Questions about the 7V trick

Shaman · Post by **Shaman** » Thu Jan 17, 2008 11:22 am

In the Get 12V, 7V or 5V for your Fans article there's a scary warning about feeding 5V back to the PSU.

Apparently there needs to be other loads on the 5V lead for it to be safe. Does this mean I need to have, for example, an optical drive in the same PSU cable that feeds the 7V fan? Or can the hardware that sucks 5 volts be on a different PSU cable?

Just out of curiosity, what happens when you feed 5V back to the PSU when nothing else is using it? Does it shorten the life of the PSU or does it explode?

Strid · Post by **Strid** » Thu Jan 17, 2008 12:21 pm

I haven't heard about anyone who actually had problems with the 7 V trick not ever, on any forum I've see. I myself have been doing it for many years...

But I'm not really sure what'd happen, so if anyone is electrically inclined, please enlighten me as well as Shaman!

Shaman · Post by **Shaman** » Thu Jan 17, 2008 12:53 pm

I also used the 7V trick many years ago on a PSU that eventually went bad, so I was wondering if the 7V trick had anything to do with it lol.

subsonik · Post by **subsonik** » Thu Jan 17, 2008 12:55 pm

Most (if not all) consumer PSU's have just one 5V line. This means that all 5V cables are tied together internally. As a result, the hardware sucking current on the 5V line can indeed be on another line.

What would happen if the load on the 5V were too low? The PSU can only source current, it can't sink current. If you try to push current into it, the voltage will rise. At a certain point (usually around 5.5 ... 6V), the overvoltage protection will trip and shutdown the PSU. No harm done.

However, if you were to have a short between the 12V and the 5V line (lousy cabling etc), the overvoltage protection won't react fast enough. There is a reasonable chance you'd blow up many PC components.

cpemma · Post by **cpemma** » Thu Jan 17, 2008 4:58 pm

subsonik wrote:However, if you were to have a short between the 12V and the 5V line (lousy cabling etc), the overvoltage protection won't react fast enough...

That's the first logical reason I've seen for taking care with the 7V trick. I've used it without a problem for many years, but now prefer to solder an approximately 5V 1.3W zener into the positive fan lead - this method does not adversely affect the fan speed signal, fan ground is still 0V.

Shaman · Post by **Shaman** » Thu Jan 17, 2008 9:56 pm

Ok thanks. If the only risk for the PSU is having a short between the 12V and 5V lines then this trick is perfectly fine, because it would take some seriously lousy cabling to make that happen, like it being wired by someone who is colorblind.

cpemma · Post by **cpemma** » Fri Jan 18, 2008 7:57 am

Shaman wrote:If the only risk for the PSU is having a short between the 12V and 5V lines then this trick is perfectly fine, because it would take some seriously lousy cabling to make that happen, like it being wired by someone who is colorblind.

I have heard of people plugging 'converted' molexes into one of their drives, to their sorrow. So add 'amnesiac' to 'colour-blind'.

jhhoffma · Post by **jhhoffma** » Sun Jan 20, 2008 5:56 pm

cpemma wrote:I have heard of people plugging 'converted' molexes into one of their drives, to their sorrow. So add 'amnesiac' to 'colour-blind'.

Which is why I always label "converted" molexes with a permanent marker (on white connectors) or tape (for the black ones). That way you don't have to tell color or remember anything.

Eagle156 · Post by **Eagle156** » Sun Jan 20, 2008 6:12 pm

You only mod the fan part of the molex though, right? So how could you plug a drive into it?