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Pulse Width Modulation explanation

  1. Feb 9, 2013 #1
    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?
  2. jcsd
  3. Feb 9, 2013 #2


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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 havent realised 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 :)

  4. Feb 9, 2013 #3


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    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.

  5. Feb 9, 2013 #4
    I did check it out but it doesn't give a sound explanation to the questions posed.
  6. Feb 9, 2013 #5


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    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.
  7. Feb 9, 2013 #6


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    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.
  8. Feb 9, 2013 #7

    jim hardy

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    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.

    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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