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Brushed DC Motor Programmable Speed Regulator

2023-08-15

Latest company case about Brushed DC Motor Programmable Speed Regulator

This article describes how to design and build a low-noise, high-performance, low-cost brushed DC motor programmable speed regulator. The regulator is immune to the temperature, voltage, and motor load variation. Also, this circuit is not sensitive to the motor coil resistance, which allows using different motors in the same application without any adjustments.

The described circuit is designed specifically for devices in which a low-power brushed DC motor is used and in which the stability of the motor speed and low noise are critical. The motor speed is pre-programmed and does not require trimming after assembly, though it is possible to reprogram it to a different speed via I2C.

Unlike traditional circuits (see Figure 1), this design does not require any feedback sensor. In this case, the motor itself is a sensor. Other, more traditional designs (see Figure 2) are dependent on the motor coil resistance. Both require post-production trimming. Some applications (for example, audio applications) are very noise-sensitive. That makes any PWM motor controllers not suitable.

Brushed DC Motor Programmable Speed Regulator
Figure 1: Traditional DC motor speed controller with current sensor
Brushed DC Motor Programmable Speed Regulator
Figure 2: Traditional DC motor speed controller based on TDA7275A
Every brushed DC motor produces interference when brushes switch from coil to coil. This is considered one of the biggest downsides of such a motor. Unfiltered, it can interrupt the normal work of the circuits powered from the same power source or simply situated close by. While it is possible to filter out to an acceptable level the interference using a capacitor connected in parallel to the motor (in some cases, ferrite coil in series), it is impossible to get rid of it completely.

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In the case of the design described in this document, the small leftover interference is used to determine the motor speed. In most cases, cheap, low-voltage, low-power brushed DC motors have three moving coils. Each of them produces interference in a form of spikes; thus, the frequency of the spikes is 3× the revolutions of the rotor. So by counting the spikes, it is possible to not only determine the speed but control it by automatically adjusting to a pre-programmed value. All this and more can be achieved by using the SLG47004, a versatile programmable mixed IC with only basic external components. See the schematic diagram in Figure 3.

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