555 Constant Freq PWM

Discussion in 'The Projects Forum' started by DC_Kid, May 21, 2011.

  1. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    638
    9
    with so many posts here about 555 PWM i cannot seem to find what i need. i need simple PWM 555 circuit to drive a triac. triac being used is logic type to keep gate power way down and is rated 10A RMS 800V. triac load will be 60Hz 120vac 1amp. i just need to be sure that the PWM freq is always greater than the 60Hz load frequency, otherwise its possible to chop off the load when it should be on, etc.

    will this work without changing frequency (or if it changes does not drop below 60Hz) when varying the pot (aka turn the knob)?
    [​IMG]
     
    Last edited: May 21, 2011
  2. #12

    Expert

    Nov 30, 2010
    16,298
    6,811
    Geez...label the pins, will ya. I feel too lazy to try to reconstruct this, then answer. The frequency circuit can stand alone and there is a control pin that alters the voltage switching points. That's how you keep the width seperate from the frequency.
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    This is a very old circuit, and I thought everyone knows the 555! :D

    555 PWM Oscillator
     
  5. DigitalReaper

    Member

    Aug 7, 2010
    70
    2
    Am I the only one who noticed he's trying to PWM a triac? Once you turn them on they don't turn off until the current drops to (nearly) zero.
     
  6. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    638
    9
    thank you Bill.

    its AC 60Hz, it drops to zero every 180 degrees (two times per cycle, or zero @ 120Hz). without gate the triac turns itself off after each half cycle.

    perhaps i am wrong in my original post, perhaps the PWM frequency has to be slower, perhaps 1Hz, this way the load freq is sure to catch a 180 degree zero point and the triac turns off during PWM off period and stays on during PWM on period. it wont be exact since the PWM freq wont be synchronized to load freq, and might lose on/off ability are the very far ends of %PWM (like 1% and 99%) because the time between on/off or off/on at these %PWM might be too fast for 60Hzac to catch a zero point, but for my simple PWM control of a soldering iron i think it should suffice....

    does this sound ok?

    and just as fyi, my cheapo soldering iron control box was using a rotary light dimmer, but now the dimmer is dead and all i get is 100%, and i already have a 555 circuit board (lots of them) that i use for other projects, etc, hence why i am traveling down this (what would be considered) poor design...... but for a $2 triac i think its ok for this application.
     
    Last edited: May 21, 2011
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