[SGVHAK] Arduino power question

[Claude Felizardo via HAK]

Anyone have any experience with powering an Arduino (Uno or Mega) with 
the 5V standby from an ATX Power supply?

 From what I've read on various websites the Arduino boards are designed 
to be powered using either the barrel connector which is connected to 
the onboard voltage regulator which outputs 5v, or the USB connector 
bypassing the voltage regulator. The output of the voltage regulator is 
available via the 5v pin on the Arduino board and the power from the 
barrel connector is available via the Vin pin.  There's some circuitry 
to disable the power from the USB if Vin is high enough as well as a 
diode to prevent reverse current flow.

You can also supply power directly via the Vin pin but it has to be 
greater than 5v.  You can also bypass the regulator and supply power via 
the 5v pin but then you have to be careful not to plug anything else 
into the barrel connector and there is a potential to damage a computer 
connected via the USB cable if the power to the 5v pin goes above 5v.

I recently ordered the Power Drive Shield kit from SparkFun which is 
designed to power an Arduino using  an ATX power supply and provide 6 
PWM outputs using MOSFETs which I will use to drive a bunch of LED strings.

https://www.sparkfun.com/products/10618

Looking at the schematic on their website and from what I can see on the 
board, Vin is connected to the 12v rail with a diode to offer some 
protection. To turn on the power supply, there is a slide switch that 
pulls the PW_ON line on the ATX connector to ground.

What I would like to do is rewire the shield to power the Arduino using 
the ATX 5V standby then allow the Arduino to turn on the ATX power 
supply when I want to turn on the LEDS.  I've experimented with this to 
a limited extent but after reading about the different ways to fry an 
Arduino I'm wondering about the best way to do this.

I thought about plugging in a dummy plug into the barrel connector on 
the Arduino board to prevent accidentally plugging in another power 
supply but what about connecting it to the 12v rail?  Would it be okay 
to connect the 5V standby from the ATX power supply in to 5V on the 
Arduino board and the 12V into Vin at the same time? I'm assuming no.

Another idea was to use a DC-DC buck booster board powered by the 5V 
standby and bump it up to at least 7 V and then send that to Vin but 
that just seems like a waste.

Oh, the reason for the standby power is this is for an exhibit that will 
be powered possibly 24x7 and I don't necessarily want the ATX fan 
running all of the time.  Also during development, I'd prefer to not 
have to disconnect the USB cable every time I want to switch to ATX power.

Any comments or thoughts?

Claude