12 to 7 V x2.. That equals??

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kuzzia
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12 to 7 V x2.. That equals??

Post by kuzzia » Tue Mar 27, 2012 9:21 am

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?

Thanks in advance.

Lithium466
Posts: 83
Joined: Wed Feb 16, 2011 11:52 am

Re: 12 to 7 V x2.. That equals??

Post by Lithium466 » Tue Mar 27, 2012 10:03 am

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 ;)

m0002a
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Re: 12 to 7 V x2.. That equals??

Post by m0002a » Tue Mar 27, 2012 10:33 am

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?

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

Mr Evil
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Re: 12 to 7 V x2.. That equals??

Post by Mr Evil » Tue Mar 27, 2012 10:48 am

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

kuzzia
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Re: 12 to 7 V x2.. That equals??

Post by kuzzia » Tue Mar 27, 2012 11:09 am

Thank you for your answers.

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. :)

Lithium466
Posts: 83
Joined: Wed Feb 16, 2011 11:52 am

Re: 12 to 7 V x2.. That equals??

Post by Lithium466 » Tue Mar 27, 2012 11:12 am

These are 56 Ohms :wink:

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