Re: Possible High LED current with long life time

On 31 Jan, 16:10, John Devereux <jdREM...@xxxxxxxxxxxxxxxxxx> wrote:
Pubs <pubud...@xxxxxxxxx> writes:
On Jan 31, 8:54 am, Pubs <pubud...@xxxxxxxxx> wrote:
On Jan 31, 7:44 am, Spehro Pefhany <speffS...@xxxxxxxxxxxxxxxxxxxxxxx>
wrote:

On Wed, 30 Jan 2008 19:39:38 -0800 (PST), the renowned Pubs

<pubud...@xxxxxxxxx> wrote:
On Jan 30, 8:00 pm, Spehro Pefhany <speffS...@xxxxxxxxxxxxxxxxxxxxxxx>
wrote:
On Wed, 30 Jan 2008 11:12:54 -0800 (PST), the renowned Pubs

<pubud...@xxxxxxxxx> wrote:
Hi,

  I have a application where myLEDcurrents are switched as in a
matrix. My currentLEDon time is 60 us and off time is 15240 us. The
intensity is problem thus I pump a higher current in that 60 us
(applied potential is arround 9V in that period of time ) and that
gives the required intensity.LEDdoee not heat up sinceLEDcurrent
duty cyle is extreamly small (rms current is  also very small too). Is
this method will reduce theLEDlife time ?.
Could some one refer some articles onLEDlife time dependability with
current.

Thank you.

Pubs

Wow. I don't like to go much beyond 8:1 to 10:1. You're doing 250+:1.
I don't think you'll get away with it unless the peak current is not
much more than 5-10x the allowable DC current (meaning the LEDs will
be relatively dim).

Best regards,
Spehro Pefhany
My design works prefectly. IT gives almost like a continoues current
because that current pulse is repeatedly applied. My concerened was
with the life time.

Yes, yes, I know, 'prefectly'.  So what does theLEDdata *** say
about the conditions you are subjecting it to? For example, here is a
data *** that specifies maximum PEAK current ("Limits of Safe
Operation") at 25°C of 150mA (only 5 x the maximum DC current) for
pulses of < 10 *microseconds*).

http://www.lumex.com/pdf/SSL-LX5063ID.pdf

It should be derated above 25°C, obviously.

Pubudu

Best regards,
Spehro Pefhany
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- Show quoted text -

So the peak current is 150 mA at less than 10 us pulses for thisLED
type. Anyway my drivers provide arround 90 mA perLED. I think I am in
the safe region.- Hide quoted text -

- Show quoted text -

MyLEDdata *** is shown below.
http://www.jameco.com/Jameco/Products/ProdDS/183222.pdf
According to it, peak current is 500 mA for pulses less than 50 ms. I
am well withing the region.

For pulses less than *1* ms, not 50.

Actually, the data*** is strange. It gives a max DC current of 20mA,
but a max pulse of 500mA at 1/20 duty. Which is a *25* mA average, and
probably much more than that in terms of effective heating.

And is has a "continuous forward current" of "100mW"!

--

John Devereux- Hide quoted text -


The whole thing is strange - maybe "pub' writes his own datasheets for
leds as the other data offered makes just about as much sense.

With a 1in 255 ratio and 90mA led current the average led current is
only 353uA yet the stated led voltage for this current is 9V!!
(unless it is meant that the average led current is 90mA whence the
led 'on' current is almost 23Amps)

Clearly the fellow has no idea what he is doing else he would not have
chosen such a ridiculous ratio to start with


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