Pulse width modulation (PWM) is ad ideal for controlling small electric PCB drills that draw currents of up to 2 A. Larger currents are possible if T2 gets additional cooling and the value of C2 is increased. The design also makes use of the fact that the drill works with a small d.c. motor whose rotary speed is a function of the voltage across it.
The design (Fig. 1) is based on astable multivibrator IC1a, whose output is low for a period determined by R1 and high for a period set by R, and P1.
When C1 is discharged, the level at the input of IC1a is below the lower threshold, so that the output of this stage (pin 2) is high. The capacitor is then charged rapidly via D1 and R1 and reaches the upper threshold in about 1.5 ms. The output of ICia then goes low,whereupon C1 is discharged via D2, R2, and P1. In the prototype, the discharge time could be set between, 0.2 ms and 25 ms. This means that the duty factor of the output signal may be varied between 5% and 90%.
The signal is inverted again and then applied to the base of T1. Transistors Ti and T2 are switched on, and the motor is energized, during the negative period of the output pulse of IC1b (pin 4). When the resistance of P1 is at a minimum, the rotary speed of the drill motor is at a maximum.