Combined e-stop and relay cut-off, mains part for UK plugs, does it exist? - Page 1
Forum section: electronics / beginners

author: Infraviolet
author_rank: Super Contributor
timestamp: March 09, 2023, 08:36:50 pm
content: This is to do with a part involved in a longer term project idea I've had. I'm just trying to get a feel if this sort of part is a commonly available reasonably priced thing, or if it would have to be self-built (in which case I'd ask for more advice on how at a later time). The overall idea relates to making a mains powered power tool digitally controlled, without having to modify the power tool itself at all.The concept is a UK plug mains "extension" lead (actual length could be minimal), where an ordinary plugged item can be plugged in to the output end, and where this extension itself is plugged in to a UK 3 pin wall socket. Maximum current drawn would be well below the 13 amp maximum for UK sockets.On this extension cord is an e-stop button (press to cut power, twist to re-enable) to cut power, obviously this means the extension either needs to be long enough in cord length to be mounted to something such that when the button is struck it won't exert torques on the actual plug in the wall socket.After the e-stop is a separate relay stop, normally closed or normally open (either is ok as this one isn't for such critical circumstances as the e-stop button), controleld from 5V or 3.3V logic wires, so a lower powered microcontrolelr device can command this on or off.Lastly is a sensor (given what I saw in another thread here a hall switch near the wire checking for the presence or lack of 50Hz current seems a feasible solution, although that wouldn't be enough to tell for certain whether the output is still active if only a low current is being drawn) which would let low powered devices know whether power is presently exiting the extension cord or not (knowing whether it is not due to e-stop or relay stop isn't important, just knowing it isn't is enough). This sensor would be expected to show an off state if there was a power cut at the wall too, so just monitoring the e-stop switch and relay wouldn't be suitable.So the "extension cord" overall has a UK mains wall plug as input, a UK socket as output, an e-stop button as an input, a digital low voltage input for relay control and a digital low voltage output for current presence sensing. Power for the low voltage stuff can all be supplied from elsewhere (batteries or a separate wall socket with 5V transformer USB wall wart).Do e-stops mounted on mains cords, in such a way you wouldn't risk bending forces on the plug socket, exist as common reasonably priced products? All I've seen is e-stops which need all wiring added.What about mains controlling relays, available as fully built parts (not smart app-controlled nonsense, plain simple exposed low voltage pins), or would need building from relay modules (many of which I suspect aren't really mains tolerant despite what they say on descriptions)?This idea is for a longer term personal project, it does not have to meet specific standards, but ought to be as safe as practical (that is to say that if it can be assembled from easily found sections with plugs already built on to them then that is preferable) for use with a human in line-of-sight (doesn't have to run unattended). If common reasonably priced and UK plug compatible (not some weird industrial huge current connector) parts could do it that's great.Thanks

author: themadhippy
author_rank: Super Contributor
timestamp: March 09, 2023, 08:52:10 pm
content: QuoteHow about a dol starter? for examplehttps://uk.farnell.com/europa-components/le1-d123u7/dol-starter-1ph-12a-5-5kw/dp/3531592

author: Infraviolet
author_rank: Super Contributor
timestamp: March 09, 2023, 09:11:38 pm
content: Not sure if that would be compatible for any generic mains evice, or only for a simple motor? And the datasheet doesn't immediately point out about "pin outs" for controlling it from other electronics.

author: Infraviolet
author_rank: Super Contributor
timestamp: March 09, 2023, 09:14:33 pm
content: It looks like there used to be a device sold by adafruit called a power switch tail, but it was built with US plugs and couldn't cope with UK voltages. A UK compatible version of that would solve the relay controlled portion of the system, but not the monitoring or e-stop.Quite insane how it is so easy to find smart RF controlled sockets, but nothing with simple digital inputs. I can't be going climbing up the OSI stack in this application, needs to be simple logic level control of a relay in an extension "lead".

author: themadhippy
author_rank: Super Contributor
timestamp: March 09, 2023, 09:17:21 pm
content: Quoteits just a relay with an overload,it dont realy care what its switchingQuotetake a look at page 3,everything you need to know is there

author: jc101
author_rank: Frequent Contributor
timestamp: March 09, 2023, 09:44:29 pm
content: Do you mean something like this?https://toughleads.co.uk/products/no-volt-release-safety-switch-with-start-stop-and-emergency-stop-button

author: SmallCog
author_rank: Frequent Contributor
timestamp: March 10, 2023, 03:59:58 am
content: I made something similar-ish for my table sawThe saw itself has the typical green to start / red to stop button set up which works quite well, however they're buried in underneath the table top of the saw and difficult to reach or find particularly if you're holding something big that you've just cut.I didn't want to mess with a brand new tool still under warranty so instead I got a foot pedal and 240VAC relay.Basically the power to the saw runs through the NC terminals of the relay, but tapping the foot pedal opens these cutting the power to the saw.The typical green/red button combo in the saw takes care of stopping the tool, if you're not aware these require a button push to restart after a power interruption, and a tap of my foot pedal is enough to turn the saw off without fumbling around looking for the stop button under the table.Whilst not fullfilling the same role as your described device it's physically kind of similar to what you're describing. I made mine from a chopped up extension cord to get the male/female ends with a small but very sturdy junction box added to the middle with cable glands. In my case buying a short extension cord with moulded ends was cheaper than buying the 2 ends with screw terminations and a piece of wire.My box has a 3rd gland leading to the pedal and the box houses the relay but your box could just as easily house the E-Stop button directly.Use a decent relay, I went with genuine Omron from a reputable source but there's any number of options in this sphere.

author: Infraviolet
author_rank: Super Contributor
timestamp: March 11, 2023, 07:44:50 pm
content: 240V ac relay, I'm aware of the rather untrustworthy looking relays often blue (possible optoisolator) boxes sold with "arduino" as a word in their names. The OMRON ones you mention are similar, but actually trustworthy quality?And the sort you've used can be controlled from minimal current 5V logic, or do they need 12V with some current for energising a coil?And the mains wiring is just done by screw terminals like replacing a plug?I've looked on RS for relays, but I must have been using the wrong sort of search term, because most of the products that came up looked like they were designed to be controlled BY 240V ac rather than for low voltage control OF 240V ac. Is there a more specific search term I should be using?Thanks

author: themadhippy
author_rank: Super Contributor
timestamp: March 11, 2023, 11:28:07 pm
content: Quotesearch by coil voltage

author: sparkydog
author_rank: Regular Contributor
timestamp: March 13, 2023, 03:45:10 pm
content: Personally, I'm rather fond of octopart for part searching; not only are its filters great, it lets you search across many distributors (including RS, but also Arrow, Mouser, Newark and many, many others, some of which TBH I don't know I'd actually want to use).Here, for example, is a list of mechanical relays with 5V coil that are rated for at least 10A/250VAC (but beware of inrush current!).An EMR is probably going to need more coil current that you want to draw straight from a microcontroller IO pin, so plan on using a MOSFET. Even a G5Q-1A4-DC5 wants 40 mA; many or even most want 72-80 mA at 5V.Note that contact ratings aren't always properly indexed; you might get more results removing that filter. For example, I happen to know that a G5RL-1A-E-TV8 DC5 might be appropriate if you need to deal with inrush current (it's rated for 16A continuous and has a UL TV-8 inrush rating, which I believe corresponds to 117A). However, while the data sheet says it's good for 250VAC, octopart doesn't seem to know that.Have you considered whether a TRIAC+SSR would be appropriate?