Slowing down a notebook fan with a resistor

[fr]

The fan in my Sony R505 makes me want to kill myself. I wouldn't mind as much if it was a constant sound, but it's a variable speed fan so it's always winding up and winding down depending on CPU usage. In the past, I've resorted to unplugging it when I have to bring to a public place, but if I go into a stuffy room, the CPU throttles down to 200MHz and it is barely useable.

The thought about putting in a resistor to at least slow it down. I tried a 1/2 watt, 68 ohm resistor, but that is too much because the fan won't start. I tried a 1/2 watt, 22 ohm resistor and I get the same result. I can't get physical access to the fan underneath so I can't see its power ratings. I'm thinking of trying a 1/4 watt, 10 ohm resistor next.

The fan still works the same as before if I remove the resistor so I didn't break anything.

 

[Jeff7]

Be careful not to lower the wattage of the resistor too much or it'll just burn up, or at least get dangerously hot. Even if it uses 10V @ 100mA, that's still 1W; a 1/4W resistor would probably snap.

 

[fr]

Thanks for the advice. Do I have any other options? If 1/4 watt is no good, the only step down left is 10 ohm, 1/2 watt. I tried a program called SpeedFan, but my computer reboots when I launch it.

 

[Jeff7]

Does the fan have any ratings on the label? Volts and amps?

I never did quite get how to use Ohm's Law to figure out the final volts and amps that would get through a certain amount of resistance. Just one thing that no matter how many times I read it, the actual usage of it escaped me, as simplistic as it seems. V=IR. Ok, so the V and I - is that before or after the resistor's added? And is it the voltage and amps dropped by the resistor, or by the load?

 

[ModeEngage]

Originally posted by: Jeff7
Does the fan have any ratings on the label? Volts and amps?

I never did quite get how to use Ohm's Law to figure out the final volts and amps that would get through a certain amount of resistance. Just one thing that no matter how many times I read it, the actual usage of it escaped me, as simplistic as it seems. V=IR. Ok, so the V and I - is that before or after the resistor's added? And is it the voltage and amps dropped by the resistor, or by the load?

V=IR

The I is the final current, including the resistance. The resistances of the resistor and the fan will add. Divide that by 12 volts (the voltage going through the fan at its maximum power) to get the current going through the circuit.

P=(I^2)R

To calculate the wattage, take the current that you got above and the resistance and use the above formula. If P > the wattage rating of the resistor, you have a problem.

That said, USE THE ABOVE INFORMATION AT YOUR OWN RISK.

I won't be held liable for stuff melting/exploding. Heh.