Why'd my LEDs pop?

M

Michael C

Guest
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?

Thanks,
Michael
 
On Fri, 5 May 2006 01:51:25 +1000, "Michael C" <nospam@nospam.com>
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?

Thanks,
Michael
Click to expand...
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
 
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?
Click to expand...
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"
I had 2 sets of 6 LEDs in series running off less than 10V.
Click to expand...

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

Your story is fake.




.......... Phil
 
"Phil Allison" <philallison@tpg.com.au> wrote in message
news:4bvjkpF13e1kjU1@individual.net...
** 6 red LEDs in series will barely even light up at 10 volts.

Your story is fake.
Click to expand...
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
 
"Tom" <tom@nospam.net> wrote in message
news:bNu6g.1414$b6.43061@nasal.pacific.net.au...
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.
Click to expand...
Does 30ohm sound ok?

Michael
 
"Phil Allison" <philallison@tpg.com.au> wrote in message
news:4bvjkpF13e1kjU1@individual.net...
"Michael C"

I had 2 sets of 6 LEDs in series running off less than 10V.


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

Your story is fake.
Click to expand...
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.
 
"Michael C" <nospam@nospam.com> wrote in message
news:445aec0f$0$24422$afc38c87@news.optusnet.com.au...
: "Tom" <tom@nospam.net> wrote in message
: news:bNu6g.1414$b6.43061@nasal.pacific.net.au...
: > 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.
 
"Anthony Fremont" <spam@nowherest.com> wrote in message
news:125m39krn11r7eb@news.supernews.com...
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.
Click to expand...
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
 
"E d" <tosser@dfat.gov.au> wrote in message
news:e3f4hc$lbi$1@news-01.bur.connect.com.au...
use a meter to measure the current in each string of leds over the
voltage range you are interested in.
Click to expand...
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 <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.
 
"Brian Goldsmith." <brian.goldsmith@NOSPAMMecho1.com.au> wrote in message
news:zui7g.24366
***** 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!!!!!
Click to expand...
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.
Click to expand...
 
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.
: >
: >
:
:
 
"Michael C" <nospam@nospam.com> wrote in message
news:445dc2e7$0$10674$afc38c87@news.optusnet.com.au...
"Brian Goldsmith." <brian.goldsmith@NOSPAMMecho1.com.au> wrote in message
news:zui7g.24366
***** 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.
Click to expand...
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:

http://electronics.howstuffworks.com/led.htm

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:

http://led.linear1.org/1led.wiz

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"

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.
Click to expand...


** Your DC voltmeter is stuffed.

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




........ Phil
 
"Phil Allison" <philallison@tpg.com.au> wrote in message
news:4c65tbF144jm6U1@individual.net...
"Michael C"

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.



** Your DC voltmeter is stuffed.

Or are you using bloody AC ??????
Click to expand...
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
 
"Phil Allison" <philallison@tpg.com.au> wrote in message
news:4c6ckbF13vhdoU1@individual.net...
"Anthony Fremont"



** FUCK the HELL OFF you STINKING ASSHOLE .
Click to expand...
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:
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?

Thanks,
Michael
Click to expand...
 
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