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 Post subject: 12 to 7 V x2.. That equals??Posted: Tue Mar 27, 2012 9:21 am
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Joined: Fri Jul 02, 2010 4:41 am
Posts: 709
Location: Denmark
Good day to you all.

I was wondering: If I connected a fan to a 12 V header but with two 12-to-7 V resistors in between.. How much will the fan then be fed with in terms of voltage? How would you calculate that?

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 Post subject: Re: 12 to 7 V x2.. That equals??Posted: Tue Mar 27, 2012 10:03 am

Joined: Wed Feb 16, 2011 11:52 am
Posts: 83
Hi,
If the "resistors" are real resistors, than you can use U = RxI to determine what voltage will be applied...but if you don't have the resisotrs value (it is often around 56 Ohms), that won't help

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 Post subject: Re: 12 to 7 V x2.. That equals??Posted: Tue Mar 27, 2012 10:33 am

Joined: Wed Feb 04, 2004 2:12 am
Posts: 2831
Location: USA
kuzzia wrote:
Good day to you all.

I was wondering: If I connected a fan to a 12 V header but with two 12-to-7 V resistors in between.. How much will the fan then be fed with in terms of voltage? How would you calculate that?

I would highly recommend Zalman FanMate-2 or a fan controller instead.

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 Post subject: Re: 12 to 7 V x2.. That equals??Posted: Tue Mar 27, 2012 10:48 am

Joined: Fri Jan 20, 2006 10:12 am
Posts: 563
Location: UK
Lithium466 wrote:
Hi,
If the "resistors" are real resistors, than you can use U = RxI to determine what voltage will be applied...but if you don't have the resisotrs value (it is often around 56 Ohms), that won't help

Actually, you don't need to know the resistance, you only need to know the ratios. In this case, one resistor gives 7V across the fan and 5V across itself. Two resistors therefore gives (7/(7 + 5 + 5)) * 12 ~= 5V across the fan.

That's assuming the resistors are in series. If they are in parallel then it will be (7/(7 + (5/2))) * 12 ~= 9V

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 Post subject: Re: 12 to 7 V x2.. That equals??Posted: Tue Mar 27, 2012 11:09 am
 Friend of SPCR

Joined: Fri Jul 02, 2010 4:41 am
Posts: 709
Location: Denmark

They are in series. I use these:

http://www.quietpcusa.com/Zalman-RC56-7 ... -P102.aspx

Perhaps I'll invest in a fan controller, though I would prefer an ASUS motherboard with one PWM header and two 3-pin fan headers. I quite fancy their fan controlling and automatic fan controlling in general.

_________________
Cooler Master Elite 341, Athlon II X3 425, Radeon 5750 (passive, fan zip-tied on), Crucial M4 64 GB.
Cooling: AC Alpine 64 Pro, rear exhaust Scythe Slipstream 800 rpm @ ~5 V

"SSD's: The difference between a casual jogger and a dog chasing a ball"

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 Post subject: Re: 12 to 7 V x2.. That equals??Posted: Tue Mar 27, 2012 11:12 am

Joined: Wed Feb 16, 2011 11:52 am
Posts: 83
These are 56 Ohms

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