Power rating on switches. AC v. DC

This may come under the "silly question" category but .......

My motorcycle runs down its battery if left unused for more than a fortnight. The clock etc. never switches off, so I'm fitting a cut off switch to the lead from the battery to the computer and instrument panel. The heaviest load would be to the solenoid relay switch when the starter is activated ( not the actual load to the starter motor)

I've loads of swiches in the junk box, but none rated for 12V. Typical would be one marked 10(3)A 250AC~.

Is it safe to use AC designed switches for DC use, as in the case above?

How do you work out what DC load an AC switch can carry?

Am I right in thinking that 10(3)A means 3A continuous and 10A intermittent use. If so, how long can the intermittent periods be?

All advice will be gratefully received.

John

Switches are typically only able to switch much lower DC voltages, as a DC arc at the contacts isn't self-extinguishing like an AC one. They usually have similar current capacity for DC or AC. A 250V AC switch will certainly be OK for 12V DC and probably for 24V DC or more.

The (3) in brackets might be for switching an inductive load, like a motor or transformer, that will generate bigger sparks.

Neil

John,

Just might be worth considering a motorcycle trickle charger. Ebay or the likes awash with them. Pays your money takes your choice on quality. Maintaining a lead acid at 13.6 -13.65 Volts is going to be better for it.

Regards Alan

The current rating for a switch is set by its ability to interrupt that current. For DC, the ratings are lower because of the increased likelihood of the contacts arcing across - you only need about 30V or so to get this to happen, or even less if you have inductance in the circuit. The limiting voltage is a function of the contact design, material, spacing etc. There are entire books covering switch design. If the switch isn't specified for a DC current, it's not a simple matter to figure out what is safe.

Looking beyond the switching current, the continuous current rating is generally set by the safety approvals bodies on the basis of thermal limitations ie how hot it gets and the materials it's made of. No idea what the marking means or what the intermittent rating would be.

In this case, you are really looking to use the switch as a "disconnector" ie you don't plan to open it under any sizeable load. In this instance, you can almost certainly use almost any switch that is rated for the continuous current. Just don't expect to be able to use it to safely turn off the electrics when it is running or if there's a fault

Edited By Muzzer on 19/05/2015 21:53:12

Why not use a proper battery isolator switch ? Plenty on ebay rated at 200 amps for about £6.00.

Nathan.

For reliability a new switch would probably be better than a junk box one.

Maplin stock DC rated ones, link here.

**LINK**

I expect most other electronic retailers stock them as well, or you could try a motorcycle dealer if you have one nearby.

Edited By V8Eng on 19/05/2015 22:38:08

Thanks to all for your input.

Neil, and Muzzer. You have given me a better idea of the "why" of it, thanks. Yes this switch will be to just cut off the current drain while the bike is stored so the contacts have just got to be robust enough to carry the load to the instrument panel and IC while the bike is starting/ running. I suppose I should check out the maximum load through it with an ammeter before I do anything.

Nathan, I have boughtt one of those switches but they are quite large and clumsy, more suitable for under the bonnet of a Land Rover!

Thank you for the link, V8Eng. I will have a look at the site. I knew though that I would learn more by throwing out the question to the Forum.

Alan, I have one, but last winter I thought that I'd top up the charge once a week or so. Well, due to senior moments in that regard I have now got a new battery fitted! You are of course right about the charging regime but I just don't like leaving electrical gadgets plugged in, but point taken.

John

Edited By Breva on 19/05/2015 23:29:07

John, have you looked at the switches that are incorporated in the battery terminal? They are also available on ebay at the same ratings.

Nathan.

Although many switches have an AC and DC rating they are often considerably different for both current and voltage. I have put a link that explains why the design of switches for AC and DC needs to be different.

Mike

.

Mike,

That's a very useful reference ... thanks for the link.

One thought regarding Breva's requirement: A good quality microswitch should be very suitable, electrically and in terms of size & durability ... all it needs is a simple mechanical actuator; and that's a nice little 'model engineering' job.

MichaelG.

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Edit: some nice 'watertight' versions, here

Edited By Michael Gilligan on 20/05/2015 07:54:49

Nathan, The only ones of those I can see if for round pole and there's little room in there for adapters.

Mike, that is exactly the info I needed. Many thanks for that link.

MichaelG, I never gave micro switches a thought even though I have quite a few stashed away. I always assumed that being small they would only be for light loads. Must have another rummage.

Very grateful again to all who replied,

John