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

Does 30ohm sound ok?

Michael

: > Michael C wrote:
: >> I had 2 sets of 6 LEDs in series running off less than 10V. The
LEDs are
: >> meant to work between 1.7 and 2.6 volts so should have been fine up
to
: >> 15.6 volts. After running for a while one set stopped working and
then
: >> the other. The legs of at least some of the LEDs got quite hot. Is
there
: >> a reason for this?
: >
: > 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.

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

Michael

"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

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.

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 <10V across 6 which is only 1.67 volts each at the most and they
were quite bright.

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 <10V across 6 which is only 1.67 volts each at the most and they
were quite bright.

Michael



***** 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!!!!!


Brian Goldsmith.

news:zui7g.24366


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.
: >
: >
:
:

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





***** Including moronic top posters!!!

Brian Goldsmith.

Its appears to be greater in this group than other's for some reason.


http://217.33.241.242/download/Firstsight%20Vision/Tech%20Tips/LED-lighting.pdf

Thanks, I'll have a look at it now.

Michael

news:445e8371$0$9697


I think I get it now. They are still similar to a standard light bulb in
that they use a certain current at a certain voltage but that current
changes very rapidly with small changes in voltage. It can vary greatly
between LEDs in the same batch. Hence you need to make a circuit to supply a
controlled amount of current, not voltage. And a plain old resistor is a
good way to do that. :-) I'd always thought the resistor was just to get the
voltage down and could have been replaced with a reg.

Thanks for all the replies.

Michael

  there are articles on why you should not run a LED of a regulator.
LED are supposed to be thought of as having a constant voltage drop
across them just like any forward biased diode.

What if your source voltage varies? It is hard to maintain a constant
current without using some form of regulation.

Simple: use a LM317 variable regulator as a current regulator.
In the NatSemi app notes you can find it, but it works by
having a resistor in series with the output terminal that
is selected to have a 1.25 V drop at the current you want to
regulate at: so 1.25/.0262%.5 Ohms. The adjust terminal is
connected to the opposite end of the resistor which is then
the output of the current regulator.

This works by having the LM317 keep a constant 1.25v across
the resistor which gives a constant 20 mA.

I'm using an lm317 to regulate the voltage.