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A cheap SSR rated for 40A is used as a switch for 2x 4500W heating elements in parallel (around 36A). It is used in 0degC temperature with a heat sink that might not be large enough.

After 2 hours of continuous usage at 36A, the heat sink is super hot. Too hot to touch more than half a second. The SSR becomes unresponsive and cannot be controlled by the inputs anymore. It fails closed. The LED still works and shows the command properly but the relay stays closed.

24h later, it works again! Back from the dead. The SSR responds to commands and is able to control the heating elements properly (only tested for a few seconds).

How could this happen? Which component is the culprit, and why? choice between BTA41 triac, MOC3063 opto and the components shown in the picture below.

There are numerous reports of fake SSR online. This one is a "genuine" cheap chinese SSR which uses a STM BTA41 triac. Here is a picture of the internals. The triac is not shown here but it is below the top pcb. SSR40DA.

The heat sink is external to the SSR and a thin layer of CPU thermal paste (brand new tube of Noctua NT-H1s). It looks like this: enter image description here

lpasselin
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  • What is a "genuine" cheap chinese SSR? Does that mean it’s from a Chinese manufacturer but not a counterfeit? How do you know it’s not a counterfeit? – Ryan Mar 29 '22 at 00:38
  • Conterfeit chinese SSR (my definition) contain parts such as a triac that are rated significantly lower than what they advertise. For example, 100A SSR which would have a triac rated for 16A. – lpasselin Mar 29 '22 at 00:46

2 Answers2

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36A requires a very large heat sink and maybe a fan, that's around 40W of heat dissipation. If the SSR loses control you're already past the point where damage occurs. The heatsink is not just inadequate but grossly inadequate.

There is not a lot of difference between "40A" triacs (none of which I - as I think most experienced engineers- would like to use over about 20-25A- which is why the counterfeits can get by with inferior parts). The interface to the heatsink may be variable- how the triac is held to the base, the flatness of the base and the thermal compound used. If that is faulty or poorly designed the triac could run hotter than necessary.

Maybe you can quadruple the size of your heatsinks and use one per heater (double the SSRs), that will probably get you into a safe range at your current ambient temperature, but that's just a WA guess.

As to "which" component, assuming you are turning the drive off completely to the SSR, the triac (likely) or the optotriac would be most likely to be causing the loss of control. That's with a complete lack of schematic or datasheet and making certain assumptions, of course. More complete information on your end might yield a better (or at least more confident) answer.

Spehro Pefhany
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  • Thank you for your comment. I fully agree with the inadequacy of the components. But that does not answer the question.

    I will update the post to specify that the heat sink is not the heat sink that comes with the SSR.

     – lpasselin Mar 29 '22 at 01:01
  • Thanks for the edit. Do you have any hypothesis at all? – lpasselin Mar 29 '22 at 01:14
  • If you heat a triac (either one) up hot enough it will turn on. That is my hypothesis, assuming your drive circuit is not faulty and leaking current. That temperature is above the temperature required to cause damage, but not necessarily permanent failure instantly. – Spehro Pefhany Mar 29 '22 at 01:16
  • What technical terms should I be looking for in the datasheet of the triac BTA41 to confirm this? – lpasselin Mar 29 '22 at 01:22
  • Maximum operating junction temperature (125°C) would be the main one. For it to turn on the junction temperature would likely have to be in excess of 160°C (typically), but 125°C is what should not be exceeded with power on. – Spehro Pefhany Mar 29 '22 at 01:25
  • "If you heat a triac up hot enough it will turn on." Where can I find more information about this? I understand if you heat it too much, it will simply melt into a wire. That is not the case here.

    I was only able to find this stackexchange post where a triac was turning on with heat. It seemed to be a faulty unit. Not a typical characteristic of triacs.

     – lpasselin Mar 29 '22 at 19:13
  • @lpasselin Where can you find out what happens when you abuse semiconductors beyond the datasheet limits? Mostly you'll find folks telling you not to do that at all. Semiconductors leak more when you heat them, an exponential increase with temperature. At some point the leakage is equivalent to the trigger current and the device turns on, when it is a thyristor. You can find reference to it in papers such as this one, but it's kind of common sense. The immunity to dv/dt is also less when it is hot, for example. – Spehro Pefhany Mar 29 '22 at 19:19
  • Thanks a lot Spehro, this is really useful knowledge for me. I really appreciate your help. – lpasselin Mar 30 '22 at 14:29
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Your failure rate of it turning on and staying on is typical of a Triac failure and or overheating. The fact that it takes that long to get excessively indicates it does not have enough cooling. I cannot see how it is mounted but that is very important. anything that will help to cool it will help at least short term. Check the instructions that came with it or look them up online and see what the heat sinking requirements are. A potentially relative simple solution would be to place each heater on its own SSR.

The ones I use use the back plate to dissipate the heat which in turn is in contact with a metal panel or something to get rid of the heat. I do not think it is a counterfeit device but a case of mis application. If you replace it get one rated about 125% of the rated load and pay attention to the heat sink requirements.

Before blaming china you might want to check where a lot of the components are manufactured and assembled that have US labels.

It worked again because it cooled down and was not destroyed by the excess heat.Additional heat sinking will keep it from reaching that temperature. Connections inside are eututectly bonded and will fail if they get to hot.

Gil
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  • Hi, thanks for the comment. I agree with the inadequacy of the components and am not blaming China at all here. I am simply trying to understand which component is responsible for this behavior (closed and unresponsive due to heat, then works again a few hours later). – lpasselin Mar 29 '22 at 01:30
  • The component is adequate, it was improperly applied. They have a maximum temperature the junction will operate, it is up to the designer to keep the temperature below that value, in this case not enough heat sink, – Gil Mar 29 '22 at 22:35