PWM Motor Control

Description

This is a circuit to control motor speed uses pulse width modulation (PWM). The PWM signal is generated by the ubiquitous 555 timer and output current amplified by a power transistor.

Notes

There are a few ways to control the speed of an electric motor. You can use closed loop or open loop systems and also basic rheostat to vary the current through the motor. An example of a closed loop design using feedback from an op-amp is my constant motor controller circuit.

In this circuit, open loop control is used. Speed variation in open loop systems is not as accurate as closed loop systems, but will be good enough providing the motor load is constant. Instead of a constant current to the motor, this circuit uses current pulses and varies the duration of the pulse to control motor speed. This technique is known as pulse width modulation (PWM), and is more energy efficient than using a constant current supplied to the motor.

The circuit uses the common 555 timer IC configured as an astable pulse generator, but with variable mark and space ratio. The timing period is set by C3 and R1, but the non symmetrical on and off times (mark and space ratio) controlled by 10k variable resistor VR1.

The output pulses are then amplified by the BD139 power transistor which controls the motor current. The direction switch is a DPDT switch and LED's D3 and D4 are there to indicate direction. C4 is there to help electrical noise and suppression.

Efficiency

PWM is more energy efficient than providing a constant current to the motor. At slow speeds, for example 10% rotation of VR1, the motor current will be supplied for 1/10 of a cycle and off for 9/10 of a cycle, see waveform below:

At half speed, for example 50% rotation of VR1, the motor current will be supplied in roughly equal parts and be on for 1/2 of a cycle and off for 1/2 a cycle, see waveform below:

At high speeds, for example 90% rotation of VR1, the motor current will be on for 9/10 of a cycle and off for 1/10 a cycle, see waveform below: