Led Confusion

[12StringStratMaker]

Ok so awhile back I replaced (after modding) the led in my Boss GE-7 with a 5mm 1.85V 500mcd red clear led.  It worked fine but wasn't as bright as the super bright blue one in my Blues Driver I had modded by Steve Dudley.

I recently found some 5mm 3.0V 4000mcd red clear leds at a local store so I picked some up to increase the brightness and when I touched it to the pads it only worked when it was backwards and when backwards it was only as bright as the 400mcd one I had in there.

It appears that there is a 3.9k resister in series with (I think) a zener diode after that.  I measure 5.2V on one side of the pad and 0.3V on the other side (when no led connects the two).
From what I read, I guess I should lower the resister to increase brightness.  Am I correct and is there a good value to try?
But why does it only work backwards?
Thanks.
-Phil

[12StringStratMaker]

Did I confuse everyone else?  

[TheBigMan]

Dunno about the backwards thing but if it's not bright enough then you probably have too high a resistor.  Remember though that lowering will increase current draw and lowering it too much will fry your LED.  It's always a good idea to get your LEDs data and calculate what the minimum safe resistor is for it.  AMZ-FX has an online calculator at www.muzique.com

[KORGULL]

When you talk about "backwards" maybe you are just experiencing the result of differing ways that manufacturers identify the anode/cathode.
I have LEDs that have the cathode lead shorter and I have LEDs that have the anode lead shorter. I guess sometimes you can't count on some marking systems. Then again I buy alot of surplus stuff so maybe the ones I have were irregular/rejects. Usually I can count on the flat side of the case being the cathode side, except I have alot of LEDs with perfectly round cases.
I guess as long as it functions properly - lights up one direction (forward-biased), blocks current the other way around (reverse-biased), you're good to go. Don't know what's up with the brightness problem.
Hope I've made some sense here - no time to re-write for better clarity.

[12StringStratMaker]

I meant backwards by the parts within the actual led... But thanks for trying to make some sense of it.

[onboard]

It appears that there is a 3.9k resister in series with (I think) a zener diode after that. I measure 5.2V on one side of the pad and 0.3V on the other side (when no led connects the two).

I'm confused. I checked a schem, and you're right about the zener. It should be zener -> 3.9k resistor -> LED. If that was 0.3mV on the other side, it would make sense.

Is there any mark at all on the LED case? A dot, a flat spot, a notch, or is one leg longer? I guess I'm not sure what "backwards by the parts within the LED" means...

[KORGULL]

I meant backwards by the parts within the actual led

Now that you mention it...I have some red super bright 5mm LEDs I got from The Electronic Goldmine last year where the internal parts are opposite of how they typically look in most LEDs.
Usually the larger "tab" or whatever it's called is on the negative lead inside the case, but these are the other way around.
It's a crazy mixed up world out there  :shock:  :?

[Hal]

too low a value will kill the battery as well as the LED - don't get greedy with brightness, its ususally not for the best....as long as you can see that its on, thats ususally the best way.

[12StringStratMaker]

I did mean 0.3V on the other side, not in between the zener and resistor (as re-reading my first post seems to indicate).  So I guess all is good then, I'll play around a little more, but also take the advice not to be greedy.  Thanks all.