# DIY 350W LED light bar! 12v DC auto question for driver and 14v alternator charging.



## ddog1741 (Mar 16, 2014)

Building a 35" led light bar consisting of 35 10W high power leds all wired in parallel the "bar" is aluminum dimensions 35"L 3"H 1"D
Cooling isn't a problem.


Leds are Chinese 10w 800-1050ma 1000 lums and will be under powered at 750-800ma 


The driver design will be super simple 
35 2w 2.7 ohm resistors
35 LM317 voltage regulators


battery-distribution block-regulators-resistors-leds


It will be coming off a 12v automotive battery my problem is the battery when not charging off the alternator is at 12.8 
when charging its at 14.5 which is where it will be at when i'm actually using the light. 


When the driver was calibrated it was for the 12v system any way to get the 
voltage after the battery and before the regulator down to the designated 12v?

http://www.ebay.com/itm/12X-10W-LED...574122?pt=US_Car_Lighting&hash=item20df60272a


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## JohnR66 (Mar 16, 2014)

So each LED gets 1" of that aluminum bar? Cooling will be a problem.
I'd skip the regulators and design for a max operating voltage of say 14.5 volts and drive the LEDs at no more than 500ma with appropriate resistor. You will still need better heat sinking.


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## idleprocess (Mar 16, 2014)

OK, so I composed my original post without looking at the LED's the OP is wanting to use, so much of it was moot.

First - the LED's. No-name Chinese LED arrays have a poor reputation - they tend not to hit their performance numbers and live short lives. _Arrays_ are difficult to focus since by definition they use multiple LED's per package. 6000K is rather cool and will visibly "flatten" a scene due to its poor color rendition, making their use in a lightbar questionable. Unless you want an uncontrolled flood of light that fades to nothingness in a couple of meters, you'd be better off using single-die LED's that have complimentary optics/reflectors available.

Second - proposed design. Passively-heatsinked designs typically strive for 10-12 square inches per watt; assuming 3x1 solid bar, you're looking at ~3 square inches per watt - effectively less if you're using 3x1 tube (heat will take its sweet time travelling to the backside surface). LM317's will buy you some immunity from voltage variation, but won't help you when your battery voltage drops below the point that you lack enough overhead for the driver to operate... and the Vf spec for your arrays is pretty wide at "9-12V".

Third - light output. Assuming you hit 50% of specified 1000 lumens per underdriving them, that's still 17,500 lumens. For comparson, a well-designed HID system with D2S bulbs will throw less than half of the combined 6400 lumens that the bulbs are rated at and _still_ produces a veritable flood of light. You're proposing to throw more than 5x as much light forward, which could flood the immediate foreground at the cost of seeing things in the distance that are more far important. I suspect that a third of the lumens with good optics could achieve your desired results.

Lastly, you should be aware of rule 11, which prohibits discussion of illegal activity. Mounting such a contraption on a road-capable vehicle will not comply with the various safety regulations regarding forward lighting and will get this thread shut down.


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## DIWdiver (Mar 16, 2014)

Assuming this is for off-road use ONLY, as on a street vehicle this would be not only illegal but horribly dangerous to you and everyone around you:

That array is fairly small, so a decent sized reflector could work reasonably well. But the OP didn't mention using reflectors, and spacing the arrays on 1" centers doesn't leave room for decent reflectors, even if you stagger the LEDs. You could probably get better illumination with fewer LEDs and some decent reflectors.

Actually, if it is 1" solid bar, you have 8 sq in per LED, because you DO count the back, top, and sides. The heat WILL travel quiet well through the bar. That's why we use aluminum for heatsinks. If it's a tube, open in the 
middle, then I wouldn't count the back. Also, underdriving them brings the input power down to around 8W, so heat generation is probably around 6W. But even 8 sq in is inadequate for 6W without serious airflow.

I'm a bit curious about your numbers. An LM317 in current limit mode with 2.7 ohm resistor should limit at 444 mA, not 750-800. I wonder if you haven't got it wired correctly.

Lastly, to your original question, you don't want to drop the voltage before it gets to the LM317. That thing needs 2-3 volts overhead to regulate correctly. In current limiter mode, you also lose the 1.2V across the resistor. So you'll be lucky to have enough voltage for it to regulate correctly even when the engine is running. Besides, the regulator is designed to do exactly what you want - take a variable input voltage and output a known, constant current into a poorly characterized load. It doesn't need any additional help doing that.

What it will need is some heatsinking. At 800 mA, it could be dissipating up to 3 watts, which none of the packages can do without some help.


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## langham (Mar 16, 2014)

Listen to DIW he knows his stuff, first of all and second how is the CRI? These leds always make me curious because the big name guys get all of the attention, for a reason, but still. The numbers that they gave were most likely based on 25C and copper mounted.


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## RetroTechie (Mar 16, 2014)

*Don't go down, but UP that voltage!*

A better setup would be to put 2-4 of these LEDs in series, and then use voltage boost circuit(s) to provide a nicely regulated output. Bonus points if configured to regulate _current_ rather than a voltage. 

That would do away with losses in LM317's (which probably won't be of much use here anyway), fluctuating battery voltage, etc.

Like others said: off-road use only. And yes you will need to think about better cooling. Sounds like a decent piece of aluminium bar, but only a small bit for each LED. That 350W you have in mind, ain't nothing.  Btw: have you done any measurements on these LEDs?


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