This simple circuit is designed for use with all kinds of DC motors up to 40 amps. Basically, it’s just a simple oscillator driving a bunch of power MOSFETs. The oscillator is a rudimentary RC type around a single Schmitt trigger device (IC1a) from a 40106 hex inverter package. When the wiper is turned towards D2, potentiometer P1 gives maximum voltage to the output. The two diodes prevent short-circuiting the output to the input. At the extremes of P1, the charge and discharge times are minimal. In the prototype of the circuit the negative-going pulse was found to be 1 µs wide, and 1.6 µs for the positive pulse. The next two inverters, IC1b and IC1c, clean up the oscillator signal, driving a buffer stage comprised of three inverters in parallel, IC1d, IC1e and IC1f. Resistor R1 was added to hold off the MOSFETs in case the 40106 is absent. The total input drive capacitance of the four MOSFETs amounts to almost 8 nF—clearly too much for the buffer to fully charge and discharge when P1 is turned to its extreme positions. That’s convenient however because in practice it allows the motor driver to manage the full output voltage span (i.e. 0–100 %). The operating frequency is in the 1 kHz ballpark. On a prototype, 1.07 kHz was measured. Diode D3 at the output suppresses the reverse energy (back emf) generated by inductive loads, which includes all DC motors.
High output currents and back emf are issues here. On an early prototype of the board, the tracks to D3 were too narrow, and when testing the circuit with one of the motors in an Elektor Wheelie one of the tracks burned out. Fully loaded, each of the two motors used in the Wheelie draws up to 20 A at 24 V. The circuit was tested at 40 A and 24 V with a resistive load. However, the PCB as designed and supplied is not able to handle such high currents. The solution is to beef up the copper tracks carrying high current with pieces of 13 or 14 AWG (approx. 2.5 mm2) massive copper wire. Possibly two paralleled pieces of 16 AWG (approx. 1.5 mm2) are easier to get into place. For this reason the PCB does not have solder stop masks. The thicker lines in the schematic provide a global indication of where high currents can be expected to flow. The supply for the 40106 is no more than a 78L12 voltage regulator (IC2) with the usual entourage of decoupling capacitors large & small. The speed control potentiometer may be mounted off the board and connected with light duty wires. The heatsink is best secured to the PCB with 3-mm (6 BA) screws. Make sure the heatsink doesn’t come in contact with the solder pads for the MOSFETs. Then determine the correct positions of the transistor mounting screws, and D3. To prevent mechanical stress within the semiconductor legs, give them a light bend—there are special tools available for this—and only then locate the positions for the holes. Tap 3-mm (approx. 1/8’’, 6 BA) threading. Don’t forget to isolate all semiconductors on the heatsink. Because of the low switching frequency there’s a good chance you can hear a whine from the DC motor—it’s pretty normal and no cause for alarm.