Re: [CAD_CAM_EDM_DRO] quickie LED lesson

In a message dated 17-Aug-02 00:31:11 Central Daylight Time,
andrewlee@... writes:

> I am trying to wire up a logic probe. I am not too familiar with the
> polarity. Which one is positive, the longer or the shorter leg?

Lloyd is right, the LONG one on a NEW LED is +. But the leads get clipped
for plugging into breadboard-sockets or soldering into a PC-board that never
happened, etc. It would be a shame to just chuck these "equal leg" LED's!
Most are clear enough that you can SEE the different electrodes inside the
epoxy encapulation. The one with the BIG gob of metal on it (with the LED
chip on top of it) is the negative. The one with hardly any globbery on the
encapsulated end - only a tiny gold wire welded to it - is the + lead.

LED's should not be thought of as "running on 2 volts" or whatever "voltage".
They are "current devices". "High efficiency" tiny red ones might be fairly
bright at 5 mA. Most T-1-3/4 size work well on 15 to 20 mA. Some might
appear to be working fine on 25 mA, but will "die" very soon if operated over
15 mA. Depends on their design and intended use!

I just got some "blinding white" LED's from one of the mailorder houses, and
that one is completely different! It is full brightness at only 10 mA, but
the measured voltage across this LED at that current is 3.0 volt! MIGHT be
bright enough to make a "penlight", if I can find some optics to fiddle with.
And a pair of 1.5 V alkaline AAA cells would power that @ 10 mA for many
hours! Hmmm...

Be very careful, as an INSTANT short to the wrong voltage/polarity without
the right current-limiting resistor can waste a brand new LED faster than you
can see what happened! And they figuring out "why it won't light" afterwards
is a frustrating mystery! (Same goes for transistors, etc., for THAT
matter!)


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