ATX Power Supply Validation

Pictured above Oscium’s iMSO-104 oscilloscope and an Apple iPad Air monitor waveforms from an ASUS ATX 600W power supply. There are two traces plotted in the display: the green trace in the analog channel monitoring the -+5Vdc output, and the orange trace is a digital channel monitoring the logic waveform PS_OK which goes high once the analog waveforms have stabilized to nominal values.

Modern ATX series power supplies provide feedback signals to the computer’s motherboard. Also noteworthy, there is an start up interlock with the motherboard that allows the operating system to shut down the computer without any hardware intervention.

The following signals and waveforms were brought out to the solderless breadboard for two reasons.

The first reason signals are brought out to the test board is to trick the power supply into thinking it’s correctly installed in the computer and connected to the case power supply switch. Pin 14, PN_ON, must be pulled low and shorted to ground. The power supply stays running (which proves some basic functionality) as long as the pin is low.

The second reason to bring the signals out to the test board is really to provide better access for probing with the oscilloscope. Oscium’s iMSO-104 digital oscilloscope has one analog scope probe and four digital channels. These capabilities allow users to simultaneously monitor transient analog waveforms and logic levels. In this instance, the analog channel is monitoring the +5Vdc analog waveform and the digital flag PS_OK that goes high once the power supply recognizes the analog voltages are stable

The figure below is the pin assignment for a generic 20-pin ATX power supply. Of particular importance for this blog are pins 4, 14, and 15 (COM).

The following diagram depicts the breakout signals from the ATX power supply and the measurement nodes used with the iMSO-104 oscilloscope. Note that PS-ON must be shorted to COM for the power supply interlock to close and the startup to commence. The signal PWR_OK (in blue) transitions from low to high once the power suplly detects the analog waveforms have stabilized.

The screenshot below is from an Apple iPad Air paired with Oscium’s iMSO-104. The first generation oscilloscope accessory for iOS devices is monitoring the analog +5V waveform and PS_OK logic signal from an ATX power supply. A detailed timing analysis of the two waveforms might be performed by zooming into the measurement. For analog measurements, timing measurements are usually made from the 10% to 90% of final voltage transition time. Logic levels, on the other hand, are judged to be high or low based on industry standards for TTL, LVTTL, LVDS, etc. voltage levels.