When the MOSFET switches off the motor current keeps flowing because of the motor’s inductance. When the drive MOSFET conducts, current flows from battery positive, through the motor and MOSFET (arrow A) and back to battery negative. If the motor is drawing any current then this current flows through the switch MOSFET when it is on – but where will it flow when the MOSFET switches off? Read on and find out! A PWM motor control circuit PWM motor control circuitĬonsider the circuit above: this shows the drive MOSFET and the motor. Inductance does not like changes in current. It is also slow enough that MOSFETs can easily switch at this frequency.

It is also a frequency above the audible range so any noise emitted by the motor will be inaudible. This is far too fast for the poor old motor to even realise it is being switched on and off: it thinks it is being fed from a pure d.c.

In a practical low voltage controller the switch opens and closes at 20kHz (20 thousand times per second). At C the motor is on for most of the time and only off a short while, so the speed is near maximum. If the motor is connected with one end to the battery positive and the other end to battery negative via a switch (MOSFET, power transistor or similar) then if the MOSFET is on for a short period and off for a long one, as in A above, the motor will only rotate slowly. To control the motor speed it varies (modulates) the width of the pulses – hence Pulse Width Modulation.Ĭonsider the waveform above. controller does: it switches the motor on in a series of pulses. If we switch the power on and off quickly enough, the motor will run at some speed part way between zero and full speed. If we switch the power off sometime before the motor reaches full speed, then the motor will start to slow down. However if you take a 12v motor and switch on the power to it, the motor will start to speed up: motors do not respond immediately so it will take a small time to reach full speed. What exactly is Pulse Width Modulation? how does PWM motor control work, and what does a PWM circuit look like? On this page we’ll go into a bit of detail to explain the theory and practise of PWM motor control.

Pwm driver circuit for stepper motor