Powering LED Flashlight cluster via 12vdc

[drventure]

Ok, this one is puzzling the EE side of me (which, admittedly, is a +very small+ side  )

I picked up a pack of those little 9 LED flashlights a while back for about 1$. My plan was to use the LED's as I might need them.

Well, in a build i'm working on, turns out it'd be nice to use use all 9 led's in the cluster to light a particular element.

No problem, but the cluster is intended to work on a pack of 3 1.5v batteries in series.

Ok. So I'm guessing the LEDs are typical 20-30ma forward current, 5v forward voltage LEDS.

There are nine of them, with all cathodes wired together and all anodes wired together.

my first thought was "I'm going to have to undo all that and hook a separate limiting resistor up to each one. Typical scenario.

But surely there's a way to connect things without undoing the existing cluster?

I've got a pile of these things so, I pull out a 1 amp fused wire, and a 12vdc powersupply, and found a 1kohm 1watt resistor (figuring they'd never light, because the forward voltage would be too low).

But, they light just fine, maybe not as bright as if connected normally to the battery pack, but as I understand it, that just means they'll last longer. The resistor doesn't even get warm.

Anyone know if this is insane, if I'm looking at a fire hazard etc.

The online LED circuit calcs don't accommodate this particular situation well, as far as I've found...

[RamjetR]

The flashlight cluster will have either a current limiting resistor pack for all those leds or most likely, it will have a switching current limiting circuit.

I cannot say with 100% accuracy about what you have in your hand as I've never seen it. But you should be able to tell if the circuit has a large or many small resistors for each LED to ground.

Otherwise look for an IC or small black mound on the pcb which could be a COB (circuit on board) IC. If a switching circuit is used to limit the current, then within reasonable limits, you should be able to connect the 12VDC to it and the switching circuit would clamp faster and limit appropiately.

Rule of thumb with single LEDs and resistors

5VDC needs 220ohm
12VDC needs 560ohm

The wattage of the resistor depends on how many in parallel you want.

[MonMotha]

Most of the cheap LED flashlights I see just have the LEDs in parallel hooked straight up to the batteries.  This is a pretty crappy design, but it's cheap.

Forward voltage on a high-intensity white LED like those used in flashlights is substantially higher than your typical red or green indicator LED.  It's something on the order of 3.5-4V.  Which is just handy if you want to hook it up to 3xAAA/AA.  Nine or so of those really bright LEDs can easily present a load of almost an amp onto the battery which drags the voltage down to something reasonable for that LED load.  The internal resistance of the battery acts as sufficient ballast to limit the current.  The LEDs are all well-matched, so they do OK in parallel.

If you want to hook it up to something else, what you have to do is add the currents up and assume a voltage of ~3.5-4V when calculating the single resistor.  The current will vary quite a bit between the cheap LED clusters and the higher end ones, even from the same formfactor flashlight (they build them to order).  I have a couple different flashlights in the same case, but one of them is quite a bit dimmer than the other.  It also has no name on it whereas the other one says "miraclebeam" and cost another dime or something.

Anyway, for a typical 9 white LED parallel cluster on a 12V supply, try about 100 ohms to start (~10mA per LED) with and maybe go down as low as 10-15 (~85mA per LED) ohms for full brightness.  1k seems inordinately high and will probably result in rather dim LEDs, indeed.

FWIW, if you wanted to be more exacting, you could rip one of the LEDs off the board and work out a V-I curve for it.

[drventure]

Thanks guys. Great info.

Yeah the lamps are a little dim, but still bright enough for my app (a power-on indicator lamp).

I needed the cluster because the lamp housing if from an old aircraft carrier instrument panel, so it's quit a bit bigger than your typical indicator light. A single LED just wouldn't illuminate it enough.

I might give a 100ohm a try, but based on what little I know, if each led is, say 5v forward voltage, with a 2ma forward current, that'll translate to almost a watt of power, so I'll need more than your typical 1/4 watt resister for the load...

I happened to have the 1kohm, 1 watt resistors lying around, but the next down is some 1/2 watts.

Thanks again though!

[MonMotha]

The 100 ohm resistor will burn ~8/10W, so you'll want a 1W minimum, probably a 2W if you can get it.  Using a 10-15 ohm will burn nearly 10W, so get a beefy one.

Note that you can easily run this off a 5V supply to drastically cut down the power burned in the resistor.  Obviously the resistor values need adjusted, then.