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I'd like to use this heater element for a project I'm working on. It looks like a good product but there is unfortunately not much documentation available, but I think it's basically a simple resistive load that can operate from 230V AC. So I started to look around to see how I could control the temperature of this heater and found an interesting circuit in Power Semiconductor Applications (chapter 6.2.2) by Philips:

enter image description here

This circuit produces the following output:

enter image description here

This looks like it could do the job for me. According to the description, it is controlled by the resistance of the potentiometer R2.

Now my question is, instead of a manual potentiometer, how can I modify this circuit to control it electrically, with an MCU (microcontroller)?

Alternatively, what other circuit could I use that can be controlled using an MCU that would achieve the same ends?

Note, I'd also prefer to keep this galvanically isolated from the MCU. I believe this could be achieved using an optocoupler, but I'm not a power expert, so I'm not sure which kind I need here.

Thank you in advance for your help!

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  • \$\begingroup\$ That kind of circuit is remarkably ill-suited to being controlled by a microcontroller. \$\endgroup\$ – Spehro Pefhany Mar 20 '16 at 21:07
  • \$\begingroup\$ @SpehroPefhany I know, that's why I'm asking how to modify it. I'm open to suggestions about alternatives, too. :) \$\endgroup\$ – Venemo Mar 20 '16 at 21:08
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Consider an SSR controlled with the MCU via PWM and with a short (~2 second) proportional cycle. You can use a temperature sensor and feedback control or open loop, depending on requirements.

The SSR provides the isolation, You'll need a large-ish heat sink and/or fan for any solid-state switching.

You should also consider what happens when (not if) the SSR or micro gets stuck on and make sure there is adequate safety provisions (maybe a thermal cutoff or other protections) to prevent injury or property damage.

What you've shown is a phase control- it will cause a lot of electrical noise and is probably unnecessary for this application.

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  • \$\begingroup\$ Good point about phase control. Would triggering only at zero-crossings be less noisy? Does the SSR do that automatically? Are there any other advantages of the SSR over a thyristor? \$\endgroup\$ – Pigrew Mar 20 '16 at 21:18
  • \$\begingroup\$ The problem with an SSR is that it's wildly expensive (and big) compared to anything that you can build with a triac. \$\endgroup\$ – Venemo Mar 20 '16 at 21:19
  • \$\begingroup\$ By the way, I added a picture of what the output is, this doesn't look like phase control for me. \$\endgroup\$ – Venemo Mar 20 '16 at 21:22
  • \$\begingroup\$ @Pigrew Yes, the zero crossing type would be less noisy- but there might be more beat problems as the cycle is shortened. An AC SSR is just a thyristor with an optocoupler, snubber, and drive circuit for the LED. The OP can build one himself. \$\endgroup\$ – Spehro Pefhany Mar 20 '16 at 21:23
  • \$\begingroup\$ @Venemo Are you planning on using it with a vacuum cleaner motor load? Your link is to an almost purely resistive heater. \$\endgroup\$ – Spehro Pefhany Mar 20 '16 at 21:33

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