Pulse Width Modulation explanation

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Discussion Overview

The discussion revolves around the concept of pulse width modulation (PWM), specifically how varying the pulse width of a waveform can generate different average voltages. Participants explore the implications of PWM in applications such as LED dimming and the underlying principles of voltage averaging in PWM signals.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about the term "NET" voltage and questions why an LED does not simply turn on and off but instead exhibits a dimming effect when driven by a PWM signal.
  • Another participant explains that PWM is a ratio of the on time to the off time, providing an example with a 10-second period and varying duty cycles to illustrate how average voltage can change.
  • A different participant suggests that PWM is primarily a coding scheme, although they later acknowledge its application in controlling LED brightness.
  • One participant describes how PWM operates by integrating charge during the active time of pulses, leading to an average voltage output that varies with the duty cycle.
  • Another participant discusses the concept of average voltage in relation to the area under the curve of a PWM waveform, comparing it to AC signals and RMS calculations.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and interpretations of PWM, with no consensus reached on the clarity of its explanation or the specific terminology used. Some participants agree on the basic principles of PWM, while others raise questions and challenge the clarity of the concepts presented.

Contextual Notes

Participants reference different aspects of PWM, including its application in LED dimming and its role as a coding scheme, indicating potential limitations in understanding the full scope of PWM's functionality. There are also unresolved questions regarding the definitions of "NET" voltage and average voltage in the context of PWM.

physio
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I have read a lot of articles on pulse width modulation and I am none the wiser as to how it generates different voltages by simply varying the pulse width of the waveform. What I read was that if you have a rectangular wave train of some amplitude and change the 'ON' period the "NET" voltage also changes! I don't understand what it means by "NET" voltage. If you have an LED and if you apply the same pulse train at some frequency then shouldn't the LED simply turn ON(+V volts) and OFF(0V). Why do we have a dimming effect? Also, based upon this explanation can anyone derive the expression of the average voltage i.e (Ton/T*V) volts. Why is it a ratio of Ton to T?
 
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hi there,

Im no expert at this and some one else may answer it in a clearer way
I will have a go with the way I understand it and what you maybe haven't realized is that a PWM signal is a ratio between the on time and the off time.
Take a PWM signal of 10 second period and a duty cycle of 100% and a supply voltage of 10V. That means the voltage, 10V, will be supplied for 100% of the time or all of the 10 seconds.
if you reduced the duty cycle to 50% then the voltage would be there for only 5 sec and off for the next 5 sec. And I suspect the avg voltage would be 5V.
Not sure if in this case avg and NET are the same thing ?

lets see what others say :)

Dave
 
physio said:
I have read a lot of articles on pulse width modulation and I am none the wiser as to how it generates different voltages by simply varying the pulse width of the waveform. What I read was that if you have a rectangular wave train of some amplitude and change the 'ON' period the "NET" voltage also changes! I don't understand what it means by "NET" voltage. If you have an LED and if you apply the same pulse train at some frequency then shouldn't the LED simply turn ON(+V volts) and OFF(0V). Why do we have a dimming effect? Also, based upon this explanation can anyone derive the expression of the average voltage i.e (Ton/T*V) volts. Why is it a ratio of Ton to T?

phisio,

Have you checked the Wiki page on pulse width modulation (PWM) yet? There find many graphic examples and descriptions how PWM is used. It has most of the important facts you need.
http://en.wikipedia.org/wiki/Pulse-width_modulation

Cheers,
Bobbywhy
 
I did check it out but it doesn't give a sound explanation to the questions posed.
 
physio said:
I did check it out but it doesn't give a sound explanation to the questions posed.

Thats probably because you are asking the wrong question. PWM is not about turning LEDs on and off, it is a CODING scheme where, as the previous posts have indicated, the information is encoded in the pulse width.

EDIT: OOPS ... I now see that it IS used in the way you indicate. Sorry. I had only been aware of it as an information coding scheme.
 
Actually, pulse width modulation simply works on the amount of Charge passed during the active ('on') time of the pulses. This charge is integrated in an RC combination to give an average Voltage - based on the average rate of charge going into the capacitor (smoothing / low pass filtering, if you like) - this can give you a continuously variable output value of Volts from a continuously varying value of duty cycle, which is very well suited to a non-linear transmission channel.
Its a good principle for making a high efficiency amplifier as there is (approaching) zero power dissipation in the amplifying device, because it is On or Off for most of the time and the oly significant power is dissipated during the switching transitions.
 
Look at any graph of voltage vs time.
"Average" is the area beneath the curve and that's why a DC voltmeter plugged into a wall outlet reads zero - a sinewave is symmetric with equal areas above and below zero so average is zro.

To get average of a PWM wave you'd just add the areas of the individual rectangular pulses.
Wide pulses have more area than skinny ones.

here's an article that makes that basic point, for backlighting LED's with PWM.
http://www.tftcentral.co.uk/articles/pulse_width_modulation.htm

In AC motor drives you PWM an AC wave, and to get power you have to take RMS instead of average .

hope this gets you started.

old jim
 

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