Looking for contactor - 12v coil, 40A 240v contacts?

There are plenty of similar contactors, from many manufacturers, that are rated the same or higher.

SteveW

Thats a fairly non DIY type of 'thing'

Thanks Steve No - I certainly don't want to be paying that sort of money (£138 !) - only needs to be single-pole anyway...

I did the obligatory search on Google, and trawled CPC / Farnell without much success..

I think it'll turn out a lot cheaper in the long run to use the 12v to switch a little din-mounted relay, and use that to switch the 240v to a £35 40A contactor. Also kinder on the circuit that's driving it..

Thanks

Yes - I know it's stretching the definition of DIY <grin>.

It's for a 'safety' relay on a glass-fusing kiln - will be wired in-series with the Big Relay that switches mains to the heating elements. In the event of the Big Relay failing 'on' (which has been known to happen), the kiln controller (12v out) can drop out the safety relay and avoid a melt-down. Safety relay needs to be rated to break 15 - 40A at

Yes - that's rather more than I was hoping to pay <understatement> - but thanks. It may be just as simple, and much cheaper, to switch a small relay with the 12v and use the contacts on that to switch mains to the Safety contactor - which can then be a £30 jobbie.

There seem to be a lot of 12v-coil 'contactors' which are targeted at the DC, high-current market - but Id want something that could interrupt 40A / 240v safely.

Thanks

Adrian Brentnall formulated the question :

I assume the concern is the Big Relay's contact becoming welded up?

Why not just have two 240v relays, both being switched on by the control supply and both having their contacts wired in series? Even if the contacts of one weld up, its very unlikely both would weld up.

I have a circuit which actually uses two in series for switching a high DC current (100+ amps). One is delayed switching, so the other relay just carries current, rather than switching it.

A separate circuit monitors the main switching relay, so that if it does break when its coil supply goes off, it prevents the circuit which normally powers coil, from powering the coil - an LED then comes on, to indicate a fault. Human intervention is then required, to unstick the relay.

Another thing you could do is use an electronic zero switching relay, much less likely to latch up. I have had one here in constant switching service for a couple of decades.

A RCD would be able to interrupt that, put the relay between L & E with a resistor to make it trip.

You would have to manually reset but thats probably best anyway.

soldi state relay is cheaper for a simgle pole but a cursory search unearthed only 24V DC control

But then you'll need a F-expensive fuse to protect the SSR :-)

How expensive can a 60A mains fuse be?

anyway you STILL need that.

Try a standard car relay? Costs a couple of quid. See if it survives. Obviously, take safety precautions in case it doesn't. But assuming it is in a suitable housing for safety purposes, chances are it will be OK.

dennis@home presented the following explanation :

Interesting idea!

As far as current is concerned, a relay will have a much easier time interrupting 40A AC than it would 40A DC. It may be worth looking at the detailed ratings of the DC devices, because if they are rated adequately for 240V the current should not be a problem for them.

I'd be dubious about puttting mains voltage into a car relay. I'd be unhappy about the insulation. You certainly could get relays to do the job, I've used them on projects, but switching 40A is probably a bit much for any of them. I think you are into the 'contactor' realm.

Really? Most have all the bits mounted on a plastic base. I'd be surprised if that wouldn't insulate 240v. Obviously it would be unsafe if not in a suitable enclosure for mains. To protect the terminals etc from prying fingers.

If it were in a box attached by a plug and flex I'd agree it was reasonable, and easy to make safe against electric shock. Although if it is supposed to break the circuit for safety some reassurance over contact spacing, internal lead spacing and the dielectric strength of the plastic used would be nice. And some note on the box to say it was not to be used with a motor or other inductive load.

However, if it is 40A it sounds as if it is to be wired-in and effectively part of the electrical installation. In that case I think one should only use type approved parts for mains voltage and the expected current.

There's one in every pre pre-engaged starter motor car. Off you go, down to your local car breakers.

Where is the arc suppression built in on 12v car relays?

Yes as others have already said, at a price.

Many industrial contactors in more recent years have been modular and you could mix and match coil voltages into a common contact base, easily adding auxiliary contacts and indicators.

For the coil 24v 48v 110v and 240v are common across a wide range of manufacturers, 12v remains extremely rare.

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