Here is a simple circuit for controlling the speed of DC operated PCB drills. The heart of this circuit is the ICLM3578 which is a very efficient integrated switching regulator that can be used for applications like this. The LM3578 has separate inverting and non inverting feedback inputs (pin1 and pin2), 1V internal reference voltage source and a built in oscillator circuit. The IC can be configured to obtain up to 90% duty cycle. In the circuit, capacitor C2 is the timing capacitor which determines the frequency of the internal oscillator. The duty cycle depends on the sum of resistors R1 and R2 and it can be varied by adjusting the POT R2. The output PWM signal will be available at pin 5 of the IC. The Q1 drives the motor M according to the PWM signal available at pin5 and thus by varying the duty cycle of the PWM wave by varying POT R2, the speed of the motor can be adjusted. The diode D1 is a freewheeling diode which protects the MOSFET Q1 from transients produced by the motor. Capacitor C3 is just a filter capacitor.
■The circuit can be assembled on a Vero board.
■The supply voltage (V+) can be anything between 6 to 30V DC.
■The supply voltage (V+) must be selected according to the voltage rating of the drill motor.
■The maximum drain current IRF540 can handle is 22A @ room temperature.
■The duty cycle can be varied from 0 to 90% by adjusting the POT R2.