Cree Modified Push Light – To fry or not to fry?

[Flight_Deck]

I'm looking for someone to look over my shoulder as it were to insure that I haven't done something stupid.

Here's the deal: I took one of the dome shaped push lights and stuck a Cree emitter in it for my daughter's reading light. Now let me tell you, we're talking SERIOUS LIGHT now, especially compared to the original incandescent dime store flashlight bulb that it came with.

The unit is powered by 4 AA batteries in series (6 volts), and the Cree emitter is from the Sandwich Shoppe mounted on the slug taken from a Golston 7W 2 x CR123A flashlight (6 volts). I included the slug to dissipate heat created by the emitter and because it houses what I measured to be a 3.2 ohm resistor. The mod was quite easy and it looks stock when off and ominously bright when on.

Now I'm not worried about this thing being hazardous or anything like that. I just want to make sure that I'm not going to fry my $10 emitter. Is it OK to direct drive a Cree emitter by 4 AA batteries in series through a 3.2 ohm resistor? Initial testing indicates that it's OK (and looks/performs way cool!), but I'm concerned that it might be history after extended use.

Thanks in advance for the advice guys. Have a great day!

 

[mikeh]

"4xAA driving a Cree LED through 3.2ohms"

So less than 1A start, settling to around 1/2A with NiMH or alkaline.
That will work so long as you have a good heatsink well attached to the slug.
You need sufficient surface area of metal to conduct heat to air.
With fresh batteries, the resistor could be putting out nearly as much heat as the emitter, but that won't last long as the battery voltage drops.
Note that torch/flashlights can rely on your hand to conduct heat away, but not this. HTH.

 

[TorchBoy]

3.2 ohms sounds good. I figure it'll start by dissipating about 0.9W (at roughly 0.53 amps), which will mean it will get quite hot. Do you really want that on the back of your LED rather than a nice cool heatsink?

To check the current isn't too much, do you have a multimeter? If so, use it to measure the current with fresh batteries. Or if you find that the multimeter leads are affecting the current (as I did with a K2 last weekend - they apparently made a difference from 1 amp to 1.2 amps) measure the voltage drop across the resistor then divide by its resistance in ohms and you get the current. My calculation was easy because it was a 1 ohm resistor but it did get very hot, too hot to touch.

Like Mike said, having a good enough heatsink is important, even though there's not going to be much convection cooling inside a push light, and make sure that the hot bits are not touching anything plastic. I was catching occasional whiffs of a melted plastic smell from what my resistor was in contact with.

Or use a bigger resistor to reduce the current some more.

 

[Flight_Deck]

Thank you very much gentlemen! The advice and observations are much appreciated.

Have a great day!
John C.