Re: Maximum current rating for blue led?

On 22 Jun 2005 05:22:59 -0700, the renowned "Tim Shoppa"
<shoppa@xxxxxxxxxxxxxxxxx> wrote:

>> and their catalog doesn't list the maximum current
>> rating for any of the leds they carry, the catalog
>> only lists the maximum voltage rating.
>
>You are not interpreting the listings correctly.
>
>Every LED in the Digikey catalog has a listed "IF (mA)". This is the
>maximum continuous current.
>
>The forward voltage drop (VF) is not the "maximum voltage", it's the
>typical voltage at the given I_F. V_F is usually not a guaranteed
>parameter, solid state physics makes determines it.
>
>Most (but not all) components in the Digi-Key catalog have a PDF data
>file on their website. Very useful.
>
>Tim.

Anyway, there's not just one number here. It depends on the nature of
the current (is it DC?) and the ambient temperature, and how close to
the maximum you'd like to run the junction. Blue (and white) LEDs tend
to have the highest voltage drop of all common colors, so they get the
hottest at a given current.

Eg. http://rocky.digikey.com/WebLib/Fairchild/Web%20Data/MV8B01.pdf

If (abs max.) is 30mA at 25°C.

But in Fig. 5 you can see that at 75°C you need to reduce that to
20mA. If you work at 50-70% of the maximum, you're at about 10-14mA.
That's probably about right for most ordinary 5mm blue LEDs. If you
put resistors or other parts nearby that raise the ambient you'll want
to reduce the current further. They probably assume the leads are
short and soldered into a PCB with certain characteristics. If its
hanging out in mid-air on long legs you'll have to further reduce it
or reduce the maximum ambient.

.

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