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Thread: Need clairification on wiring on/off switch

  1. #1
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    Need clairification on wiring on/off switch

    Ive done a ton of searching on this subject but I still cant find the answer Im looking for. Here's what I want to do....
    I want to put a master switch for my computer up front near my dash. As long as the switch was on, my computer would kick on as soon as the ignition key was turned to the "on" position. And would shut down when the key was shut off. If the master switch was off, then the computer would not come on when I started the truck, until the switch was turned on.
    Here were Im confussed...
    I took apart an old PII computer I had sitting around to see how the switch and power supply worked. The power switch on the PII has four wires running to it(I thought it would only have two), all coming from the power supply. I was going to use a simple on/off switch. But I read that you need a momentary on/off switch(like a doorbell switch). The switch from the PII is an on/off switch so thats why Im confussed. Do the new motherboards only need a momentary power signial??? I also thought that at the power switch the power would be DC, not AC, but its not. Please excuse my ignorance but I was under the impression that PC were run on DC power after the power supply, but I measure 120V AC at the switch???? Since I thought it was all DC power after the power supply, I was going to run a relay setup to basically replace the power switch for the computer so I can turn it on when the car starts, and shut it down when its off, all automatically. I hope this makes sense...

    So if you guys could give me some clairification on these things I mentioned it would help out alot. Thanks and appreciate any insite ... Fern

  2. #2
    FLA gospeed.racer's Avatar
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    Newer motherboards (ATX) will use a momentary switch. Simple single pole single throw momentary. So are you using the mobo out of this older case? If so you may have a little work ahead of you....especially when it comes to a dc power supply for an older mobo (AT)
    2005 Ford Focus ST

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    Thanks for the reply, I bought a Mercury KOB 740 FDMx motherboard, so this would most likely use a momentary switch setup. If that was the case, then to turn the PC on, you'd press the switch, which would close the contacts, right??

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    FLA gospeed.racer's Avatar
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    Exactly...if I had a $.10 for every time I "hotwired" a pc with a screwdriver to the FP connector...i'd have about a buck by now

    Just look in the manual, youll see the 2 pins, short them to turn on, and if it is set up correctly, short them for a soft shutdown.
    2005 Ford Focus ST

  5. #5
    Constant Bitrate P4_2.66GHz's Avatar
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    If you want just a master switch, you can simply route your computer's power through a switch. Use one that is more than 10A rated. The soft power up/ shut down can be done using a momentary switch button, separate controller, or Opus' built-in one.

    In this setup, if the switch is off, the computer will draw no power at all.

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    Is the switch setup on the new mobo just a two wire connection thats joined and broken(by the switch). So for the second part of my question, Is everything after the power supply DC volts??? Fern

  7. #7
    Constant Bitrate P4_2.66GHz's Avatar
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    Quote Originally Posted by syclone477
    Is the switch setup on the new mobo just a two wire connection thats joined and broken(by the switch). So for the second part of my question, Is everything after the power supply DC volts??? Fern
    Yes and Yes.

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    Perfect, it all makes sense now. What really screwed me up was that my old PII switch and power supply was a differant setup. Thanks alot. So I guess using the power supply from the old PII is out of the question then, have to buy one thats compatable with the mobo??

  9. #9
    Constant Bitrate P4_2.66GHz's Avatar
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    Quote Originally Posted by syclone477
    Perfect, it all makes sense now. What really screwed me up was that my old PII switch and power supply was a differant setup. Thanks alot. So I guess using the power supply from the old PII is out of the question then, have to buy one thats compatable with the mobo??
    If your old PII was AT style, then yes. Even the connectors are different.
    ATX switch started happening around PII time.

  10. #10
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    It does have differant mobo connection, so I will have to but a new supply. Thanks again for your help. Fern

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