for example, In CW or In CCW – the corresponding transistor, Q2 or Q4, conducts, with base current limited by R1 and R4. The pulse signal to the base of Q1 or Q3 short-circuits to 0 V, thus shutting off Q1 or Q3. On the opposite side, Q4 or Q2 does not conduct, but Q3 or Q1 receives pulses from the chopper through D2 or D1 and R6 or R5. Thus, Q3 or Q1 conducts each time transistor Q5 is on. The chopper uses a 555 timer circuit, IC1, connected as an astable multivibrator. The zener diode, D7, and R10 limit IC1’s power-supply voltage to the maximum allowable: 15 V. The timing capacitor, C2, charges through R11, the upper part of the potentiometer R12, and zener diode D7. The discharge takes place through the lower part of R12. Using β for the position of R12’s wiper (middle: β = 0.5; down: β = 0; up: β = 1), the charging time is
and the discharge time is
The total time of one period of the chopper is thus
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| Figure 2. | The position of the wiper on the speed-control potentiometer determines the duty cycle of the chopper circuit. When it is fully down, β = 0 (a); when it is fully up, β = 1 (b). |
The output signal on Pin 3 is a square wave with nearly fixed frequency and adjustable duty cycle. In Figure 2a, the potentiometer’s wiper is fully down (β = 0). In Figure 2b, the wiper is fully up (β = 1). Q5 and Q6 adapt the voltage level to drive the bases of Q1 and Q3, which conduct only in the time when the output (Pin 3) of the 555 is high (TON). This conduction adjusts the rotational speed. Diodes D3 to D6 protect the transistors Q1 to Q4 against inductive voltage peaks. The fuse, F1, protects the whole circuit against overcurrent conditions. Capacitor C3 between VCC and ground acts as a kind of energy tank that filters out the current peaks. The circuit was a help in determining speeds or gear ratios to use during test and adjustment of prototypes on small machine tools. The transistors should preferably be Darlington types, adapted to the power-supply voltage and the motor current. (Don’t forget the high inductance of the motor.) Select resistor R10 and the zener diode according to the power-supply voltage, VCC.
