Help finding AC mains disconnect device - Page 1
Forum section: electronics / projects

author: Gary350z
author_rank: Regular Contributor
timestamp: May 13, 2022, 06:53:44 pm
content: I am looking for a device to connect between the AC mains and my test equipment, that in the event of a power failure will completely and permanently disconnect the test equipment from the AC mains, until it is manually reset. This is to protect the test equipment.The reason for this is that when my power goes out, it goes off and on many times, and the voltage fluctuates greatly. I don't think my test equipment likes this.I could make one, but it will save me time just to buy one that is fully compliant with all the regulations.1) Does such a device exist (I assume it does)?2) What is this device called?3) Where can I buy it?4) Should it disconnect both the live and neutral, or just the live?5) Please post any thoughts you have on this device.Thanks

author: Gyro
author_rank: Super Contributor
timestamp: May 13, 2022, 07:08:23 pm
content: In the UK it's called a 'no volt release'. These are common (obligatory) on power tools, where you don't want spinning blades etc starting up again after a power interruption. You should be able to find them very cheaply.

author: themadhippy
author_rank: Super Contributor
timestamp: May 13, 2022, 07:11:58 pm
content: A DOL contactor without overloads will do what you want,

author: andy3055
author_rank: Super Contributor
timestamp: May 13, 2022, 07:16:21 pm
content: There are so many to choose from on eBay. But I would go to a local electrical supply store - not the hardware stores - and get something that can support your total power requirements. Run a power strip off of that to the test equipment.

author: bdunham7
author_rank: Super Contributor
timestamp: May 13, 2022, 07:19:26 pm
content: I haven't seen a 120V standalone wallsocket device like that in the US, but they do show up as industrial controls. It's just a relay that is wired to keep itself on plus two pushbutton switches for on/off, so easy to make. However, the problem for a test bench is that small power blips will shut it off. This is OK for a table saw, but problematic for an electronics bench. For this specific use, you'd be way better off using a sinewave UPS set to not come back on if the battery runs out. This is what I have on my bench, and it will ride through whatever it can (and it is pretty big). If the power is off long enough to deplete the batteries, it shuts off and then when the power comes back on it recharges its battery, but does not repower the bench until I turn it back on.

author: themadhippy
author_rank: Super Contributor
timestamp: May 13, 2022, 07:26:58 pm
content: Quoteyou've reminded me of another option,a non latching rcd/gfi socket.although im not sure if there common on your side of the pond.

author: DavidAlfa
author_rank: Super Contributor
timestamp: May 13, 2022, 08:05:11 pm
content: You need a latching power switch like these:They're not simple switches, there's a coil that activates when pushing ON, holding the power until you stop it, also releasing when the power goes off.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 13, 2022, 08:47:39 pm
content: DavidAlfa showed a nice circuit diagram, but it's quite possible that either or even both switches are not electrical but only mechanical and directly push the relay armature on or off.

author: Zero999
author_rank: Super Contributor
timestamp: May 13, 2022, 09:15:00 pm
content: Note that you should buy something with the relevant approvals, rather than make something yourself out of relays and push-buttons, as it's being used for a safety critical application.

author: jonpaul
author_rank: Super Contributor
timestamp: May 13, 2022, 09:30:27 pm
content: not needed, place a UPS so load is immune to mains interruptionin USA the APC UPS2200 are $50..200 used, new battery set perhaps 150moreJon

author: james_s
author_rank: Super Contributor
timestamp: May 13, 2022, 11:12:07 pm
content: Quote from: Zero999 on May 13, 2022, 09:15:00 pmI wouldn't call this a safety critical application, it's not even required at all, it's just a convenience thing he wants in order to protect the test equipment. Personally I have to agree with the other reply, just use a UPS, that will disconnect and switch to battery any time the line voltage is out of spec, low or high.

author: Zero999
author_rank: Super Contributor
timestamp: May 14, 2022, 09:04:51 am
content: Quote from: james_s on May 13, 2022, 11:12:07 pmYou're right. I misread the OP. He's not using it on a test bench, but test equipment, so no one will die if it fails.

author: vu2nan
author_rank: Contributor
timestamp: May 14, 2022, 01:42:57 pm
content: Hi Gary,Quote from: Gary350z on May 13, 2022, 06:53:44 pmHere's the schematic.Circuit breaker / fuse is not shown.K1 is the switching contactor.All components / cables are to be adequately sized.The unit is to be wired in an enclosure as per applicable electrical standards.Nandu.

author: nightfire
author_rank: Frequent Contributor
timestamp: May 14, 2022, 02:27:15 pm
content: Depending on the way of implementation, i could think of two ways:a) Modifying the mains distribution circuit with a standard undervoltage relay- these are quite commonly used in industrial applications. Disadvantage: You have to work on your mains gear and a single power drop will cut off your bench. Alternatively get some box with a DIN Rail, install this and a circuit breaker in it together with a reset button, and put some cables (one plug and one outlet) and you are good to gob) Use of a PRCD-S - those devices also protect against leakage currents and other stuff, and are standard equipment on construction sites- and also trip with undervoltages. I admit, that I don't know about the situation in the US, as the OP apparently is from...