Power Supplies: An Overview
In late 1995, Intel
replaced the "AT" standard
and it's two separate "P8" and "P9" connectors with a single piece main connector from the power supply to the motherboard. Enter
PSU is "LIVE", but not "On".
ATX power supply is always "live", even when it's "off", when it's
plugged in and it's rear switch (if applicable) is switched to the "on" position. .
+5V standby is always coursing through your motherboard whether
your PC is on or not. This
is how the CMOS keeps it's settings with or without it's CMOS battery
and wake on LAN, wake
on ring and keyboard power on is accomplished.
power supply is instructed to energize the other power leads throughout
the system when the "Power On" lead (the green wire on the main
connector) is grounded. Without
grounding the green wire on the power supply, you will not be able
to test ANY of the power leads on the power supply except
for the +5V standby.
test the power supply, you can do this: Unplug the PSU from the wall and unplug the power connector
from the motherboard. Connect the green and black wires on the
connector together with a paper clip or a piece of wire. It's ok... this is a signal lead and not a live lead. This will tell the PSU to energize once you plug it back in. Now, using a multimeter, the 12V should
read 12V, the 5V should read 5V and the 3.3V should read 3.3V
(within +/- 5% per Intel specifications.)