On/off switch


Old 09-05-12, 12:50 PM
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On/off switch

Hey guys,
I need some help making an automatic On/Off switch << my appology if it has a different name; but basically I want to make a switch that turns on when it gets power and turns off when it loses power. This might sound confusing but I'll be using it in a computer I'm building and basically I need something on a circuit that activates another circuit when it's active and diactivate that circuit when it's inactive, but it has to be automatic, i don't want to use a manual switch.
note: each circuit will have it's own power supply.
I'll be using it for 12Volts not more that 500Watt and it has to be portable, I'll be putting it in my desktop case. I'm willing to get the tools of ebay or if the actual automatic switch exists I'll purchase it off ebay but I need to know how it works and will probably need help installing it.
I would really I preciate if you'd use simple terms as I'm a biginner at this.
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Old 09-05-12, 02:15 PM
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Sounds like you need a N/O (normally open) SPST relay with a 12 volt coil. Hook the relay coil to one of the fan terminals on your mother board. Hook the power to your second load across the relay.
Old 09-05-12, 02:28 PM
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Hi and welcome to the forum. Well if you are using this inside of a computer case then I would not bother with a switch as your motherboard will handle all of the power issues. On all of my desktops everything is hooked up either to the motherboard or directly to the power supply and and I let those two things handle all of the power issues to my fans.
Is there a particular reason you want the extra switch like maybe something broken on the board or something as I have never seen anyone add a switch in. Only switch I have ever had problems with is the power switch in a used computer I bought many years ago are you referring to the switch on the front of the case?
Old 09-05-12, 02:36 PM
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Your question is a bit vague to really be able to help.

You said you will use it on 12 volts but are you switching twelve volts, operating the coil at 12 volts or both.
And, what part does the computer play in this, are you energizing the relay with the computer, switching power to the computer or what???

Tell us exactly what you are trying to accomplish with specific details.
Old 09-19-12, 12:20 PM
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Curious, are you trying to build a carputer? This setup sounds familiar...

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