This simple circuit drives DC motors with a maximum current of 1 A and can be built with readily available components. The output voltage is adjustable between 0 and 14 V and the polarity can be changed so that not only motor speed but also rotation direction can be adjusted by turning a knob. The circuit is also ideal as a controller for a DC model railway or small low-voltage hobby tool. Power for the circuit is supplied by a 18 V mains transformer rated at 1.5 A. Diodes D1 to D4 rectify the supply and capacitor C1 provides smoothing to give a DC output voltage of around 24 V. A classic ‘H’ bridge configuration is made up with transistors T1/T3 and T2/T4. Transistors T5 and T6 together with resistors R7 and R8 provide the current sense and limiting mechanism.
The maximum output current limit can be changes from 1 A by using difference values of resistors R7 and R8:
Iout = 0.6 V / R
where R gives the value for R7 and R8. For increased current limit the mains transfor-mer and diodes will need to be changed to cope with the extra current as well as the four transistors used in the bridge configuration. Motor speed control and direc-tion is controlled by a twin-ganged linear pot (P1). The two tracks of P1 together with R1/R2 and R3/R4 form two adjusta-ble potential divider networks. Wiring to the track ends are reversed so that as the pot is turned the output voltage of one potential divider increases while the other decreases and vice versa. In the midway position both dividers are at the same voltage so there is no potential difference and the motor is stationary. As the pot is rotated the potential difference across the motor increases and it runs faster. The vol-tage drop across D5 and D6 is equal to the forward voltage drop VBE of the bridge transistors and ensures that the motor does not oscillate in the off position with the pot at its mid point.