[Laser] Diode Care and Feeding (Was: How to Collimate LED?)
David Willmore
[email protected]
Sun, 09 Feb 2003 19:18:26 -0500
> > They're predominately current mode devices, so don't worry too much about
> > the voltage, regulate the current. You can use an LM317 design for
> > reasonable sized arrays or some switching arrangement for larger ones.
> If I understand right, the issue with LEDs is primarily one of junction
> temperature as a result of average power at the junction.
Hmmm, two answers. One is for the little 3mm and 5mm epoxy cased type that
most of us are familiar with. They tend to be limited by short term thermal.
They have very poor thermal junctions to their leadframe and the leadframe
to ambient is a joke. The former limits the short term power and the latter
limits the average power.
> I have a tiny array of 3 Radio Shack IR leds rated for 100 mA that I pulse at
> about 1 amp, ~800 Hz, 8% duty cycle. They seem quite happy, without any heat
> sinking.
I've seen the data sheets for similar IR LEDs made by optronics and they're
pretty fully charactarized for pulse mode operation. There is a chart of
permissable duty cycle and pulse rate vs current.
Your application seems a little heavy for the wide pulse width. Depends on
how I read it, I guess. Is that 1A for the whole array to share or is that
1A through all of them? If the former, then you're cool, if the latter, I
think you might be on the border. But, those charts are put together for
people much more concerned about long term reliability than we are. :) If
one of your LEDs fails, you can just put a new one in and a QSO gets delayed.
If it's in consumer electronics (or medical equiptment), things get worse.
The limitation for power into LEDs that is *not* thermal (as far as the LED
cares) is the fuse current of the wisker contact on the upper ohmic bond.
That little guy can go at 1-2A on normal LEDs, but is generally rated up to
10A for the IR pulse diodes like you're probably using.
Take a look at optronics web site and peruse some of their data sheets. It's
good reading. :)
Cheers,
David N0YMV