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 Post subject: Posted: Sat Sep 25, 2004 8:14 pm

Joined: Fri Jul 23, 2004 11:10 pm
Posts: 657
looks right as far as i can tell, going to be kind of confusing typed out like this compared to written on paper, but anyways:

V = Volts, I = current in Amps, R = resistance in Ohms, P = power in Watts

V = IR -> R = V/I
so for your fan, R = 12V/0.15A = 80 Ohms

resistance for series circuit = R1 + R2
so to get down to 7V, you want to make the fan's resistance equivalent to 7/12 of the total resistance of the circuit
80 * (12/7) = 137 Ohm total -> resistor needs to be 137-80 = 57 Ohms

80 + 57 = 137
so total circuit: 12V = I * 137
I = 0.0876A
or you can calculate series current like this:
1/I (for overall circuit) = 1/I1 + 1/I2
1/I = 1/0.15 + 1/0.21 (since the resistor would be drawing 12V/57 Ohms = 0.21A current if it were by itself)
1/I = 11.4, I = 0.0877A

this is just so we can calculate the power dissipated by the resistor, if you have a big fat 10W resistor or something (or if this makes absolutely no sense) you can ignore everything below "57 Ohms"

P = I^2R
your resistor needs to be able to dissipate 0.0877^2 * 57 = 0.438W of power, so look for one that can handle say 0.5W (and 50 or 60 Ohms is probably about the closest you'll easily find).
buying one resistor at a time usually gets pretty expensive, if you can just buy a mixed bag of them from say 20-200 Ohms, rated for a watt or so, you can mix and match them to get whatever voltage you want for any fan.

edit: as you can see i assumed that "max" current = current draw at 12V; i'm not sure whether manufacturers base maximum on the rated voltage or the point at which the fan explodes. too lazy to check other fans and see what seems reasonable... if that max current is actually current at 13.8V then you'd need a resistor something more like 66 Ohms to end up really close to 7V.

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 Post subject: Posted: Sun Sep 26, 2004 5:03 am
 SPCR Reviewer

Joined: Sat Nov 23, 2002 6:33 am
Posts: 8636
Location: Sunny SoCal
simms wrote:
Ok, I'm outfitting my entire computer with Nexus fans. Now, here's the thing. If I want my fans at 7V, what resistor do I need to wire between them?

How about using Kostik's nifty Fan Resistor Calculator that's been talked about here on SPCR?

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 Post subject: Posted: Sun Sep 26, 2004 7:08 am

Joined: Tue Aug 24, 2004 12:16 pm
Posts: 41
Wow, awesome! That tool really helped out.

Now that I've got 4 80mm Nexus fans running at 7V.. 92mm Nexus on the HSF....sealed up all the holes in the case... dremeled out my grills... suspended the HDD's... got rounded cabling and rounded PSU cables...

Where do I go from here? Insulation?

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On 2CPU/AMDMB/Anandtech/Agora/Forum OC/Got Apex/[H]ardForum/Heat/PCA Forums/Tech Zone as 'simms'
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 Post subject: Posted: Sun Sep 26, 2004 10:17 am

Joined: Fri Jul 23, 2004 11:10 pm
Posts: 657
kids these days, so lazy!
forgot i had fancalc sitting on my desktop too.

if everything moving is decoupled then yeah, some sort of sound absorber could help now, tons of diff threads on that though.
rounded cables aren't that hot though - can't really fold them behind motherboards or drive cages etc. i still usually use them myself, and had to use a flat cable when moving some stuff yesterday, not for cablegami, but because a rounded cable just couldn't bend the way i needed it to, go figure.

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 Post subject: Posted: Sun Sep 26, 2004 4:18 pm

Joined: Tue Aug 24, 2004 12:16 pm
Posts: 41
I guess this thread is coming to an end. Turns out I could mount a 92mm exhaust on the TOP of the case. So, I've got a 7V Nexus (thanks to fancalc) in my PSU, a 12V 92mm Nexus on the top of the case, 12V 92mm Nexus on the CPU, 80mm Nexus on the Exhaust, and an 80mm UV CCFL Sunbeam @ 7V just for show (and it's quiet too!)

Cut out the holes today. I'll be taking pics shortly, plus my own custom modded top exhaust of which I'm very proud of. Thanks to everyone at SPCR for great advice!!!

By the way, if anyone knows where I can get a few 80mm chrome fan grills, that would be great.

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On 2CPU/AMDMB/Anandtech/Agora/Forum OC/Got Apex/[H]ardForum/Heat/PCA Forums/Tech Zone as 'simms'
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 Post subject: Posted: Sun Sep 26, 2004 5:17 pm

Joined: Tue Aug 24, 2004 12:16 pm
Posts: 41
I'd also like to say that my temps dropped 10 degrees when I installed my 92mm Nexus on top of the case. Or maybe it was the 80mm one in the PSU. Either way, I'm happy =)

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