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Reverse Current?

Discussion in 'Electronic Basics' started by The little lost angel, Jan 20, 2004.

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  1. One of the "neat" tricks being used on the PC PSU is changing the
    ground connection of fans from gnd to +5V so that the result is +7
    across the fan and makes the fan slower/quieter. Some folks say it's
    dangerous, others say it's fine, some say as long as the net draw on
    the +5V is positive, it's ok.

    Can anybody enlighten me about whether is it really safe or are there
    conditions attached?

    Thanks again!


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  2. There's a website that tells how to do this. Back in the '90s I had
    problems with some PCs not turning on (turn the switch on, and nothing
    happens) and I found that when I unplugged the serial cable, they
    would turn on. What happened was the voltage, something like 10 to
    12V, was feeding back into the PC and causing it to be receiving power
    when it was still off. When it was turned on, that voltage at turnon
    was enough to prevent the PC from getting a power up signal from the
    power supply. So what I'm saying is that it's not a good idea to feed
    the 12VDC power back into the 5V power. So I don't recommend doing
    this. I don't believe it's totally safe, either.

    One other important point: slowing the fans down reduces the cooling,
    and that's _not_ good for the PC. Your PC already has more than
    enough heat inside, you don't need to make it worse. People just do
    _not_ understand that the hotter a piece of equipment gets, the
    shorter will be its lifetime. If you want the fans to be quieter,
    then buy some quiet fans that will do the job properly. Just my
    humble opinion. Read this. http://www.directron.com/quietpc.html

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  3. I wouldn't do it. When you put two ideal voltage sources in series, the
    voltages are added or subtracted depending on their polarities. The above
    trick subtracts 5V from the +12V rail to give 7V for the fan. By doing this,
    the +12V rail is sourcing current which the +5V rail must then *sink* as it
    is now the fan's GND point. Now, the +5V rail will already have a lot of
    heavy loads attached (motherboard, HD, etc.), which will do the necessary
    sinking, so that the "net draw on the +5V is positive". In this case it
    should be fine, but if the fan is the primary/only load, then the current
    sinking issue becomes important.

    I don't know much about computer PSUs and their specs, I guess the trick
    must work since many people are doing it. But I don't think it's a good
    thing to do. Think about what will happen if the fan gets damaged and shorts
    itself. The 12V and 5V rails will be shorted together. A good PSU will
    probably sense this and power down, but still it's a dangerous thing, esp.
    with cheap PSUs.

    A really easy, cheap and very safe method to drop the fan voltage is to
    simply use a few diodes in series with the fan connection. Each diode will
    drop about 0.7V, so you'll need about 7 diodes in series (anode of first
    diode to PSU's +12V, cathode of last diode to fan's + terminal). Or use a
    single 4.7V Zener, but this time the cathode goes to +12V and the anode to
    the fan. If using a Zener, make sure it can take the current drawn by the
    fan. In this way, the PSU rails are not coupled in any way. What's more, you
    can connect a switch in parallel with the diode(s) that effectively removes
    them from the circuit by shorting them, thus getting the full fan speed
    during a hot summer day when max. cooling is desired. You can improve this
    even further by using a temperature-controlled switch, so the fan speed is
    controlled automatically depending on the case temperature.

    You can buy continuously variable temperature-controlled fans if you want to
    save yourself the trouble of doing the above, but you sound like someone who
    likes to build thinks by him/herself, so there you go.

    cheers,
    Costas
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