Looking for inline GFCI module
Peter Walsh wrote 06/03/2023 at 02:11 • -2 pointsI'm putting together a power supply "board" for dangerous projects, such as those that use 120 VAC. I'm literally planning to have a wooden board with an ATX power supply mounted and the voltages brought out to a banana plug jumper board, two duplex 120V outlets, two SSR units that can disconnect the power almost instantly, an E-Stop switch in the most convenient location, and an onboard arduino that can monitor external things and control the SSRs if any external (to the board) device detects a fault.
The board is to be used for prototyping experiments. After the experiment is done and disassembled, the board stays intact and is used in the next experiment.
I'd like to have a GFCI in the circuit somewhere, but can't find anything that's both inline and cheap. Obviously I could run the input through a GFCI outlet, or I could plug the board itself into a GFCI outlet or GFCI extension cord. Or mount a GFCI breaker on the board.
I'm basically looking for a GFCI box with connectors on each end that would go inline on the board itself, probably with a TEST and RESET switch and maybe an LED power indicator.
The cheapest one on DigiKey (that I could find) is $150, which doesn't track with an inline extension cord ($20) or a duplex outlet ($10), so I must be missing something.
Where can I find an inline GFCI module for prototyping? Is there a special search term I'm missing?
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Another user helpfully suggested adding "DIN" to the search terms, and this comes up with a bunch of "DIN rail mount" GFCI devices, which are also priced about right and would do the job.
And this leads me to using DIN rail mount for some of the components, which I was familiar with but wasn't thinking about until someone suggested it.
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Don't forget the SSR's leakage current. If the SSR has an internal snubber, there's a bit more leakage from that. Use a conventional relay in series with the SSR. (Also helpful should the SSR fails "on" ...)
An isolation transformer would be a nice addition, too.
How resistant to EMI is the 'duino? Can you depend on it running correctly to cut the power after you've dropped a screwdriver on the work board?
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Thank you so much for your post! It got me thinking in new directions.
I was unaware of SSR leakage current, did some research, and... yep good call. Relay with the SSR is the way to go.
I have a medical grade isolation transformer that's a separate box with plugs and outlets, but after reading your post I looked around and found that you can purchase one as a component for $25, and that's a more elegant solution. Can't forget to plug it in if it's a part of the system. The ATX power supply is *theoretically* isolated, but I don't trust it, which is why I was planning on using the isolation transformer in the first place.
It turns out that Isolation transformers have leakage current as well, due to interwinding capacitance.
I'm aware of EMI issues generally, was planning on "just trying it" with the Arduino and making changes if any problems cropped up. Optoisolated inputs for the arduino, and of course all the connections on the board will be shielded.
After reading your post and thinking through the design choices, I'm thinking of writing this up as a project. I haven't seen anything similar and the information would be useful to others.
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That was quick.
A helpful user IM'd me and suggested a "Dead Front" GFCI, which was the needed search term. It's like a GFCI receptacle, but doesn't have the receptacle slots.
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