24V switches

spandit · Post by **spandit** » Fri Jun 04, 2010 4:32 am

Am I right in thinking that since 24V systems can use thinner cable than 12V, using a 12V switch is no problem for switching, say, spotlights?

Just ordered a bunch of 2mm cable which should suffice for most of what I want to do

krick3tt · Post by **krick3tt** » Fri Jun 04, 2010 8:20 am

I had a few left over toggle switches from stripping out my UniMog radio box that I used for my spotlights but I would thing you are right.
There might even be a truck supplier in your neck of the woods that would have 24V stuff...no worries there.

spandit · Post by **spandit** » Fri Jun 04, 2010 12:01 pm

I normally switch mine through relays anyway, so probably not a problem

edzz · Post by **edzz** » Fri Jun 04, 2010 12:13 pm

Generally speaking yes you can, just remember it should carry no more than one half of the rated 12 volt amperage. i.e. if rated for 12vdc @6amp rating for 24vdc would be no more than 24vdc @ 3amp.
To find the required amp rating of your switch add the total wattage it is expected to carry (i.e. two 55 watt lights would total 110 watt load) now divide the load by 24 for the amperage (110watt(P)/24volt(V)=4.6amp(I)) P/V=I
http://en.wikipedia.org/wiki/Joule%27s_laws

Ed

spandit · Post by **spandit** » Fri Jun 04, 2010 12:26 pm

This confuses me - if a lamp is rated at 55W, at 12V it would draw 4.6A but applying the law P=VI (P/V=I) then it implies at 24V it would draw 2.3A so why can't I connect 55W bulbs from my 12V car straight onto a 24V supply?

I know you can't and I'm not going to...

edzz · Post by **edzz** » Fri Jun 04, 2010 1:35 pm

spandit wrote:This confuses me - if a lamp is rated at 55W, at 12V it would draw 4.6A but applying the law P=VI (P/V=I) then it implies at 24V it would draw 2.3A so why can't I connect 55W bulbs from my 12V car straight onto a 24V supply?

I know you can't and I'm not going to...

The lamp filament is not rated for 24 volts. A 12volt 55 watt light bulb will have a fixed resistance rating of about 2.6 ohms pumping 24 volts into it will apply too much power to it and it will burn out. If I apply 12 volts to a 2.6 ohm load (the light bulb) It will draw about 4.6 amp. If I apply 24 volts to a 22 ohm load (the light bulb) it will draw about 9.2amp. So by doubling the voltage across a fixed resistance (the light bulb) the amperage also doubles. This causes the wattage applied to the fixed resistance (light bulb) to double to 110 watt’s the light bulb being rated for only 55 watts would then glow very bright for the very brief time it took the element to burn out.

http://www.ohmslawcalculator.com/ohms_l ... ulator.php
You can however link two 12 volt lights in series however if one bulb burns out it breaks the circuit and both lights will be dark.

Ed

spandit · Post by **spandit** » Fri Jun 04, 2010 1:54 pm

Thanks, Ed, knew one of the variables had to be fixed.

spandit · Post by **spandit** » Sat Jun 19, 2010 5:39 am

Sorry to open an old thread but I have a 12V relay, rated at 30A. Does this mean on 12V it can take 360W but 720W on 24V? I'm intending on switching it with 12V but running 24V across the power terminals - no more than 100W per application so think it should be OK, but thought I'd check

audiocontr · Post by **audiocontr** » Sat Jun 19, 2010 10:04 am

half. not double

edzz · Post by **edzz** » Sat Jun 19, 2010 11:25 am

If it were me I’d be looking for 24v relays, better safe than sorry.
In my early days I had the joy of an electrical fire

in my series 3 Landrover during an outing in the Sultan basin. Happily the fire went out on its own after consuming the auxiliary lighting wires. As it was also on this occasion that I noticed that someone had felt it more important that they have a fire extinguisher than I.

The lesson for me was never to trust a previous owners wiring job and always check my emergency gear before an outing.

Ed

spandit · Post by **spandit** » Sat Jun 19, 2010 3:09 pm

I can't find relays that are switched with 12V (which they need to be as they have indicator LED's that are 12V only) but can SWITCH 24V. If a 30A relay can take 15A at 24V then that's 360W which is fine for 100W of lights). I carry an extinguisher and the electrics will not be hidden away

w5yk · Post by **w5yk** » Sat Jun 19, 2010 10:19 pm

edzz wrote:Generally speaking yes you can, just remember it should carry no more than one half of the rated 12 volt amperage. i.e. if rated for 12vdc @6amp rating for 24vdc would be no more than 24vdc @ 3amp.

There's some confusion creeping into this calculation. If a switch can take 6 amps, for example, then it can do so regardless of whether it's in a 12V vehicle or a 24V vehicle. Your calculation is assuming that there is 12V or 24V across the switch. That's incorrect. The voltage across the switch is 12V or 24V when it is open, but when it is closed, that drops down to almost nothing, and the voltage is transferred to the load.

So a 6A wire or a 6A switch is still good for 6A regardless of the vehicle battery voltage. A lamp switch from a 12V vehicle would be perfect in a 24V vehicle, since it would be capable of carrying a much higher current than the 24V lamps will use.