# Converting a LED flashlight



## Hellspawn (Oct 13, 2006)

I want to convert several LED flashlights so that they can be ran on permanent power (cpu power supply) for next year, I wanted to know if anyone has done this before and can offer some advice?

Before I fry something, I thought I would ask, no sense in experimenting and losing a flashlight when I could ask the experts here.

is it as easy as calculating the amount of voltage the flashlight is currently operating on? (3 AAA batteries, 1.5 volts each = 4.5 volts) and then finding a transformer or power source that closely matches that number?

so in my case, I want to use a PC Power supply to run all my lights, that power supply will run (IIRC) 3, 5 and 12 volts so ill run these lights off the 5v terminals, should I add a resistor to the flashlight? or is .5 too much power enough to worry about?

Anything im missing here?


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## Otaku (Dec 3, 2004)

Yep, you got it. Just match the voltage and you're good. The LEDs probably need only 20-30 mA to operate, and the extra 0.5VDC shouldn't bother them. If you want to be safe, find an LED resistor calculator online (Wolfstone has one on his site) to determine the resistor to add. If you plan to run more than one flashlight on the circuit and they're wired in series, you'll have a problem if you use the 5VDC power tap. I assume that the power supply has a couple of amps output at 5VDC. Wire the flashlights in parallel - that way you'll get 5VDC to each light and you will still have plenty of amperage headroom.


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## Hellspawn (Oct 13, 2006)

Otaku,

You seem like the one to ask.

Last night I pulled out one of my powersupplies and hooked up one of the flashlights, I started with +3.3v and it was pretty dim, went to the +5v and it lit up great, only thing I worried about is that it got pretty hot, is this a sign of bad things?

I mean, it got really hot, I was worried about keeping it going, would a resistor fix this? another concern I had was it seemed to heat up alittle even using a 3.3v source.. any ideas?


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## hawkchucker (Oct 5, 2007)

He gives you sound advice, but also try here.
http://www.candlepowerforums.com/vb/
These guys can really do anythign with a torch.


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## Otaku (Dec 3, 2004)

Yeah, hot is not good. LEDs should run cool, if the LED itself is heating up it's being overdriven and will fail. How many LEDs are in the flashlight?


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## Hellspawn (Oct 13, 2006)

There are 9 leds in each flashlight and each flashlight runs off 3 AAA batteries. I could be confusing myself with having that 3.3v current causing the leds to be overdriven, but it could have just been warm from having it on the 5v source before that.

I had litterally minutes to experiment with them last night.


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## Otaku (Dec 3, 2004)

Are there any resistors in the flashlight that you can see? They may be very small (SMT) devices.


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## Hellspawn (Oct 13, 2006)

im not the brightest bulb on the line when it comes to electronics, and a small resistor was the first thing that I thought of, I havent looked again to see if I can see anything in there (not sure I even know what im looking for) but if I remember right, looking at the pcb that the leds are attached to, I didnt see any small chips or anything of the sort, just a PCB with traces to connect all the leds.

Problem that I have is that I dont know the specs of these leds, I know they are ultrabright though.


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## TommaHawk (Sep 18, 2007)

Very strange that they'd heat up. That they're built to run off 4.5 volts and the amps only 3 AAA batteries put out, I'd put a resistor in line to drop the volts to 4.5 - try to mimic the "original power source".


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## TommaHawk (Sep 18, 2007)

Forgot this: http://led.linear1.org/led.wiz

I'd say you should use your 9 LEDs as 1 LED in the wizard, drawing probably 270 mA, 3.3v of forward current, 5v power source. This produces a 2.2 ohm resistor - am I kosher here?


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## Hellspawn (Oct 13, 2006)

Im really wondering if in the battery harness there is a resistor of some sorts, can you wire several small resistors in line? I could just keep adding a resistor untill they stop heating up or the light starts to dim?


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## gmacted (Jan 18, 2006)

These flashlights probably have a DC voltage boost circuit in them. This would also explain why they are getting hot with a 5V input. I'm sure they wouldn't get hot if you supply 4.5 volts to them. There are ways of producing 4.5V with a DC to DC converter, but you'd have to build a small circuit.


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