Personal computing discussed

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voltage of CoolMod

Mon Jan 13, 2003 10:58 pm

I just got a Volcano9 heatsink with the coolmod. I want to plug the Blue LED's into my case power, rather than into the motherboard. That way, I can still have the case light on. Do the LED's run off 12 volts or 5?

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Sat Jan 18, 2003 4:14 pm

LED's are current type devices and if you feed them more current than they can take they will fry. It's a pretty sure bet that increasing the voltage from a design value of 5V to 12V will increase the current through the LED to fatal levels. The simple solution is to put a resistor in series with the LED to keep the same current at the 5V level when you increase the voltage to the 12V level. I recently found design info on LED's on the web, but can't remember where now. You could put different resistors in series with LED until you find one that gives you the limiting current, which for most LED's is about 15 to 20 mA, but try to stay down at about 5 mA for safety. You should start with a resistor of about a 1,000 ohms and then work you way down. I have to go now, but there is a mathematical way to accurately estimate the resistor you need. I'll try to post again later.
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Sat Jan 18, 2003 6:29 pm

JimH, you mean Ohm's Law?

Basically, Voltage = Current x Resistance...

Resistance, then is Voltage/Current

On the 12V rail, you want a current of at most 15 mA, or 0.015A

Therefore, reistance must be at least 12/0.015 Ohms, or 800 ohms...

For a safe level of 5mA, you'll need a 2,400 ohm resistor

Hope this helps,
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Sat Jan 18, 2003 7:07 pm

The Coolmod comes with its own power connectors should you decide you don't want it put on your motherboard header..
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Sat Jan 18, 2003 9:03 pm

IntelMole, yes that's the one, but it's not quite so simple a calculation because the LED will have it's own inherent voltage drop which can vary from a low of about 1.7 V for a red LED to a high of somewhere around 3.5V for a blue LED. What's weird is that this voltage drop is fairly constant, no matter what the current going through it is, so it throws off the simple Ohm's Law calculation. But, I did find a site with a neat calculator for the resistor to use in simple LED circuits, check this out:

This should work for anyone who is not sure about how to calculate the value of the current limiting resistor.

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