The motherboard contains the current limiting resistor for it.
So you don't need a "voltage LED".
Automotive LEDs, they consist of a LED plus a series resistor.
The assumption is, 12V will be applied. If the LED has a forward
voltage of say, 3V or so, then about 9V is dropped across the
internal resistor. For the privilege of adding a $0.05 resistor
to the light package, they charge many times the price of the
LED itself.
Computer LEDs run off 5V. And there is a reason for that.
Commodity LEDs are rated for 5V PIV. That's "peak inverse volts".
If you "put the LED in backwards", you're carrying out a
peak inverse volts test, unintentionally. LEDs are polarized
and only light up in one direction. Since the motherboard
maker powers the LEDs from 5V, it means the user can't
burn out the computer case LEDs! Pretty neat. If the LED
doesn't light, simply rotate the 1x2 LED cable connector
180 degrees, and try again. So reversing the LED wires,
doesn't hurt the LED, because the computer case LEDs
have a 5V PIV rating.
And LEDs don't "run off voltage". They take current, and
a current limiting resistor is placed in the path, so a
defined current is fed to them. With no resistor in place,
if you stuck a LED with a Vf of 3V across a 5V power source,
an infinite amount of current would flow, there would be
a brief flash of light, and then the LED would be dead.
Placing a resistor in the path (there's one on the motherboard
per LED already), prevents an infinite amount of current
from flowing.
Another issue, is LED color. The motherboard has a resistor
selected, assuming perhaps a red LED is on the computer front
panel. But the forward voltage drop is a function of color.
A blue LED has a higher Vf than a red LED, and for the
same "dropping resistor" on the motherboard, this results
in a dimmer output. So you can use blue if you want, but
in an ideal world, you'd change the resistor value on the
motherboard, to give a more pleasing light level.
After having said all that, let's pick a LED. I think the
size is T1-3/4. I bought a bag of 50 of those the other
day, so I have lots of them to look at now
I bought
that many, because I got tired of being gouged by
local retailers when I want a couple.
Geez, $1.69 for a $0.05 LED! And it's not even a particularly
bright LED. At least my local specialty electronics store
sells good LEDs, but they're no cheaper than Radio Shack.
The LEDs I bought on the Internet, cost me $0.22 each,
and the seller still made a tidy profit from them.
http://www.radioshack.com/product/index.jsp?productId=2103802
The longer leg should be (+). That's only important if you
wanted to get it right on the first try. A red LED like that
will be at least 5V PIV, so it doesn't matter if the LED
is reversed. The motherboard header likely marks the polarity,
if your purpose was to get the wiring right on the very first
try.
There are a few, expensive LEDs, where reversal is not
recommended at all. I have one here, and the first thing
I did with that one, is protect the LED with a schottky diode.
(That's to clamp it, if a reverse bias is applied.)
The data sheet isn't clear on how many applications of a
reversed potential it takes to damage a LED like that.
These are LEDs that cost between $5 and $20 each, and
are white in output color. And unlike the RadioShack product,
they actually tell you how much current it is safe to use.
My LEDs are standard 20 milliamp LEDs. They're bright enough,
that you can make flashlights with them. That isn't necessary
on a computer case LED, because you don't want to "burn out
an eyeball" with an indicator light. Imagine having
a computer in your room, where a front panel LED kept
you from getting to sleep... Using that wimpy RadioShack
LED, should work out just right.
Paul
