110Vac heater control, bangbang or PID?

  • my bed heater is 110v 1000 watt, omron ssr, can this be used with thermocouple and daughter board controlling it via PID or would it be better as a bang-bang, same question for my chamber heat, 500 watts 110vac but it is a PTC type heater with a 24vdc fan built into it. suggestions on best ways to control from Duet2 wifi, i also have the duex2 board as well as one each pt100 and thermocouple daughter boards.


  • as long as you specify a reasonably low switching frequency PID probably works better as it allows you to autotune the heater.

  • Could you get some weird output oscillations if your PID frequency and mains frequency weren't in sync?

    With DC you just cut the supply as you like but with AC wouldn't you need it to be in sync with the supply and cut at a specific voltage level, resetting on zero voltage cross over?

  • I could see that happening as i have done some PLC programming and with timers you need to debounce it or weird things happen, even with DC. so is the PID just pulse width modulation or does it also control current? maybe i need to look at installing some type of voltage control so that you could cut the voltage down so that it would not cycle too fast while maintaining target temp.

  • As far as I'm aware it is just PWM control on the heater/fan outputs on the duet. Someone else may well confirm/correct.

    For AC I'm guessing you would need to do an add on module that is controlled by a PWM signal. At a guess voltage clipping would be easier, but it wouldn't necessarily be linear with power output, so some tuning/correction would be required - but then again the PID autotuner may cope with that.

    All that said a large thermal mass on the heatbed may well be enough to smooth over the issues of pwm on AC.

    Edit 2: Add on module = inverter?

  • Most SSRs do zero crossing switching, and I believe RRF has a default 10hz frequency when using PID for the bed in recent versions to avoid problems with the SSR actually switching on the 50/60hz zero crossings instead of exactly the PIDs PWM frequency.

    I didn't have any issues with a 230V/50Hz heater with a zero crossing SSR at least.

  • @bearer interesting, thanks for the background info. My vapour build is still alternating between the AC and DC heaters - a balance of dangers of AC and the costs and complexities of dealing with high current DC.

    Still, not as bad as planning for a 12V winch on the truck. Got an awesome looking 1000A busbar in the garage but still need the massive cable crimps and cutters! 😄

  • my bed plate is 310x310x1/4in thick cast alum jigplate i have a pc of 1/16 garolite under the silicone heater, and 1/8 cork under that. the lift platform is 1/4 alum swiss cheesed for weight and airflow0_1562664143554_heat bed exploded.jpg

  • With proper grounding and leakage current protection I'd say the AC is probably safer for higher wattage than 0000 gauge DC cable carrying 1000A.

  • @bearer the 1000A was forthe winch on my truck, not heater. 1000W at 12V would be around 83A. Although that's high enough to need really good crimps and joints.

  • Yeah I got that, but 83A will still maintain a hell of an arc if something happens, not to mention the cost of power supply, and wiring will buy you a lot of RCDs

