Versatile DC Motor Driver Circuit: Basics and Oscillator
This straightforward circuit is tailored for various DC motors, handling currents up to 40 amps. Essentially, it functions as a basic oscillator driving multiple power MOSFETs. The heart of the oscillator is a simple RC configuration built around a single Schmitt trigger device (IC1a) sourced from a 40106 hex inverter package. Potentiometer P1, when adjusted towards D2, maximizes the output voltage. To prevent output short-circuiting to the input, two diodes are strategically placed. At the extremes of P1, the charge and discharge times are minimal. During prototype testing, the negative-going pulse was measured at 1 µs wide, and the positive pulse at 1.6 µs.
The subsequent inverters (IC1b and IC1c) refine the oscillator signal, driving a buffer stage comprising three parallel inverters (IC1d, IC1e, and IC1f). A resistor (R1) was included to hold off the MOSFETs if the 40106 is absent. Given the nearly 8 nF total input drive capacitance of the four MOSFETs, the buffer cannot fully charge and discharge when P1 is turned to its extremes. However, this proves convenient as it enables the motor driver to manage the entire output voltage span (0–100 %). The operating frequency hovers around 1 kHz, with a measured frequency of 1.07 kHz on the prototype. To suppress reverse energy (back emf) generated by inductive loads, including all DC motors, diode D3 is positioned at the output.
Handling High Currents and Back EMF
Addressing high output currents and back electromotive force (emf) poses challenges. In an early prototype board, the tracks leading to D3 were too narrow, causing one of the tracks to burn out during testing with one of the motors in an Elektor Wheelie. Each of the two motors used in the Wheelie draws up to 20 A at 24 V when fully loaded. The circuit underwent testing at 40 A and 24 V with a resistive load. However, the originally designed and supplied PCB cannot handle such high currents. The solution involves reinforcing the copper tracks carrying high current with pieces of 13 or 14 AWG (approximately 2.5 mm²) solid copper wire.
Alternatively, using two paralleled pieces of 16 AWG (approximately 1.5 mm²) wire is a more manageable approach. Consequently, the PCB does not feature solder stop masks. Thicker lines in the schematic indicate areas where high currents are expected to flow. The 40106 receives power from a 78L12 voltage regulator (IC2) with the necessary assortment of large and small decoupling capacitors. The speed control potentiometer can be mounted off the board and connected with lightweight wires. When securing the heatsink to the PCB with 3-mm (6 BA) screws, ensure it does not contact the solder pads for the MOSFETs.
Determine the correct positions for the transistor mounting screws and D3. To prevent mechanical stress within the semiconductor legs, lightly bend them using specialized tools before locating the hole positions. Tap 3-mm (approximately 1/8’’, 6 BA) threading. Remember to isolate all semiconductors on the heatsink. Due to the low switching frequency, it is normal to hear a whine from the DC motor, which poses no cause for concern.