24v to 12v dropper wiring HELP! :) - Motorhome Forums, Motorhome Discussion, Motorhome Chat

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post #1 of 8 (permalink) Old 21-11-2010, 16:31 Thread Starter
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24v to 12v dropper wiring HELP! :)


I have a 24v chassis, and seperately a 24v camper battery set up charged from the alternator, mains and solar as and when. Within the camper are two 24v to 12v droppers put in by a sparky for several 12v circuits via a fuse box.

Where should the 12v dropper circuits be earthed to?

I can't tell where the 12v earth stuff ends up, would it be back to the inverter, the chassis, the camper batteries?

I want to put a 12v feed into the cab but can I earth it in the cab?

Cheers for any help


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post #2 of 8 (permalink) Old 22-11-2010, 09:50
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Hi Jason
Do you have or know how to use a multi metre
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post #3 of 8 (permalink) Old 22-11-2010, 12:20 Thread Starter
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Yes, but not with enough knowledge to suss this out!


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post #4 of 8 (permalink) Old 22-11-2010, 14:56
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It should just earth to the chassis, which is probably where the 12v batteries are earthed to!!

The supply goes from the 24 - 12v dropper, to the appliance (or what ever) then from that to the vehicle chassis (earth) circuit complete, gizzmo works, simples
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post #5 of 8 (permalink) Old 22-11-2010, 15:33
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You can also just take 12 volts from the battery that the earth is attached to as well saves putting a dropper in.

We use to do this to run 12 volt fridge fans and electro clutches.


There are no typing, spelling, or grammatical errors in my post. What you are seeing is the evolution of the English language in action.
This post may be unsuitable for overly sensitive persons with low self-esteem, no sense of humour or irrational religious beliefs,if you are please disregard the above post. Andy
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post #6 of 8 (permalink) Old 22-11-2010, 15:46
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the reason you use a 24 volt to 12 volt converter is because you use both batteries and the total current supplied to the load is from both batteries so they both discharge at the same rate, if you do use the "earthy" battery all the current is used out of one battery and thus you get charging problems and finding one battery is discharged and the other is not. When charging both batteries they both charge at the same time, so its not good practice to just use the one battery... as far as earthing always bond the -ve battery supply to the chassi and in the 24volt supply its the "earthy" battery -ve you bond to the chassi
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post #7 of 8 (permalink) Old 22-11-2010, 16:49 Thread Starter
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The cab did have a supply coming from the earthed starter battery. After about 2 years one battery was knackered (not holding a charge, sitting below 12v while the other stayed full), needing 300 to replace both (5.7l diesel needs decent batteries). Perhaps if I'd swapped them around a bit that may have delayed that event if I had known about it before. Therefore I now have droppers!

Some info I've found says the dropper circuit needs to go back to the dropper not vehicle earth, but I can't see anything that suggests that is the case with these ones. The 12v returns go behind a bit of wall with two 24v circuits returns and I can't see what then happens. The sparky who did it is no longer trading, but there isn't a problem with what he did, just what I want to do with it.

Having done the cab feed, taking the earth back to where I think the rest of the 12v dropped circuit goes to, it occasionally blows a fuse.

Sometimes the 8 amp fuse in the "earth" side goes, sometimes the 5 amp in the positive fuse box side. There is mostly 0.2 amps from a rechargeable torch only. I just put a new 5 amp in, and turned on the two cab fans and a plug in worklight thing which will pretty much the most there will be and drew 2.9 max settling to 2.6. No blown fuses. Could I just put bigger fuses in, say 10amp?

Before I found and replaced the blown fuse I measured the voltage in one of the sockets to see if the torch wasn't charging or there wasn't any power at all. At first perhaps 4 volts dissipating down to nothing and staying there. Wait 10 minutes and it'll do the same. Why?!

I could just take the cab feed from the back of a 12v socket the sparky put in since if we're in the cab we won't be using a camper socket, but I'd be happier if I knew what the dropper circuits actually did and how!



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post #8 of 8 (permalink) Old 22-11-2010, 18:54
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It is a pity you are stuck with a system with 2 voltages. We had problems on some pleasure boats which had been converted from commercials.
The biggest problem we found was that the 12v +ve was 'hot' to both the 24v +ve and the -ve. The vehicle mechanics we used found this out amid much sparking and swearing.
I would recommend that you find the negitive used by the original spark and use only that - even if it means a bit of deconstruction to find it.
As a matter of interest, do the droppers have a big parasitic load?
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