Power Supply Voltage Tolerances

Proper voltage ranges for ATX power supply voltage rails

Tim Fisher has more than 30 years' of professional technology experience. He's been writing about tech for more than two decades and serves as the SVP and General Manager of Lifewire.

The power supply in a PC supplies various voltages to internal devices in a computer through power connectors. These voltages don't have to be exact, but they can only vary up or down by a certain amount, called a tolerance.

If a power supply is providing the parts of a computer with a particular voltage outside this tolerance, the devices being powered may not work properly—or at all.

Below is a table listing the tolerances for each power supply voltage rail according to Version 2.2 of the ATX Specification (PDF).

To help when testing a power supply, we've also calculated the minimum and maximum voltages using the tolerances listed. You can reference our ATX Power Supply Pinout Tables list for details on which power connector pins supply which voltage.

Power Good Delay is the amount of time it takes a power supply to start up completely and begin delivering the proper voltages to the connected devices. According to the Desktop Platform Form Factors Power Supply Guide [PDF], Power Good Delay (called PWR_OK delay in that document) should be 100–500 ms.

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