Power LED pins on motherboard not working... is there a fix or alternative?

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Kusaywa

Weaksauce
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Mar 11, 2003
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I bought a used motherboard off of ebay, Zotac GF9300-g-e. The power LED pins on the motherboard don't work. I hooked up the power LED to the hard drive LED pins and it worked. I also hooked up the hard drive LED to the power LED pins and that didn't work so I know the problem is with the motherboard. Tried switching the polarity, nothing. Can this be fixed? If not, can I hood the LED up to a power source so I can have the pretty little light? Thanks
 
LED Diode + 100ohm resistor (give or take a few ohms) across a +5V line gets you a power LED, if you want to be technical. The resistor on the mobo is probably just blown. It's possible to fix it, but given that it will be tiny and surface mounted, a bit difficult with a Weber home kit.

If your just looking for a light. get a USB LED or something and plug it into an internal USB header.
 
If I plug my existing LED into the USB will it blow the light? Do I need a resistor?
 
Kusaywa said:
If I plug my existing LED into the USB will it blow the light? Do I need a resistor?
Click to expand...

USB is supposed to prevent itself from overload, but I wouldn't count on that to not burn something up if you tried that - either the LED, the USB port, or something else... A basic LED itself is like a direct short - it needs a little resistance there. The less resistance, the brighter the light, but you hit a point where if you don't have enough resistance, something will burn out because it can get to be a lot of current.
 
Brian_B said:
LED Diode + 100ohm resistor (give or take a few ohms) across a +5V line gets you a power LED
Click to expand...

Not for long it will, using a 100 ohm resistor across 5 volts will net you 50 mA, grossly exceeding the current capability of standard T1¾ LEDs. The older GaAsP technology LEDs will burn up quick, while the newer white/blue LEDs with a phosphor will quickly go dark and eventually burn out.

Here are proper resistor values for most generic T1¾ LEDs to run them on 5 volts.

250 ohms = 20 mA (maximum brightness)
270 ohms = 18 mA (very bright)
330 ohms = 15 mA (bright)
470 ohms = 10 mA (mid bright)
680 ohms = 7 mA (dim)
1k ohms = 5 mA (very dim)

I would recommend staying in the range of 330-680 ohms as it ensures the longevity of the LED and isn't blindingly bright.

Kusaywa said:
If I plug my existing LED into the USB will it blow the light? Do I need a resistor?
Click to expand...

If you put a standard 3v LED across a 5v source, it will almost immediately burn out. You'll probably see a flash of light and then a black spot of death inside the LED where the die is. LEDs do exist that run straight off of 5 volts, but they're more expensive. it's easier and cheaper to just put a resistor in line with the LED.
 
GiGaBiTe said:
Not for long it will, using a 100 ohm resistor across 5 volts will net you 50 mA, grossly exceeding the current capability of standard T1¾ LEDs. The older GaAsP technology LEDs will burn up quick, while the newer white/blue LEDs with a phosphor will quickly go dark and eventually burn out.

Here are proper resistor values for most generic T1¾ LEDs to run them on 5 volts.

250 ohms = 20 mA (maximum brightness)
270 ohms = 18 mA (very bright)
330 ohms = 15 mA (bright)
470 ohms = 10 mA (mid bright)
680 ohms = 7 mA (dim)
1k ohms = 5 mA (very dim)

I would recommend staying in the range of 330-680 ohms as it ensures the longevity of the LED and isn't blindingly bright.



If you put a standard 3v LED across a 5v source, it will almost immediately burn out. You'll probably see a flash of light and then a black spot of death inside the LED where the die is. LEDs do exist that run straight off of 5 volts, but they're more expensive. it's easier and cheaper to just put a resistor in line with the LED.
Click to expand...

Not that this matters too much, since we are talking a $.10 LED. I don’t have recent first hand experience, I haven’t done any electrical work like this for years, but the article I linked sounds halfway legit. And seems to imply your forgetting about the diode bias voltage drop in your calculation.
 
In looking on eBay, the resistors come in watts... What wattage should I use? And thanks to everyone for your help and replies!!!
 
that depends on the listing, they are normally rated in ohms. use google to do a conversion...
 
I'm looking at 470 ohms but they come in 1 watt, 5 watt, 1/4 watt, 1/2 watt, 2 watt, etc.???
 
pendragon1 said:
that depends on the listing, they are normally rated in ohms. use google to do a conversion...
Click to expand...

Resistors have both a resistance rating (ohms) and watts rating (how much current the resistor can dissipate before burning up.) Common hobbyist resistor wattages are quarter, half and one watt.

Kusaywa said:
I'm looking at 470 ohms but they come in 1 watt, 5 watt, 1/4 watt, 1/2 watt, 2 watt, etc.???
Click to expand...

If you're only driving a single T1 (3mm) or T1¾ (5mm) LED, then a quarter watt resistor (1/4W) will probably be fine. You can bump it up to a half watt (1/2W) if you want some extra margin. Using anything larger will just be a waste of space and money.
 
Wanted to try this out before I posted a response. Bought a 470ohm 1/2watt resistor. Hooked the LED from the power button up to the USB on the motherboard and it worked perfect. Not too bright or dim. One thing I would like to point out... I tested this on an old eMachine motherboard first. The USB on that motherboard is constantly on, so the power led would stay on. Luckily, the USB on the motherboard with the led problem didn't, so the power led went on/off when the PC is powered on/off. Thanks everybody for your help... such a pretty blue light!!!
 
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