View Full Version : Blown LED - but why?

11-06-2007, 08:47 PM
I blew a red Lux III just a few minutes ago. 4.8v pack fully charged (so around 5.2 volts). 1 Ohm 10 watt resistor (that's what was for sale at Radio Shack). The LED blew. Voltage measured across the LED read 5.05 volts. Current to the LED read 1505mA.

Somebody clue me in.

Lord Maul
11-06-2007, 08:52 PM
Was it in a saber? If you didn't put thermal tape on the bottom, it could of grounded out. Been know to happen, but I'm sure you would of taken that precaution.

11-06-2007, 08:58 PM
Sounds like it grounded out. I've had it happen to me for no apparent reason.

11-06-2007, 10:06 PM
Well, that sucks. Does everything else seem like it's alright? I mean, I know the resistor is limiting current, but is the voltage (5.05v) too much? I'd hate to buy another LED and pop it too.

11-06-2007, 10:46 PM
It shows in the LED resistor chart that you should use this (1.2 Ohm 3 Watt) not sure if your using a 1 ohm 10 watt is the problem?

11-06-2007, 10:55 PM
You can always use a higher wattage resistor. The 1 Ohm resistor I used reduced the current to the LED to about 1505mA, which is not too far off, and only overdrives the LED a tiny amount.

For grins, I hooked an old white Lux III to the setup, and it worked like a charm.

11-07-2007, 01:33 AM
Jon, I have to chime in here...

Ask Corbin about this (fairly odd) anomaly. Indeed, if you are not careful, Red and Red-O Lux IIIs CAN ground out via the LED star through the body of the saber.

Corbin (unfortunately or fortunately--depends on how you view life) gained EXTENSIVE knowledge and understanding about this when he built the saber staff for BOP I. He kept "losing" LEDs for no good reason. Long story short... my guess is you are probably getting the same thing.

The guess is--the body of the saber is sending the full battery voltage to the LED (somehow), since the LED touches the heatsink and the heatsink touches the saber body.

I had problems like this as well when I re-designed the saber staff for BOP II... but I (luckily) did not burn out LEDs... one simply kept shutting off. They were grounding out via the SWITCH bodies, if you can believe that. I had isolated EVERYthing else.

My guess is--if you isolate EVERY SINGLE SOLITARY thing with either thermal tape, or rubber or close off "touching" parts with non-conductive materials... you should be OK.

It's a mystery though. STRANGE things occur. The Red-O and Red LEDs are apparently constructed differently than the "light side" versions. Maybe even the Amber Lux III.

I don't know WHY... but... there it is.

11-07-2007, 09:26 AM
The electronics were sitting on the workbench. I was testing them before mounting it up in the saber. The LED was mounted on the heatsink.

I looked at the emitter of the LED, and one of the thin wires is indeed broken. Looks like a new LED is in order. I was so hoping to have this pair of sabers done this weekend.

11-07-2007, 11:01 AM
1500ma is the leds absolute max rating, not the recommended rating. You have just shown why they give ratings. As with all things manufactured there are differences, the one you got must have been on the bottom of the spread so bumping it even a little over the MAX blew it.

Another might run fine on the same setup, then again it might just pop again.

I prefer to run them a little below max as generally you cant see the difference and its safer and increases the lifespan of your led. Generally 1200 - 1300ma is more than enough for a red.

11-07-2007, 11:50 AM
Sounds reasonable to me. I'll pick up a 1.5 Ohm, 5 watt resistor on my next trip through the store.

11-07-2007, 11:38 PM
Neo... thanks for confirming that about the Max. Cont. Current thing... I was beginning to wonder about what Eandori was doing regarding over-driving a Lux V to 1000ma (instead of 700ma).

He says it is working fine, and he sees a distinct brightness difference, but... I wonder when or if the thing is going to start losing its brightness... and/or simply blow!

For a Lux III or K2... big deal, $6 to $8 gamble. For a Lux V... I don't know if it's worth $24 every time one would blow.

SO... the quote of the day is:
~~ Give your LED what it WANTS... no more, and preferably a bit less... ~~ :)

11-08-2007, 04:56 AM
It depends on what he is doing when it comes to overdriving. In certain circumstances you can drive an led alot. For example using a pulsed source the led is only on for a very short space of time. Ive pulsed normal 5mm IR leds that have a 100ma max current at 1500ma before but only because the led was being driven for a fraction of a second with a 12% duty cycle, so they can cool down between shots (it was for a outdoor lasertag system)

In simple terms the death of any led is heat. Heat is caused by the flow of current. With Jonitus's example he could see the thin wire was broken. This is what happens with a fuse, too much current for too long heats up the wires and they pop. Thats good for a fuse, not so good for an led :)
Alot of power for a very short duration and things dont get hot enough damage it. Thats why pulsed operation works but the figures can look misleading to someone who doesnt follow exactly whats happening.

With our use for sabres a puck/driver of the proper current rating is best, and if you have to use a resistor and cant find the exact resistor value you want use the next value UP, not below.

11-08-2007, 01:30 PM
Oops, I forgot... good point! Eandori is using CF, so... that's right, as long as he doesn't keep it on the "solid blade" mode for too long (or just always uses the flickerdepth modes alot), gotcha... should be good. Thanks again...