Dist+ Eating LEDs

[Deep Blue]

I built an MXR Distortion Plus, and it keeps frying LEDs.  It works for a while, but eventually the thing just fries the LED.  It like flashes red, and then stops working.  I don't know what to do.

I'm using an 8.2k resistor, like GGG says to.  Any suggestions on what is going on?  Everything is wired correctly.

[aron]

>Any suggestions on what is going on? Everything is wired correctly.

Which LED is this? The bypass LED?

Increase the value of the resistor to 10K. What kind of LED are you using?

[idlefaction]

yeah, definately try changing the 8k2 resistor.  does the red LED glow orangey red?  that's a sure sign of too much current.  it's damn easy to get the resistor colour code mixed up!  so many times i've fried things cos i got brown and orange wrong.  

[Mark Hammer]

There are very few LEDs that will glow sufficiently brightly to be seen or to burn up with a current-limiting resistance over 20k and a 9v+ supply.  So, here is what you do.  Since LEDs of different sensitivity can end up in the same parts drawer and pretty much look the same, make yourself an LED testing jig.  Take two 10k trimpots and solder them end to end to form a variable resistor that can go as high as 20k.  Solder an alligator clip to one end and solder the other to a 1k resistor (for safety) and from there to a 9v battery clip.  On the other battery clip contact, solder either a wire and alligator clip or just the alligator clip alone.  Set both trimpots to maximum resistance and connect the LED.  Adjust the first trimpot downward (towards 0 ohms).  If you still don't get decent illumination, start to reduce the second trimpot until you see something usable.  Measure the total resistance at that point (1k plus trimpot/s) and identify the nearest safe value of resistor to use in the circuit (e.g., if you get decent illumination at 3.5k, you can either use 3.6k is you can tolerate a bit less light, or use 3.3k for slightly brighter).

[Deep Blue]

Alright, I'll definetly rig something up like that at some point, Mark, thanks.

But in the meantime, I need to get this Dist+ to my friend before his next recording session, and I'm not going to ship it across the country with a broken LED.  What's a simple solution?  10k resistor?

[R.G.]

You're right not to ship it. Either there's something simple you're missing or there's a massive problem lurking in there.

LED's are almost universally rated for a DC current of 20ma. The forward voltage of most LED's is 1.5-2.5V, with the reds being at the low end and greens at the high end. Some of the new exotics get to 4 or 5V.

But if all you have is a 9V battery, you can't get more than 9/10 of a milliampere through a 10K resistor, even if the LED it's nominally limiting is shorted. Mother Nature won't allow it. Even more importantly, the LED eats up maybe 2 of those 9 volts, and so you only have 7 volts for the resistor - a 10K resistor will keep the LED current to 7/10 of a milliampere, and I can't think of an LED anywhere that would fry on that.

The 10K resistor is not your problem, I don't think. A 10K just won't let any normal indicator LED fry. It can't. Raising it to 1M probably won't help, unless it clears the other problem you're not seeing. As Conan Doyle once said through Sherlock "eliminate the impossible, and whatever remains, however improbable, must be the truth."

I'm guessing that the 10K is not really in series with the LED, and that there's something else that's letting current into the LED.  I can only guess, but some possibilities are:

- connection to the wrong end of the 10K limiting resistor, so the LED is getting 9V straight. I've done worse when flipping boards upside down and rightside up
- solder thread, ball, or stray wire contacting the LED with raw 9V
- some other part or lead letting +9V into the LED.
- the LED itself is not connected up the way you think it is.
- it's not a 10K, but some lower value, like 10 ohms.

Something like one of these must be true. MOther Nature speaks softly most times, but she never lies.

[Deep Blue]

At the moment, I'm using an 8.2k resistor, not a 10k.  The LED is blue waterclear from Small Bear.

[jimmy]

how long does the led work for before it burns out? if its only a very short while, its somehow shorted to 9v+ but if its longer i think theres more to this problem than we know. try increasing the limiting resistor to 10k, and if it is still dodgy, change the led to a different type, like red. another way to tell if a leds about to burn out is if its a defferent colour to what its supposed to be. i learnt that lesson testing a green led straight onto a 9 volt battery, and wondering why it was orange. and then theres this weird smell. good times.

[Mike Burgundy]

What's the voltage across this LED? As RG stated, you must be putting 9V on it in some way.

[downweverything]

yeah i definately think something else maybe fishy.  maybe you are looking at the wrong resistor or it is hooked up wrong (not to 9v).  the led should be hooked up to 9v in series with a resistor on the order of 240-470ohms.  most blue leds take 20ma typical with a forward voltage of about 3.5v thats 9 - 3.5=5.5V.  then divide 5.5V by 0.02A and you get a resistance value of approx a 270ohm resistor.  an 8k resistor probably wouldnt even give enough current to light the led if in series with 9v and the led.

[R.G.]

OK, lets try this.

Blue LEDs are not really like other LEDs in that they are not exactly diodes that just happen to emit light. They have a reverse voltage of about 5-7V. A 9V battery will break them over in the reverse direction, which is not what ordinary red/orange/yellow/green ones do.

What you said about the LED makes sense with this. You said the LED glows red and then orange. That is consistent with having the LED in backwards, breaking it over and causing too much power dissipation. The glow in that case would be from pure heat. I think that you may have the LED in backwards, as well as not having enough resistance in series with it.

Did it ever glow blue?