by Karlsweldt » Wed Dec 03, 2014 11:58 am
If an ATX PSU, there is a 'standby' +5 volts present at all times when mains power feeds the PSU, and its power switch (if present) is "on". That is a violet wire at the back of the main ATX plug.
There is a fuse inside the PSU case, for protection. But hazardous voltages are present inside, so give extreme caution on not touching any exposed components inside! Easy to test the fuse with a neon socket tester or volt meter to case ground. If blown, it is for a reason. They only blow as a last resort. May be time for a new PSU.
Switching a PSU to 115 volt input if on 230 volt would definitely pop a fuse.. or fry the main control transistor that creates the chopped DC for the core transformer! If on a 115 volt feed and switch is set to 230 volts, insufficient circuit voltage to become active.
One other main reason for 'no-start' conditions is the BIOS battery being weak. It is a redundant backup for when there is no PSU +5 volt 'standby' feed when shut down. The BIOS battery keeps the real time clock proper, as well as retaining memory settings in the CMOS feature. If the real time clock is not functioning, bus and other clock circuits may not be active. Have seen that scenario too many times.
Any type of switching PSU needs a minimal load on the +3.3 or +5 volts, and the +12 volts to become operational. A dual-filament headlight bulb would be a safe load of about 3 amps each. Normally a 3 ohm 20 watt resistor is used for the +3.3 or +5 volt loading, and a 6 ohm 20 watt resistor for the +12 volt loading. But they get very hot very quickly!!
There is one critical tiny component on the PSU board. About 1/4" square with 4 leads. That is a photo-isolator diode, which interacts between the output and input control circuits. If bad, no activity.
F@H.. to solve mankind's maladies.. in our lifetimes!