How to drive 3x Cree XR-E constant current regulated off of 12v?

Veto · #1 04-26-2007, 10:15 PM

My 3 Cree XR-E light assemblies arrived. These are the regulated 900ma animals that take between 3.6 and 9v. I'm wondering how to connect them to run off of a 12V powersupply / lead acid battery. Should I run them in series or parallel? I'm really at a loss due to the constant current regulation.

On a side note, my 3 Cree stars and Artic Silver adhesive also arrived. Looking forward to attaching them to a nice piece of aluminum and doing some testing with various resistors to see what works best.

Led_Blind · #2 04-26-2007, 11:30 PM

Hey Veto, seeing they are current regulated you will need to supply the right Vin, you cannot connect them in series, should be in parallel.

Use 6V SLA's

or use a different power source.....

Veto · #3 04-27-2007, 09:41 AM

Okay, so it looks like I'll have to feed them with voltage in the range (3.6 - 9.0), but I'm pretty locked into having a 12v supply, so it appears I'll need to have a 12v -> 6v converter and then run them in series @ 6v. Since they say they are 900ma driven and I think they are around 4v drop, would it be right to assume that I'd need a supply that can do with 6v @ 1.8a? (4v * 900ma * 3 ~= 6v * 1.8a)

soffiler · #4 04-27-2007, 11:40 AM

Ideally, you want them in series, since you have a 12V source available, which means you'd be providing about 4 volts each - which is within the 3.6-9V input spec. The one big problem with connecting them in series is that you will be forced to electrically isolate them... in other words, they can't all be attached to the same heatsink.

yellow · #5 04-27-2007, 12:19 PM

You mean its possible to hook up three step-down converters in series, as if they were led or resistors, or ... ?

soffiler · #6 04-27-2007, 01:04 PM

Quote:

Originally Posted by yellow

You mean its possible to hook up three step-down converters in series, as if they were led or resistors, or ... ?

Yep.

The input side of the converters will look more or less like three resistors in series as far as the 12V source is concerned. Because they are in series, the current through the input side of all three will be identical. Assuming all three have the same electrical characteristics, they'll each get 1/3 of the 12V source. But since they won't be EXACTLY the same (due to natural variations that occur in production) the exact voltage each one sees might be slightly different, but of course the three will always add up to be equal to the source voltage.

It is important to remember what it means to hook in series: source (+) goes to (+) on #1; (-) on #1 goes to (+) on #2; (-) on #2 goes to (+) on #3; (-) on #3 goes to source (-).

Since these drop-in modules use the body as (-), it means you cannot just mount all three of them to the same heatsink... this is because the body on #1 is actually at ~8V, and the body on #2 is actually at ~4V.

Veto · #7 04-27-2007, 01:26 PM

So I can do them in series off of 12v, excellent. So I'll just need to make sure that the bodies are isolated from each other in the mount which means no mounting them to an aluminum plate directly which would put me way over their capabilities and the magic smoke would escape.

I just want to double check before I hook them up in a test harness and hit them with 12v...I'd hate to destroy ~$40 worth of assemblies!

soffiler · #8 04-27-2007, 02:29 PM

Quote:

Originally Posted by Veto

So I can do them in series off of 12v, excellent. So I'll just need to make sure that the bodies are isolated from each other in the mount which means no mounting them to an aluminum plate directly which would put me way over their capabilities and the magic smoke would escape.

I just want to double check before I hook them up in a test harness and hit them with 12v...I'd hate to destroy ~$40 worth of assemblies!

Those things run, what, 900mA output? That's a lot of power. I am reasonably sure they are going to need a good heatsink, such as the aluminum body of the flashlight they are designed to drop into. I realize you're doing something different with them, but I strongly suggest you think about heat sinking... it'll have to be three separate ones that are electrically isolated from each other.

Veto · #9 04-27-2007, 02:55 PM

Steve,

I'm planning on mounting them to either 3 separate aluminum plates or possibly 3 separate cheap aluminum heat sinks for CPUs turned over. I'll drill a hole in either one to be able to get at the (+) terminal, which is in the middle. Then I'll take those 3 sub-assemblies and mount them to a non-conductive carrier (probably wood).

2xTrinity · #10 04-27-2007, 03:21 PM

Quote:

Originally Posted by Veto

Steve,

I'm planning on mounting them to either 3 separate aluminum plates or possibly 3 separate cheap aluminum heat sinks for CPUs turned over. I'll drill a hole in either one to be able to get at the (+) terminal, which is in the middle. Then I'll take those 3 sub-assemblies and mount them to a non-conductive carrier (probably wood).

Before you go to all sorts of trouble buildin a complicated setup, I'd test to see whether the drivers actuall work hooked up in series, just with some loose batteries, some jumper wires, and a multimeter.

I woudln't be surprised if they don't run very well in series as most drivers don't behave like simple resistors -- they work by switching themselves off and on in order to limit the current -- and if you "feed" the second and thrid drivers in the string with a pulsating current from the first, they may not work well. A lot of that may depend on the design of the driver though.

I'd just try out two in series, on your desk, and see if they light up evenly, with no flicker etc. before building anything permanent. Ideally, the drivers would be hooked up in parallel. The Ideal way to hook up a string of LEDs in series would be to just wire the LEDs themslves together, and get a single DC-DC converter that takes in an unstable 10-14V signal (typical for cars), and outputs a constant 750mA-1A at up to 12V.

soffiler · #11 04-27-2007, 03:56 PM

I totally agree with 2xTrinity as far as testing things before you spend any time and effort fabricating.

Veto · #12 04-27-2007, 08:13 PM

Indeed, I will setup a test harness first before spending time on fab. Thanks for the suggestion.

It's not actually going to be a car that's driving these things in their final setup, but it will be 12v batteries, charged by solar. These lights will be on a switch and run from time to time to spot part of a fishing camp..well, that's the plan at least.