Mixing 12V & 24V on relay

[spandit]

I want to install some spotlights using LED toggle switches but the ones I like, you can only get in 12V. My Pinzgauer has a voltage dropper installed, so a 12V source is no problem, but can I put 24V across the heavy terminals of a relay (85 & 86) whilst putting 12V across 87 and 30?

The reason I don't want to do everything on 12V is that my voltage dropper is only 15A and with 50W bulbs, that's only 3.6 bulbs (and I need to switch at least 6!)

[edzz]

Yes you can (should) use a relay(s) to control your lights. The relay must be rated for the voltages and amperage you intend to use. The control voltage and the controlled voltage need not be the same. It is common to use a low voltage to trigger a relay that controls much higher voltage / amperages. The relay has two separate considerations. The electromagnetic coil (controller section) and the contacts (switch section). A relay could be rated to use 12 volts to control the coil section and have a switch section rated for 220 volts at 100 amps. Find an old relay to take apart and look at how it works.
Also two 12 volt lights (same wattage) can be run in series from a 24 volt source, the only down side to doing this is if you have one bulb burn out it will break the circuit and both lights will go dark.

Ed

[spandit]

Thanks, Ed.

I'm planning on having 3 sets of spotlights, independently switched - the front set will have on/off/switched with main beam, the two side sets (for off-road challenges) will just be switched through relays. Each set will be 100W or thereabouts (here's a picture of one attached to my Land Rover: http://i277.photobucket.com/albums/kk76 ... G_0434.jpg)

I think I'll look for relays that can be controlled with 12V so I can use the fancy LED switches (although I have found them in 24V, they are expensive: http://www.autoelectricsupplies.co.uk/product/820) but powered by 24V - the bulbs I have in my spotlights are currently 12V as easy to obtain, but I think 24V MR16 bulbs are available

Here's a picture of the spotlights I made:

[Jimm391730]

Considering the difference between 12V and 24V, switches and relay contacts are really only defined by their current rating. Voltages create sparks and arcs that gradually erode the contacts, but the addtional 12V that a 24V switch or relay must deal with is a very minor issue and probably only needs be considered if you attempting to use a switch or relay at their full rated current (and you should "oversize" these components by about 50% for reliability and longevity, meaning use 15A contacts or greater for a 10A expected load).

However, relay coils should not be powered by voltages higher than their rating if they are expected to run for more than a few seconds.

Know also that incandescent bulbs can draw as much as 5-10 times their rated current at the instant they are switched on; after 1-2 tenths of a second their current should have fallen to the rated current. So relay and switch contacts take a bruising every time they are turned on. 100W of 24V bulbs is about 4A, so expect the initial current surge to be 20-40A. Obviously a higher current rated contact will deal with this better.

Inductive loads (like motors) are very hard on contacts due to the arcing that occurs; most switches/contacts are "de-rated" to lower currents for these types of loads, similar to what I would expect will happen when using a 12V rated contact for a 24V load. Most are derated by about 25% reduction in current, so getting a 20A or better contact rating should probaby be the minimum you should choose.

But I think that your design should be OK as long as you don't try to use wimpy 5 or 10A rated relays.

Jim M.
712W and 710M

[edzz]

Absolutely agree with what Jim said.
Nicely worded Jim.

Ed

[spandit]

I think the relays are rated at 30A. Just got to decide where to mount the switches now!

Thanks all for help!