Slowing down a 120mm case fan

[Assimilator1]

In my 2nd PC I've got a 120mm intake fan currently running off 12v & its too noisey.
I know I could connect it upto the 5v line but I feel that this would slow it down too much.

I remember some people saying that you could have the fan feed on 12v & connect the '-' to the 5v live giving you 7v ,or to the 3.3v live giving you 8.7v.
This would give me the options I need but I've never tried it & I'm somewhat dubious connecting one live to another different live rail!
I'm no expert on PSUs ,so are my fears unfounded? or is it a good way to short out your PSU!?:Q

 

[shabby]

http://www.ocforums.com/showthread.php?t=372297

You're not shorting out anything, everyone does this and it works.

 

[Zepper]

You can get pre-assembled adapter cables just for the 7V trick. I prefer fan controllers like the Zalman Fan Mate 2 or the Sunbeam Rheobus so you get a range of control instead of just one or the other. Check jab-tech.com or SVC.com for the adapter cables and/or fan controllers. You could also use a mobo fan header if you have a free one that has the control function.

.bh.

 

[Assimilator1]

Thanks guys:thumbsup:

Fan has a molex plug on it ,I could adapt it ,but just plugging it into a different rail saves me buy anything extra

Though I'm curious as to why this mod could kill cheap PSUs ,why's that? does it make harder for the PSU to regulate the voltage on the lower voltage line?Am I right in saying that you're only 'back feeding' the amount of power that is drawn by the fan??

Hmm ,the PSU in my 2nd PC is a cheapo unit ,ColorsIT IIRC.

Any technical explanations about the possible risks of this?

 

[Zepper]

Better PSUs have reverse current protection - bottom feeders may not. IAC, unless your fan is fairly high resistance (low current = 200mA or less), I wouldn't do the 7V trick with it. Rheobus is only $13. new from jab-tech and the fanmate type control is only $5. or $6. - May find both for less on fleaBay...

.bh.

 

[Navid]

Originally posted by: Assimilator1
Any technical explanations about the possible risks of this?

You connect the fan between the 12V and 5V lines.
So, the voltage across the fan will be 12 - 5 = 7V.
Your fan has an effective resistance. Let's say it is 50 Ohms as an example.
Then, it will conduct a current equal to 7V / 50Ohms = 140mA.

The 140mA current enters the 5V line.
Your hard drives draw more than 140mA from the 5V line. So, there should be no problem.

If there is no load on the 5V line in your computer, the 140mA current must be absorbed by the 5V voltage regulator (part of the PSU). In other words, the regulator must sink current (140mA).
Any voltage regulator can source current.
Not every voltage regulator can source current as well as sink current!

If a voltage regulator cannot sink current and you inject current to its output with no load, the output voltage will go up (an absolute unacceptable condition for a voltage regulator)!

 

[Xvys]

I would just turn down the fan speed in the bios (QFan with Asus) to 75% or use SpeedFan to reduce the fan speed. Then you can easily increase it back to normal if overclocking or gaming.

 

[Assimilator1]

I can't do that unfortunately, like I said the 120 fan is powered via a molex connector.

Thx for the info Zepper & Navid
Not sure what I'm going to do atm

 

[toadeater]

I never had a PSU fail from volt-modding. Even when I accidentally shorted the wires. I have two volt-modded fans running off a cheap Rosewill PSU right now.

But I guess due to the laws of irony, my cheap PSU can handle it, but any 1000w, gold-plated, four-rail, bullet-proof, LED-flashing, 25% efficient, global warming-causing monstrosity is going to crash and burn the second you attach a volt-modded 40mm fan running at 10rpm.

 

[Assimilator1]

lol

How long have you been running volt modded fans off that PSU?

[update]Well after all that I thought I'd at least try 5v 1st just to see its effects ,I was guessing CPU temps would go up a bit ,but I was wrong in a good way!
CPU temp is core temp measured by speedfan ,CPU was running DPAD for 100% load.

120mm fan @12v

CPU temp 51-52C
Case temp 26C
HD0 temp 33C

120mm fan @5v

CPU temp 51-52C
Case temp 28C
HD0 temp 33C
GPU temp 39C

Room temp 24C (I didn't measure it before dropping the voltage unfortunatly, however it was only about 15mins at most between the 2 setups so I doubt it changed much if at all).
Took readings when they rose no further.

This is great! ,I've got a quiter PC & it cools the CPU just as well (looks like the fan was 'spinning its wheels' to some degree) ,though I'm guessing when the graphics card is loaded too that it would run a little hotter....... darn wish I'd tested that too ,guess I still could.

Btw though I probably won't be using the 7v mod for this PC the info was certainly useful & will no doubt use it for future modding

[update2]
Running ATITool to load CPU & GPU @100% (as much as I know for the GPU)

120mm fan @12v

CPU temp 53-54C
Case temp 28C
HD0 temp 33C
GPU temp 72C
Room temp 25C

120mm fan @5v

CPU temp 53-54C
Case temp 30C
HD0 temp 33C
GPU temp 73C
Room temp 24.5C

No real loss their either ,tiny increase on case & GPU temps.