rtbarton Posted February 8, 2006 Share Posted February 8, 2006 Has anyone used 12v car relays to switch AC mains? I'm working on a project where I need to switch low powered mains motors using 12 volts control voltage. I have loads of car relays kicking about, so I thought I'd ask the question before finding out the hard way Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted February 8, 2006 Share Posted February 8, 2006 I'd hazard a guess that it'd be a Bad Thing, they're not insulated for 240v. Proper jobs aren't much money anyway - it's not like you're going to be switching 40A @ 240v (or at least I flippin' hope not!) Quote Link to comment Share on other sites More sharing options...
simonr Posted February 8, 2006 Share Posted February 8, 2006 I'd hazard a guess that it'd be a Bad Thing, they're not insulated for 240v. Proper jobs aren't much money anyway - it's not like you're going to be switching 40A @ 240v (or at least I flippin' hope not!) Like Mr Freezer, I would not reccommend it on the grounds of the insulation not being rated for mains. However [Whisper] I may have done this onceor twice [/Whisper] If you put the whole thing in a decent plastic box - it should be OK. Most electrical factors sell very good (and cheap) plastic enclosures which are rated for mains voltage. One of these would be ideal. Si Quote Link to comment Share on other sites More sharing options...
minivin Posted February 8, 2006 Share Posted February 8, 2006 As a comparator, lets have a look at Leach relays as they're one of the big'uns: http://www.leachintl2.com/english/index.html They generally list their relays as 28VDC or 50VDC, however when you look into the datasheets they are also capable of ac, lets have a look at the generic M300 which is a very useful relay: M300 10Amp Relay As can be seen, the 200Vac current capability is the same as if it was 28VDC, however, the moral of this story is do your research and you may be happily supprised what you find Quote Link to comment Share on other sites More sharing options...
simonb Posted February 8, 2006 Share Posted February 8, 2006 The contact rating (amps) depends on ac or dc. On ac the arc will extinguish each time the voltage passes through zero (50 times a sec for uk mains), on dc the arc is more severe. As a result a relay which can switch ac or dc will have a lower rating on dc. Contact material is usually also different for an ac vs dc relay. So not a good idea to use a 12vdc relay to switch 240v ac. Now 10a@12vdc is 120w, which is only 0.5A at 240v ac... Not withstanding a car relay may not be insulated to 240v (mains relays usually have a 3mm contact gap for safety).. Look on the maplin or RS web page, there are plenty of relays with 12v coils that are designed to switch ac mains and they start at a few squid each. Quote Link to comment Share on other sites More sharing options...
rtbarton Posted February 8, 2006 Author Share Posted February 8, 2006 The contact rating (amps) depends on ac or dc. On ac the arc will extinguish each time the voltage passes through zero (50 times a sec for uk mains), on dc the arc is more severe. As a result a relay which can switch ac or dc will have a lower rating on dc.Contact material is usually also different for an ac vs dc relay. So not a good idea to use a 12vdc relay to switch 240v ac. Now 10a@12vdc is 120w, which is only 0.5A at 240v ac... Not withstanding a car relay may not be insulated to 240v (mains relays usually have a 3mm contact gap for safety).. Look on the maplin or RS web page, there are plenty of relays with 12v coils that are designed to switch ac mains and they start at a few squid each. Pretty much what I assumed, just wanted some experience. Being a Yorkshireman didn't want to have to buy in what I've already got a drawer full of! BTW on a 50 Hz supply, the voltage crosses zero 100 times per second, Once on the way down and again on the way up. Quote Link to comment Share on other sites More sharing options...
dollythelw Posted February 9, 2006 Share Posted February 9, 2006 You could try Ross or if you want something a little more chunky try HERE I play with these toys during the day time, they make purdy lights and scary noises Quote Link to comment Share on other sites More sharing options...
robhybrid Posted February 9, 2006 Share Posted February 9, 2006 I use OEN 12v relays at work that are rated to 240v. They are automotive relays that have 2 contact points on them the first takes the initial ark and is made from a different material from the 2nd which takes the main current. I work at a 10-550 KVA generator manufacturing factory. Quote Link to comment Share on other sites More sharing options...
VERY OLD DISCO Posted February 10, 2006 Share Posted February 10, 2006 I have got 3off 12v relays, 230v 16A switching contacts Normaly open or normaly closed, made by Omron. You can have em if you want? You would need to solder them onto Veroboard or similar to make the 230V connections absolutley safe and then insulate and earth them properly. Geoff Quote Link to comment Share on other sites More sharing options...
rtbarton Posted February 10, 2006 Author Share Posted February 10, 2006 I have got 3off 12v relays, 230v 16A switching contacts Normaly open or normaly closed, made by Omron.You can have em if you want? You would need to solder them onto Veroboard or similar to make the 230V connections absolutley safe and then insulate and earth them properly. Geoff Hi Geoff, thanks for your offer - I'm sure I've got some somewhere myself from years of hoarding stuff! It's just that the car relays are easier because of the mounting brackets and push-on connectors. I'll think I'll take one to bits and check the contacts &c. Quote Link to comment Share on other sites More sharing options...
02GF74 Posted February 10, 2006 Share Posted February 10, 2006 Pretty much what I assumed, just wanted some experience. Being a Yorkshireman didn't want to have to buy in what I've already got a drawer full of!BTW on a 50 Hz supply, the voltage crosses zero 100 times per second, Once on the way down and again on the way up. that is right. also the maximum voltage measured from 0V is 120 V; it is just that is reverses. It is not ideal but I reckon it would work; just make sure your life insurance premuims are up to data and ditto for our car/house/cat/ Quote Link to comment Share on other sites More sharing options...
rtbarton Posted February 10, 2006 Author Share Posted February 10, 2006 that is right. also the maximum voltage measured from 0V is 120 V; it is just that is reverses.It is not ideal but I reckon it would work; just make sure your life insurance premuims are up to data and ditto for our car/house/cat/ Or is it 120 x 1.14 or suchlike, the 240 volts is rms, not peak. Quote Link to comment Share on other sites More sharing options...
02GF74 Posted February 10, 2006 Share Posted February 10, 2006 Or is it 120 x 1.14 or suchlike, the 240 volts is rms, not peak. yep; school boy error, it is friday after all; (240 VAC rms) Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted February 10, 2006 Share Posted February 10, 2006 IIRC it's about 380V p-p. For the sake of a few pennies is it really worth using something that's not right for the job? If you were using mains relays to switch 12V I wouldn't grumble, but the other way round safety becomes an issue Quote Link to comment Share on other sites More sharing options...
minivin Posted February 11, 2006 Share Posted February 11, 2006 IIRC it's about 380V p-p. For the sake of a few pennies is it really worth using something that's not right for the job? If you were using mains relays to switch 12V I wouldn't grumble, but the other way round safety becomes an issue The rms value is the 3dB value of the peak voltage, or 0.707*Vpeak to get a very good result, so reverse the equation and we get Vrms/0.707 = 340Vpeak for 240Vrms That's first year ONC Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.