The design presented PCB enables the PCB drill to be controlled with only one preset. as an aside. the rotary direction of the drill may be reversed. When the preset is at the center of its travel, the drill stops.
The power supply delivers (immediately after the bridge rectifier) a voltage of about V for the drill, and, by means of resistor-R19 and R20, and zener diodes D6 and a symmetrical voltage of ±5 V for the circuits. The ±5 V voltage is smoothed by C6 and C7. Diodes D4 and D5 prevent the motor of the drill being connected to the ±5 V section.
Speed control is effected with P1, while the rotary direction is determined by comparator IC1a. This opamp ascertains whether the preset is to the left or to the right of its center. On the basis of this, relay Rei is energized via T1 or not, and this decides whether the drill turns clockwise or anti-clockwise. The speed control operates with pulse width modulation. The control (direct) voltage set with P1 is applied to D2 and IC1b-D3. This latter combination functions as a rectifier. Whether the voltage at the wiper of P1 is negative or positive. it will appear as a positive potential (minus the 0.6 V forward bias of the diode) at the junction of D2 and D3.
The modulator is formed by 1C1c. This stale is designed as a rectangular-wave generator, which causes C4 to be charged and discharged continuously via R11. The setting of P1 determines (via R9) in conjunction with R10 how much additional direct voltage is applied to C4, and thus the pulse/spacing ratio at the output of IC1. Inverter IC1d enhances the edges of the signal, which are thereupon used to switch the power FET (T4) via T1.
When the drop across R4, which is in series with the drill motor, rises to slightly more than 0.6 V (that is, when the current through the motor is about 5 A), T3 will switch on, whereupon T5 will reduce the pulse width slightly.