I'm trying to build a micro-controller controlled PID. So far I have found a few recipes for high powered AC voltage ( 10 - 15 Amps ). The first recipe I found involved Using A potentiometer, a capacitor, a resistor a triac an a diac as illustrated below, however this is an analogue controlled system.

The second system involves measuring the 0 crossing with the controller, and using a opto-isolated Triac. The issue with the example shown is that it is designed for control a lamp ( 100 watts ) and I'm looking to control a 1500 watt heater. Can someone recommend a suitable opto-isolated Triac. I was considering using a relay, but I want smoother control over the heat, and I would be concerned that the relay would wear out pretty quick if switching on and off so frequently.

enter image description here


If you are using heater, then a better solution is to use PWM technique with zero cross SSR / triac, rather than dimming with phase control. The entire process won't "see" the difference if you choose for example a 10s PWM period and turn the opto triac with zero cross with a timer controlled output. The phase control also produces EMI and has nonlinear output vs. control phase angle, additionaly you need a zero cross detector. It makes sense to use phase control for dimming lights or for motor control.

EDIT: Difference between phase angle control and PWM control of a triac/SSR enter image description here

  • \$\begingroup\$ I'm not sure how phase control is different from what you are recommending... Are you talking about tail edge control vs leading edge control of the waveform? \$\endgroup\$ – Andrew Spott Sep 30 '16 at 21:21
  • \$\begingroup\$ @AndrewSpott I have added the depiction of the difference. \$\endgroup\$ – Marko Buršič Sep 30 '16 at 21:38
  • \$\begingroup\$ Ah, that makes it more clear why you referenced the heater at the beginning. \$\endgroup\$ – Andrew Spott Sep 30 '16 at 22:34
  • \$\begingroup\$ Any recommendation on what components to use ? \$\endgroup\$ – user379468 Oct 1 '16 at 16:40

The circuit you have shown is for a simple dimmer switch. It is not suitable for your application.

The simplest and safest solution is to use an opto-isolated solid state relay (SSR). A 1500 W heater is likely to have a thermal response measured in many seconds so we can use a simple on-off control system with variable duty-cycle to control the average power being sent to the heater. By making the switching cycle short (say 2 to 5 s) relative to the response time of the heater (tens of seconds) tight control of the temperature is possible.

enter image description here

Figure 1. Adjusting heater power by varying the duty cycle of a mains on-off controller. The waveform is the 50 or 60 Hz mains voltage applied to the heater.

So using the scheme of Figure 1 we can adjust the power from zero to 100% in steps of one mains half-cycle.

enter image description here

Figure 2. A typical industrial grade SSR by Crydom. These are available in zero-cross and non-zero-cross versions. (This one has an AC control input. For a micro-controlled application a DC input type would be used.

For your application you want a zero-cross type which will, when triggered, wait until the next zero-cross of the mains before turning the TRIAC output on. This will minimise electromagnetic interference with nearby devices.


simulate this circuit – Schematic created using CircuitLab

Figure 3. Sensor, micro, SSR, fuse, supply and load.

Normal safety and good layout rules apply.

  • Keep mains away from low-voltage circuits.
  • Mount the SSR on some sort of heatsinking.
  • Don't skip the fuse.
  • \$\begingroup\$ To be honest, when I look up the industrial SSRs with a 10A - 15A rating, they are in the 100 dollar range. I should have been more specific, it's a heating coil within a popcorn popper, and it's actually more like 900W. It really has very little thermal mass, so I don't think the long duty cycles will work, also there is a fan involved so it makes that issue worse. Why woud one choose a SSR over a octo - triac setup? \$\endgroup\$ – user379468 Oct 1 '16 at 16:38
  • \$\begingroup\$ "I should have been more specific ..." Agreed. "Why woud one choose a SSR over a octo - triac setup?" It's an opto-triac. Look at the current ratings on the datasheet. \$\endgroup\$ – Transistor Oct 1 '16 at 17:14
  • \$\begingroup\$ No need to condescending, it was a typo \$\endgroup\$ – user379468 Oct 2 '16 at 16:23
  • \$\begingroup\$ What gave the impression of condescension? I have no way of knowing whether it was a typo or whether you thought that was the correct name. I have given you an amount of my time answering your question and you should have read the comment in the light of my answer. Bye. \$\endgroup\$ – Transistor Oct 2 '16 at 16:37

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