Power_OK or Power_Good

Myth or Fact

The primary purpose of the PC power supply is to supply stable current and voltage of differing levels to the motherboard and various peripherals attached to our PC's. The ATX design guides are very specific with respect to voltage levels, acceptable variation, overload circuitry, plug design and even screw-hole placement on the PS cases themselves. As many of you are aware, the personal computers today do not function well with fluctuating voltage levels and may be damaged if operated in a less than ideal electrical environment.

One of the design specifications ensures that the system will not function if voltages are not sufficient to operate the system properly. In short, the PS is designed to complete a series of Power On Self Tests (POST) before the motherboard starts to powerup. These tests determine if all the voltages are up to design specification and stable before sending a signal to the motherboard. If the POST completes successfully, a POWER_GOOD or POWER_OK signal is sent to the processor over the POWER_GOOD line. This signal must be continuously present for the system to operate and, if withdrawn (due to a brownout, for instance), will generate a system RESET. The system will remain in a continuous RESET mode until the signal is restored. Since the processor initializes in the RESET mode, the system will not start until sensing the PWR_OK signal.

This signal is a +5V active (nominal) high, usually present within 100ms to 500ms after applying A/C to the powersupply. Active high means that as long as the PS is functioning properly (active), the signal can be measured. There may be some variation in the voltage level on this line, but ranges of +2.4V to +6.0V are generally considered to be sufficient to force the processor out of RESET.

Since the signal is generated by the PS for use by the MPU, the PWR_OK (gray) wire should not be grounded, attached to any of the other output lines or tied to a resistor. It is not required for the PS to function -- its sole purpose is to allow the motherboard to initiate the boot process and to continue to function in the absence of unstable or improper power levels.

Could this signal be of any use when converting a PC powersupply to desktop usage? Realistically, the answer is YES. A voltage on the PWR_OK line indicates that the PS has completed a successful POST and that the output voltages are stable and within design specification. If you wanted to use an LED (light emitting diode) as an indicator that the PS is on, rather than tie it to one of the +5V or +12V lines, attach the PWR_OK line to the anode (+) side of the LED and place a 220 ohm resistor on the cathode (-) leg before grounding it. The cathode leg is normally shorter on new LEDs -- if the legs have been clipped, the cathode will be on the same side as a flat spot on the LED base. The LED should be bright for normal operation -- it could possibly glow faintly if the PS has withdrawn the signal simply due to bleedover.

After having read several news groups and had some inquiries relative to this function, I thought I would add this short section to clarify the purpose and operation of the PWR_OK signal. I hope it has been of use.