7volt mod the safe way?

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A

aggressor

Platinum Member
Oct 10, 1999
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I have two Panaflo 120mm fans that I want to volt mod down to 7v. After wondering why my PC was so loud with nothing but Panaflo fans (1x92mm, 2x120mm, all low speed models) I started messing around and found out it was the 120mms causing 90% of the noise. I did the potentially dangerous method of the 12v - (+5v)=7v mod and I am pretty pleased with the results.

I want to continue running them this way, but I'd rather be safe about it. The question is, what do I buy? I am not interested in any kind of fan controller, as I want them to always run at 7v.

Side question...why are they so loud at 12v? Is the noise just from pushing so much air? The 92mm is rated at 28db, and the 120mm fans are rated at 30db, so it can't be that much of a difference.
 
Zap

Zap

Elite Member
Oct 13, 1999
22,377
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Panaflo fans aren't necessarily quiet at full speeds. They have a good reputation with the SPCR crowd not because of their stock noise levels, but because of their smooth bearings making for a quiet fan when undervolted (or so I understand).

If you want something to buy, just get a cheap fan controller and mount it somewhere inside the case. I did that once before, when I wanted to just set the speeds and forget about it. Mine was a Nexus 305 or something, normally fit in a 3½" drive bay. I mounted it on top of the PSU in an Aspire "alien" case with double sided tape.
 
J

Jhhnn

IN MEMORIAM
Nov 11, 1999
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What you've done isn't unsafe, at all. It's fairly common practice in all kinds of electronic equipment to create usable voltage differences in such a fashion.

Or you can use something like a zalman fanmate and whatever wiring mods/ adapters you need, or even go to something like this, which I've never used-

http://www.xoxide.com/sunbeam-theta-tp101.html

Noise measurements aren't linear, they're logarithmic, so the two 120mm fans create ~33 db together at 12v, perceptually a lot louder than the 28 db of the other fan.

 
A

aggressor

Platinum Member
Oct 10, 1999
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Everywhere I look there are warnings against doing this type of mod, so I dunno what to think. I'm using good fans with a good power supply (500w Forton) and nothing has blown up, but I just don't want to risk anything. I'm trying to do the cheapest mods possible, so that software controller is a bit too pricy :( Getting a fan controller and hiding it doesnt seem like a bad idea, but I wish there was something more elegant I could use.

And I knew noise measurements were logarithmic, I was just shocked at just how much louder a single 120mm fan was compared to the 92mm model.
 
Z

Zepper

Elite Member
May 1, 2001
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You can calculate the resistance of the fan using the formula: I=E/R (where I=amps, E=volts and R=resistance - just rearrange the formula to put r on one side of the equals - R=E/I) and use just a resistor in line with the fan's + lead to drop the voltage to 7. so you'd want a resistor in the same ratio 5 is to 7 as x is to fan resistance in Ohms.

That's the same thing that Nexus and Silen-X does with their fans...

Or you could get a Zalman FanMate instead...

.bh.
 
V

vanvock

Senior member
Jan 1, 2005
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Originally posted by: Zepper
You can calculate the resistance of the fan using the formula: I=E/R (where I=amps, E=volts and R=resistance - just rearrange the formula to put r on one side of the equals - R=E/I) and use just a resistor in line with the fan's + lead to drop the voltage to 7. so you'd want a resistor in the same ratio 5 is to 7 as x is to fan resistance in Ohms.



I'm trying to do the same thing but just can't seem to get it right. Havent used Ohm's law in a while & I'm rusty. My fan is .37a which means 32ohm & 4.44watt, 12v of course. I've tried a 10, 100 & 1k resistor on the P/S plug & checking with meter shows virtualy no voltage drop. How about it Zepper(anyone?), could you help me out with the math?
Click to expand...
 
Z

Zepper

Elite Member
May 1, 2001
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vanvock,
. To give you ~ 7V on the fan, the resistor you need is 22 Ohms with anything over a 1.1Watt rating (you could probably get away with a 1Watt resistor, but to be safe, I'd go with a 2 Watter). The resistor goes IN LINE (in series with the fan) with the red fan wire, NOT across the fan wires.
. When you put a resistor across the fan wires (in parallel with the fan), the fan is still getting 12V no matter what size the resistor unless you go so low and so big (watt rating) that you're nearly causing the PSU to fail. It would definitely fail before you loaded the 12V rail down to where it could only put out 7V...

.bh.
 
N

Navid

Diamond Member
Jul 26, 2004
5,053
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The warning you have seen is because in this arrangement (12-5=7), current flows from the 12V rail through the fan to the 5V rail.

Regular power supplies are designed to supply current, not to receive current.
The current that flows through the fan has to be less than the current that the components draw from the 5V rail. Then, as far as the supply is concerned everything is fine and it supplies current.

However, if the fan current is more than the total current your computer components draw from the 5V rail, it effectively is like the 12V rail is charging the 5V rail. The supply may not like that.
The possibility of such a problem goes up as you increase the number of fans since you increase the overall fan current.
 
M

Meuge

Banned
Nov 27, 2005
2,963
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Originally posted by: Navid
The warning you have seen is because in this arrangement (12-5=7), current flows from the 12V rail through the fan to the 5V rail.

Regular power supplies are designed to supply current, not to receive current.
The current that flows through the fan has to be less than the current that the components draw from the 5V rail. Then, as far as the supply is concerned everything is fine and it supplies current.

However, if the fan current is more than the total current your computer components draw from the 5V rail, it effectively is like the 12V rail is charging the 5V rail. The supply may not like that.
The possibility of such a problem goes up as you increase the number of fans since you increase the overall fan current.
Click to expand...
If the 5V rail supplies ANY components, then the chances of a 1-2W fan pushing any current into the PSU are nil.
 
N

Navid

Diamond Member
Jul 26, 2004
5,053
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Originally posted by: Meuge
If the 5V rail supplies ANY components, then the chances of a 1-2W fan pushing any current into the PSU are nil.
Click to expand...

It is not a matter of chance! It is a simple equation.
It is also important to note that at bootup, the components don't all turn on at the same time.

I did not say that there will definitely be a problem. I just pointed out why some people don't like the 7V mod and prefer the insertion of a resistor in the 12V path instead.
 
J

Jhhnn

IN MEMORIAM
Nov 11, 1999
62,365
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From vanvock-

"I'm trying to do the same thing but just can't seem to get it right. Havent used Ohm's law in a while & I'm rusty. My fan is .37a which means 32ohm & 4.44watt, 12v of course. I've tried a 10, 100 & 1k resistor on the P/S plug & checking with meter shows virtualy no voltage drop. How about it Zepper(anyone?), could you help me out with the math?"

If there's no load, there's no voltage drop across a resistor... your meter isn't a load. As zepper points out, the proper method is to put the resistor in series with the fan. power> resistor> fan> ground or power> fan> resistor> ground...

The reason some people don't like the 7volt trick is because they don't understand the way a psu works in the first place. It actually reduces the load on the 5v rail slightly. With normal computers and fans, that amounts to a small % of the total load on the rail, and harms nothing... It can even be beneficial on some older systems where the 5v rail is heavily loaded, running virtually everything on the motherboard...
 
Z

Zepper

Elite Member
May 1, 2001
18,998
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You don't want to put the resistor in the ground lead of a fan as it might screw up the tach signal (because you would be raising the level of ground w/ respect to the sensor and it's signal is generated relative to its ground). So the sensor signal would be X volts higher than normal relative to system ground. If the fan has no speed sensor, then it doesn't matter in which lead you put the resistor.

.bh.
 
S

Slickone

Diamond Member
Dec 31, 1999
6,120
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Don't they have something like the Fanmate that gets it's power from a 4 pin connector instead? I want to connect several fans to it, which would be too much to take from the mobo fan header.

With that Spire Dual Speed, could I connect 5 fans (4 pin) to one side (Antec fans, .2A I think, so still under the 1.2A max), and my CPU fan to the other side, and have RPM monitoring for that one fan?


Zepper, how did you get 22Ohm? If he wants 7V and his fan is .37a 7/.37 is 18.9 ?
Also can I use one resister but run several fans off of that?

I don't fully understand the potential 7V problem but if I have six 80mm fans running at 7V on say a 40W Antec PSU, could I have problems?
Also, is it OK to run a hard drive of a power lead from the PSU, then after that run a fan at 7V on the same lead?

BTW, this mentions the problem (in light blue).
 
N

Navid

Diamond Member
Jul 26, 2004
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Originally posted by: Slickone

Zepper, how did you get 22Ohm? If he wants 7V and his fan is .37a 7/.37 is 18.9 ?
Click to expand...

12V / 0.37A = 32.43Ohms
So, the fan has a 32.43-Ohm resistance.

If you want 7V across the fan, knowing its resistance, you can calculate its current:
7V / 32.43 Ohms = 0.22A
So, the current through the fan is a function of the voltage across it. When they say the fan draws 0.37A, they mean from 12V not from 7V.

Now, you need a resistor in series with the fan. The current of such resistor will be the same as the current of the fan (series). So, the resistance can be calculated since you have its voltage (5V) and its current (0.22A).

R = 5V / 0.22A = 22.7Ohms. A 22-Ohm resistor is the closest standard (10% tolerance) size resistor to this value and pretty damn close in size.
 
Z

Zepper

Elite Member
May 1, 2001
18,998
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As Navid said in his usual lengthy manner... ;)

It's a ratio the voltage is 5 (drop across the resistor) to 7 (drop across the fan) so the resistance ratio is also 5 to 7 (resistor to fan) 32/7 = 4 4/7 * 5 ~ 23 and the nearest standard resistor value is 22 Ohms.

.bh.
 
G

GalvanizedYankee

Diamond Member
Oct 27, 2003
6,986
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http://www.cpemma.co.uk/diodes.html

Gotta love the Brits for DIY articles.

Read the Resistors section in total. At the bottom he speaks of the reverse polarity diode
that will present it's own voltage drop. I never knew that till I found this site.

This easy, cheap way, to get solid voltage drops for fan control is to buy a Sunbeam Rheobus
for < $10 and put it in a 5.25" bay.


...Galvanized
 
S

Slickone

Diamond Member
Dec 31, 1999
6,120
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Originally posted by: Navid
Now, you need a resistor in series with the fan. The current of such resistor will be the same as the current of the fan (series). So, the resistance can be calculated since you have its voltage (5V) and its current (0.22A).
Click to expand...
Why is the voltage of the resistor 5, and not 12?
 
S

Slickone

Diamond Member
Dec 31, 1999
6,120
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Is it OK to run a hard drive of a power lead from the PSU, then after that run a fan at 7V on the same lead?
 
N

Navid

Diamond Member
Jul 26, 2004
5,053
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0
Originally posted by: Slickone
Originally posted by: Navid
Now, you need a resistor in series with the fan. The current of such resistor will be the same as the current of the fan (series). So, the resistance can be calculated since you have its voltage (5V) and its current (0.22A).
Click to expand...
Why is the voltage of the resistor 5, and not 12?
Click to expand...

Two components in series have the same current. They do not necessarily have the same voltage. Link
Two components in parallel have the same voltage. They do not necessarily have the same current.
 
J

Jhhnn

IN MEMORIAM
Nov 11, 1999
62,365
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From Slickone-

" Is it OK to run a hard drive of a power lead from the PSU, then after that run a fan at 7V on the same lead?"

Sure- no problem at all.

Always alter the fan side of any plug situation rather than the psu side. If the fan has a pass-thru connector, alter the output side so that it has a standard configuration... otherwise, it's easy to forget, plug something new into the non standard plug.. disaster ensues...
 
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