SSR failed close and did not respond to commands (possible overheat). A day later, it works again. Why?

Question

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. .

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:

Answer 1

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.

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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.

Answer 2

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.