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Why do I need 12v when on 240v - question ?


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Silly question but why do I have to switch to 12v to power pump, TV, satellite? It seems counter intuitive to be hooked up to 230v yet have to turn on the leisure battery. 
 

Edited by MartinJB
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  • Martin changed the title to Why do I need 12v when on 240v - question ?

Because essential facilities in the van are powered by 12v so you can use them off grid when 240v are not available.

Life is not a rehearsal . . .:)

Porsche Cayenne S Diesel & Knaus StarClass 695. Previously Audi S4 Avant & Elddis Super Sirocco

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Because some things work on 12V DC power only and not 240V AC and they are powered by 

a) the leisure battery

b) the car battery

c) the mains-driven power-supply/leisure battery charger

 

depending on different setups/situations:

 

a) with no mains hookup or the charger-psu switched off when on hookup

 

b) when the towcar is connected and engine stopped/ignition off (e.g. in a layby for a cuppa/loo break)

 

c) when on mains hookup and charger-psu is on

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Thanks. I understand that it is sotheycanbe used off grid. I just don't understand why there isn't a 230v override facility when hooked up. To avoid battery being used. 

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Kirsten, take the water pump as an example - it’s 12volt so that you can go without mains electric if you want to - but 12v pumps won’t work on a 240volts supply - so you need the transformer/battery charger to convert 240v down to 12v. The battery isn’t ‘used up’ because it’s constantly being re-charged.

 

Edited by -Jim-
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The battery doesn't lose any power. When on mains, the 240V keeps the battery charged and supplies 12V to the rest of the caravan. You generally can't turn off the battery. It is just there. 

Graham

Unless otherwise stated all posts are my personal opinion 

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3 minutes ago, Kirsten Statham said:

Thanks. I understand that it is sotheycanbe used off grid. I just don't understand why there isn't a 230v override facility when hooked up. To avoid battery being used. 

That would mean you would require a 240v water pump same with the majority of the lighting & heating control system. It's much easier and cheaper to have  everything 12v D.C. and run off the battery and keep it charged up through a battery charger when on 240v or a solar panel when off grid. The heating system is dual, gas or 240v also the fridge. They may be  some lighting 240v only. 

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basically, anything with a plug on is 230v,  however, items like the fridge , oven and heating, even when it's running on gas or electric still needs the 12v to make it work. It may not make any sense but that's how it's made.

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Modern caravans often don't use a 'charger' per se but rather a device known as a switched mode power supply or SMPS. The SMPS is set to give out (usually) about 13.6V but it can supply 20A+ which is more than enough to power all of the 12V items in your caravan*. It is connected in parallel with the leisure battery in a format known as 'float charging.' Time was years ago when the power supply was not a SMPS but a true charger which had to have a battery connected to it for it to work. The high demand/short duration items such as the main water pump which needs a quite high current to start it but less to keep it running actually drew power from the battery as the charger couldn't handle the current, but as soon as you stopped using the pump the charger pushed current into the battery to replace what the pump had used albeit over a much longer period than you had been using the pump.

*As already noted the fridge will work off mains or gas for cooling when on site, it only runs on 12V (to keep it cool rather than to re-cool) when being towed. Same for the heating if you have Alde, gas or mains on site but it needs 12V to run the circulation pump just as some fridges need 12V to run the temperature control system.

 

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Historically vans used battery power and gas, mainly as there wasn't the vast array of hook-up points available then, a lot of caravanning was done off grid. Things have changed dramatuically for most folk over the years but there is that bit of that's how we've always done it, but you know, it works pretty well. The main reason it is like it is,  must be down to cost, but functiona;;ity plays a part, with both systems you have the best compromise for everybody.  you couldn't have a 12 volt only van and a 230 volt van.  Things like pumps are smaller and lighter as 12V, plus I don't fancy putting a 230volt pump inside my aquaroll, 12v DC is just fine thanks.

 

Dave

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Hi Kirsten, lots of European vans have 12v equipment too-they just don't have a battery!

Many are 240v only with the supply to all the 12 v equipment coming from a 'damped transformer' to give a consistent supply.

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simple answers, cost and weight. The caravan already has a method of supplying 12v to the accessories which need it by way of the battery and charger which are necessary to make the caravan function when not hooked up

 

why add cost, complexity and weight adding a secondary system to supply something the van already has?

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1 hour ago, Allan Guest said:

lots of European vans have 12v equipment too-they just don't have a battery!

Older Hobby vans had no battery and no charger ( possibly other European ones ? ) but utilised a 12v DC supply from the tow car for comfort stops, minimal lighting, pumps and a supply for 12v ignition for space or water heating by gas, though a battery and charger could be fitted as extras.

 

The odd thing about these was they had two12v systems for when on hookup, 12v DC for such as pumps, anything with a motor that will only run on DC, the comfort stop 12vDC items and the 12vDC comfort stop lighting.

 

However they also had 12vAC  circuits, I kid you not, which was for most of the other 12v lighting, filament bulbs which are not bothered if is AC or DC.

 

Yet the supply for these wasn't from a single 12v source as to be expected, each of the round multi 12V  bulb ceiling fittings had a 240v AC to 12v AC transformer sat in the ceiling, only found when the round light fitting was unscrewed, guess how I know ?

 

Yes, I did  bit of head scratching, more than a bit, on my Hobby at the time on a seasonal pitch in an area subject to power cuts, when I wanted to fit a battery and charger, the task wasn't helped by Hobbies refusal to supply any meaningful wiring diagrams and to wire virtually everything 12v, AC or DC in white or black wiring with very few exceptions. 

 

 

It used to, still possibly does, cause much head scratching, when early LED's , DC only, were fitted in the van, worked OK in some of the lights  but not in others, similarly if another item, radio, was piggy backed into the AC circuit and wouldn't work, or if the 12vDC system packed in, yet some 12v lights still worked, the presumption being that 12v always means DC.

 

Beggard if I know why this complicated system was used when DC could

 

 

 

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Could it be anything to do with their country of origin?  ;)

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  • 4 weeks later...

I would love to know what the colours are on the 12 v coming out of my broken transformer so I can wire them to a leisure battery 

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14 minutes ago, Vinny278 said:

I would love to know what the colours are on the 12 v coming out of my broken transformer so I can wire them to a leisure battery 

It might not be as simple as this. Chances are the transformer is 230 v AC input and maybe 14 v AC output. If so, after the transformer the 14 volt AC will be converted to DC so you would need to connect in after this point and remove the transformer and its connection to the 230 volt supply. If the transformer is faulty I suspect you can just continue to use the caravan leisure battery without needing to do anything. When the transformer (charger) on our 2000 Elddis failed I did not need to do anything in a hurry since the 12 volt stuff continued from the battery. The replacement charger was a different make but was a direct fit without any changes needed.

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Thanks for your reply. There isn’t a battery on my hobby prestige. Apparently this is common problem and others have fitted a battery and charger  

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I have just seen your other topic on your transformer problem so ignore my post here. 

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