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Discussion Starter #1
A few people have been asking lately about using LED lights for the inside of their vans. Now the main problem with trying to use LED`s is their operating voltage is usually lower than the voltage normally given out by the vans battery/alternator. The battery is rated at 12v but when its not under load or being charged by the alternator this can rise to about 13.8v. This could be enough to seriously shorten the life of most 12v rated LED`s ie IKEA ones. There are lots of lights that operate well below this as you`ll see by the ones i`m using. They are IKEA RAMSTA battery operated ones and cost me the princely sum of £1.69 each.



Now if you where to connect these to 12v they`ed probably blow straight away, so we need some way of reducing 12v down to their original operating voltage of 4.5v. For this we can use a little circuit that`s based around an LM317 voltage regulator.

Circuit diagram.



The cost of the parts for this is about £2.70 plus another pound or so for the actual circuit board (strip-board).

If we rearrange the circuit so it fits on the strip-board it`ll look like this,



R1 and R2 are used to set the output voltage Vout and in this case are 15K and 30K respectively. Ci is a 0.1microF 35v cap and Co is 1.0microF 35v cap. You can get away with not using the caps but they make the circuit more stable and less prone to interference.

so all we need to do is take the components,



and solder them into the strip-board so they match the rearranged circuit,



next connect the input and output wires,



and its ready to test. The only battery I had to hand (as I didnt fancy lugging the leasure battery out of the van) was an 18v one off my cordless drill. The maximum input voltage that the regulator will take is 37v so its still well inside its `safe zone`.



Moment of truth,



I`ve cut the board down to size and put a small heatsink on the LM317 as its generating about 6W of heat as it drops the voltage from 18 down to 4.5 (i`ve actually set it to run at 3.9 to make life easier on the LED`s)



I`m now going to leave it on test for a couple of days to make sure all is ok.



All thats left to do is mount it in a suitable little box and connect it up to the lights in the van.



I hope this will be of use to people.
 

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Thats an interesting post, thanks. Im not massively clued up when it comes to 12v electrics as you will discover from what im about to say. I Purchased some decking LEDs from B&Q, cut off the connector cable from the lights to the transformer and then wired them straight into the back of my Zig style equivalent unit with a fuse. I figured if they didnt come on then, ok they didnt come on! if they blew, then voltage etc was all wrong, or simply if they worked, all good!. i have sat in the van for a few evenings now with the lights on for quite a few hours while im working. Am i to assume they are ok, bearing in mind past experience has showed me that assumption is the mother of all [email protected]*kups. I dont know what the amps/ volts etc is of the lights at moment as im at work but i can look later. what do you think, do i need to do what you have suggested or have i just been very lucky getting a good match of lights etc
 

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Discussion Starter #3
It`s an unanswerable question really. If you`ve just had the lights on when the engines not running you`ll probably get away with it for a while but the LED`s are more than likely getting too much voltage, they`ll work ok but the life expectancy will probably be reduced. If they`re 12v LED`s then you could get away with a straight 12V regulator like a TS7812 , http://www.maplin.co.uk/1a-positive-fixed-voltage-regulators-46475. That`ll stop anything other than 12v getting to the lights and you dont need any other bits to make it work. The advantage of using an LM317 is that you can set it to slightly under-run the LED`s so that theres no chance of burning them out.

Sounds like your one of the lucky one`s where you just wire it up and it works straight away.

:):):)
 

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Discussion Starter #5
Yes the output is determined by the values ofR1 and R2.

If you have a specific voltage you require I can tell you the values you need.
 

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Interesting post Gromit, I'm about to embark on fitting LED's to my T4 and will more than likely just cut the transformer connection off and "hard-wire" in to the roof lighting circuit. I've had a look at the Maplin website and the voltage regulators seem cheap enough (if I'm looking at the right things - about a quid each?), but I'm a little unsure as to how they're supposed to be wired in.
Any advice would be helpful - I really don't want to buy the lights and then blow them up the first time I switch them on.

Thanks.,
 

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Discussion Starter #7
The 12v regulator only need 3 wires connecting to them, input, output and a common. If you use a TS7812CZ (maplin #QL32K) theres a data sheet available here, http://www.datasheetcatalog.com/datasheets_pdf/7/8/1/2/7812.shtml.

To wire it up to your lights the connections are as follows,

Pin 1 - Input -12v from battery

Pin 2 - Gnd - both battery ground and lights ground

Pin 3 - Output - 12v to lights.

Your regulator will have a TO-220 case btw.

HTH
 

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sorry for sounding like a noob, but all I need is this?



12v in to leg 1, earth leg 2 and also lights earth cable and output to the LED + on 3? and that then gives me clean voltage?
 

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Discussion Starter #9
Basically, yes. Apart from the one you`ve got in your pic is a 7805 and gives 5v as an output. Just make sure the one you get has 7812 on it for a 12v output. If you don't fancy soldering wires onto the regulator you can always butt crimp connectors to hook everything up with.

Dont forget to stick an in-line fuse in the 12v supply from your battery too, a 5A one should surfice.

T:
 

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I'm in complete agreement about the need for the regulation..

However, you may be better off with a LM2940, rather than a 317 or 7812 - The LM317 suffers with a nasty voltage drop (anything up to 3v) and also track the input voltage - so for example set to provide a 12v supply from a 14v input, if the input varies then so will the output, dropping to 10v for a 12v input and so on, including when going 'over', so a brief 15v surge would send 13v to the lights..
The 7812 also has a fairly harsh voltage drop, so only works well with an input of >14v etc.

It's worth looking at the LM2940 12v 1A regulator as an alternative - it's ideal for an LED supply. the voltage drop is only 0.5v, and it therefore will provide a stable 12v output with any input from 12.6v to 26v. It doesn't 'track' the input voltage, unless the input drops below 12.5v (ie, a heavily discharged battery) - its heat output is low too.

Here is a really good reference article on these: http://www.reuk.co.uk/LM2940-12V-1A-Low-Dropout-Regulator.htm

Here they are ready-made, with PCB mounted fuse and connectors for both supply and switch for just £6.79 - hardly worth the effort of making it yourself for that! If you're gonna spend £40-£80 on LED fittings, seems sensible to protect them with something like this. http://www.reuk.co.uk/buy-12-VOLT-REGULATOR-WITH-FUSE-AND-SWITCH-TERMINALS.htm
 

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Do I need to fit regulators to any other equipment powered off the leisure battery? Such as:
  • Stereo Headunit
  • Eberspacher
  • DVD Screen
  • Water pump
I'm a bit worried as I've got a sterling battery to battery and it can put over 14v in the leisure battery when the engine is running so this seems a bit high for equipment rated at 12v

Thanks in advance
 

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Discussion Starter #12
I would have thought that most equipment designed to run off 12v has some form of internal regulation.
 
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