I wired up a solid state relay to only turn on the hot water tank when electicity is cheapest. Using a 25 amp relay to switch about 22 amperes. I guess that was too close to it’s rating, and eventually it failed spectacularly

Comments:

“wow. you’re brave. I usually don’t go over 2/3 of what the spec sheet says something like this can take.”

“It decided to become a plasma state relay.”

“This is an application where a standard mechanical relay would be better. A SSW is more suited to situations where the load needs to be switched on/off rapidly or in short intervals, such as in the case of a PID controlled heater application.”

“Reading the comments in the original video someone mentioned that these knockoffs tend to catch fire….lol.”

“I also warned of the dangers of those knock off Foteks. They’re great for some experimenting but there is not a chance I would leave one hooked to mains unattended.”

“SrMofo Exactly. He was lucky this time but next time it may burn down his house. I do not trust their DC SSR for the same reason because, typical of the Chinese, they claim an Amp rating but you better halve, a third, or a fourth of their number to get a margin of safety. Spend the money for a real one (they are expensive but for a reason) as life is too precious to screw around with like this.”

“You’re supposed to use these with a properly rated heatsink.”

Kudos to you for publicizing the failure, and not just brushing it under the carpet.

But I guess the “safety know-it-alls” have a point sometimes, eh?

The tests performed were:
1- Temperature rise test 250Vac 10A
2- Endurance test 250Vac 10A resistive
3- High voltage 2500V for 1 minute (FAIL)

Yes it is. When buying off eBay, aliexpress, bangood or whatever, you need to know that you don’t pay a fraction of what a brand product cost and expect same quality.
I don’t even think they are (intentionally) lying when it is branded 25A. It probably started out as a design with a beefier triac, but then costs have been cut to meet a price point.
What is good, is that they are reasonably well designed and built (that is not immediately dangerous, which there are a good deal of products that are). And they are really, really, REALLY cheap.

So, bottom line, no they are not comparable to quality SSR’s and no they should no be used for everything. But, if you know what to expect, they are very good value for money.

I just got the 4 fotek ssr’s that purchased, they were obviously cheapo copies. I installed one the first one, it started out fine, the led on it was flashing according to the pulses from the rambo. But then i noticed a burning plastic smell. And i noticed that it was stuck on the open position. Tried again, it was dead. The moment you give it a little pulse it just switches to the ON position and never goes off. Shut off the power when bed reached 120 degs.

After finding out that the absolute minimum input voltage was closer to 5.0-5.3 volts than the stated 3 volts, the doubts began to surface — but what could go wrong? The use case was marginal, but not unreasonable. The system operates at 208V; the relay is rated for 380V. The operating current is less than 48A, with a cold current less than 53A. The relay is rated for 60A.

A bit embarrassed and reluctant to spend a (relatively) lot of money for a Crydom or Gavazzi, I headed back to the internet for options. Somewhere along the way, a chisel and bandsaw attacked the failed SSR and a few interesting details dripped onto the pavement around its body.

The PCB traces carrying the main current path are 1mil thick, 150 mil wide. To some degree, the solder from hand-assembly does serve to increase sectional area, but I can’t imagine anything more than 25A wouldn’t start melting things.

The output stage is composed of discrete thyristor packages screwed to the backplate.

It’s two CTA24-800 clones (triacs) in parallel without any means of enforcing current sharing (other than a common heat sink). I should point out that even in the ideal scenario, that only amounts to 25Ax2=50A worth of current capacity, not 60A. Curiously, the FOTEK product pdf for SSR50DA has transient current capacity curves strikingly similar to twice that of a mounted CTA24-800. I’m curious to know if they also use paralleled discrete output triacs or if that’s just a consequence of a coincidentally similar thermal impedance.

The remnant internals of the triacs can be inspected and tested to clearly indicate that they are the insulated-tab variant.

With an input of 1.2 W/A at 40°C, one needs a 0.3 K/W heat sink to keep the junction temperature safely below 125°C at the limiting current of 50A.

My casual guess at what happened was that one of the triacs remained on after the input went low, a condition made more likely if one had been hogging current. This resulted in the failure of the stuck triac. When the input signal reappeared, the remaining device destroyed itself, blowing out the optocoupler in the process.

Interpreting the schematic from the board is trivial, though a few curiosities appear.

It’s also worth noting that the trace spacing reduces the input-output isolation distance to roughly 20 mil. I haven’t checked, but even under thin coating of epoxy, I doubt that meets the 2.5kV isolation spec.

So what are the actual capabilities of this thing? If one is able to trust that the components (triacs, optocoupler) meet their individual specifications, I would still feel reluctant to trust this relay above about 20A at 240V. I don’t trust the configuration to share current reliably between triacs, so I’d derate it to the capacity of a single component. Even still, one needs to consider the added thermal resistance caused by the packaging as mentioned before.

In the end, the device design is a functioning SSR, but it is not capable of performing to the specified capacity.

There are other stories online of fake FOTEK SSR’s sticking on or melting