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I have a 500W (230V) cartridge heater element and a K-type thermocouple. I want to use the feedback from the thermocouple to be able to vary the temperature of the cartridge heater. I have no concerns about being able to implement the thermocouple circuitry and software, however I have never used a high power heating element before.

I'm looking into the control of the element. I personally don't have an issue with an on-off control (as in, I'm okay with applying some hysteresis and having, say +/- 10°C from the desired value, or using PWM), however maybe there are some other considerations as to why this wouldn't be a good idea (switching life cycles for example). If there is a component to safely vary the power to the heating element, so I can implement PID for example, then I would also be interested in that.

As I want to control the heating elements from a microcontroller (preferrably 3v3, but can level shift to 5v if needed), I've been looking into relays. However, I am struggling with the power requirements - from what I find, 500W seems to be a lot of power for a PCB based relay (although since I've never used a relay before, maybe I am misinterpreting the specifications). I'm also concerned about the actual traces on the PCB - I'd much prefer to have the relay controlled by PCB traces but the heating elements just connected to the relay by wires, if such a device exists.

I've looked into latching, non-latching, solid state relays, SCRs, triacs, but I feel like the more I look into it, the less sure I am. I'm now at the point where I feel like there must be a standard way to do this, but I'm just getting lost in the sea of options.

So, is there a standard circuit/device to safely control the temperature of 500W (230V) heating element using microcontroller-level voltages? The cheaper/simpler method is usually better in my eyes, as long as it's safe.

EDIT:

Thanks all for the comments and answers so far. Looks like I'll go with some type of relay, and possibly some backup protection if I want to use a latching relay. One of the main issues I'm having with finding a suitable relay is geting the specifications correct. For example, the following is taken from a relay datasheet:

enter image description here

The first thing I see is that max switching voltage and max switching current are both high enough - great. Then I saw the rated load box, when confirms this. However, what confuses me is the 'Max. switching power' box. I'm already a little bit hazy on the difference between VA and Watts, but the fact that at has a maximum of 300W, while my element is rated at 500W, confuses me. Can anyone help explain this?

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  • \$\begingroup\$ Have you considered something along the lines of an old-fashioned thermostat? Bimetallic strip and a magnet, that's about all you need. No active electronics at all. \$\endgroup\$
    – Hearth
    Jun 3, 2020 at 16:58
  • \$\begingroup\$ I haven't considered that yet. Interesting thought. I'm hoping to have software control over the temperature, so I'm guessing I would sacifice that with such a system? \$\endgroup\$
    – rbv
    Jun 4, 2020 at 10:43
  • \$\begingroup\$ Yes, software control of the temperature would be hard in such a system. \$\endgroup\$
    – Hearth
    Jun 4, 2020 at 11:22

3 Answers 3

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500 watts at 230 volts is only 2.17 amps. If you don't want to have the power continuously on at that power level, you might want to design for 2.5 to 3 amps. An PCB mounted electromechanical relay for that current rating and a resistive load should not me difficult to find.

You should also consider using a solid state relay to trial with zero crossing control. The idea would be to pass a certain number of full cycles of AC line current and block a certain number, varying the on/off ratio under PI or perhaps PID control. Regulating to only +/- 10°C seems like quite a pessimistic expectation. The old-fashioned thermostat suggested by @Hearth would do better than that.

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  • \$\begingroup\$ Thanks for your answer. I've editted my question with an example relay datasheet. While I'm hopeful that I will find something suitable, I have some concerns which the edit show. Can you help at all with my understanding of what specifications I should care about when looking for a relay? I'll look into the zero crossing thing too, thank you very much \$\endgroup\$
    – rbv
    Jun 4, 2020 at 11:01
  • \$\begingroup\$ That is not a very clear spec, but I suspect that the switching values in parentheses are for DC. \$\endgroup\$
    – user80875
    Jun 4, 2020 at 13:18
  • \$\begingroup\$ The values in parantheses are for "Bifurcated crossbar contact type" relays - the datasheet is for multiple models. Sorry, I should have included that in the screenshot. I'm mainly confused about the 2,500VA (which is way more than needed for my 500W element - great), and the 300W (which is obviously no good) being in the same "Max. switching power" specification \$\endgroup\$
    – rbv
    Jun 4, 2020 at 13:45
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    \$\begingroup\$ Given that the parentheses have been explained, I would propose the 300W as the DC switching rating. \$\endgroup\$
    – user80875
    Jun 4, 2020 at 14:17
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The easiest way, and probably the one that will yield the best control, is to buy an industrial temperature controller with some kind of serial interface, and switch either a power relay or an SSR with the output. That buys you the thermocouple front end, properly isolated from ground so a fast sensor can be used, an auto-tune PID algorithm and a user display.

Power relays are inexpensive and reliable, however they wear out after about 100,000 operations at full current. At 20 seconds per cycle, that's a couple years operating 24/7. If you derate the contact capacity you can get considerably longer life.

SSRs generate heat (albeit not much at only a few amperes, but will generally require a heatsink) and tend to fail more randomly, and generally fail 'on', which is often an issue.

In either case, if the controller being stuck 'on' or 'off' could possibly cause significant injury or damage you'll need a redundant backup such as a thermal fuse or an independent limit controller.

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  • \$\begingroup\$ I'm not so interested in a pre-built controller, I already have a thermocople circuit from a previous circuit which I know allows me to read the temperature accurately. And the project has a microcontroller used elsewhere so it's no hassle to add in some PID to the software. Thanks for the info about relays and SSRs. 20 seconds per cycle running 24/7 would reach 100,000 cycles in around 24 days, no? But I think that 100,000 cycles will be enough for this projects lifetime. \$\endgroup\$
    – rbv
    Jun 4, 2020 at 10:51
  • \$\begingroup\$ I've editted my question to add a relay datasheet. Is there any more advice you can give on power relays to help my confusion? \$\endgroup\$
    – rbv
    Jun 4, 2020 at 11:00
  • \$\begingroup\$ Can you link the actual relay datasheet? \$\endgroup\$ Jun 4, 2020 at 12:11
  • \$\begingroup\$ docs.rs-online.com/51dc/0900766b80dbf2c8.pdf It's a datasheet for multiple models but the screengrab was from page 4 \$\endgroup\$
    – rbv
    Jun 4, 2020 at 13:42
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    \$\begingroup\$ The bottom line refers to switching a DC resistive load, so you can ignore that in your application. DC is more troublesome to switch because there are no zero crossings to help interrupt the arc. VA=W for a resistive load. \$\endgroup\$ Jun 4, 2020 at 13:50
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There are many ways to skin a cat, as you've found...

If your heating load has a large thermal mass, and takes a while to heat, then the bang-bang on/off model is simple and you can use a relay. You can get relays with faston tabs, so that you aren't running the current through your PCB, although 2.2Amps isn't that difficult to do in a PCB. See example image below, though this specific relay may not be what you need, it shows the fast on mounting.

enter image description here

Being that it's only 2.2Amps, you can also control it with Triacs. This would allow much finer control, like PWM but cycle by cycle instead. There would be higher losses on the load side compared to a relay, but the relay coil is also a loss, so you'd have to see if one wins out over the other for your situation.

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  • \$\begingroup\$ You're right in that I've been able to find many relays which are reated for 2A, however I'm struggling because a lot of them seem to go on to say they are raed for less than 500W. I've editted my question to illustrate this, can you help with this at all? \$\endgroup\$
    – rbv
    Jun 4, 2020 at 11:02

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