ATX Power Supply Turn-on

Pictured above Oscium’s iMSO-104 is used in troubleshooting a faulty ATX power supply circuit in a desktop PC. The ATX motherboard’s standby power mode green LED illuminates when the power supply is switched on, but why won’t the computer turn on, with all the hard drives, CD ROMs, and spinning fans, when the ON button is pressed. Keep reading to find out why and how the problem is resolved.

Dead PC – Swapping Power Supplies Didn’t Fix It

Desktop computers have smart power supply systems. They don’t operate like a simple light switch on the house wall, for example. Computers can shut themselves off, enter a hibernation mode (for power saving or unnecessary frustration, depending on when this happens), and rely on circuitry in the motherboard for power supply control.
When this computer suddenly didn’t power up one morning, it was unclear if the problem was with the power supply itself, a loose/floppy connection, or a move across the country damaged the motherboard itself. Simply swapping one power supply for another did not fix the problem.

Digging In – Making Voltage Measurements

First, thank you Wikipedia and its contributors for placing every bit of useful information on the world wide web (WWW) that was once only contained in user manuals and instruction manuals. This pin diagram for an ATX power supply is courtesy of Wikipedia and can be found at http://en.wikipedia.org/wiki/ATX.

Fortunately, this free online pin diagram matches with the wiring of the ATX motherboard and power supply that is being worked on. Wikipedia has useful information but it isn’t always directly applicable to the immediate situation. This is an especially fortunate set of circumstances.
Pictured below is Oscium’s iMSO-104 digital oscilloscope paired with an Apple iPOD Touch, 2nd Generation (old, but useful) measuring the output of the suspect ATX power supply on the PS_ON pin.

The power supply wants to work. This is good news. When the power supply is switched on, the PS_ON output goes to the nominal +5V output. If there had been a problem with the supply itself, the PS_ON pin would not be a logic high at initial power up. The image below is a screenshot captured with the iPOD Touch and iMSO-104 measuring the PS_ON pin.

What’s Next?

The original ATX power supply appears to work. Simply swapping out one power supply for another didn’t fix the problem of the computer not powering up when the ON button is pressed. Following the binary search approach, if it’s not the power supply then the problem must be with the switch or with the motherboard itself. On the surface, one problem sounds like a cheap quick fix while the other sounds expensive and time consuming.
Pictured below is the ASUS ATX motherboard, purchased right after they embedded the WiFi cards in the motherboard. Yeah, it’s seen a couple of years.

Pictured below is a closer look at the motherboard, video card, CPU cooler, and MOST importantly, the illuminated green LED that indicates the power supply is in standby mode.

Lastly, here’s a photo of the ATX power supply connector on the motherboard. If all else fails it will be necessary to tap into some of these connectorized contacts to monitor voltages during the power up sequence. It’s a messy practice and one best avoided, but when other measures fail, it’s a proven way to monitor analog voltages and digital outputs.