how to pick a fet

[cashkennedy]

I'm making a small board to sense 5v from a computer and turn on a xbox 360 power supply if the 5v is present.

I need a transistor / FET that can be controlled by a gate voltage of 5v or less, and can handle very tiny currents.

The 360 power brick has a 5vsb and expects 3.1V on the power_on and uses 75mA in the detection of power_on I assume id want to use a roughly 2000 ohm capacitor to drop the 5vsb down to 3.1v, assuming the fet had no resistance / current drop.

Anyways how should I go about picking a transistor / fet for this?
If I go to mouser and look at mosfet's I assume I need n-channel, gate-source threshold voltage around 1-1.5v? (since source will be at 3.1v after the 2000ohm resistor) What else should I be worried about?

[momaka]

Re: how to pick a fet

You don't need *any* of that ^.

Xbox 360 power supply can be turned on directly by inserting 3 to 5V on its PS_ON pin.

So, simply connect a 100 Ohm to 1 KOhm resistor (any value in that range will work) between Xbox 360 PS_ON signal and 5V rail of the computer you want to make the Xbox PSU turn on. And that's it!

Now, if you are trying to use an Xbox 360 motherboard to turn ON a PC ATX power supply, that is different. Let me know, I have a schematic and post here on the forums for that as well.

Can you also please tell us what you are trying to do here?

[cashkennedy]

Re: how to pick a fet

You are right. Just need to either run 3.3v or 5v + resistor from the power supply driving the rest of the system to the ps_on pin on the 360 brick.

I am building a itx build in a 360 case, using a pico 160w power supply for the motherboard and a xbox 360 power supply for the graphics card pcie power connector.

Should I be worried about using 2 different power supplies to drive parts of a computer?

[momaka]

Re: how to pick a fet

I am building a itx build in a 360 case, using a pico 160w power supply for the motherboard and a xbox 360 power supply for the graphics card pcie power connector.

Should I be worried about using 2 different power supplies to drive parts of a computer?

In that particular setup you have, I think it should be fine. *Most* graphics cards keep the 12V rail from the PCI-E motherboard connector separate from the PCI-E power connector. Same with the 4-pin 12V CPU connector on the motherboard and the ATX connector - they are almost always separated (I say almost always, just in case there is some crazy motherboard manufacturer out there that didn't that I don't know about).