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Does undervolting fans place strain on the PSU ?

post #1 of 9
Thread Starter 
My local shop sells these cables for undervolting fans, 12v - 7v
It´s simple enough and I will do it myself.
I was reading somewhere that the extra 5 volts goes back into the PSU , is this correct, just wondering if it places anymore strain on the PSU ? I will be doing it on 2 case fans.




please no fan controller suggestions.
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post #2 of 9
No, undervolting doesn't damage your PSU
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post #3 of 9
I'm not a good enough electrical engineer to know if grounding +12V current through your +5V will cause damage to the PSU. Most of the people who do know more than I do aren't even sure.

I would be extremely cautious. Don't do it to every fan in your case, only the ones that really need it.
post #4 of 9
No strain there, Its just using the 7v line instead of the 12v of the PSU (I think ...by looking at the picture)

The extra 5v isnt exactly 'going back into the psu' as it dosent work like thats, Its perfectly safe.
If you have a normal molex > 3pin converter just switch around the pins in the molex connector to convert it to 7v (Just then make sure you dont artatch a hdd or something to the other end of the molex extension

The only other way is to have one that has a resistor.
http://www.mnpctech.com/7volt_fan_re...pctech.com.jpg
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post #5 of 9
Quote:
Originally Posted by antipesto93 View Post
No strain there, Its just using the 7v line instead of the 12v of the PSU (I think ...by looking at the picture)
There's no 7V line. There's going to be current going from 12V through the fan down into the 5V (and presumably into devices that take 5V).

The deal is that the 12V and 5V sources from the PSU aren't ideal sources. There is a lot of feedback, rectification, etc. that goes into maintaining those values as near 12V and near 5V so it's hard to analyze what will happen--well, at least for me, since I'm definitely not expert in electronics, much less SMPS design.

By doing this, you are connecting the 12V and 5V subsystems in ways that are not intended. You have part of the 12V output feeding the parts that take 5V (in addition to the 5V output that's supposed to be taking care of them).

Of course, with just a couple fans, you're not going to be drawing much current so there shouldn't be much of a difference. Theory aside, it seems a lot of people do this and their PSUs haven't all stopped working.
post #6 of 9
Using an in-line resistor would be better.
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post #7 of 9
Quote:
Originally Posted by Arctucas View Post
Using an in-line resistor would be better.
Or a Zener diode.


One you could use:
http://www.newark.com/solid-state/1n...o-4/dp/10P4371

6.8V, 10W
Edited by Phaedrus2129 - 3/12/11 at 10:49am
post #8 of 9

Amusingly a 2 fan controller is cheaper than a simple diode


(afaik this is a simple adjustable resistor, which I personally like as a fan control option)
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post #9 of 9
Thread Starter 
Thanks for the replys.

I made a mistake and ordered a few MB´s which only had 2 headers , 1 for the CPU and 1 case fan (The later seems to run at full speed) I didn’t want to get lumped with the cost of three fan controllers, hence the search for a cheaper solution.
But, seeing how there is no definite answer, and these are for customers, I think I’ll just go for those fanmates, as I can get a few of those for the price of one normal fan controller. I wasn’t aware they existed.
They look like they should be able to handle a couple of case fans each, and it would save me rewiring for six fans.
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