Frequency control of AC

Discussion in 'The Projects Forum' started by zeusy, Mar 26, 2014.

  1. zeusy

    Thread Starter New Member

    Mar 26, 2014
    Hi all,
    New member here and glad to be around others of similar inclinations. Normally I have been able to solve issues I've had with any projects or google has helped me but I'm trying something and I'm just not sure.

    To simplify it, I've got a 240vac heating element that I want to control with 24vdc pwm. Can I do this by running the active for the heating element through a solid state relay controlled by the pwm? If not, any other suggestions on how to use the pwm to control the AC?

    Also, I'm using PID to control the "on" time of the pwm so the supply comes on full until it get closer to reaching the desired temperature at which point it reduces the % of "on" time until it reaches the desired temp. I've played with the PID settings to simulate it doing what I want but I don't fully understand how the settings affect the output of the controller. Anyone got a good link (or can explain here) that would help explain how the settings affect the output?

    Thanks all
  2. crutschow


    Mar 14, 2008
    You can use a solid-state relay to control the heating element using PWM but the frequency of the PWM should be low, perhaps no faster than a few Hz (sometimes called Bang-bang control which is used to control environmental ovens). This is because the solid-stage relay stays on until the end of a half cycle of the mains frequency so your switch frequency should be much less than the mains frequency for good proportional control.

    Here's some info how to tune PIDs. How much you need to tune the PID depends somewhat on the accuracy of the temperature you want to control.
  3. zeusy

    Thread Starter New Member

    Mar 26, 2014
    Thanks crutschow. I'll see whether it's possible to alter the frequency of the pwm with the controller I am using.
    Also, I have the option of a current output which is 24vdc up to 5A. Is there some device anyone knows of that will "dim" the 240vac using a current input?

    Thanks again
  4. strantor

    AAC Fanatic!

    Oct 3, 2010
    I have used this in the past, works great. Don't forget the SSR and transformer that goes with it.

    But I really don't think you need it. The thermal mass of [just about anything] is such that cycling on/off like heater controllers do, is just fine. The only time you really need "dimmer"-type (phase angle) power control like I linked to, is when the thermal mass of your workpiece is so low that it cools instantly when power is removed; like a thin piece of wire in a hot wire cutter (that's what I used the part in the link for).
  5. zeusy

    Thread Starter New Member

    Mar 26, 2014
    Thanks for your help.
    I can adjust the frequency output. Our AC is 50hz so what frequency should I use? 30?
    Also, I'd prefer to use this "dimming" setup at the heating element (a ceramic heat emitter), from what I've heard, are prone to a shorter life with a standard on/off style thermostat. They're not the cheapest heating devices in the world so if I can prolong it's life by using this setup, then I'd like to go that way.
  6. MaxHeadRoom


    Jul 18, 2013
    As strantor mentions, High wattage heating control by SSR is usually done with burst firing, on/off periods of varying length, I don't see any advantage to PWM unless the reaction time requires to be fast as in foam cutting etc.
    Opto22 make suitable SSR's.