PWM, not working properly, need advice!

Discussion in 'The Projects Forum' started by bytraper, Apr 4, 2012.

  1. bytraper

    Thread Starter Active Member

    Sep 28, 2010
    126
    4
    Bill,
    I took a video of the PWM... at the very beginning and end the triangle wave seems to go haywire. The squarewave is pin 7 and the triangle wave is pin 6


    http://youtu.be/Lpgv3nfWcho

    Hope this helps ?
     
  2. bytraper

    Thread Starter Active Member

    Sep 28, 2010
    126
    4
    I still can't figure this out... any help will be appreciated!
     
  3. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    What is the load on your PWM? Is it a mosfet gate? If so it could be ringing on the gate back feeding in to your circuit.
     
  4. bytraper

    Thread Starter Active Member

    Sep 28, 2010
    126
    4
    No, it feeds straight to a driver IC
     
  5. crutschow

    Expert

    Mar 14, 2008
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    The waveforms look more or less normal to me. At the maximum and minimum duty-cycles you are going to 0% or 100% output with circuit noise causing some sporadic pulses. The "haywire" is likely from the oscilloscope sync. If you sync on the sawtooth waveform you should get a stable sync and will better see what is happening.
     
  6. bytraper

    Thread Starter Active Member

    Sep 28, 2010
    126
    4
    No, its not normal, where the pwm freaks out is where the mosfets or the driver blow up.
     
  7. bytraper

    Thread Starter Active Member

    Sep 28, 2010
    126
    4
    Just to let everyone know what happened with this, there was no problem with the pwm, the error was completely mine.

    I made a really bad error that I didn't pick up on for ages. Floating inputs on the 5v rail cause random noise.... should have realised when I put my hand near it and it started to go wonky. Stupid, stupid stupid.

    Didn't see it on the schematic because I'd drawn it so compact.
     
    Wendy likes this.
  8. Wendy

    Moderator

    Mar 24, 2008
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    Happens to us all. And remember, experience is when you make the same mistake a second time.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    Or as I heard it "Experience is what allows you to recognize the same mistake twice". ;)
     
  10. wayneh

    Expert

    Sep 9, 2010
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    "Education is what you get reading the fine print. Experience is what you get when you don't." Pete Seeger
     
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