Resistor help

[SXRguyinMA]

Ok, I want to put inline resistors in the cpu fan in Red Comet so a) it won't sound like a jet engine and b) I don't have to hook up the stupid zalman fanmate and have one more piece of hardware to sleeve and mount.

I've calculated that I need a 68 Ohm 1/2 watt resistor. Being that I don't have any 1/2 watt resistors laying around (mostly 1/4 and 1/8), how can I hook them up in series to get the needed wattage and resistance?

For instance, I know that if I hook up two 68 ohm 1/4 watt resistors in series, the voltage drop will be the same, but I don't know how that effects the wattage?

[blueonblack]

If you hook two 68-ohm resistors in series you'll have 136 ohms of resistance. No idea on the wattage, but resistors in series add together.

[SXRguyinMA]

so I'd want to do 2 32-watt in series then, but still don't know if I put 2 1/4 watt resistors in that it will equal 1/2 watt. :think:

[nevermind1534]

nope; if you have that value, you'd need two series of that value, hooked up in parallel. You'll have a total of four resistors. Or you might be able to just get away with the 1/4 W resistor.

[SXRguyinMA]

well the fan is .15A @12v. I want to slow it to ~7v, and I calculated it'd take 68ohms with .4w, so I'd need at least a 1/2 watt or the resistor will burn out.

I guess I'll just take a ride to radioshack and get the right ones. I was trying to figure out how to do it without making a special trip, but one resistor will look nicer than 4 lol

[nevermind1534]

Or you could run two resistors of around twice the resistance in parallel, and it will accomplish the same task. A single one probably would look better, though.

[SXRguyinMA]

so if I ran 2 ~136 ohm 1/4 watt resistors in parallel it should work fine?

[nevermind1534]

Yes, that would give you 68 ohms, 1/2W.

[Oneslowz28]

FWIW, Noctua uses 1/4w resistors in their low speed adapter cables.

[SXRguyinMA]

really? maybe I'll give it shot with the 1/4 then, if it fries it fries lol

[nevermind1534]

really? maybe I'll give it shot with the 1/4 then, if it fries it fries lol

Just disconnect it if it starts glowing. Or if it's that hot, it will probably just desolder itself.

[Konrad]

I've never thought of using low-temp solder as a fuse before ...

You can always use a variable pot, not as elegant as pwm control, but the amount of wasted power (say, maximum of 5V@0.2A = 1W) is a negligible amount of heat for such a physically large component to dissipate. This approach creates a tiny amount of electrical noise (in the form of impedance oscillating at half the fan rpm frequency), but any not-cheap-crap mobo and PSU can easily clean and filter it out. I wouldn't personally worry about any of these things unless the resistor/pot is part of a fanbus that drives more fans or fan power.

My Noctua "Low Noise" adapter has a 70Ohm 1/4W (probably 68Ohm with loose tolerance, I haven't removed the heatshrink), and the "Ultra Low Noise" adapter has a 120Ohm 1/4W.

[SXRguyinMA]

I ended up using a 1/4W one and it worked just fine :D thanks though!