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For those, who do not want to spend a lot of money on fan controllers!

post #1 of 6
Thread Starter 
Hi all,
Fan controllers have never been my favourite, as the cabling for them makes my case even more messy than it was before.
There is however another way of slowing the fans permanently down...

You remember those little cables you get with some of the fans to reduce their speeds, they are nothing else than a normal cable with a resistor.
So simply by placing the required resistor in line between the socket and the fan you could slow the fan down, by reducing the voltage which goes to it. You ask me what resistor..., that varies, but there is a way of calculating what you need:

Let say you have got Akasa Apache 120mm like I have got and you do not want to run it at full speed as it is too noisy and you want to reduce the voltage to let's say 10V so what we do:

Original fan voltage: 12V

Original fan current: 0.33A (Normally placed on the fans sticker)

V=IR (Ohm's Law), so 12 = 0.33R => R = 12 / 0.33 = 36.364

Fan resistance: 36.364 Ohms

Target voltage: 10V

V=IR (Ohm's Law), so 10 = 36.364I => I = 10 / 36.364 = 0.275

Target current: 0.275A

Voltage dropped by a resistor: 12V - 10V = 2V

V=IR (Ohm's Law), so 2 = 0.275R => R = 2 / 0.275 = 7.273

Target resistance: 7.273 Ohms

Power dissipated by resistor: 2V * 0.275A = 0.55W


So to run Akasa Apache 120mm from 12V rail at 10V you need 7 Ohms 1 Watt resistor... Have fun...
That should take us from 1300 to about 1000 rpm, which is noticeably quiter!

God bless Georg Ohm
Edited by chrisf4lc0n - 8/18/12 at 11:27pm
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Viggen Blue
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post #2 of 6
This is overly complicated, and on top of that the resistor will heat up.

It's much better to use diodes, two generic rectifying diodes in series (such as any from the 1N4001 - 1N4007 range) will reduce the voltage from 12V to 10V, as one diode drops the voltage by about 1,0V (varies by type).

Pros:
1) you don't need to make new adapters when you change the fans, diodes will work the same for any fan
2) the diode will generate virtually no heat, unless you drive it close to its maximum specified current (1A in case of the above type)
3) much smaller than a 1W resistor
4) no calculations needed, apart from "You want to drop X Volts, you use X diodes"

Cons: none
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post #3 of 6
In what way does dissipating energy through a diode differ from doing the same through a resistor ?

Assuming 2V dropped thus power wasted is equal in both cases.

dunx
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post #4 of 6
Sorry for late response, I haven't been around.

Based on your asumption:
You need to drop 2V, you use 2 generic rectifying diodes in series. Each of them will drop around 1V in a wide range of the current flowing through them, up to 1A. In other words, regardless of the load (as long as it's mA not uA or pA).
One problem with a resistor is that to dissipate 2W from it, it has to be big and bulky. You could use the same thing as above: two 1 Ohm resistor in series instead of one 2 Ohm. But more important thing is that any change in the load (current flowing through the resistor) will change the resulting voltage. With a 2 Ohm resistor, at 1A you'll be dropping 2V, but at 0,5A it's only 1V at so on.
U = I * R where R is constant since the resistance doesn't change, therefore with the changing I (varying load), the voltage lost across the resistor will change. That's why it's not wise to use resistors to control the voltage directly, unless the load is constant. With a diode the load can vary without that effect.
Most 1N4001 for example are rated at 3W and 1A maximum, so they won't have a problem with dissipating the 1W that you'd see at 1A.
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post #5 of 6
Thread Starter 
If it is not wise to use resistors to drop the voltage, so why all the so called low noise adapters are built around resistors?

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post #6 of 6
As usual, money.
It's cheaper and "not too bad" or "good enough", however you want to put it.
The adapters you're probably talking about are usually used to lower the voltage from 12V to 7V for a single fan. It would be more expensive to make such an adapter with diodes, but on the other hand, it wouldn't matter if you connect 1 or 5 fans to it, the resulting voltage would be the same (+- a fraction of a Volt). With a resistor adapter, what will be the voltage (and heat dissipated) if you connect not 1 but 6 fans of the same type (same current rating)?

Assuming an average 0,2A per fan, for a single fan the resistor - to drop from 12V to 7V - would have to be 25 Ohms and it would dissipate 1W.
What would happen with 5 fans?
Edited by wlw wl - 12/3/12 at 8:27am
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