PWM, not working properly, need advice!

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

  1. bytraper

    bytraper Thread Starter Member

    Joined:
    Sep 28, 2010
    120
    3
    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 ?
     
    #21
  2. bytraper

    bytraper Thread Starter Member

    Joined:
    Sep 28, 2010
    120
    3
    I still can't figure this out... any help will be appreciated!
     
    #22
  3. shortbus

    shortbus Senior Member

    Joined:
    Sep 30, 2009
    3,334
    923
    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.
     
    #23
  4. bytraper

    bytraper Thread Starter Member

    Joined:
    Sep 28, 2010
    120
    3
    No, it feeds straight to a driver IC
     
    #24
  5. crutschow

    crutschow AAC Fanatic!

    Joined:
    Mar 14, 2008
    8,834
    1,817
    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.
     
    #25
  6. bytraper

    bytraper Thread Starter Member

    Joined:
    Sep 28, 2010
    120
    3
    No, its not normal, where the pwm freaks out is where the mosfets or the driver blow up.
     
    #26
  7. bytraper

    bytraper Thread Starter Member

    Joined:
    Sep 28, 2010
    120
    3
    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.
     
    #27
    Wendy likes this.
  8. Wendy

    Wendy Moderator

    Joined:
    Mar 24, 2008
    19,940
    1,986
    Happens to us all. And remember, experience is when you make the same mistake a second time.
     
    #28
  9. crutschow

    crutschow AAC Fanatic!

    Joined:
    Mar 14, 2008
    8,834
    1,817
    Or as I heard it "Experience is what allows you to recognize the same mistake twice". ;)
     
    #29
  10. wayneh

    wayneh AAC Fanatic!

    Joined:
    Sep 9, 2010
    9,790
    1,630
    "Education is what you get reading the fine print. Experience is what you get when you don't." Pete Seeger
     
    #30
Loading...