SSR teardown

A solid state relay (SSR) is an industrial component used for interfacing low voltage control signals to high current mains voltage loads. The SSR may be designed to switch either AC or DC to the load. It serves the same function as an electromechanical relay, but has no moving parts. You can control SSRs easily for example with Arduino:

They sell them on eBay at a ridiculously low price, so it’s time to see what’s actually inside one of these eBay units. Teardown of an eBay 25A Solid State Relay. (SSR) video shows what is inside cheap device that is marketed with name FOTEK:

UL has a warning about these fake relays:

UL warns of solid state relay with counterfeit UL Recognition Mark (Release 13PN-52)

Note that the Model SSR-25 DA relay with the counterfeit UL Mark is similar to Model SSR-25 DA that is authorized to bear the UL Mark. The relay that is authorized to bear the UL Mark is marked “Rated: 25A max.” and “Taiwan Made” on the label, these markings do not appear on the counterfeit product.



  1. Tomi Engdahl says:

    Electronically controlled LED lamps glowing when off.

    LED lamps (and some fluorescent ones) may glow or flash when they’re supposed to be off when used with electronic switching devices like solid state relays and dimmers. Here’s why.

  2. Tomi Engdahl says:

    25A Solid State Relay. (SSR modification 30amp to 50amp )

    DC Solid State Relay Problem Explained

  3. Tomi Engdahl says:

    The Inner Workings of Counterfeit FOTEK SSRs

    Not too long ago we had a serious warranty issue with some of the controllers we manufacture. The controllers were passing test and would initially work for our customers, but would eventually fail. And this failures occurred after a long run of controllers that did not fail, so I knew that something changed. We have used FOTEK SSRs in our controllers from day one and they have turned out to be very reliable. One change we made is we switched from buying directly from a Chinese supplier to buying through USA based suppliers selling on

    Suspecting that the SSRs I bought through was the problem, I broke apart a 25 Amp rated part and a 40 Amp rated part and what I found surprised me. I expected junk but both SSRs were built very well, but they were both built with under-rated TRIAC’s.

    The 25 Amp SSR was manufactured with a BTA12-400C TRIAC. I did a quick online search and discovered that the part is rated at a continuous load of 12 Amps and a non-repetitive load of 126 Amps. In other words, the TRIAC used in the 25 Amp SSR is only rated for 12 Amps!

    The 40 Amp SSR was manufactured with a BTA20-600C TRIAC. I did a quick online search and discovered that the part is rated at a continuous load of 20 Amps and a non-repetitive load of 200 Amps. In other words, the TRIAC used in the 40 Amp SSR is only rated for 20 Amps!

    And even though some of the counterfeit claims are likely failures caused by improper installation, apparently counterfeit SSRs are quite a problem! My next question was – are there even legitimate FOTEK SSRs out there? Yes there are and here is their Web Site.

  4. Tomi Engdahl says:

    UL Warns of Solid State Relay With Counterfeit UL Recognition Mark (Release 13PN-52)

    The following is a notification from UL that the Solid State Relay identified below bears a counterfeit UL Recognition Mark for the United States and Canada. This Solid State Relay has not been evaluated by UL to any Standards for Safety and it is unknown if this product complies with any safety requirements.

    Note that the Model SSR-25 DA relay with the counterfeit UL Mark is similar to Model SSR-25 DA that is authorized to bear the UL Mark.

  5. Tomi Engdahl says:

    I suspect that these counterfeits are nothing more than lower current models re-labeled as higher current models. If this weren’t so I believe the counterfeits I disassembled would be poorly built. There is a huge financial motivation to do this. Lower current models cost less than higher current models and any re-labeled product would result in instant extra profit!!!!


  6. Tomi Engdahl says:

    Fake SSR Burned Up

    Within a minute it’s getting close to 100ºC but I’m smelling the fiery death of electronics and sure enough the SSR’s plastic case is beginning to melt. That’s when I realize that the label on this particular SSR is NOT the same as the rating/picture on the one from Amazon! Different ratings and the stickers show different polarity… WTF!!! I google a little and immediately find someone else explaining they had a similar burnout and they suspected someone was shipping knockoff components or some such.

    This seems to be a common problem with this package of the SSR’s. They generate enough heat from internal resistance that they MUST be mounted to a substantial panel or a fairly large heat sink.

    I would not rule out the possibility of a knock-off but a heat sink is a necessity.

    my first Fotek with 40A rated burned also in less than one minute. Ordered a second one which is working fine with PID. But even on a heatsink its at 70C when its working. Only 26amps drawing and its rated to 40Amps. Due to the fast switching on PID its getting hot.

  7. Tomi Engdahl says:

    How to destroy a fake FOTEK SSR

    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

  8. Tomi Engdahl says:

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

    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.

  9. Tomi Engdahl says:

    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.

  10. Tomi Engdahl says:

    Actually if you get real FOTEK SSRs then they are fully capable of handling their rated current. The reason they have a bad reputation is that there are a lot of fakes out there.

  11. Tomi Engdahl says:

    Fake Fotek SSR-25DA FAIL

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

  12. Tomi Engdahl says:

    Spectacular solid state relay failure

    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


    “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?


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