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What is the advantage of PWM vs supplying variable voltage? Answered

Friend and I are very new to electronics and we were dimming LEDs in completely different ways I think. We both defended the way we had learned to do it without really knowing why :) Any thoughts?
Also, with a servo, can you do both methods as well? And is there a way to tell a servo to go to a specific "spot" on its turning radius, or is it all relative, like supply a low voltage and it will turn a little? Do you just sort of calibrate it, so you can figure out how much voltage at how much time goes a constant radial turn?




5 years ago

If you ever played with an electric train

You will notice it is hard to slow a train to stop exactly at a station or unloading platform that is wanted.
Because a plain voltage control, lowers the power at the same time as it lowers the speed.
The train will stall and jog as you play with the control.

However with PWM every pulse activates the full power of the motor.
This kind of speed control allows the train to smoothly creep into
every station position stop no matter the number cars attached.
And put on a smooth un-jerked train start as the control is increased.

That is the Advantage of PWM (  Pulse-Width-Modulation ) versus the
simple analog voltage control...


In addition to previously mentioned things PWM allows for smaller form factor than variable voltage in general, because of heat dissipation reasons. In PWM generation a small transistor is switched between fully on and fully off, therefore it has minimum resistance or is not conductive. If you hold a transistor in between, in partially conductive state, then it has a resistance and heat is dissipated on it. Same applies for voltage divider like a potentiometer. And if there is heat you need bigger form factor and possibly a heat sink.

PWM is easier to control in a digital control environment.

The term "servo" is badly abused by many people. It is NOT just the little things that sit in model planes, but its ANY system which guides an output variable to the demands of an input variable using feedback.

Model servos RELY on the PWM input signal for their operation, it can't use an analogue input signal, but IN GENERAL many servo systems WILL accept an analogue input signal.

Analogue Model servos in generally work like this:


To expand a bit on the other answers PWM - Pulse Width Modulation - Provides the full voltage but for a vairable amount of time.

This means that whilst the motor/LED is on it is receiving the full voltage and current required and so has it's normal torque/brightness available. If you simply vary the voltage, because power - watts - is volts x amps you will reduce the power available and hence reduce the efficiency of the device - this is most noticeable in motors which have much reduced torque at low voltages and LED dim much lower and better with PWM. If your using a microprocessor or equivalent electronics then PWM is the way to go.

Your hobby servo however works in a different way - see other post - The width of the transmitted pulse controls the position - This pulse must be transmitted on a regular frame as expected by the servo mechanism.

I keep directing people to these 3 PDF manuals as the principles apply to all/most microprocessors and it is well explained. http://www.picaxe.com/Getting-Started/PICAXE-Manuals/