2 channel relay to cut both LIVE and NEUTRAL AC line? - Page 1
Forum section: electronics / beginners

author: elcrni
author_rank: Regular Contributor
timestamp: December 16, 2021, 05:44:24 pm
content: hi guys,First fo all apologies for posting Fritzing "schematic" i know a lot of people hate it, but needed something sketched fast as a proof of concept.Basically, my question is simple, i am building a "power ON/OFF" module for custom built 3D printer, so the concept would go like this:Rasberry Pi runs on separate power supply, momentary switch is connected to raspberry pi GPIO3 (pin 5) and GND, with a simple script, shorting these two will power ON or OFF the RPi.Once RPi is booted it would trigger GPIO signal to then trigger the relay and allow AC current to go to Meanwell 24V PSU. When shutting down, pushbutton is pressed again and while RPi is shutting down it sends the signal to relays again, cutting off the AC power to printer components.Now, this all works and is clear but what i wanted to do is to cut both LIVE and NEUTRAL line, instead of just one, so my question is:Can i use 2 channel relay (2 relays) to separately cut LIVE line with one and NEUTRAL line with the second relay. Is this OK?Many thanks for any help in advance,Alek

author: fordem
author_rank: Regular Contributor
timestamp: December 16, 2021, 05:45:41 pm
content: A two pole relay will do what you want.

author: elcrni
author_rank: Regular Contributor
timestamp: December 16, 2021, 06:55:39 pm
content: many thnaks but can i use a 2 channel relay? is that safe to use?

author: Zero999
author_rank: Super Contributor
timestamp: December 16, 2021, 08:59:24 pm
content: Quote from: elcrni on December 16, 2021, 06:55:39 pmWhat do you mean by two channel relay?The only safe way to do this is with a double pole relay.There needs to be adequate creepage and clearances between the DC and mains.

author: bdunham7
author_rank: Super Contributor
timestamp: December 16, 2021, 09:26:29 pm
content: Quote from: elcrni on December 16, 2021, 05:44:24 pmYes, but why? You can use a two-pole (DPST) relay just as easily.https://www.mouser.com/ProductDetail/Panasonic-Industrial-Devices/DSP2A-DC5V?qs=UO%2Fx91QLkSAKSTb%252BBH4SDQ%3D%3D

author: elcrni
author_rank: Regular Contributor
timestamp: December 16, 2021, 09:28:41 pm
content: i mean this, image attached. i need 5v control voltage while all 2 pole relays are 24+v, as far as i found. many thanks

author: themadhippy
author_rank: Super Contributor
timestamp: December 16, 2021, 10:10:47 pm
content: Quotemaybe,but its safer ,and part of the uks wiring rules, to have both poles to be broken by the same actuator.Quotetry dpdt instead of double pole,theirs 1000's of em out there,for examplehttps://uk.rs-online.com/web/p/non-latching-relays/1762768

author: SL4P
author_rank: Super Contributor
timestamp: December 17, 2021, 07:59:07 am
content: Reply #3excludes a LOT of ebay / chinese hobby modules.They aren’t designed or manufactured to *any* safety standards for mains voltage isolation.The individual relays may be ok, but the assembly / pcb is rarely compliant to any recognised safety regulations.

author: elcrni
author_rank: Regular Contributor
timestamp: December 17, 2021, 09:05:35 am
content: many thanks everyone!i would prefer DIN rail mount relay or at lest one with screw terminals and current rating of at least 10A, and of course trigger 5V. seems not easy to find, but also i am not really into relays so any advice about possible specific model would be highly appreciated. thanks, Alek

author: Siwastaja
author_rank: Super Contributor
timestamp: December 17, 2021, 09:14:28 am
content: Please reconsider working with mains. It is really lethally dangerous, so you really need to know what you are doing.A complete certified brand-name module would be safest, but cost is high. But that's the cost you pay for safety.Chinese PCB modules off Ebay are not usually safe for mains, even if the relays used in them are.

author: david77
author_rank: Frequent Contributor
timestamp: December 17, 2021, 09:40:59 am
content: Do NOT use two single pole relais to switch your load. That could lead to a dangerous situation!If the relay in the neutral wire for some reason does not switch on, but the one in the live (Phase) line does switch on your load will not work. But it will have a live wire attached to it. One might assume the power is off and touch the live wire, which is dangerous.Bitte halte dich von Netzspannung fern, wenn du dir nicht sicher bist, wie man SICHER damit umgeht.

author: elcrni
author_rank: Regular Contributor
timestamp: December 17, 2021, 10:53:23 am
content: Many thanks everyone for your concern and help.I have been working with mains on hobby level for the past 20 years, electronics in general, but i always ask question when dealing with mains to stay within safety standards, thus this relay question.I have issues finding 2 pole relay with current rating of 10A and control voltage up to 5V, with screw terminals so any specific recommendation would be great.I do not have a problem with 20-50 EUR relay as long as i am on a safe side.I have to agree, ebay/hobby relay modules are not to be trusted, i have used them for tests/concept designs for low power LED light bulbs and sometimes for low current DC.Many thanks,Alek

author: Kjelt
author_rank: Super Contributor
timestamp: December 17, 2021, 11:08:11 am
content: 5V -> That is because for mains switching you want some real fast force to pull the relay in, preventing arcing.That means in professional applications you mostly see 12V and 24V relays for mains power switching.I would recommend using at least 12V, you can then use a fet or transistor or ic (uln2803a) to switch the 12/24V relay.Further I agree with the above posters for mains switching preferably do not use pcb relays that are open, preferably use a din relais or socket that is isolated and wires with ferrules.

author: elcrni
author_rank: Regular Contributor
timestamp: December 17, 2021, 11:29:55 am
content: Quote from: Kjelt on December 17, 2021, 11:08:11 amThat makes sense. I can easily implement 5V FET to switch 12/24V relay, the only downside is i would need another DC source for switching the relay.So in this case i would have AC directly connected to 5V Meanwell PSU for RPi and another PSU 12/24V for triggering the relay, everything else would be disconnected, OR i could use single 12V PSU, step down to 5V for RPI and use 12V for relay.All my ferrules and connections are isolated for both AC and DC, i tend to be very cautious about that.So, this one should do:https://www.conrad.at/de/p/finder-22-22-9-012-4000-industrierelais-nennspannung-12-v-dc-schaltstrom-max-20-a-2-schliesser-1-st-506058.htmlMany thanks again for all your help.

author: themadhippy
author_rank: Super Contributor
timestamp: December 17, 2021, 12:43:21 pm
content: 5v coil-yepmains rated-yep10A minimum switching capacity-yepscrew terminals-yepreputal brand whos products wont burn the house down?- yepDin rail mountable-be surprised if it ainthttps://www.bpx.co.uk/store/product/lc1d12al

author: rpiloverbd
author_rank: Regular Contributor
timestamp: December 17, 2021, 01:11:26 pm
content: '2-channel' relays are for driving two AC loads. Suppose, you want to drive to lightbulbs or one bulb and one fan. Then you will use two channel relays. Your requirement can be fulfilled by DPST relays that many have suggested here.

author: CaptDon
author_rank: Super Contributor
timestamp: December 17, 2021, 02:13:33 pm
content: I am not sure if they make a DPST SSR (solid state relay) but if they do it may help you. Most SSR's operate from 3-30vdc at around 10ma. You should be able to drive it directly. They are commonly 240vac capable and many easily do 10a or more. A double pole mechanical relay will insure both sides on or both sides off 'unless one contact welds' which can happen with highly reactive loads, and a double pole SSR would insure both sides on or both sides off unless the internal device failed 'blown SCR or triac' which could also happen. Design for safety but anticipate failures.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 17, 2021, 02:25:36 pm
content: Actually, all this safety talk about needing double pole relays in this thread is just BS.You just do not assume a plugged-in device is potential-free, never, ever. Whether it has single pole switch - completely normal in any consumer device with a plug, typical example would be desk light - double pole switch, single pole relay, double pole relay, or two separate single pole relays, SSR or nothing at all, is utterly irrelevant. Fuse can be blown, and that always disconnects only one of the lines. Half of the countries use nonpolarized plugs so you don't know which one.Even a massive contactor rated for 1000VAC could be accidentally driven on by firmware during servicing, which is why both wires will be disconnected by physically unplugging the device before servicing it. You just can't assume a device that seems non-operational is free of dangerous voltages and start working on it, that is a recipe for disaster.

author: elcrni
author_rank: Regular Contributor
timestamp: December 17, 2021, 02:50:55 pm
content: Thanks everyone!Ok, so perhaps a bit of further clarification:1. My AC entry is high quality/reputable one with filtering, physical switch and dual glass fuse2. I NEVER EVER work on anything while connected to mains, physically disconnect the cable and then work on things. So the fact that relay can froze/stuck or anything else is irrelevant as its not my main safety barrier.The whole point of this relay is daily printer shut down, perhaps sometimes once a week or so, when i am not using it, so i dont have to flip the main AC entry switch behind the printer every time i want to shut it down. Also, need a soft RPi shut down and then disconnect SSR that controls the heated bed and main board of the printer from mains, with this new relay.This relay is NOT my safety barrier or failsafe, its should serve as a switch to disconnect mains when printer is not in use, and yes, since mains plugs are non polar in EU, disconnecting just one line is not an option as the other line can always be live or neutral, depending on how the plug is plugged.I would also be happy to find SSR with dual pole if possible, but any other reputable/reliable would do.now looking at this one as an option:https://www.conrad.at/de/p/finder-58-32-9-012-0050-relaisbaustein-nennspannung-12-v-dc-schaltstrom-max-10-a-2-wechsler-1-st-1217734.htmlMany thanks for all your help.Alek

author: Siwastaja
author_rank: Super Contributor
timestamp: December 17, 2021, 03:15:20 pm
content: SSRs are not that great. They have significant leakage current and dissipate more power than a physical relay due to 2-3V voltage drop in TRIAC. Use SSRs for heaters you want to PWM at significantly high frequency (many times every minute), where mechanical relay would wear out.I would use just a normal DPST (or DPDT) relay because they are not much more expensive than SPST/SPDT relays, one from a reputable manufacturer, with proper mains ratings (10A 250VAC for example), then make sure the PCB includes proper creepage/clearance distances. Not going deep into legislation and different standards, generally a 8-10mm distance on bare PCB, or maybe 4-5mm if you add a routed cutout (physical gap on the PCB material), is more than enough. This is the distance between the mains contants and the control side.

author: bdunham7
author_rank: Super Contributor
timestamp: December 17, 2021, 03:38:54 pm
content: Quote from: elcrni on December 17, 2021, 02:50:55 pmMany EU non-polarized devices do use double-pole switches to interrupt both lines, but I don't really see why you would need to for your application. But since it is easy enough to do, I suppose you may as well do it the way you want to.What you need is available, just a suitable DPST or DPDT model that meets your specs. They're readily available. As far as mounting options, just use a plug-in relay and get the appropriate socket for it.https://www.mouser.com/ProductDetail/TE-Connectivity/KUP-11D15-5?qs=WGv6JurbshcsYMj0X1dPJQ%3D%3DAsSiwastajasays, I wouldn't bother with SSRs. Especially cheap ones.

author: elcrni
author_rank: Regular Contributor
timestamp: December 17, 2021, 03:54:20 pm
content: Awesome, this is really helpful guys, thank you!I did not know about "contactors" and whats the difference compared to relays...I have found one triggered with 12V:https://www.conrad.at/de/p/finder-22-34-0-012-4640-schuetz-2-schliesser-2-oeffner-12-v-dc-12-v-ac-25-a-1-st-504725.htmlI think i will go with this one.Another new thing learned! :-)