Power LED advantages?

[Olen]

Short question:

Are there any advantages in using the power LED to turn on power LEDs over just butchering a molex? I'll be running a bit over 20 LEDs to show power. I could fiddle with a transistor to make the power LED connector turn on and off the LEDs while they take their power from a molex but I don't know if there would be any advantage.

What voltage is the power LED?

Cheers.

[Luke122]

Molex gets my vote.. I think that the onboard power LED is either 3 or 5 volts.. I made a post somewhere about it before where I actually took a meter to it to check.. but I'll have to dig it up for you.

[Spawn-Inc]

well if you were using 1 or 2 LEDs then i would say yes. you wouldn't need any resistors but with 20 i would go the molex route.


900th post!

[Olen]

Is there any difference? When the computer comes on the molexes will be powered and fans will buzz - what is the purpose of the power LED?

I suppose I should stop being lazy and dig out the voltmeter.

[Spawn-Inc]

the purpose of the power meter is is exactly that, to tell you there is power. granted for use super geniuses we can tell when the pc doesn't have power without some LED telling us if its on or not. to be honest i'm not really sure what its meant for. couldn't find anything on google.

[Bucko]

the power LED plug will supply the correct voltage for LEDs. However it's the current it draws that is the problem. It wouldn't be designed for the extra power the 20 LEDs would pull.
It may well work if you wired the LEDs in series, as long as you are willing to risk your motherboard possibly frying from the extra current draw on the circuit.

[Olen]

You couldn't wire 20 in series. The voltage drop each one gives (0.3 or 0.7 volts) would be far to huge before taking resistance into account. Did you mean parralel? As you say I wouldn't trust it.

I wanted to know voltage because I have 2.5V LEDs so 10 parralel sets of 2 with the 5V wire on a molex should be fine. If, as I should think it is, the LED connector is between 0 and 5 volts wiring a transistor would be so easy there would be no reason not to if it is 5V things get more awkward as my collection of resistors is lacking. But then why waste a transistor when it has utterly no purpose?

[xmastree]

I suggest you search for the LED tutorials first, to find out what makes LED's tick. Forget voltage, they run off current...

FWIW, the easiest way is to run them from the 12V supply available from the nearest molex. But don't put them in parallel, put them in series in 'strings'.

http://i50.photobucket.com/albums/f341/chrispollard/LEDs.png


I'll try to find the relevant threads and edit this post once I have them.

Edit:
http://www.thebestcasescenario.com/forum/showthread.php?t=2669

[Olen]

You get a voltage drop over a PN junction. If you wired all 20 in series and they were germanium (I can't remember what substrate they use) then the voltage dropped overcoming the barrier would be 0.3V for each. 0.3*20 = 6V. I can't remember the exact effect this has but I'll avoid it.

Seeing as they are rated 2.5V two on the five volt line should only need a very small resistance to protect them... but what you say makes sense, I use rubbishy batteries (like worse than you can imagine) normally which have a fair internal resistance (like a few ohms when new) so yes I will use a resistor with each parralel series set.

With 2.5 ones would you reackon 3 or 4 in each string? With four the protecting resistor would be 2/0.03 = 67 ohm. 75 ohm should do there shouldn't it? Or would I be better with each string being 3 LEDs and 2 75 ohm resistors (I have a bunch of 75 ohms...).

But back on topic is there no difference in the behaviour of a molex powered LED and a power LED jumper powered LED?

[Bucko]

Did you mean parralel?

:redface: I did. It was a typo putting series, you know they keys are right next to each other.

[xmastree]

With 2.5 ones would you reackon 3 or 4 in each string? With four the protecting resistor would be 2/0.03 = 67 ohm. 75 ohm should do there shouldn't it? Or would I be better with each string being 3 LEDs and 2 75 ohm resistors (I have a bunch of 75 ohms...).
So, you're going for 30mA? 4 LED's and 75 ohms should be fine.


But back on topic is there no difference in the behaviour of a molex powered LED and a power LED jumper powered LED?
The power LED header has the resistor on the board, so you just connect one LED directly across it.
Molex? Well you wouldn't want to connect one LED directly across that... :dead:

[Olen]

Sorry that wasn't clear - I meant that once you had the LED/s functioning they would light and turn off at the same time regardless of whether you used a molex or the mobo jumper. It makes the mobo jumper seems somewhat redundent.

And yes 30mA. Would there be a noticble difference in light intensity running them at 26mA instead though (2/75 = 26.6)?

[xmastree]

Ah, I understand now.
I have seen some boards which flash the power LED under certain fault conditions, so that's a difference.

I'd just string them across the 12V supply.
26mA should be fine, why not just wire some up and test it? If they're bright enough then go for it.