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Why did my LED burn out??

post #1 of 18
Thread Starter 
I bought a 3V LED because i could not get a 1.8V for a CMOY amp. I applied a 9V to test the power circuit on my amp as per Tangents guide. In about 3 seconds the LED burnt out. I tested the resistance on of the resistor and it was i think 22 ohms. i am confused.

can someone tell me ohms law and how it works, i used to know it.
post #2 of 18
22 ohms is way too low! It should be 10k ohms (according to cmoy's schematic)!
post #3 of 18
It is safe to assume that LEDs are in the 1.5 to 1.8 volt range for red through the red to green range. They will have some resistance from 5 to 40 ohms, which you often can (and should) ignore. So your LED was exposed to nine volts, huh? Let's see, 9-1.8 is 7.2. 7.2V/22R is 327mA, about ten times what it might reasonably withstand. Even with it's inherent resistance, you're still talking about 116 to 267mA which is also way too much. Ten kay will limit the current to 7.2/10000 or 720uA, good for battery powered indicators. Does this help?
post #4 of 18
Thread Starter 
thanks for the help, i think i accidently put a 10ohm resistor in instead of the 10K, i didn't have a meter to check before i did it. i think my meter was off also because when i checked later it registered 10 ohms.

the 3v LED i bought because i couldn't get a 1.8V, what size resistor should i use. When i tested it, it wouldn't work with 1.5V only worked with 3V.
post #5 of 18
Time to learn Ohm's law: V=IR

You can algebraically rearrange that to I=V/R -- current equals voltage divided by resistance. Since the LED drops the voltage going across it by 3V, current is 6V divided by 22 ohms, which equals 273 milliamps. That LED was surely rated for only 20 or 30mA.
post #6 of 18
Thread Starter 
i think i accidently put in the wrong resistor, so that is my bad.

Tangent, thanks for the help with ohm's law. it will probably come in handy later.
post #7 of 18
I have a Blue LED that almost burnt out, but I caught it. The plastic in the lens melted and turned brown right above the LED itself, so now I have a weak, but very cool looking, aquamarine LED.
post #8 of 18
Oh, damn, now you've done it! People are going to start talking about LED burn-in! "Your amp just can't possibly be sounding as good as it could, the LED isn't burned in yet!" Because as we all know, the better the amp looks, the better it sounds.
post #9 of 18
Quote:
Why did my LED burn out??
I think it was heat exhaustion.
post #10 of 18
Is there a blue LED from digi-key that uses the right voltage for a Cmoy?
post #11 of 18
We just had this a few days ago. Check the thread here.

The voltage spec on an LED is forward voltage. This is the voltage drop across the LED. The rest of the voltage (source voltage minus forward voltage) powers the LED. Forward voltage is not very critical in choosing an LED.

The current rating on an LED is maximum current. More current = more light. You don't want max current to flow through it...it's much brighter than you need and it drains your battery fast.
post #12 of 18
Quote:
Originally posted by sonichead
Is there a blue LED from digi-key that uses the right voltage for a Cmoy?
Check Tangent's Meta42 parts list.
post #13 of 18
Quote:
Originally posted by MisterX
Check Tangent's Meta42 parts list.
There arn't any, but I found a really cool green one with a clear green lens, also it's low current and 5mm! Thanks anyway.
post #14 of 18
Quote:
Originally posted by sonichead
...I found a really cool green one that's clear until you give it current, then goes green, also it's low current and 5mm!
Those ones are cool. I used a blue one for a guitar amp a while ago. There's a nice texture to the LED when you look at it. Unfortunately, it has a 5V forward voltage, so it will probably go out before the battery's dead. What's the forward voltage on the one you found, and where did you find it?

Since LEDs are spec'd with forward voltage, you just need a number lower than your minimum voltage. For a single 9V battery, which will drop to 4-5V before it's dead, you need a forward voltage less than 4V. Significantly less, hopefully. Most LEDs have a forward voltage around 2V, which is "right" for just about any battery-powered circuit you'll find around here.
post #15 of 18
Quote:
Originally posted by stereth
Those ones are cool. I used a blue one for a guitar amp a while ago. There's a nice texture to the LED when you look at it. Unfortunately, it has a 5V forward voltage, so it will probably go out before the battery's dead. What's the forward voltage on the one you found, and where did you find it?

Since LEDs are spec'd with forward voltage, you just need a number lower than your minimum voltage. For a single 9V battery, which will drop to 4-5V before it's dead, you need a forward voltage less than 4V. Significantly less, hopefully. Most LEDs have a forward voltage around 2V, which is "right" for just about any battery-powered circuit you'll find around here.
Well it looks like I lied! oops It has a clear green lens, but runs at 2V max, and 25mA, but will light with current as low as 2mA. Sorry about lens color, but I was pretty impressed that one that size can light with that little current, and still give decent light.
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