Question about mobo LED voltage

[Dash20]

After lurking these boards for a good while, I figure it's about time I get involved a bit. I'm not too new to the modding scene, and I've just about finished my second mod. I figured I'd keep it simple for mods and go for the expensive "automotive" finish paint job using spray paint (thanks to Paul and Jon for writing the book featuring this guide, if you don't have it GET IT :D).

To keep with the black and blue colour theme, I decided to change the power and reset LEDs to blue and white. I have some extra ones without tails floating around, and I wanted to solder them to my existing tails. I have a problem, however. These LEDs don't have built-in resistors. Can anyone tell me what the voltage supply is on a mobo to the power/reset LEDs? Is it 12V, 5V, or 3V? I'd really appreciate any help on this, thanks guys!

[Crimson Sky]

Hey there, welcome to TBCS--thanks for the kudos on the book, glad you enjoyed it!

the voltage to the reset/power headers is about 2.3-3vDC depending on the mobo. To find the proper resistor, try Linear's Wizard (http://led.linear1.org/led.wiz)--I doubt you'll need a resistor for such low voltage--even the 3.3Volt super bright (6000+ mCd) leds would work just fine

[Dash20]

Thanks for the quick response Paul, and after looking at the voltage requirements for the blue LED, you're right, a resistor isn't necessary. The required voltage for it is actually a little higher than the supply, so I'll do a test to see if it will light at all. Thanks for the calculator too, it's probably one of the better ones I've seen on the net. :)

As for the book, I must say that I'm STILL enjoying it, there's more unique info in there than I thought! I figured I was taking a risk buying a mod book because they all say the same thing, but luckily I was wrong this time. Again, great stuff, and now you can lecture me if I ask a question on here that's already in the book, lol.

[Malatory]

Thanks for the quick response Paul, and after looking at the voltage requirements for the blue LED, you're right, a resistor isn't necessary. The required voltage for it is actually a little higher than the supply, so I'll do a test to see if it will light at all. Thanks for the calculator too, it's probably one of the better ones I've seen on the net. :)


As long as you have the power going the correct way through the LED you should have no problems with it. Mobo headers are designed to work with the LED with out resistors. I could see a problem with computer builders if they had to figure out what resistor they had to add just so they could have the case power light on, so much for hiring a high school student for the dull work after school.

[Xato]

Its just that diff types of LED's have different voltage requirements, and have higher lower draw. It doesn't matter too much, but it could blow out a lower resistance LED.
I think case makers just make use LED's of the same voltage for their cases, so it doesn't need to be changed for every mobo that goes into the comp...

Although i could be wrong.

[Frakk]

most of the LED's require 2-3 volts for their optimal output. the header provides 2.5-3volts and it will not do any damage to the LED's because they have a volt or so overhead. if you put a super bright diode on them that needs 5V, it will still work but a little dimmer than its made for.

[Dash20]

That's exactly what I thought. Fortunately, a loss of half a volt doesn't dim the LEDs noticeably. I just tested both and they work just fine, and they're actually too bright for me to look at, heh. Now to clean off my desk (shelf now, lol)...

[Guttenaffe]

Normally the LED headers on a Motherboard are run off 5V with a 333Ω resistor.

Also note that measuring the "voltage" across the headers will give you what we call a "false" meter reading. This is due to the internal resistance of the meter. Most DMM are 5MΩ to 20MΩ internal resistance, because of this touching your probes to the header pins will put the meter in "series" with the motherboard’s resistor and we all know you can’t read voltage in series.