Mains Rated Solid State Relay Test Box


We like our nice, safe, 5V prototyping projects where the only thing that might get fried is a chip. But there are times when you want to switch appliances for one reason or another and then you’re going to want a mains rated relay. [Viktor] got tired of having exposed high voltage on the bench during the prototyping stage of these projects so he recently built a solid state relay test box.

The only thing he bought for the project was the SSR itself. To act as an enclosure he used the brick from an old laptop power supply. This is perfect for a couple of reasons. First off, it’s designed to contain high voltage if there is ever a short or other problem. Second, it’s already setup for incoming and outgoing power. He just needed to remove the guts and mount the relay. Notice that it comes with a clear plastic shield that physically separates the high voltage side from the low voltage control end. This, along with the cable routing, keeps the dangerous stuff on one side to ensure you won’t get an arc to the low voltage portion of the project.

30 thoughts on “Mains Rated Solid State Relay Test Box

    1. Metal is good.

      The HaD descriptions mentions that a plastic laptop power supply brick is ideal because it is made to withstand high voltages.

      I saw this and scratched my head, especially since so many folks here get all wonky when ever it comes to a project involving mains voltage.

      It seems so much more durable and safer to just get a $1.39 handy box at the hardware store and assemble it all into that, and ground it using a #10-32 screw into the tapped hole already supplied in the box…

      Ugly? Perhaps. Safe? Very. Cheap? It’s not always “I had it laying around already” cheap, but it’s pretty darn close. And being a metal box, the relay will be cooled better (which is important).

      Or, as you say: A metal power strip would also work nicely, and safely. (This is actually on my shorter list of useful things to build someday.)

  1. ER….. some clarification needed… please tell me there is a Mix up with the cable colours and that [Viktor] is not just switching the Neutral side of the circuit. Because if he is that box could be lethal I.E. Risk of death etc.

      1. Yeah – I’m not too experienced with Mains hacks but if my memory of basic plug wiring serves, he really is switching the neutral, not live. That seems like a lot of faith to put into the isolation of the enclosure.

      2. Yeah – I’m not too experienced with Mains hacks but if my memory of basic plug wiring serves, he really is switching the neutral, not live. That seems like a lot of faith to put into the isolation of the enclosure. I think the output wiring is correct, though.

    1. However, he lives in Greece and in most part of europe we don’t have polarized wall outlets so there’s no easy way to know which wire is hot or neutral. I would prefer to switch both wires on/off instead of just one of them.

    2. OK, you need to switch the “hot” line instead of neutral, but since an SSR doesn’t isolate galvanic like a “real” relay, you will still be shocked when you touch the output. Even when it’s switched off.

    1. I agree. Most people I know don’t understand the difference between maximum recommended ratings and normal operating specs. My company went through this with a couple of customers who kept complaining that the relays were burning up within a week of installation. Even though they were the same model they’ve bought for the past 25 years, they figured there was a design change and kept sending them back for warranty exchange. I finally got into their plant and found that they were not only ignoring the heat sink integrated into the machine, they were letting the relay dangle in free air with red electrical tape (the kind with really gooey adhesive) around it to “insulate” it from the chassis.

  2. I do think it’s kind of silly, people getting worked up about wiring like this when they use something as outright insane as the standard NEMA 1-15 (Type A).

    It does look like he’s switching the ground, though.

  3. Don’t purchase your SSR from eBay.

    Some Chinese engineer actually designed and produced a board with *fake* SSR guts and put a whole bunch of them inside authentic FOTEK enclosures.

    These things look real, will test real on the bench, and burn out when used to switch a heavy load.

    “BuyInCoins” is one eBay seller to avoid, probably many others as well.

      1. I have such an Ebay solid state relay installed as a remote turn-on for an amplifier. Says Fotek, 25A, etc — just like the one you show.

        I don’t know what constitutes switching a “heavy load,” but it’s been switching the good-sized linear power supply of that amp without episode, several times a day, for a year or so.

        Big transformer, big diode bridge, big 120,000uF Sprague caps at 75V. The lights in the house dim for a moment when it turns on.

        I have it nut-and-bolted to the steel chassis with heatsink goo, and the chassis is itself fan-cooled and normally cool to the touch at that point.

        Perhaps cooling is an issue for some applications. I note that some Ebay sellers have a matching heatsink…

        But based on my experience, I wouldn’t hesitate to use another one just like it for any manner of normal plug-in 120V device, at least in the realm of hacking. (I try to stay away from unusually-cheap Ebay goods when I’m selling a project for a profit, just on principle.)

        1. Er. Off by a factor of 10 on the caps — 12,000uF, not 120,000uF (which would be a mighty capacitor indeed). But, anyway, it’s still working fine, and I don’t notice any difference between the cheap SSR and the front-panel switch that bypasses it.

        2. Replying to myself after 2.5 years:

          I now have before me a dead Fotek 25A SSR. It failed short while switching what must have been a few hundred amps of inrush, repeatedly, over the course of a minute or two.

          While a teardowm isn’t something I’m willing to do (it’s potted in black goo), I did do some research and discover the following:

          This clearly shows a “good” Fotek relay, vs a “bad” Fotek relay. The important distinction between the two photos is that the housing is keyed for the sticker, so that the sticker can’t be rotated 180 degrees (swapping the input and output labelling) on accident, nor can the plastic mounting plate for said sticker be rotated. The fake one has no such keying.

          Searching Ebay for things like “fotek ssr taiwan” did get me a few genuine-looking items, all around $50 (which is to be expected), and a whole lot of cheap fakes which had “Taiwan” printed on the sticker, and the corner notched out of the sticker, but the housing was not (!) keyed.

          Since I still need another SSR for my amplifiers, I’ve decided that I’m sticking with buying used/surplus Crydom units, which seem to be more popular in the US anyway.

          That said, this fake Fotek SSR never even got warm doing what it was doing. And it was perhaps being used well beyond its intended surge current. And it had successfully moved enough sustained current to trip a new 20A Square-D HACR-rated circuit breaker. It was something like $7, shipped. It didn’t kick my dog, rape my wife, or burn my house down. But it only lasted about a year…

  4. You are NOT supposed to solder on wires to the tabs. You’re supposed to use crimped connectors such as abiko because if a fault occurs and there’s a short, the solder on the ground/zero wire may melt and the wire can slip off, leaving you with a lethal device.

      1. nope, they have an eye so that the dimple on a matching connector sits in it and helps keep it in place.

        same reason that decent spade connectors have a hole, to that the mating female spade connector is attached more securely.

        that said, I’d have soldered it too.

    1. Right on! My local hardware store has them on sale for $8.99, and I bought the last three they had. Yea, this is the little town I live in, and my hardware store is the most awesome I have found no matter where I have been in the country. They stock almost everything one can imagine, in a town of 2500 people(nearest large town 20 miles away), even stuff that Home Depot and Lowes don’t carry(even some electronic components!), at prices usually lower:

      XLR to 1/4″ phono adapter – $1.99
      HDMI to Mini-HDMI adapter – $3.99
      BNC – F adapter – $0.99
      25′ HDMI cable – $14.99
      RJ-45 crimp connector – $0.29

      Tons of extruded aluminum and steel, threaded rod, smallish gears, and a small spring selection upwards of 200 different items…

    2. Also, about these wireless switches, the channel they operate on is selected by a set of pins on the controller that are connected by copper traces to ground. In production, apparently a worker cuts one of the traces(appears to be with a rotary tool and cut-off disk) to set the channel that the transmitter transmits on, so that the transmit channel could be selected by cutting all of the traces on the transmitter and connecting them to output pins on the controlling device in order to select between multiple receivers. I’m working on this right now using opto-isolators, but this is standard procedure, and hardly worth documenting and posting…

  5. I really like his text: ” I remembered how much pain it was to test the solid state relay (SSR) with naked mains cables running around” ……

    ….. he must have touched some live wires if pain was involved….

  6. I have a whole bunch of old CAD machine screen savers for this purpose.

    They attach between computer and keyboard+”mouse” and shut off the screen’s 230V power if the computer has not been used in a while. They contain an RC clock, a 10, 11 or 12 (selectable) bit divider and an optically isolated triac so it’s basically a solid state relay with computer power passthrough.

    I just cut out the timer and attached a resistor and leads directly to the optocoupler and got a neat filtered 3A SSR

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