Pulse train back to straight DC?

Discussion in 'General Electronics Chat' started by kenw232, Feb 20, 2015.

  1. kenw232

    Thread Starter Active Member

    May 18, 2009
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    If I apply PWM to a straight DC signal, can I not just take that resulting square wave pulse train and rectify it back to straight DC? But this resulting straight DC out is not as powerful because it was limited with the PWM. Kind of like a poor mans current limiter. Correct?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    No. The PWM output duty-cycle controls the average voltage, not the current.
    The current is limited by the PWM power supply and the switching transistors' current capability.
     
  3. kenw232

    Thread Starter Active Member

    May 18, 2009
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    I'm completely lost. Its the same thing. Average voltage X current = watts. if I lower the average voltage with PWM using a low duty cycle then wattage drops accordingly. Of course the current is limited by the switching mosfets, thats how it works.
     
  4. #12

    Expert

    Nov 30, 2010
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    Lower voltage causes lower current, but the math works better in terms of voltage difference.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Careful. Vrms • Irms ≠ Prms
     
    cmartinez likes this.
  6. WBahn

    Moderator

    Mar 31, 2012
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    I'm not sure what it means to "apply PWM to a straight DC signal". Your PWM pulse train generally IS a DC signal (in terms of being of a single polarity), so what is there to rectify. You can filter it to retain only (or at least mostly) the DC (i.e., 0Hz) component.

    As to what is the limiting factor, that's a separate issue. One of the advantages of PWM is the ability to deliver high instantaneous power with low average power. This can be handy when working with servomotors, for instance, because if you want to turn the motor very slowly or get a smooth start, it is hard with a DC supply because you have low torque at the low voltages and so you get a jerky start-up. But with PWM you get a whole bunch of high torque "kicks" that get past the stiction more smoothly.
     
  7. crutschow

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    Mar 14, 2008
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    No it's not the same.
    You stated that the PWM acts as a current limiter.
    It does not, expect at the power supply or MOSFET limits.
    The PWM adjusts the average voltage with the current determined by the load resistance.
     
  8. crutschow

    Expert

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    What? :confused:
     
  9. WBahn

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    Power factor.
     
  10. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    you could electronically calculate your avg. power by integrating the output of a difference amp with your PWM on one input and s DC level equal to the pk voltage of your PWM on the other.

    Rectification will have losses that prevent a true conversion to your avg power by equivalent DC voltage levels
     
  11. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    PWM does nothing to limit current. You need some other element to limit current.

    "the current is limited by the switching mosfets" is not true either, the fets are just turning on and off a voltage.

    Look this way: Say you put 10 volts into a dead short circuit; You get an amount of current that approaches infinity.

    Now turn theat voltage on and off to pulse width modulate it; you now get pulses of current that where each pulse approaches infinity.
     
  12. crutschow

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    Mar 14, 2008
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    I don't think Power Factor is a factor in a PWM output generating DC. ;)
     
  13. WBahn

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    Mar 31, 2012
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    Both current and power factor are primarily dictated by the load.

    As an example, consider driving an inductor or a capacitor with a PWM waveform. You will get a Vrms and an Irms, but you will still be dominated by reactive power yielding very little Prms.
     
  14. ian field

    Distinguished Member

    Oct 27, 2012
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    I would have put it slightly differently...............

    In the usual case the voltage output is sensed, and that is what's used to control the PWM generator.

    Its not impossible to sense current and use that to control the PWM.
     
  15. crutschow

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    Mar 14, 2008
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    Of course, but I don't see how that applies to the subject of this thread. :confused:
     
  16. wayneh

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    Sep 9, 2010
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    I only brought it up in #5 because of the statement by the TS in #3.
     
  17. alfacliff

    Well-Known Member

    Dec 13, 2013
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    also, motor speed and starting wold be affected. pwm pulses start up motors more easily than giving them a low voltage dc that you got from filtering the pwm pulses.
     
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