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Switching 2 mainboards with one momentairy switch

  1. Mar 8, 2017 #1
    hey guys,

    to be hounoust i'm kinda ashamed to be asking this, but a simple thing is turning out far less simple then anticipated and i don't seem to be able to find the solving clue.

    i am in the situation that i want to switch on and off two mainboards (one pc mainboard and one soft-switch board for an hypex amplifier) with one momentairy switch.

    normal situation, both board have a separate switch, upon short suppresion the board will turn on, and a second or long suppresion will turn it off.

    as i would like to have both pieces of equipment to be turned on simultaneously, i intended to connect both to the momentairy switch in parrallel manner. in principle this seems to be very simple, but.. it turned out not to that case. when i added power to the soft-switch and mainboard the following situation occured:

    1) the soft-switch board for the amp turned on.. and didn't turn off either, not even after 5 sec (usually it should turn off when in a normal situation a momentairy switch would be suppressed for more then 5 secs.

    2) the pc mainboard didn't turn on at all.

    to me the odd thing here is that neither the mainboard nor the soft-switch board give polarity on the connectors to which the switch is connected. Normally i'd expect that there would be a small current being 'shorted' upon suppressing the switch, but this would mean polarity ofcourse. If this would be true and i would mix them up one board would turn on the other board and visa-versa.

    but as said, only one board turned on and the other didn't.

    so basically my main / first question is: how does this switchery stuff actually work if it is not polar..
    and if it is polar... what would could have been the 'cause for the odd behavior described above
    and where do i go wrong with the above thought train and second (but i might be able to figure that out myself after i find the correct answer on the first question).

    kind regards

  2. jcsd
  3. Mar 8, 2017 #2


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    Gold Member

    Soft power buttons like that work as follows:
    Short two pins together and some voltage level on one or less likely both pins changes.

    Many times one pin is system common and the second pin is some positive voltage when the button is not pressed. Pressing the button will cause the both pins to be at system common. However you may also make something like that where one pin is some positive and pressing the button makes both pins that voltage. More creative designers may do something completely different.

    The positive voltage may or may not be the same for both pieces of equipment. One piece of equipment's voltage may in fact be low enough to be sensed as low on the other.

    The exciting situation occurs if one device is powered and the other is not. Current may flow from the powered one into the un-powered one. This could damage the un-powered one. Most exciting is where you get into a situation called latchup. Current flows in the un-powered equipment in an unwanted way. On powerup that equipment shorts out internally and you get a nice cloud of smoke (or possibly flames) and it stops functioning.

    There a number of other ways this can go terribly wrong.

    If you have to have one button find one that has two switches internally. This is called double pole.

  4. Mar 9, 2017 #3
    Well Bob, thank you very much! that indeed explains a lot!

    i understood it needed to be something like that.. just couldn't put my finger on it...

    fortunately i have varified that both pieces still work, so had no fireworks or smoke signals ruining the build.
    i'll go and have a look for a double pole momentairy switch (with angle eye ofcourse)

    and indeed that will solve any problems in a simple matter.

    Again, thank you for your response

    Kind greatings

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