Is there any reason why I shouldn't use a 12V car-relay to switch mains? - Page 1
Forum section: electronics / beginners

author: Re
author_rank: Newbie
timestamp: September 13, 2014, 07:58:12 pm
content: This question has been on my mind for a long time and I haven't had a good place to ask.I have a bunch of 12V car relays rated to 12V @ 40A. I've thought of the idea of using an MCU to switch them (indirectly) in order to turn on/off light switches in my house, But I haven't ever done it because, of course, the 12V rating of the relay. I'm not sure if it's a good idea to use car relays to switch mains power.Is there any reason why I shouldn't use these relays? Considering the current limits of the relay it should be fine, but I really am quite a n00b in the field and I've always assumed there would be a reason why they are rated as such.I would understand why not to use it to switch way higher voltages, like in the kilovolts, due to the gaps of isolation being too small to prevent arcing, but I don't seem to think that this is the case with just typical 120V mains. Again, I'm just going by intuition here and not any logical basis.Perhaps it would also be dependant on the size of the relay, and how closely the different terminals are to each other externally and internally. Perhaps one of you know the answer?Thanks.

author: SeanB
author_rank: Super Contributor
timestamp: September 13, 2014, 08:09:49 pm
content: 12V relays typically will arc over between the frame ( the common contact in most cases) and the coil at around 60VAC, so you basically will be applying mains direct to the coil with no ability to control it. As well the contact separation is only rated to work at up to 48VDC, at mains voltage the contacts will arc on opening, and this arcing will continue even as the relay has the contact open. This will destroy the relay in short order.Short and simple is use them at 12V only.

author: Neilm
author_rank: Super Contributor
timestamp: September 13, 2014, 08:16:19 pm
content: Where to start?A relay for an auto application is rated for 12V - there will be some transient allowance on that but basically not much more.A relay for mains obviously requires mains voltage 120V or 230V. However, on top of that are transients - for mains that is several kV. Assuming you are talking after the socket this from memory is about 4kV - your relay will have to survive the mains voltage + the transient to the coil and across the contacts.Contacts. Mains is a much higher voltage and you can draw an arc as the relay opens. This will seriously degrade the contact material and it will break in no time.In short - no you can't

author: Re
author_rank: Newbie
timestamp: September 13, 2014, 08:22:18 pm
content: Ah, degrading the contacts from arcing as it closes. Simple answer, Thanks!

author: Monkeh
author_rank: Super Contributor
timestamp: September 13, 2014, 08:29:49 pm
content: Quote from: Re on September 13, 2014, 08:22:18 pmAlong with the multitude of other reasons you can't get away with it..

author: tautech
author_rank: Super Contributor
timestamp: September 13, 2014, 08:42:21 pm
content: Short and sweet, if you can find a spec/datasheet that says they are suitable for mains then yes.Otherwise you risk your house and yourself.However you could cascade them with a suitably rated 12 V coil mains relay/contactor.

author: German_EE
author_rank: Super Contributor
timestamp: September 14, 2014, 09:13:29 am
content: The breakdown voltage between the coil and the chassis. Out of curiosity I just tested one for you now and the coil flashed over to the chassis at 410V. I have not pulled it apart (it is sealed) but I bet the coil is wound directly on the iron core.

author: Re
author_rank: Newbie
timestamp: September 23, 2014, 11:56:12 pm
content: Quote from: Monkeh on September 13, 2014, 08:29:49 pmThanks for the needlessly condescending and unhelpful comment, jerk.Quote from: German_EE on September 14, 2014, 09:13:29 amI'm not exactly sure what you're referring to. What do you mean by "chassis"? The part that closes the circuit between the switch?

author: Monkeh
author_rank: Super Contributor
timestamp: September 24, 2014, 12:05:05 am
content: Quote from: Re on September 23, 2014, 11:56:12 pmOther people made perfectly useful, clear posts on the matter, which you ignored, and I'm a jerk for pointing it out?Okay, I'll reiterate the reasons and you can call me a jerk for it again:Insufficient open contact clearance.Insufficient coil/contact insulation.Insufficient coil and contact insulation from the relay body.Inadequate arc quenching.Insufficient inrush current capacity.There are probably more which aren't jumping to the front of my mind.For more jerk-munition: The ratings exist for a reason, not just for fun.

author: tautech
author_rank: Super Contributor
timestamp: September 24, 2014, 12:29:02 am
content: Quote from: Monkeh on September 24, 2014, 12:05:05 am+1Rewhen super contributors with their considerable knowledge give advice....listen.Although detailed in-depth explanation was not forthcoming, it was for good reason:none of us would consider using ny device near/with mains unless rated for it.

author: David_AVD
author_rank: Super Contributor
timestamp: September 24, 2014, 01:49:00 am
content: Quote from: Re on September 23, 2014, 11:56:12 pmThe reply was helpful. He pointed out that there were other reasons not to use that relay on mains, not just the one you seemed to single out.You've done yourself a great disservice by replying in that manner and may not find help as forthcoming in the future. Read,think, then reply.

author: sunnyhighway
author_rank: Frequent Contributor
timestamp: September 24, 2014, 02:43:43 pm
content: Quote from: Re on September 13, 2014, 07:58:12 pmThe mains in The Netherlands is 230V.Either your flag is wrong or you have a lot to learn.

author: jlmoon
author_rank: Unknown
timestamp: September 24, 2014, 03:35:19 pm
content: As Clint Eastwood would say: "A man's got to know his limitations"

author: Monkeh
author_rank: Super Contributor
timestamp: September 24, 2014, 04:13:45 pm
content: Quote from: Mechatrommer on September 24, 2014, 04:12:25 pmWhatever for?

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 24, 2014, 04:18:51 pm
content: Quote from: Monkeh on September 24, 2014, 04:13:45 pmno thanks you deleted it making the world less informative even though entropy will continue to keep rising. whatever.

author: Monkeh
author_rank: Super Contributor
timestamp: September 24, 2014, 04:33:44 pm
content: Quote from: Mechatrommer on September 24, 2014, 04:18:51 pmI didn't delete your post. I deleted the quote after you deleted yours.I'm sorry to say that advising people to do stupid things is not informative.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 24, 2014, 06:26:04 pm
content: Quote from: Monkeh on September 24, 2014, 04:33:44 pmtalking out of arse is not helping either. moreover when we are talking about a "fact" as "stupid". surely i cant prove or show you the deity but... this is one i can...for the record, since you have deleted the quote of mine, let me re-emerge it again to avoid confusion to non-live-viewers... (and increase the entropy even further what a waste)Quotei deleted it because i was too lazy to prove it by finding one of the car relay, i thought i've used them all up in my car and i thought to put some credibility to some commenters above. but lucky you i found one the relay. thank you for wasting my few minutes making the setup, and about an hour uploading it (and 60MB of youtube space) to prove we have a bunch of <insert your favorite word here> in this forum...MODERATOR EDIT: Dangerous advice video removed.

author: Monkeh
author_rank: Super Contributor
timestamp: September 24, 2014, 06:29:36 pm
content: Quote from: Mechatrommer on September 24, 2014, 06:26:04 pmWait, it's my fault for deleting a quote of a post you deleted?QuoteWell, we can't see your video, and nobody asked you to make one.12V relays are not designed or rated for switching LV. There are reasons for this, and just because you get away with it for five minutes without things going wrong doesn't make it safe.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 24, 2014, 06:41:15 pm
content: i think its public now i forgot to click the button (see below)... btw i agree on using the proper rated relay. why use 12v relay when the mains rated one is so cheap?MODERATOR EDIT: Dangerous advice video removed.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 24, 2014, 06:44:48 pm
content: Quote from: Monkeh on September 24, 2014, 06:29:36 pmyou are right... just because a device or component is rated as "safe" in laboratory doesn't make it, it cant kill you, nothing is absolutely safe and eventually everything will be dead. its just a matter of how you get by it

author: Christopher
author_rank: Frequent Contributor
timestamp: September 24, 2014, 06:49:16 pm
content: Cringed watching the video! Just because you can doesn't mean you should.You buy relays to switch 12VDC for a reason. One of those reasons isn't switching 240VAC!Advising a (possible) beginner to do the same is pretty much batshit crazy.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 24, 2014, 06:55:12 pm
content: Quote from: Christopher on September 24, 2014, 06:49:16 pmyeah palestine batshit, but thats not the real life scenario, just a 5 minutes quick setup dedicated specifically for this thread, by a highly cautious "non-certified" personnelin real life you dont do that, you cover all those with rated tape, put all the things in non-combustible chamber and be doneedit: and what you dont see back there is a fully functional circuit breaker box rated as "safe" just in case if shit happened

author: Monkeh
author_rank: Super Contributor
timestamp: September 24, 2014, 06:57:25 pm
content: Quote from: Mechatrommer on September 24, 2014, 06:55:12 pmYes, because tape and a 'non-combustible chamber' solve all the evils of using inappropriate parts.

author: SeanB
author_rank: Super Contributor
timestamp: September 24, 2014, 07:06:52 pm
content: Shafri, it is fine doing it with 15W CFL lamp. Now repeat with a 3kW load and see how many cycles it takes to burn the relay out. I have killed plenty of them at 12V with a 10A load, though admittedly they were no name relays bought at the auto spares shop. Branded Bossch, but inside just a tiny set of contacts on a small frame, not like the real Robert Bosch relay with a heavy copper coated steel frame, a coil on a good quality former and a large silver button on both sides og the moving contact and the NO and NC contact posts.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 24, 2014, 07:07:17 pm
content: Quote from: Monkeh on September 24, 2014, 06:57:25 pmand also solved all the evils if the appropriate parts fail (it happened to me alright) safety is few fold of layers, you dont leave the layer count to 0, usually every material around you is already engineered to be non-combustible to handle appropriate and inapproprite parts. you can do it but make sure a layer of safety surrounding it. what i saw in this thread is... "you cant do it" i dont agree with that. even dave of taking things apart is not apropriate or batshit things for beginners from the point of view.Quote from: SeanB on September 24, 2014, 07:06:52 pmsorry i cant find 3KW light bulb for home usage as the OP describe my friend.

author: Monkeh
author_rank: Super Contributor
timestamp: September 24, 2014, 07:11:33 pm
content: Quote from: Mechatrommer on September 24, 2014, 07:07:17 pmTape and a box aren't safety layers, they're what irresponsible, uncaring imbeciles wrap their stupidity in to make themselves feel good.If you want to do things properly and safely, you cannot use a 12V relay for 240V. I do not care how many arguments and 'layers of safety' you want to wrap it in, you don't do it.

author: SeanB
author_rank: Super Contributor
timestamp: September 24, 2014, 07:19:17 pm
content: Few lights can range from 15W to 3kW, it depends just how many lamps you consider to be room lighting and controlled by the same one switch. 12V relay will do for a while with 15W, but for higher wattage, and after multiple cycles you have no warranty that it will not either arc shut or arc over.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 24, 2014, 07:23:28 pm
content: ok ok now i've seen the light! we should use this...no further argument needed we should all agree with this (even though i already failed few of those. thank you! for the discussion its very informative

author: Lightages
author_rank: Unknown
timestamp: September 24, 2014, 09:55:27 pm
content: Telling anyone that a 12 volt relay is safe to use for 240 is not only irresponsible but also criminal IMHO. It is especially so for unattended home automation. If someone wants to experiment at their own risk and properly informed, then that is their problem.Do not use 12 volts relays for 240. It is as simple and clear as that. A beginner should not be experimenting with safety.

author: tggzzz
author_rank: Super Contributor
timestamp: September 24, 2014, 10:25:39 pm
content: Using a 12V DC component with 240V AC is very dangerous. Full stop. Anyone that follows your advice is in danger of damaging themselves and/or their houses.Encouraging others to do this iscompletelyirresponsible.If you believe otherwise then clearly you are so dangerously ignorant that you don't even understand the externt of your ignorance.OK, so you (mechatrommer) have not found any problems when testing a lashup in limited circumstances for a limited time. THAT DOES NOT PROVE IT IS SAFE. It may be accepted practice in Malaysia (but I doubt it), but it most definitely is not elsewhere.Your statements are similar to telling people that there's no problems walking across a busy road without looking. And then illustrating that it is OK by walking across a small backstreet once.You really should retract your statements.Anybody that believes this is dangerous might like to consider leaving a comment on the youtube video, and clicking the "report to moderator" link on the message.

author: Andy Watson
author_rank: Super Contributor
timestamp: September 25, 2014, 01:24:00 am
content: Quote from: tggzzz on September 24, 2014, 10:25:39 pm+1

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 25, 2014, 04:13:55 am
content: well, the burden of proof lies in you, i have done my part. "if i dont see it... i dont believe it", thats the culture i believe. "the culture" of believing mere lips statement from unknown members doesnt blend with me very well. and by following any labels in most electrical equipments (only for certified personnel) "law", then this forum should (1) not exist, (2) validate a person's competency before joining as member or specifically watching the video, because some noobs will like to follow whats done in this forum and risk damaging or killing something. but well, this "free to watch hazardous materials in this forum" may be acceptable to the "more civilized" world, you may make the complaint to youtube hmm. and was that somebody suggesting that me is like suggesting its safe to cross road without looking? who said its safe to cross road without looking? otoh i also have read more and more "view" toward asian/china "culture" in this forum coming in. meh whats this forum got into? more civilized? no offense just trying to be academic

author: timb
author_rank: Super Contributor
timestamp: September 25, 2014, 05:13:26 am
content: Technically, everyone was saying it can't be done and that the contacts would instantly flash over, causing your house to instantly vaporize. He did prove that to be demonstrably false. He also said that a proper mains rated relay should be used.So, he's not factually wrong, if you want to be pedantic.Sent from my Smartphone

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 25, 2014, 07:05:52 am
content: well glad someone with competent english tried to clear that up. i said earlier, the reason to not to do it is if you dont have enough knowledge, so try to gain knowledge (or experience) before doing it. and besides my short video, its backep up by facts and science (unless you have something factual to say otherwise). you can google (i forgot what too lazy to find) how much clearence before 240Vac arc, iirc its so tiny to care of or compared to clerarance in common devices or pcb. and i'm also talking about low wattage load, assuming thats what the OP meant. agreed we dont usually engineer things close to its rated capacity, esp if we dont have enough experience and using no-brand-name devices. but lighting up 18W bulb with 30A relay? come on! thats only 18W/240V = 75mA load, thats 400X safety factor. if i'm not mistaken, 30A rating is based on how much the contacts can take before arc deposition problem become "out of spec", granted higher voltage will create longer arc during transient hence faster deposition rate, but what the hell, 400X safety factor should compensate, no? everything will eventually die, even the rated one.just because you have seen tragedy doesnt means it always apply to other scenarios in not so similar manner, abandon ship of electrocutephobia, use your senses and use by knowledge. i also crynged when seeing video like photoinduction and mikeelectricstuff, but they eventually did it didnt they? but that not necessarily means to justify its not safe or safe, its just because i dont have proper knowledge for it or no guts enough to take the risk, hence i'm not doing it, i want to do it? i better gear up, thats the first thing you should do. something is not safe because its not well understood, correct? once you have the knowledge, you may forget the paperwork, thats only for muggles who are ignorant enough to race for the knowledge, those paperwork is made by those "knowledgable people anyway" for a very specific (well understood) condition to occur... no?

author: Simon
author_rank: Unknown
timestamp: September 25, 2014, 07:11:08 am
content: I've not read the whole thread but i am getting reports. Please can we calm the thing down. The definitive answer is: "thought shalt not no anything that the datasheet does not explicitly tell you you can do" Unless your a rocket scientist with kit and knowledge to try your crazy misuse of a part out in a controlled and safe manner. END OF.And yes in my opinion trying to use a 12V rated relay for mains is a very bad idea. 40ADC on relay contacts is quite something, it has to have been achieved with compromises!

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 25, 2014, 07:30:08 am
content: the problem is... there is no datasheet, there is no mechanical drawing to be more specific, this is pretty much an open question with variety of answers. everybody is like absolutely sure "it cant be done", so i'm providing another variant (based on provided clue by the OP)

author: Simon
author_rank: Unknown
timestamp: September 25, 2014, 07:32:41 am
content: Quote from: Mechatrommer on September 25, 2014, 07:30:08 amAutomotive means 12 or 24V NO HIGHER. The relay will most likely state on it's body that it's for 12 or 24V so there is no case to go higher. If you buy a 240VAC relay it will say on it what the coil voltage is and what the switch voltage is, it's that simple.

author: rolycat
author_rank: Super Contributor
timestamp: September 25, 2014, 07:34:12 am
content: Quote from: timb on September 25, 2014, 05:13:26 amNo-one said it couldn't be done, they said that doing it would be extremely unwise and incredibly dangerous.QuoteNo, initially he said:QuoteQuoteFor once I don't want to be pedantic, I just want people to stay alive.You do not exceed maximum rated values when building circuits for domestic use at mains voltages and currents. Period.If you want to live dangerously that's your decision. Endangering others is criminally stupid.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 25, 2014, 07:51:16 am
content: Quote from: rolycat on September 25, 2014, 07:34:12 amdefine "endangering others".... people drove while drunk its endangering others but they will get the punishment until they did kill someone right? by getting caught (drive while drunk) all they get is a ticket isnt it?, then why do you say selling beer is not endangering others? and whats endangering about my statement you quoted? i did warn them didnt i? about knowledge.how about making campaign about banning photoinduction, mike and dave to be "criminally stupid" posting highly evolved videos? then why do you say they are not endangering? whats fun about this thread is the quote of burning house and persistent 240Vac arc on disconnected relay contacts which i never come accross, please prove me wrong with a very good link or video.

author: Simon
author_rank: Unknown
timestamp: September 25, 2014, 07:55:53 am
content: I am looking this thread as clearly it will go nowhere other than a mud slinging match.As i said further back unless you have the equipment and knowledge to further test a component beyond it's rated specs you are not in a position to recommend exceeding the specification particularly if your talking hazardous voltages which is anything over 50V.If it's rated for 12V then only at your own personal risk do you put 240V through it.