What obvious and stupid thing have I missed

[dekekun]

So I'm a noob to all this, learning as I go, so sorry if this is so blatantly obvious to everyone else but me, infact I'll be relieved if it is.

Anyway, just started wiring up my LED saber. My components are as follows:
- Luxeon Rebel Star Amber (3.6V 700mA recommended)
- Single AA Trustfire 3.7V

Now I will have more to add to this circuit later, switch, soundboard etc, but I thought that, as a start, simply wiring a battery straight to the LED would power on the LED. I'm using quick-connects, so just wired a pair to the battery and a pair to the LED. Red wire on the plus on both the led and the battery pack, black wire on the negative on both.

The LED does not power on.

[TuxedoCartman]

Off the top of my head, two things...
1) ALWAYS use a resistor! Doesn't matter that the voltage is the same, you need a resistor on there. It looks like you can get away with a 1-wattt, 1ohm resistor. (http://led.linear1.org/1led.wiz?VS=3.7;VF=3.6;ID=700)
2) And this one is probably why you're not getting any results... have you hooked the Trustfire up to a charger yet? They're shipped with the PCB in a safety mode, where they won't provide any juice until connected to a charger.

[dekekun]

Isnt there an entire thread on here dedicated to how to run LEDs without using a resistor? The gist of which was that as long as the V and mA are within tolerances it should be safe. Again this isn't permanent, I just wanted to test that it worked (which I thought it should) before I wired up the much more complex bits like the soundcard. Obviously the soundcard has resistors on it to run the LED.

And the trustfire is charged, I tested it in one of my other sabers, and it powers on normally.

[Silver Serpent]

A picture of the wiring would be helpful. For testing LEDs, I usually use a pair of AA alkalines. 3v is usually enough to get them to light up at least a little bit, and you don't run the risk of popping the LED from too much current.

Show us a picture if you can, and we'll see if there's anything we can suggest for you to do differently.

[IndustrialAction]

The amber rebel has a 3.2 Vf not 3.6 with a maximum of 3.51V

[Darth Invictus]

You might want to check out this thread ( http://forums.thecustomsabershop.com/showthread.php?12033-Resistor-with-a-Li-Ion&p=186225#post186225 ) It includes some good information and the following quote.

Novastar
Novastar is offline Council Member My location

Join Date Mar 2007
Location San Jose / San Francisco, CA
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I know some people go resistorless when the voltages match, so I am looking for voices of experience to chime in. I'd use a 14500, but I'm looking for run-time and I haven't found one that actually puts out a steady 1000mAh, yet.
Basically, if the rated voltage of the cell is equal to or a bit less than the forward voltage of the LED... you should be good without a resistor. I know this for a fact because I've been doing it with no problems for the last 5 years with multiple sabers, all with differing setups... but with the same basic principle: batt v = (or ~=) LED fwd v

This, however, is all about driving an LED with no special board. That is to say... if you're not using an FX board, a "Corbin" board or an "Ultrasound", CF, etc. etc. In the case of most of the common boards we all use... some kind of controlled and/or fixed current is being supplied, and a resistor is not needed--because it's already present on the driver board in a certain fashion.

Still... what Rhyen is saying is also true--you take a risk if you have more voltage from the battery source over what the LED "wants" (fwd v). But even with THAT said... you have to remember:

* The fwd voltage of an LED is basically the voltage at which it lights up "well" or "properly". You may be able to have it light up with less voltage... and more voltage too! Within reason (say + or - ~0.5v)
* As an LED attempts to draw more current from a cell... it requires more voltage to do so. Because voltage and current are relational. If the cell cannot provide more voltage... the LED will be unable to keep drawing more and more current... and you won't run into "current over-run".

Hope it helps... I know it can be "hard" to understand. But one thing is a good bet--if you "match" voltage & current to the best of your ability... a resistor may not be needed. And having less resistance means less waste, which provides more runtime in the long run. Good luck!

[dekekun]

Ok so that's all well and good, but the thing I don't understand - even if it was too much voltage, wouldn't the LED have just come on for a second and then popped? Thats how LEDs I've used in the past have worked. The fact it never even turned on is what concerned me...

[amwolf]

So, this may be a ridiculous question, but did you check that you didn't cross wiring paths? My last saber I was absolutely sure I wired the neg wire to the neg pad, no need to check that... and 30 minutes later guess what the problem was?

[dekekun]

Brought the multimeter home from work - plugged it in and found zero current reaching the LED.

The battery pack positive contact had a thin film of transparent plastic over it on the INSIDE. Sigh!

Works great now...