Electronics What resistor should I use to lower fan speed?

I have a 12v fan - 0.30A. What resistor should I solder in to lower the speed about half or sth..? I believe it goes in red cable.. right?

 

15Ohm, 1.5W

V=IR, so the voltage across the fan will be 12-0.3x15=7.5V

That should make it run pretty quietly but not stall!

Power dissipated by the resistor will be P=I²R=0.3²x15=1.35W

Therefore you need a 1.5W resistor to stop it burning up.

hth
ch424

 

I have only 0.5W resistors... ok, so need to buy 1.5W one..

thanks!

 

But the current won't still be 0.3A at the reduced voltage.

First calculate the fan resistance, 12/0.3 = 40 ohms

So a 30 ohms resistor in series will take 30/(40+30) X 12 = 5V approx, leaving 7V for the fan, the current will be 12/(40+30) = 0.17A approx, so power wasted by the resistor = 5 x 0.17 = 0.88W

You can use a couple of equal 0.5W resistors in series or parallel as a 1W, they share the heat.

To save getting the calculator out, I posted a link here only yesterday.

 

If you want to go even simpler then just splice the wires with a USB cable and then you have a 5 volt USB fan

 

If this is a 4-pin molex connector (but only uses 2 wire) internal case fan, than just flip the plug on reverse and force it in. Like so: http://www.kenable.co.uk/images/case_fan_80mm_4pin_molex.jpg

That's what I did. You get half speed right there. Just DO NOT plug anything on the other end of the power plug of the fan, as IT WILL DAMAGE it. So make sure at 100% that you use that ONLY for your plug. I won't be responsible for any damage.