T. Ment wrote:
On Thu, 27 Jun 2019 13:05:02 -0500, VanguardLH wrote:

If the 2-pin LED mobo header connects to a voltage regulator that
provides a constant current, maybe your old LED is okay.

Doesn't matter. I got a pack of 25. I can start over with a new one.


I haven't
looked at the mobo circuit diagrams to see what the LED mobo header
connects to. For the old LED that failed, was there a resistor for that
one?

The original case LED is identical in appearance to the ones I bought.
They both have the two legs, one longer. There is no resistor in the
connecting wires. They both insert in a small plastic housing the same
way. And then you push the little connectors on the ends of the wires
inside the little housing to make contact with the LED legs. It's
designed so the LED is easy to replace.

The new one is just too bright. I have a second identical case, the two
sitting side by side. In one case the original LED still works. It's not
a matter of viewing angle. The replacement is LED much brighter than the
original. It drowns out the HDD LED below it.

You say a resistor too high will drop the voltage too low for the LED to
light. But I thought Paul suggested 4mA.

Much of the discussion is over my head, so I will just have to try some
resistors and see what happens. But that may take some time, I have none
handy right now.

At 4mA, the LED will still light.

Those LEDs will light at 1mA!

The voltage will still be at the 2V range.

Even if you dropped to 100uA, there will still be
around 1.9V or so. But you would need to darken the
room, to see a dull green glow.

*******

On high power LEDs, running them at extremely low current
flows, causes the color to shift. The LEDs "look unhealthy"
in such cases. And the manufacturer mentions this, as
a limit to running them too low. It's a cosmetic issue.
For color purity on white LEDs, it doesn't pay to go too
low. The color comes from the phosphor on a white LED,
which is part of the problem. For high power LEDs,
you make sure to buy the right LED for the application.
(Don't buy a 200W array when a 3W array is the right one.)

That should not be an issue in this case. Green, red, and
yellow indicator LEDs are "pure" GaAs devices doped with particular
things, and no phosphor is involved. On the "pink" LEDs,
those are phosphor based. The white LEDs use phosphor too.
The "original colors", before phosphors came along,
are pretty good (200-300 nm "line width", they're
not "lasers").

*******

I built a logic probe once, with darlington transistor
pairs for high impedance. And the LEDs on that ran at
1mA to save power. The display of ones and zeros was
plenty bright for home lab work.

Paul