Controlling the PCB Drill with a Single Preset
The presented PCB design offers the capability to control the PCB drill using just one preset. Additionally, there is an option to reverse the rotary direction of the drill. When the preset is positioned at the center of its travel, the drill comes to a complete stop.
Power Supply Configuration for the Drill and Circuits
Immediately after the bridge rectifier, the power supply provides a voltage of approximately V for the drill. Additionally, resistor R19 and R20, along with zener diodes D6, generate a symmetrical voltage of ±5 V for the circuits. This ±5 V voltage is stabilized by capacitors C6 and C7. To prevent the motor of the drill from being connected to the ±5 V section, diodes D4 and D5 are incorporated into the design.
Speed and Rotary Direction Control
Speed control is achieved using P1, while the determination of rotary direction is entrusted to comparator IC1a. This operational amplifier evaluates whether the preset is positioned to the left or right of its center. Based on this assessment, relay Rei is either energized via T1 or not, thereby determining whether the drill turns clockwise or anti-clockwise. The speed control mechanism relies on pulse width modulation. The control voltage, set with P1, is directed to D2 and IC1b-D3, functioning as a rectifier. Regardless of 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.
Pulse Width Modulation and Signal Enhancement
The modulator is established with 1C1c, designed as a rectangular-wave generator, continuously charging and discharging C4 via R11. The configuration of P1, in conjunction with R9, determines how much additional direct voltage is applied to C4, consequently influencing the pulse-to-spacing ratio at the output of IC1. Inverter IC1d enhances the signal’s edges, which are then employed to activate the power FET (T4) via T1.
Current Monitoring and Adjustment
When the voltage drop across R4, situated in series with the drill motor, exceeds slightly more than 0.6 V (equating to a current of about 5 A passing through the motor), T3 switches on. Consequently, T5 adjusts the pulse width slightly, ensuring efficient control of the drill’s operation.