# Need resistance help for a computer fan



## guy2 (Apr 30, 2009)

Building a laptop cooler for moms acer laptop ,it always overheats and shuts down.

Iv already built one for my self i like to use power supply's instead of usb ,mine i used a DVD player power supply that puts out 12V at an unknowable amperage because i didn't have a good meter then and now that i do iv found out that the power supply i plan to use for my moms is 15V at 5.?-6amps ,it really pushes a 12V fan ,power supply is from an old garbage laptop i sacrificed. 

Iv got a few 12V fans all at different amp ratings but none are rated any where near 6 amps and 15V.

Is it possible to make or scrap a resistor that would work even at this power supply rating with out destroying good fans ,i can get allot of them but never really had to use em and not sure how to match the resistor rating with the one of the power supply to the fan.

Im not even sure if i need just 1 for the positive or 2


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## dustyjay (Jan 24, 2003)

Try using a potentiometer rated at 15V.


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## Frank4d (Sep 10, 2006)

You really do need a voltmeter for this, because DC power adapters aren't well regulated. You might find your 15V adapter is 17V with no load and 16V with a medium load.

But let us assume it really is 15V, and your 12V fan uses 0.30 amps. You need to drop 3 volts (15 -12 = 3) across a resistor in series, so you need a 10 ohm resistor (3 volts / 0.30 amps = 10 ohms). And you need it to be rated at least 1 watt (3 volts X 0.30 amps = 0.9 watts which is close to 1 watt).

You may not find a resistor that exactly matches your calculations, so using a higher resistance and wattage rating will work. You could for example use a 12 ohm, 5 watt resistor in the above.


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## guy2 (Apr 30, 2009)

I do have a good 35$ meter with auto ranging ,i managed to drop the 15v-13.5 but the amperage stays the same ,does the amperage matter if i get the voltage down?


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## dustyjay (Jan 24, 2003)

The fan will draw what amperage it needs. If you keep the voltage with in range you should be alright.


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## Drabdr (Nov 26, 2007)

dustyjay said:


> The fan will draw what amperage it needs. If you keep the voltage with in range you should be alright.


:up: :up: I was about to post the same. Amperage is a pull; not a push.

As long as the power supply has large enough amperage capability to provide for the fan, you're fine. As dustyjay states, the voltage should be the same, or awfully close.

I might consider putting a fuse on it.


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## JohnWill (Oct 19, 2002)

guy2 said:


> does the amperage matter if i get the voltage down?


If the voltage goes down, so does the current, assuming a constant load.


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