Pulse Width Modulation (PWM)

I’ll go into a few basics regarding Pulse Width Modulation PWM.

The problem

To understand PWM I’ll use as an example a situation where you want to control the speed of a motor. The easiest way to do that is by varying the voltage going to that motor. Let’s say the motor runs on 25V DC and we want it to go half speed. To go half speed, we would need a lower voltage and one way to do this is to have a transformer that gives us one output for 25V and another for the lower voltage. Now let’s say we want the motor to go a third speed the transformer would now not only be more complex by having three outputs but it would also be heavier. And now consider we want to be able to continuously vary the motor speed not just have set speeds. It’s getting complex, isn’t it?

Rheostats – one solution, an old one

A much better way this was done for decades especially in large industrial motors was by using a rheostat. This is basically a variable resistor that can take high power loads. The problem with this approach is that power is being wasted on the resistor itself, and this is before taking into account that rheostats are big and heavy themselves.

PWM – the modern solution

The ultimate solution to all of this was using PWM. Pulse Width Modulation is modulating pulses of differing width to (in the case of a motor) make it behave like we are applying a diverse range of voltages to it. So basically, we are taking a fixed voltage and connecting and disconnecting it really fast to our load. This image should help:

Wiki Media to the rescue with an appropriate image
Duty Cycle

As you can see in a green cell the pulse of power can take up anything from nothing to the whole duration of the cell. How much time the power is on is called the duty cycle. Also, the green cell is called a period. So, in every period the same thing happens, the power is on for a while, rolex replica watches our motor is spinning then it is cut off. This does not mean that the motor stops and starts every period, at least not in a way that can be observed. The motor keeps spinning because of inertia. Modern electronics are so good there’s no vibration caused by this process at all. Another thing that helps is that the frequency at witch these cycles happen is high.


The interesting part is that PWM is used all around us. Computer fan controllers regulate fan RPM. A lot of modern motors use PWM because it increases the efficiency of a control circuit. In lights, especially LED lights, this is used because LEDs do not have any inertia, the inertia here is in the human eye. So when a LED is dimmed it is actually flashed at an extremely high frequency so that to the human eye it seems like it’s continuous but in fact the LED is not always on. And where do we use LEDs? In monitors, they provide the backlighting, dimmable LED lightbulbs use this principle, RGB LED strips change color by having each of the primary colors run brighter of dimmer etc.

And that should cover the basics of PWM.