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Modding fans to 10V

post #1 of 11
Thread Starter 
I thought this was done before, but I can only find a Xbox fan mod.
It seems rather simple. If the voltage is 12V and it needs to be 10V and the amps stay the same the calculation is easy with this formula:

R=U/I

But I don't know how many amps go through a fan header. Does anyone here know?
Then I'll buy some extensions and resistors and solder those resistors in.
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post #2 of 11
I'm not positive on this, but I read somewhere a fan header pushes about 0.8-1.2 A. I could be wrong, though. There's got to be a source somewhere that is more reliable than my word tongue.gif
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post #3 of 11
I'm no physics expert (no really I almost failed my Physics A level) but I believe it is not the mobo you need to look for but the fan(s) as tey will only draw the current it needs.

Basically use the amps of the fan (normally around the 0.2 mark).
post #4 of 11
Thread Starter 
Quote:
Originally Posted by bajer29 View Post

I'm not positive on this, but I read somewhere a fan header pushes about 0.8-1.2 A. I could be wrong, though. There's got to be a source somewhere that is more reliable than my word tongue.gif

But that would be diffrent per motherboard. If that is the case, there should be diffrent 7V adapters for every motherboard, right?
Quote:
Originally Posted by KaffieneKing View Post

I'm no physics expert (no really I almost failed my Physics A level) but I believe it is not the mobo you need to look for but the fan(s) as tey will only draw the current it needs.

Basically use the amps of the fan (normally around the 0.2 mark).

These are 0.38 A. But if that is the case, why do 7V fan adapters work with every fan?

EDIT: They are 0.083A
Edited by mrgnex - 2/12/16 at 6:23am
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post #5 of 11
Quote:
Originally Posted by mrgnex View Post

These are 0.38 A. But if that is the case, why do 7V fan adapters work with every fan?

I did say I almost failed tongue.gif

You must be right, maybe you only use the fans amps when working out resistance caused by the fan (or how many fans you can power).
post #6 of 11
Thread Starter 
Quote:
Originally Posted by KaffieneKing View Post

I did say I almost failed tongue.gif

You must be right, maybe you only use the fans amps when working out resistance caused by the fan (or how many fans you can power).

Hmm, that could be. I could try that. If it fails I can desolder the resistor.
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post #7 of 11
I'm trying to understand what your saying. Current is determined by the resistance of the load. Not by the fan header. Unless you''re wondering what the max current that the fan header can provide. If you want to run the fan at 10v on a 12v source just put a resistor in series with the fan. Measure the resistance of the fan and add a resister that has 16.7% (10/12) of the resistance of that fan. Do your wattage calculation of the fan I*E=W. Make sure you use a resistor of equal or greater wattage rating so it doesn't burn up.
Edited by Thick8 - 2/11/16 at 5:55pm
post #8 of 11
Thread Starter 
Quote:
Originally Posted by KaffieneKing View Post

I did say I almost failed tongue.gif

You must be right, maybe you only use the fans amps when working out resistance caused by the fan (or how many fans you can power).

I was looking around, and I think you are right. I found some led resistor calculators (which essentialy use the formula I used) and found that I need 27 Ohm resistor:

R=U/I=(12-10)/0.083=24 ohm but you need the next bigger resistor which is 27 ohm.
Quote:
Originally Posted by Thick8 View Post

I'm trying to understand what your saying. Current is determined by the resistance of the load. Not by the fan header. Unless you''re wondering what the max current that the fan header can provide. If you want to run the fan at 10v on a 12v source just put a resistor in series with the fan. Measure the resistance of the fan and add a resister that has 16.7% (10/12) of the resistance of that fan. Do your wattage calculation of the fan I*E=W. Make sure you use a resistor of equal or greater wattage rating so it doesn't burn up.

Ah yes, I have found that eventually, thanks.
So that makes P=U*I=10*0.083=0.83 Watts.

EDIT: This site recommends 1/2W or greater. So that means I can use these.
Giving the fans 10V will make them spin at: 10V/12V=0.83333333333 so 1850 rpm*0.83333333333=1542 rpm
Using 9V will result in: 0.75 so 1850*0.75=1387.5

I think that is fine too (only a 155 rpm diffrence).
I will need a resistor that is: (12-9)/0.083=36 ohm. The led calculator suggest 39 ohm.
That makes P=U*I=9*0.083=0.747W but the site mentioned earlier recommended 1/2W for this as well.
Edited by mrgnex - 2/12/16 at 9:49am
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post #9 of 11
Fan speed is not directly linear to voltage applied. Why do you want to slow down the fans? How slow do you want them to spin?
post #10 of 11
Fan headers don't actually have an official spec for current, but most should be able to do 500mA if not 1A. I've got four on a single header pulling 0.28A stock and, with a resistor to drop the voltage, ~0.18A modded. No problems there other than the noise - ball-bearing, 40mm, and under $5 each mean they're a bit noisy.

As for 7V adapters. What they do most often is replace the 0V reference GND pin with a 5V source pin. Since voltage is a potential difference, we get 12V - 5V = 7V across any fans used. I would advise against those for one simple fact: a voltage source is not a voltage sink. In many cases, this works alright and you get 7V out. But depending on the PSU, you could cause damage from sending higher voltage current in the wrong direction.

Quote:
Originally Posted by Thick8 View Post

Fan speed is not directly linear to voltage applied. Why do you want to slow down the fans? How slow do you want them to spin?

It is, actually, assuming everything is perfect and friction doesn't exist, etc. DC motor rotation speed is proportional to voltage applied. A 12V fan stepped down to 7V should spin at about 58% the speed. Additionally airflow is proportional to rotation speed, so we can also say airflow is proportional to voltage. Pressure, I'm not entirely sure of, but I don't think it's linear.
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