7v fan trick...safe?

[thrawn86]

I have a few fans that I'm needing to quiet down a bit, and I was reading about the 7v fan trick. the article said that it could be dangerous to feed current back into the psu (because you're using the 5v line as the ground for the 12v). If i'm only using the 7v trick on , say, 2 fans, that would only be about 200-300mA of current. Would that be safe, or should I not risk it.

[muisejt]

I have a fan that has been running for about 3 years like that without problem. I took a couple years of electronics and couldn't think of any reason that it would hurt anything.

[thrawn86]

yeah it seems safe enough. here is the quote from silentpcreview.com:

Caution! There are people, who will say "You can't feed current back into your power-supply" and they are right! If you plug something between the 12V and 5V lead, you have to be absolutely sure, that no current is fed back into the PSU. Which means, that you need to have other loads on that 5V lead, which suck out more current than you feed to that lead from the 12V lead. For example: Your fan uses 100mA at 7V. You take 100mA from the 12V lead and feed them into the 5V lead. If you have a different fan (or whatsoever), that sucks out 200mA from the 5V lead and feeds them to ground, you're fine. 100mA are sucked out of the 5V lead, 100mA come from the 12V lead and 200mA go to ground. Since usual fans only use 1...3W, you are usually perfectly safe with using the 7V trick there. There a lots of devices in typical computers, which draw much more power out. Peltiers or things with power consumptions running up to dozens of watts, require careful calculation of what goes out of the 5V lead and what's going in there from the 12V lead.

[Smythe]

I can't attest to its safety, but just make sure to only hook fans to them

[Ashpool]

Why not just get a rheobus instead? Things are $20 or less today.

[Colossus]

I have been using the 7v mod for about 8 months now without an issue...

Works great!

[thrawn86]

Originally posted by Ashpool
Why not just get a rheobus instead? Things are $20 or less today.

can I just get some insulated wire and make extensions if the fan headers can't reach?

[thrawn86]

well, I finished it. I have a molex y splitter with two 7v fan tails. It works beautifully. Now if only my watercooling parts would ship, I'd be all set to go.

[thebove]

most fans will still kick over and run ok using as little as 5v. i have 3 120mm fans on a switch wired for 12v and 5v. when on 5v, the fans are very quiet, i'd say completely inaudible when compared to the rest of the system, but still move enough air to make it worth while.

so, what i'm saying is try the fans you wanna quiet down on the 5v line instead (just reverse the pins in the molex). and see how they run there. this way you'd still quiet them down, but wouldn't be feeding back into the 5v.

[thrawn86]

thats an interesting idea. however, these 80mm fans I'm using it on wouldn't really move any air at all at 5v. They move enough to make it worthwhile (barely) at 7...

My dad the electrician doesn't think 300mA of extra current would hurt anything....and I see other people doing the same, so I think im just going to try it out for awhile.

[vertices]

I've used the 7V mod for years on lots of systems and have never ever seen a problem arise form it.