Originally published by Paul Stenning in Electronics in Action, February 1994
One problem with the smaller PCB drill machines is that the speed drops rapidly as the load increases. This can be overcome to some extent by increasing the supply voltage, however the off-load speed is then too high.
The speed controller in this article overcomes this problem by raising the voltage to the drill as the load is increased. The unit uses a basic Pulse Width Modulation (PWM) regulator to reduce the power dissipation.
This unit was designed for the Expo Reliant drill. This popular low cost tool is ideally suited to light duty use.
The controller can power other small PCB drills, as well as other devices containing small 12 Volt DC motors, although some changes to component values may be necessary.
The circuit diagram is shown in fig *.
IC1 acts as an oscillator, running at several KHz. The pulses from this briefly turn on TR1, which discharges C4. Between these pulses C4 charges via R5, producing a rough sawtooth waveform.
This sawtooth is connected to one input of a comparator circuit IC3, while the other input is fed from a reference voltage. The output of the comparator controls the switching transistor TR5, via TR3 and TR4.
Thus as the reference voltage is increased, the switching transistor is turned on for a longer period, and the drive to the drill is increased.
VR1 is the front panel speed control, the voltage from which is buffered by TR2, and fed to the reference input of the comparator circuit by VR2.
R15 senses the current flowing in the drill. The peak voltage across this is amplified by IC2, the output from which is also fed to the reference input of the comparator via VR2.
Thus as the load on the drill increases, its current consumption raises, which in turn causes this unit to increase the drive to the drill. VR2 sets the ratio of control to feedback.
FU1 will blow if the drill is stalled for long period, or if the output is short-circuited. Since the circuit can withstand the short circuit current until the fuse blows, a more elaborate over-current circuit is not necessary.
Statement: This post is only the personal view of the author and does not represent the opinions of ALLPCB.com.