Dave90 · Oct 14, 2012

Hello all, might be a bad time to ask about complicated electrical stuff, but I've managed to confuse both my dad and myself with this

I've been looking at some momentary switches on eBay that are rated to 3amp @ 250v AC, Does anybody know how many amps they will take @ 12v DC?
They have a LED illumination ring that works only on 12v DC so they must sell them for cars or other 12v applications. The contacts don't seem to be very heavy duty, but I was just wondering if anybody knows of a formula, or has had previous experience with them?

Cheers

top-cat · Oct 14, 2012

if theyre not dc rated i wouldnt use them.
a dc load would burn the contacts in no time at all

Dave90 · Oct 14, 2012

top-cat said:
if theyre not dc rated i wouldnt use them.
a dc load would burn the contacts in no time at all

Thanks for the heads up

Wonder why the LED works with 12v DC?

Geep · Oct 14, 2012

LEDs don't use any current, you're talkin Millie Amps, so I reckon your 3 Amp switches will have no problem. Just don't use them to directly switch the main bulb, just stick a relay in.

JoelC · Oct 14, 2012

As far as I remember, it is ok to use AC switches for DC applications provided the voltage is substantially lower, using DC on AC switches adds additional stress to the contacts, so lowering the voltage helps get round this

Amp rating should be the same for AC or DC, you should be ok using switches rated for 230ac at 12dc

J

Dave90 · Oct 14, 2012

Thanks for the replies. I wasn't planning on using them to within an inch of they're lives, I was going to use one for the rear wiper, and another for a couple of 55w fog lights using a relay, hopefully won't be a problem

TheKeymeister · Oct 14, 2012

AC fuses should be de-rated by 50% for DC (or so I was taught), I would apply the same for current carrying capacity.

What are you planning to run off it? I'd run it through a relay as a matter of course if it's anything too big

TheKeymeister · Oct 14, 2012

Rear wiper draws quite a bit more than 3 amps anyway so I'd use a relay for that. It'll handle a relay coil (milliamps) easily

Dave90 · Oct 14, 2012

TheKeymeister said:
Rear wiper draws quite a bit more than 3 amps anyway so I'd use a relay for that. It'll handle a relay coil (milliamps) easily

Yeah would be a good idea, the terminals on it don't look too meaty, suppose I could kill two birds with one stone and use a timed relay on the wiper

Just a thought however, seeing as it's rated to 3 amps @ 250v, wouldn't it take more amps at the lower voltage? Even taking into account the 50% reduction you mentioned? Obviously it's better to be safe than sorry though

TheKeymeister · Oct 14, 2012

I think its something to do with AC being an alternating sine wave (i.e. not always 'all' of the amplitude) and DC uses all of the amplitude all the time...but that could be a load of rubbish!

dogsbody · Oct 14, 2012

TheKeymeister said:
I think its something to do with AC being an alternating sine wave (i.e. not always 'all' of the amplitude) and DC uses all of the amplitude all the time...but that could be a load of rubbish!

dead right, plus the switch-on surge

ratty · Oct 15, 2012

Dave90 said:
Yeah would be a good idea, the terminals on it don't look too meaty, suppose I could kill two birds with one stone and use a timed relay on the wiper
Just a thought however, seeing as it's rated to 3 amps @ 250v, wouldn't it take more amps at the lower voltage? Even taking into account the 50% reduction you mentioned? Obviously it's better to be safe than sorry though

watts divided by volts gives you the amps.

The maths says the switch MUST be used to operate a relay rather than directly powering the wiper and/or spots.

I can't comment on the reasoning behind the pros and cons of the AC side of things but I would be looking for a switch designed for powering DC if I was doing it.

Using a VW number 99 relay will make the wipe speed adjustable anywhere between summit like 2 and 45 secs.

Dave90 · Oct 15, 2012

ratty said:
watts divided by volts gives you the amps.

The maths says the switch MUST be used to operate a relay rather than directly powering the wiper and/or spots.

I can't comment on the reasoning behind the pros and cons of the AC side of things but I would be looking for a switch designed for powering DC if I was doing it.

Using a VW number 99 relay will make the wipe speed adjustable anywhere between summit like 2 and 45 secs.

Ah yes, makes sense, I'll probably just leave these switches and go for some 12v 20amp switches without illumination, and if I want a timed relay then look into the VW one you mentioned, at the moment my rear wiper is on a momentary switch and because I haven't a clue if it has a park switch or if it's even working, many thanks

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