The power supply in a PC provides different voltages to internal devices in a computer via power connectors. These voltages do not have to be exact, but they can only vary up or down by a certain amount, called a tolerance.
Power supplies explained
If a power supply supplies a computer's components with a voltage that is outside of this tolerance, the devices it powers may not work properly or may not work at all.
Below you will find a table with the tolerances for each supply voltage rail according to version 2.2 of the ATX specification (PDF).
To aid in testing a power supply, I have also calculated the minimum and maximum voltages using the listed tolerances. You can consult the ATX Power Supply Pinout Tables for details on which power connector pins provide which voltage.
