Underdriven non-heat-sunk Cree

[Fallingwater]

I remember posting about this before, but I can't find the thread.

I have a small pocket flashlight that belongs to my girlfriend, it works with two AAs and drives one of the dimmest bulbs it's been my displeasure to see.

I'd like to LED-ify it with a Cree Q5 I have (or a P4 if I manage to find it), but with no drivers - the thing is all plastic, so it can't heatsink worth a damn and a properly driven Cree would just bake itself to death.

I'm thinking about driving it directly from the two AAs.

Connected to my lab power supply at 3.0V, the Q5 drew about 70mA and gave respectable light (relatively speaking). Is this low enough to run it with no heatsinking whatsoever?

If it isn't I'll just use a Nichia GS, which when underdriven still outshines the original incandescent bulb.

But I'd really like to Creeify the little thing.

Thanks.

 

[stephan2148]

You should be safe there. 70ma is nothing like the crees are used to.
Worst case would be typically 1.6V per cell freshly charged (correct me if I am wrong pleez), and I think 3.9V is a typical Q5 voltage at 350ma.
If it doesn't get warm on good batteries, don't worry about it.

 

[LukeA]

The Cree will be fine at that current with no heatsinking. A 5mm LED would fry but high-flux emitters can handle the temps produced by 70mA no problem. There's less heat per unit area because of high-flux LEDs' larger dies.

 

[rolling]

Worst case would be typically 1.6V per cell freshly charged (correct me if I am wrong pleez), and I think 3.9V is a typical Q5 voltage at 350ma.

I have a Q5 that is 3.9V at 1000ma.

 

[Jarl]

3.2V is typical at 350ma. Bear in mind the alkalines will droop at that current level- their voltages will drop, and so will the drive current.

Could you not even stick a few coins on the back of the star/LED? That would still help a lot.

 

[Fallingwater]

I found the P4. I'd rather use that, as I'd like to save the more expensive Q5 for more deserving lights.
I think it's because of its lower Vf, but when connected to two full alkalines through my multimeter it drew about 130mA. Is this still safe?

 

[Jarl]

No idea. Leave it on for 5 or 10 minutes, come back and poke the LED. If it's warm, it's fine, if it's hot, it's not fine, and attach additional heatsinking (ramsinks, coins, bits of metal, etc.)

 

[Fallingwater]

The construction of the light prevents me from sticking anything at all under the led's board without cutting off the entire lamp assembly, which I'd rather avoid doing as the light isn't mine.

 

[LukeA]

I found the P4. I'd rather use that, as I'd like to save the more expensive Q5 for more deserving lights.
I think it's because of its lower Vf, but when connected to two full alkalines through my multimeter it drew about 130mA. Is this still safe?

Could resistor it.

 

[Fallingwater]

I have no appropriate resistors, the electonic stores around here only stock a very small number of values and I don't want to wait weeks to get them with an online order.

 

[jzmtl]

FWIW I have a dollar store flashlight, and I cracked the PR base incan bulb and soldered a luxIII I had in it, directly driven by two carbon zinc D cells and it seems to work fine. The only "heat sink" is a ball of rolled up aluminum foil I stuffed into the can and glued with heat sinking compound.

 

[IMSabbel]

I have a Q5 that is 3.9V at 1000ma.

A random Q5 from DX i just tested come to 850mA.... (and that doesnt include the 15mV Voltage drop from the probes...)

Edit: And how much is your time worth?

You can get a usable AAA light with driver and a Cree for a few (10? 12?) bucks. Undeniable better than an all-plastic AA light.

Is it really worth it breaking your head for that?

 

[Fallingwater]

There are several reasons.
1) the light has sentimental value for my girlfriend.
2) I like to modify stuff.
3) I don't currently have to work (with any luck this situation will last), so I have time to dedicate to pretty much any project that crosses my mind.

 

[LEDite]

Fallingwater;

The Cree will be dissipating a little under half a watt @ 130 ma.

It will get warm, so you may need to put a IR thermometer on it to check temp after it runs for thirty min.

I would shoot for 140° F at the LED.

Larry

 

[Fallingwater]

Warm it gets; after about thirty seconds the underside of the LED is noticeably warm, although I wouldn't call it "hot". Would the LED manage to survive at 140F?

 

[Jarl]

A good rule of thumb is that if you the heatsink is getting too hot to touch, you're going damage to the LED. In this case, the LED is the heatsink, so leave it for ~5 minutes (though terminate beforehand if necessary) and see what the temperature is.

 

[enLIGHTenment]

My suggestion would be to buy her one of these for next birthday/Christmas/whatever. Shiny and expensive beats sentimental attachment all the time--and it's a very good light, too.

 

[Fallingwater]

My suggestion would be to buy her one of these for next birthday/Christmas/whatever.

Never in a million years am I spending that much for a light that doesn't have a HID lamp in it. The $50 I spent on my NDI is pretty much the absolute limit of what I'm willing to shell out for a light that doesn't make people fifty metres away go "ouch, turn that thing off!".

Shiny and expensive beats sentimental attachment all the time

Thankfully, my girlfriend is of the rare kind who genuinely do not care how much one spends on presents. I built her a relaxing colour-changing lamp with $10 worth of parts and RGB LEDs and she was enthusiastic. Sniny and expensive is not required.

By the way, I did the mod by crushing the existing bulb out of its screw socket, soldering the LED's contacts to the socket and inserting it in the original holder. Then I glued the LED on with two-part epoxy.

Not very bright, but far more useful than before.

Now I just need to find something to glue on as protection, or possibly a small reflector/optics to concentrate the beam if my gf should prefer something with less flood.