Pulse Width Modulation & Duty Cycle

[tahir4awan]

The term Pulse Width Modulation often confuses me. Because it is a modulation technique use in communication as it name implies. But why do we use this term in power control circuit.For example to increase and decrease motor speed we increase and decrease duty cycle of the voltage. We are only specific to duty cycle and there is no term modulation is used then why do we use this term PWM?

 

[Tahmid]

You are changing the width (the duration) of the pulse on-time. Let's say you have a signal with frequency of 50Hz, upon which you will carry out PWM. Then time period is 20ms. If you provide power to the motor for 15ms and keep power off for 5ms, you are controlling the power to the motor and restricting it to (15/20)*100% = 75%. The longer the pulse the greater the power. The smaller the pulse, the lower the power. By changing the width (duration) of the pulse, which is what the duty cycle indicates, you are controlling power.

What is modulation? According to https://searchnetworking.techtarget.com/definition/modulation

Modulation is the addition of information (or the signal) to an electronic or optical signal carrier. Modulation can be applied to direct current (mainly by turning it on and off), to alternating current, and to optical signals.

So, PWM is carried out by turning the switch on and off.

Hope this helps.
Tahmid.

 

[tahir4awan]

In your motor power control example you are talking about signal width control i.e changing its duty cycle. My question is why do we call it Pulse Width Modulation while it is simply a signal width controlling and there is no modulation involve in it.
In communication PWM is a technique in which an analog signal is modulated with a carrier so that a PWM signal generated.

 

[Tahmid]

See here (point 6): https://www.thefreedictionary.com/modulation

The meaning of the word modulation is, variation of the property of a wave or signal. In pulse width modulation, by changing the duty cycle, we are changing a property of the signal: the on-time, which is the pulse width. That's why it's called pulse width modulation - we carry out modulation (change in a property of the signal) by changing the pulse width. In amplitude modulation, you change amplitude; in frequency modulation, you change frequency. Similary, in PWM, you change pulse width.

Hope this helps.
Tahmid.

 

[tahir4awan]

You right that modulation is a variation of the property of a wave or signal but the property of the wave is changed with respect to other signal.For example in AM and FM modulation the analog (message) signal causes the change in carrier signal.
But if you see a 555 timer based PWM duty cycle is changed by RC time constant. We can not change RC time constant value by electrical signal its mean it is a fixed duty cycle generator. What I mean for a modulation process there must be two signals i.e the signal to be changed and the signal that causes change.

 

[keith1200rs]

In PWM there is a second signal that causes modulation - the signal that varies the duty cycle. Without that it wouldn't be PWM - it would simply be a fixed duty cycle.

Keith

 

[Tahmid]

There obviously is a second signal that allows change in duty cycle. Unless it was there, you'd just get a fixed duty cycle, since, like you said, RC time constant is fixed - it's a constant.

 

[BradtheRad]

Pulse width modulation might refer more specifically to something like a signal to control a servo. The servo is positioned by a pulse which varies from 1 mSec to 2 mSec. The frame rate is usually about 50 per second, but this spec is not critical.

When we vary the length of the pulse, there is a sense in which we are changing the duty cycle. However duty cycle is not the relevant factor controlling the servo.

 

[Tahmid]

Of course, it depends on which device is being controlled. For a DC motor, though, duty cycle will be the relevant factor. But in all cases, it's the pulse width that's being altered, be it a servo or a regular DC motor or something else being controlled.