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wiring for idiots guide

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So i have a small project i am trying to do, but i dont really understand electrics so its making my head hurt.

just bought a mini mud pod and some carling switches, mostly on/off

i want to run a couple of work lights off this, one per switch for left and right.

bought suitable sized wire - double checked with friend who understands these things

got fuse holders and suitable fuses,

have 2  4 pin relays, one per light - lights are led so low current draw i belive

so i have guessed,  

1 pin from relay to a suitable earth

1 pin from relay to a permenant live from aux battery, including fuse on this line for safety

1 pin from relay goes to live in terminal on carling switch

1 wire goes from live out on carling switch to led light 

1 wire goes from earth on led light back to suitable earth on landy


1 wire goes from earth terminal on carling switch to suitable earth in dash, probably near fuse box mount points - 2003 td5 defender

need to take a live feed from fuse for side lights, including a fuse i suspect as electrical coward up to terminal on carling switch for sidelight/illuminate


so on relay i have 85,86,,87,30

85 - earth

30 live from aux batt in

87 to light

86 - god knows?


So looking for clear, idiot proof guide if possible please, i dont understand the pretty symbols etc so its really a case of is what i listed above ok, and if not why in idiot proof terms - cant offend me on this honest!

idea is to have side lights same as rear work lights ( which i didnt wire) that permenant live off aux batt and flick a switch and they come on.


over to smarter people


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Carling Tech Condura II switches, they type Mudstuff sell, are rated for 20a, so unless you are wiring aircraft searchlights, you won’t need relays for them.  Have a look at the wattage of the lamps - divide by 12 and that gives current draw (in practice, with engine running, the current will be slightly less as the voltage goes up to 14ish, but dividing by 12 gives the current when running on battery, and is the most conservative estimate from a safety perspective).

The colour of the lamp won’t affect current with an incandescent bulb.  With LED lamps, current draw will be a lot less, so would definitely not need relays, but current may differ with colour because different colour LEDs use different voltages (the LEDs themselves, not t.he lamp assembly), so that may affect current too.  I’m not sure.

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35 minutes ago, Snagger said:

in practice, with engine running, the current will be slightly less as the voltage goes up to 14ish, but dividing by 12 gives the current when running on battery, and is the most conservative estimate from a safety perspective).

No, wrong. If you up the voltage the current goes up.

V = IR       ===>      I = V/R

R (resistance) is fixed (well, not QUITE true with incandescent lights... but we can assume it to be so for this example.)

V (voltage) is what you feed it, in this instance.

I (current) is relative to voltage. 


P = IV (in DC applications)

P = power in watts.

Components are often quoted with power at 12V, in real life situations the in a car at 14V with the engine running the current will rise and give you more power. Think headlights with engine off and on, they are always brighter. Brighter light = more power.

Additionally, from the equations above you can rearrange to:

P = V2/R

So you can see, the power goes up with the square of the voltage.

20A Carling switches are quite well rated, and can negate the need to use relays in some applications. But without knowing the lamps (LED? HID? Halogen?), or if they have a pre-made loom for example, I couldn't comment on their suitability here.


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running a pair of about 40 watt (from memory) led lights, like wee light bars so not massive.   

no pre made loom just a pair of wires out of each as is normal, red/black  so suitable for whatever application hopefully


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so i built the circuit today on bench - aka coffee table using a battery charger in place of battery live/earth and after some rude words it worked almost fine - only difference i could see was i ran a wire from live battery to sidelight pin on switch so was just on, normally this would i assume work when i had a sidelight on.   then i took it apart pin by pin and listed stuff so i could do in car later.

excuse wording its what made sense to me mixed in with some info from carling wiring off mud site.

So I have, Earth from light to bulkhead bolt, used as earth

Live from light to 87 on relay Live Inc fuse from batt to 30 on relay

Earth from batt to 85 on relay

86 on relay to switch terminal two, which is power out Pin 2 linked to 3 on switch (carling say to do)

Live from battery to pin 1 on switch, which is power in Pin 7 on switch to earth at fuse box bolt

Pin 8 on switch to side light live


so god knows why in blue






Edited by western
it's not blue anymore,
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