LED problem. Can somebody explain this?

[Burstbucker]

I built a Treble Booster about a month ago and it worked great but all of a sudden the LED wasn't coming on.  The first thing that I suspected was a faulty 3PDT stompswitch but it turns out that the switch wasn't the problem after all.

With the pedal on, I wasn't getting any voltage on the two legs of the LED but I was getting the full 9V on the current limiting resistor, a 3.6k resistor. Before this problem cropped up, I found that the LED was sufficiantly bright and the total current draw of the pedal was only appx. 2mA.

So I thought that maybe I had an open resistor but that checked out ok.

Here's where things get a little weird, I decided to take a jumper wire and bypass the current limiting resistor for just an instant, then all of a sudden the LED is working fine again, like it did originaly.

Afterwords, I took out the 3.6k resistor and replaced it with a 1.8k which is what I usually use and have never had a problem. The pedal now draws about 3.6 mA which is totally acceptable.

So, it seems to me that the LED had shorted out and when I put the full 9v on it, it somehow reset itself?  Could this have been caused by too low a voltage on the LED initially or is this LED garbage?  I've never seen this before, any thoughts?

[Gilles C]

Here's where things get a little weird, I decided to take a jumper wire and bypass the current limiting resistor for just an instant, then all of a sudden the LED is working fine again, like it did originaly.

The last time I connected a LED directly to 9V, very shortly and by mistake...  :shock: , it burned right away.

So if yours is still working, I am wondering if you have some resistance in series somewhere in the path. That would explain it. Or a resistive contact at the switch.

Or maybe that it's onlt that I wasn't lucky or fast enough and you were...

You could measure the current flowing through the resistor with a meter, and compare it with what it gives you when you check the voltage between the 2 legs of the resistor.

(V/R=I) should equal (I) measured.

Gilles

[brett]

Have you got 7V across the 1.8k resistor?  If not, there's some extra resistance in the path.  (Assuming the battery = 9V and it's a red (2V)LED (9-2=7)).  LEDs blow quickly when forward biased with 9V  :twisted:

good luck

[niftydog]

I can't explain it, but LEDs are just diodes essentially, and a low forward voltage on a diode results in a high resistance path. I guess it's conceivable that there was some miniscule short circuit path between anode and cathode and a blast of high current burnt it away...

Happens in TV tubes all the time, but they're TUBES not semis!

spooky.

[Mike Burgundy]

I have also seen LED's that were dead "light" on high voltages: you're using the gold wire that connects the top of the die to ine of the legs as a filament. Not for very long, though...