Video powered case fans



  • Friend bought in a computer complaining it wouldn't start up. I decided to use my own monitor to test it.My own personal WTF resulted in me unplugging the cord from the monitor and plugging that into the computer. So the DVI cable was running from my (still powered on) computer to his computer.

     Messed around trying to figure out why it wasn't showing anything on the screen then while I was marvelling at my own WTF, I noticed something odd.

    The case fans(which had pretty blue LED's, and were connected to the PSU)  and CPU fan were still turning. Not so odd you might think, until I saw that I had also unplugged the power cable. The ghost fans stopped when I unplugged the video cable, and started again when I plugged it in.

    I'm just a humble programmer (no electrical engineer)  but this made me go WTF?!.



  • This isn't too surprising, recently (I have an EE degree but don't work in the field). Most electrical circuitry (as opposed to electronic circuitry) works both ways round unless you really go to effort to make it asymmetrical; as a simple way to demonstrate this, you can try plugging a cheap pair of headphones (with no electronics, just speakers) into your microphone port and shouting into them, which should make the computer pick up a microphone signal. In this case, the monitor needs to get power from somewhere, and even though it's probably using an independent power supply, there's going to be a small power supply provided to it from the motherboard just so it knows how to interpret the signals (given logic levels meant to indicate 0 and 1, which in practice means two different voltages, which you can draw power from). The circuitry for providing that sort of simple DC power down a wire is identical to the circuitry for receiving it, so everything in the case that uses the same voltage as the monitor does will be powered. (Things using different voltages won't be; DC voltage regulators, being electronic, aren't typically reversible (not even the simple ones), and the massively complex sort used on modern motherboards definitely aren't.)



  • @ais523 said:

    This isn't too surprising, recently (I have an EE degree but don't work in the field). Most electrical circuitry (as opposed to electronic circuitry) works both ways round unless you really go to effort to make it asymmetrical; as a simple way to demonstrate this, you can try plugging a cheap pair of headphones (with no electronics, just speakers) into your microphone port and shouting into them, which should make the computer pick up a microphone signal. In this case, the monitor needs to get power from somewhere, and even though it's probably using an independent power supply, there's going to be a small power supply provided to it from the motherboard just so it knows how to interpret the signals (given logic levels meant to indicate 0 and 1, which in practice means two different voltages, which you can draw power from). The circuitry for providing that sort of simple DC power down a wire is identical to the circuitry for receiving it, so everything in the case that uses the same voltage as the monitor does will be powered. (Things using different voltages won't be; DC voltage regulators, being electronic, aren't typically reversible (not even the simple ones), and the massively complex sort used on modern motherboards definitely aren't.)

    Nah. Definitely a ghost.



  • Pin 14 of a DVI cable supplies +5v power to a monitor for use when in standby. If you connecta DVI cable from one PC to another then you link the +5v busses of each computer. Any devices powered from this 5v bus would try and use power via the DVI cable.

    Actually I am pretty certain that the way modern PSUs work not all the 5v supplies are directly linked, so this is not guaranteed to happen depending on which output is used for each device - but it does not surprise me at all.



  • Not really that surprising - there's some current leakage through the cable, and the 5V rail is often shared. I've seen USB hub power the chipset fan myself.



  • @ender said:

    Not really that surprising - there's some current leakage through the cable, and the 5V rail is often shared. I've seen USB hub power the chipset fan myself.
    I wouldn't exactly call it "leakage" the OP has connected a cable that is used to provide a 5v power supply to one device to the 5v rail of another device. He may as well have clipped a bench PSU to a 5v test-point.



  • @GettinSadda said:

    Pin 14 of a DVI cable supplies +5v power to a monitor for use when in standby.
     

    to a monitor (from a GPU card), I can understand. But from a monitor to a GPU in such a way that it powers up case and CPU fans?

    I get the leakage story... but if it'd happened to me I'd have been surprised also. Not to mentioned concerned that I may have drawn power off a cable I shouldn't have.



  • It wasn't powering the motherboard from a monitor; it was powering the motherboard from a monitor cable connected to the video output of a different motherboard.

    I agree that you wouldn't want to do that in practice, though; tolerances are typically large enough that the motherboard's 5V power supply could supply all the 5V needs of two motherboards, but it's not guaranteed, and the power supply might well end up overloaded.



  • @ais523 said:

    It wasn't powering the motherboard from a monitor; it was powering the motherboard from a monitor cable connected to the video output of a different motherboard.
     

    Which is what the original post said.

    Which I completely missed.

    TRWTF is me.

    Yet again.

    LART?

    ta.



  • @GettinSadda said:

    Pin 14 of a DVI cable supplies +5v power to a monitor for use when in standby. If you connecta DVI cable from one PC to another then you link the +5v busses of each computer. Any devices powered from this 5v bus would try and use power via the DVI cable.

    Only one problem: fans are almost always powered off the +12V line.



  • @Carnildo said:

    Only one problem: fans are almost always powered off the +12V line.
    Either that, or a truckload of hot dogs and beer.



  • @Carnildo said:

    @GettinSadda said:
    Pin 14 of a DVI cable supplies +5v power to a monitor for use when in standby. If you connecta DVI cable from one PC to another then you link the +5v busses of each computer. Any devices powered from this 5v bus would try and use power via the DVI cable.

    Only one problem: fans are almost always powered off the +12V line.
     

     So since you are always correct, the reasonable assumption is that this is a real ghost




  • @Carnildo said:

    Only one problem: fans are almost always powered off the +12V line
    The large ones are. Small chipset fan and fans on cheaper graphic cards are often 5V.


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