# How to drive 20 CREE XP-E in series parallel



## jungleb0y (Dec 1, 2009)

So heres the deal we have 12v and need to run 20 CREE XP-E's in series or parallel and it needs to be reliable with low heat.. Just wondering what everyones thoughts would be. Small design as well.:thinking:


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## qwertyydude (Dec 2, 2009)

Is it exactly 12 volts? If so you may be able to connect them direct drive in sets of 3 in series and each set of 3 in parrallel. 12 volts would run each led at 3 volts, minimal current so no heat issues. Maybe you can add another 1 to make 7 sets of three at 21 leds to even it out.


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## space (Dec 2, 2009)

You need to specify more about your application:
More info on the power supply (what kind, max/min voltage)
How hard are you planning on driving the LED's? / Do you have good heat sinking in the application? 
Do you need to have constant output? (Constant current to LED's?)
Do you have good heat sinking in the application?
Do you need to be able to adjust the output? If so, do you need to adjust all simultaneously? 

space


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## jungleb0y (Dec 2, 2009)

qwertyydude said:


> Is it exactly 12 volts? If so you may be able to connect them direct drive in sets of 3 in series and each set of 3 in parrallel. 12 volts would run each led at 3 volts, minimal current so no heat issues. Maybe you can add another 1 to make 7 sets of three at 21 leds to even it out.


Well it will be running off of a car battery so around 12.6 volts at it's max if the battery is in top shape. would like to stay away from the resistor's and use a driver possibly 2 drivers if I can find one that would work.


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## jungleb0y (Dec 2, 2009)

space said:


> You need to specify more about your application:
> More info on the power supply (what kind, max/min voltage)
> How hard are you planning on driving the LED's? / Do you have good heat sinking in the application?
> Do you need to have constant output? (Constant current to LED's?)
> ...


I'm planning on flashing the LED's so I'll have to decide on the heatsink option if needed I would like to flash them at 1000ma. I will be flashing each of the array's we make via a PIC16f887. I would like to use a driver if I can find one that will take the 20amps or possibly 2 10 amp drivers. 

I don't have much experience at all with using LED driver's usually just needed to do the resistor thing which always worked for me but want to use the driver's because I hear they produce less heat and are more efficient.


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## clint357 (Dec 2, 2009)

You must use a constant current driver for vehicle applications because there is way too much variation in the voltage. There is actually an automotive section on this forum as well.


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## mudman cj (Dec 2, 2009)

I would use 2 CCHIPO drivers from taskLED to each power 10 of them wired in series. Assuming 85% efficiency (it's at least that high), the driver will pull about 3.3 Amps from 12V.


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## mds82 (Dec 2, 2009)

it would be better to run them in 2 sets. each set would be 3P 3S, push 3 amps to each string, each LED gets 1amp. you would only use 18 LED's then, not 20


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## bshanahan14rulz (Dec 2, 2009)

for your application, it sounds like you are making modules for flash-bars? are they internal or external?  You will need a driver, because a resistor would get very very hot. Plus, you'll need big fat honkin resistors, not just your run-of-the-mill 1/2W peanuts. 

Things to consider:
heat - an external solution so that air can pass through fins of a heat sink. throwing ideas out there, you could mount them all in a row facing up, put reflectors over them to redirect the light forwards. This way, your heatsink will be in a position to get lotsa fresh air, instead of sitting in the trailing vacuum. 
directing light - do you need focused spots somewhere, or just very high visibility (like for the assumed flasher bars)? Need varying brightness, or just bright or off?
power - Why not try the standard LM### voltage/current regulator? relatively simple to wire up, and cheap! 

I guess that's it. More info on your project would be interesting and helpful for those trying to give some pointers.

(it is also assumed that these lights will be used in accordance to any laws in your area. yeah. [/disclaimer])


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## Al Combs (Dec 2, 2009)

The new Hyperboost, also from TaskLED, might be better than two CCHIPOs. It's fairly new so there isn't a pdf in the Technical table to download. But it looks like you could run all 20 XP-E's in series on a single driver. Max output voltage is 80 volts. The Tech page says, "Input_current should be around 5A or less for optimal performance". There is an example on the same page close to 90% efficiency with 57 watts power out. It has a spot on the board to wire an external trim pot. Also two CHIPPOs cost $90 and one Hyperboost is only $40. 

Use an example of 700ma as the rated max current of an XP-E and 3.4 volts to reach that level. The string of 20 requires ≈ 68 volts, or a boost ratio 5.67 from 12 volts. Allowing 10% for inefficiency would mean about 4.4 amps. That's well within the rated max. I'm sure George would help you out either here on CPF or the contact link on TaskLED.


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## Illum (Dec 2, 2009)

driving LEDs in parallel without a current limiter is a recipe for thermal runaway:green:


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## space (Dec 4, 2009)

What is meant by flashing? For how long will it be on? And how often? 
Will the leds be run when the battery is charging (in a running car for instance)? (Means that ~12v will get up to 14.4v.) or maby mostly when the battery is charging? Is it ok that the output drops off abit when the voltage goes down to and under 12v?


Parallelling LED's without current blancing systems is usually unoptimal, espesially when you are running them at maksimum output. But if flashing means a few seconds (or less) now and then, then thermal run away should not be the biggest problem if you have good cooling and match the LED's. 

The Hyperboost looks like a really good solution for the descibed application. 
Another posibility is low drop out linear voltage regulators. They should manage to run 3 LED's in series each at regulated 1000mA and efficiency from 70% to 80% (14.4V to 12.2V input). One candidate might be LM1086. 

space


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## jungleb0y (Dec 7, 2009)

Think I'm going to be using the LM3429 to drive LED's and use a 7X2 array.

It's not for a light bar it's for my wing tips. NAV/POS/Strobe

Each side is going to have 2 circuit's in them one for NAV (green or red facing fwd) and one for POS (white facing aft) the center and center of fwd and aft will also have 2 strobe led's sitting in the array to get the 180 deg viewable angles for strobe. Going to be using the same cirucuit to power the white POS as is powering the red and green NAV.. Found some matching LED's that are only off .04fv at 700mA dont think it should be noticable.

I havent decided on the sequence of the flashes but they will be flashed max twice a second and pause for maybe 2 seconds which I'll controll and sync via my pic.

RR-------------WW <--R&W are within the same circuit
SSSSSSSSSSSSSS <-- seperate circuit
RR-------------WW
|
sync
|
GG-------------WW <--G&W are within the same circuit
SSSSSSSSSSSSSS <-- seperate circuit
GG-------------WW


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