7v mod causing damage to PSU?

S

slaya

[H]ard|Gawd
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Nov 10, 2002
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Hey, I've heard stories about the 7v mod causing damage to the PSU, because the extra 5v's are being sent back to the PSU which is not good.. Co-winkda-dinkly, I want my CPU fan to run at 7v and my VGA fan to run at 5v, could it be wired so that my CPU fan draws 7v's of current and the remaining 5v's are sent to my VGA while preventing any damage to my PSU?

Thanks
 
Yes, you could wire it that way. With one caveat: You should probably put a capacitor bridging the 5V and GND lines of the VGA fan. This is because DC motors are notorious for having all sorts of back EMF, and the capacitor will make sure that the voltage stays a little smoother. However, you don't really have to avoid the 7V/5V mod.

You see, the potential damage to a PSU from doing "the 7V mod" has been vastly exaggerated. Most PSUs today won't allow themselves to get damaged this way--they'll just refuse to run if there's more current coming back in the 5V line than is going out. If a fan wired to the 12V and 5V lines shorts out, your PSU might fry, but that's a very remote possibility.

Case in point: I set up a spare PSU with a 120mm fan to create some air circulation by a big window in our apartment to clear off condensation. The fan was originally wired for 7V. I turned on the psu (yay for paper clips!), but when I plugged the fan in, the PSU shut off. I modded the fan back to 12V, and all was well.

In a normal computer, you probably have ten times as much current going to your CPU via the 5V line than comes from a typical fan wired to the 12V and 5V lines. So your PSU doesn't even know you have a fan wired that way. If the fan shorts out, chances are your PSU will panic and shut off.
 
A 7V mod won't hurt a loaded PSU.

It's simple - suppose you have a power supply that's putting out 10A on 12V, and 10A on 5V. You hook up a delta screamer that draws 1A at 7 volts, using the "7 volt" trick. Your power supply will now be putting out 11A on 12V, and 9A on 5V.

If you run a power supply by itself with a 7-volt-trick fan and no 5V load, then problems might happen. So don't do that.
 
Ah, thanks a lot guys.. I was a little worried about hurting my PSU.

I guess I will wire it normally.

I was thinking about putting all my fans on 1 lead from the PSU. I have this Zalman thing which allows me to connect a two 5v fans and two 12v fans, I am going to connect 2 fans to the 12v connectors and 1 fan to the 5v connector, and on the sam PSU lead I will also have a 7v mod'ed fan, is this safe or will I be overloading the wire?
 
Where have you heard these stories from? I've been building baybus' and fanbus' for people for over 5 years. Most of these machines have been running for years without problems. I personally have 3 machines with fans all running at 7v on 24\7 for 2 1\2+ years and I have yet to have any problems. IE don't worry about it.
 
A 7V mod won't hurt a loaded PSU.
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That's what I was trying to say. Except it took me a lot longer.

Today, even an unloaded PSU won't be hurt by a 7V mod, (possibly) unless it's some really cheap yum-cha PSU. Any decent modern PSU will just refuse to work if all it has is something going from 12V to 5V.

If you have anything drawing significant current from the 5V line (like a hard drive), the psu will be happy. Like Zero_Distortion, I've been running a computer for over a year now with a couple of fans at 7V with no adverse effects.
 
Teancum said:
Like Zero_Distortion, I've been running a computer for over a year now with a couple of fans at 7V with no adverse effects.
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I've been running one 7v modded fan in my parents comp for a while now, with a cheapa$$ PSU and its been fine.
 
Teancum said:
Most PSUs today won't allow themselves to get damaged this way--they'll just refuse to run if there's more current coming back in the 5V line than is going out..
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It doesn't "come back in the 5V line".

Current from the +12V line goes through the fan, meets the +5V line and flows on to ground through the components using 5V. Plenty of them in a system. Voltage across fan = 7V. The "normal" 5V line current is reduced by the value of the fan current, usually only by a very small percentage.

Means you can't do the 7Volt trick without something attached to the psu using 5V, which only arises if you have a second psu running just fans & other 12V stuff.

Worst that can happen is that the fan motor introduces some noise into the 5V supply, but I've had no trouble in several years doing it.
 
i'm confused...

if you need 7V why not use a voltage regulator... feed it with a 12V line fmor the PSU.. and drop it to 7V??
 
RancidWAnnaRIot said:
i'm confused...

if you need 7V why not use a voltage regulator... feed it with a 12V line fmor the PSU.. and drop it to 7V??
Click to expand...

Because it's way, way easier just to switch two wires around and be done. :p
 
waddles said:
Because it's way, way easier just to switch two wires around and be done. :p
Click to expand...

Exactly.....my 7v mods take like 5 seconds.....just pop the two wires out, switch 'em ...and you're done. :)
 
Cool, thanks guys.. Yeah I do have a few fans running at 5V's so I should be fine..

Thanks again.
 
When using a voltage regulator to drop 12v down to 7v you are also dissipating (5v * the amps the fan draws) watts in heat. It's not all that much heat, but there's no reason it needs to be there at all.
 
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