Executive summary: The circuit only connects to low voltage points on the motherboard and presents no hazard to the installer.
This circuit connects in parallel with the low voltage front panel switch on your computer, not the mains power switch which is sometimes integral to the power supply in the back of the computer.
Nothing on a computer motherboard has 120V on it. The power supply in a PC takes 120 (or 240) and typically converts it to +5, +12, +5 standby, 3.3V , -5V and -12V. None of these are especially dangerous. The voltage on the power switch is low voltage and high impedance as well. The motherboard uses the +5V standby to provide a logical high voltage (i.e. 5 volts) to one side of the power switch header on the motherboard through a pull up resistor or transistor current source. The other header pin is ground. When the power switch is pressed, the high side of the line is shorted and a sensing circuit turns on the power supply through the supply's PS_ON control line. Thus, the highest voltage on the power switch pins on the motherboard is +5 volts and even that is through a limiting resistor or high impedance current source. You cannot hurt yourself touching +5 volts, so you do not have to worry about plugging in the header in my circuit in parallel with the power switch . The +12 volt fan connection is at a higher voltage and lower impedance but still is not very dangerous. It is current flow that harms you but you have to get the voltage high enough to force that current through the human body, 12V is too low to do that. (But if you ever touched your tongue to a 9V battery you got a tingle because the wetness and salinity of your saliva allows some current to flow and your tongue is very sensitive.)
See [Link: hyperphysics.phy-astr.gsu.edu]
Nevertheless, I always recommend unplugging the power cord when working inside of any mains connected device (after watching my dad melt a notch in a stainless steel butter knife trying to get a piece of bread out of the toaster when I was ten years old).
Last edited by dabrams on June 24, 2009 23:01.