Questions about the 7V trick
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Questions about the 7V trick
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?
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?
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.
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.
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.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...
I have heard of people plugging 'converted' molexes into one of their drives, to their sorrow. So add 'amnesiac' to 'colour-blind'.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.
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.cpemma wrote:I have heard of people plugging 'converted' molexes into one of their drives, to their sorrow. So add 'amnesiac' to 'colour-blind'.