Why'd my LEDs pop?

Find a decent LED supplier such as Kingbright or Stanley, and download a LED datasheet. All the information you need is in there. LEDS require a constant current, not voltage.

-Andrew M

Use resistor to limit the current, typical small LEDs should work at

10-20 mA, they shouldn't work from voltage source.

Tom

"Michael C"

** 6 red LEDs in series will barely even light up at 10 volts.

Your story is fake.

......... Phil

I dunno phil, I measured 9 point something volts and there was definately quite a lot of light coming out of them. They are meant to run at 1.7V min.

Michael

Does 30ohm sound ok?

Michael

I've got a pile of bright blue LEDs that have a Vf of 3.3V when passing say 10mA. They will clearly illuminate with only 2.5V applied. I discovered this when I place one on the output of an op-amp that was biased at 2.5V. The LED was intended to clip the signal and illuminate only when that happened. However, it stays clearly lit all the time w/bright flashes during clipping.

: > Use resistor to limit the current, typical small LEDs should work at

10-20 : > mA, they shouldn't work from voltage source. : : Does 30ohm sound ok? : : Michael

use a meter to measure the current in each string of leds over the voltage range you are interested in.

There is a bit of an anomily in what I measured. The LEDs start putting out light around 1.6v but it's very dim. Even at 1.7v it's quite dim but I measured

30ohm gives up to 30milliamp and adequate brightness for me.

Michael

"Michael C" wrote

There is a bit of an anomily in what I measured. The LEDs start putting out light around 1.6v but it's very dim. Even at 1.7v it's quite dim but I measured

You may well be right, I am a bit confused about what's going on here, although there's no need for your childish rudesness. The reason I'm asking here is because I'm trying to get a better understanding. Surely the voltage across the LED affects the amount of current going through it, hence they will light up once a certain voltage is reached? Why isn't a household lightbuld considered a "current operated device", surely it lights after a certain current is reached.

there are a number of rude posters here, just like anywhere. but anyway....

this appears to be an informative article you may want to read: http://217.33.241.242/download/Firstsight%20Vision/Tech%20Tips/LED-lighting.pdf

"Michael C" the pompous tosser said... : > ***** You really haven't got an effing clue,have you.Do you think you : > might : > be able to getit through your thick skull that LEDs are CURRENT operated : > devices,measure the current Luke,measure the current!!!!! : : You may well be right, I am a bit confused about what's going on here, : although there's no need for your childish rudesness. The reason I'm asking : here is because I'm trying to get a better understanding. Surely the voltage : across the LED affects the amount of current going through it, hence they : will light up once a certain voltage is reached? Why isn't a household : lightbuld considered a "current operated device", surely it lights after a : certain current is reached. : : >

: >

: > Brian Goldsmith. : >

: >

: :

asking

voltage

LEDs are quite different to incadescent lightbulbs in that they are semiconductors, and the thing you need to understand is that they have a (more or less) constant forward voltage. This isn't a bad summary of how they work:

Without a resistor to limit the current in the circuit, putting even a fraction higher than the forward voltage through a LED will result in the current going through the roof and instantly blowing the LED.

All you need to do is work out what resistor you need - you can do this empirically using an ammeter in circuit as you try different resistors, or if you know the forward voltage and the supply voltage, you can work out what resistor will give you the required current using Ohm's law, or an online calculator like this one:

Oh, and try to ignore the malcontents such as Mr Goldsmith and friend. Unlike them, most of us are here to share and learn and are happy to do so.

"Michael C"

** Your DC voltmeter is stuffed.

Or are you using bloody AC ??????

....... Phil

putting

but I

Test it yourself, maybe you'll learn something new. Like I said before, I can apply 2.5V without any current regulation to the blue LEDs that I have (Vf=3.3). They light up (dimly but still quite visible) and current is apparently limited by the LED itself.

"Anthony Fremont"

** FUCK the HELL OFF you STINKING ASSHOLE .

...... Phil

I guess you tried it for yourself. Enlightening wasn't it?

"Anthony Fuckhead Fremont"

** FUCK the HELL OFF you STINKING YANK ASSHOLE .

...... Phil

Michael, There is a possible explanation for what you've experienced.

All LED's do not have an equal operational voltage, even those of the same type. It varies with manufacture tolerances, some higher and some lower, hence the datasheet gives a range of operation.

Those with a higher operational voltage will at any current of operation, dissipate more power, turning it into heat and light. Thus some LEDs get hotter than others.

If your circuit was not running off pure DC then it's possible that when your meter shows less than 10v the RMS voltage value was much higher, thus overdriving your LED chain and cooking the one with the highest Vf or lowest tolerance for overdriving first.

To solve your problems, make sure you have pure DC, as much as possible, then measure current of your chain running it through various resistors, start with a value about 1K and reduce the resistor value until you get to about 20mA through the chain. The use this value to run the LEDs permanently.

As the current will now be roughly constant in the chain, all the LEDs will live happily ever after.

Best Regards, Brenden Ede

Michael C wrote: