PWM Heating Element Frequency

Discussion in 'General Electronics Chat' started by Stuntman, Dec 6, 2012.

  1. Stuntman

    Thread Starter Active Member

    Mar 28, 2011
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    I have a meat smoker that utilizes a 120V heating element. However, this particular model does not have any kind of temperature control, this means lots of deviation in smoking temp when the outside temperature fluctuates.

    I am planning on doing a simple variable duty cycle PWM scheme with an SSR. (Yes, I may design an automated control system at some point)

    1.) Is there an optimum frequency that a PWM like this should be implemented?

    Given the slow reaction of the heating element, I could see something on the order of seconds being acceptable... That said, I wonder if letting the element cool too long between "on" cycles decreases element life?

    2.) I'm assuming that given an SSR is basically a Triac with an isolated gate circuit, it can handle this kind of on-off operation. Anyone care to deny or confirm this?
     
  2. #12

    Expert

    Nov 30, 2010
    16,321
    6,818
    All changes in temperature cause changes in pysical dimension. 30 seconds off would allow a lot of cooling. I'm thinking 10 Hz would be about right for a power line regulator. If you're going to use DC, you could go to 1000 or 10,000 Hz and not hurt anything. Somehow I'm believing you are going to use the power line for energy.
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    We used the same system to control the temps in our burn in chambers. A controller setup ran a SSR which controlled the AC going to the heater elements. I remember they pulsed at maybe 2 Hz, you could watch the LED indicator flash.

    The SSr's blew out a lot. Hopefully they make them better now. They are sensitive to dV/dt rate and sometimes need an R-C snubber on the load side.

    I scrounged a couple of the SSr's and built a system to turn my hot steam vaoprizer on and off during the night. SSR's blew up at random times. Worked for months then died, no reason.
     
  4. Stuntman

    Thread Starter Active Member

    Mar 28, 2011
    181
    47
    Good information.

    The nice thing is, probably the lowest duty cycle I will run will be around 80% (I will always need ample heat to smoke) so the cooling, even at low frequencies, shouldn't be too terrible on the element...

    However, keeping that element warmer will alleviate bad inrush currents and hopefully keep this SSR working longer.

    I'll try something around 5hz and see how she works.
     
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    I don't think it was current killing the SSrs. It was the dV/dt rate on the load side. If it's too fast, the voltage couples through the internal capacitance and breaks down the gate and kills it. I remember they ended up with an R-C snubber bolted right to the SSR terminals and they lived longer but still died regularly. Hopefully, they make them sturdier now.
     
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