Triac control with PWM?

Discussion in 'The Projects Forum' started by barbababa, Mar 8, 2012.

  1. barbababa

    Thread Starter New Member

    Mar 8, 2012
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    0
    Hello all!

    I am still a bit novice about electronics so i thought if someone could give some comments about this.

    I want to control a resistive heating element according to it's temperature. I have a working circuit with picaxe 20x2 to measure and display the temperature. Also it can output a PWM signal.
    I thought that the current source (max 20 A) would be connected to the element and a triac in series and the PWM signal would control the triac.
    Is this a proper way do deal a problem like this?

    If the current source works with 5v 50 Hz, would it work if i use a let's say 1 kHz PWM to control the current flow across the element?

    Or would high power mosfets fit to this problem better because of the zero crossing problem?

    Very huge thanks for any interest!

    Cheers,
    barbis
     
    Last edited: Mar 8, 2012
  2. Experimentonomen

    Member

    Feb 16, 2011
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    46
    You cannot use PWM with a triac, triacs are used in phase angle controllers for AC, PWM can only be used with DC.
     
  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    You can use phase angle control with a triac. The control signal has to be synchronized with the 50Hz frequency (in your case) , this way you can achieve a controlled switching of the triac.

    That would be a 100Hz control signal. Kind of a PWM, at very low frequency. :)

    Or the easy way: http://search.digikey.com/us/en/products/S108T01F/425-2395-5-ND/720455


    You can also use MOSFETs (2 of them) for AC switching, yes.
     
  4. barbababa

    Thread Starter New Member

    Mar 8, 2012
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    0
    thanks for the replies!!

    I like easy ways...
    About these ssr components, all that i found had a output voltage rating
    of 110 or 230 V. Minimum recommended was 80V.
    Does this mean that these won't work with just 5 or 12 ?

    or...
    Could you give some circuit examples of how to use two moswets to control ac current with PWM???


    huge thanks!
     
  5. praondevou

    AAC Fanatic!

    Jul 9, 2011
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  6. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    What is the AC supply voltage for the heating element? I see no reason that PWM cannot be used with an SCR by using an optically coupled SCR driver. PWM cycle rate about 1 sec allowing 0 to 50 cycles per period or 0 to 98% power.
     
  7. barbababa

    Thread Starter New Member

    Mar 8, 2012
    3
    0
    Great!

    Thanks alot!

    the AC supply voltage will be either 5V or 12V.
     
    Last edited: Mar 10, 2012
  8. John P

    AAC Fanatic!

    Oct 14, 2008
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    I think if you have a low voltage to control, you'd be better off rectifying it and switching it with a transistor. Then you wouldn't need to worry about frequency or phase of the power line.
     
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