ATX PSU turns itself off when uploading sketch to Arduino

Question

I'm trying to power a stepper motor with the 5V rail of an ATX PSU and control it using arduino. To keep the PSU running, I have shorted the PS_ON pin to ground. Also, I have connected PSU ground to arduino ground. The arduino is powered over USB.

The problem I'm having is that the PSU turns off automatically when I upload a new sketch to arduino. I can reproduce the problem with these steps:

1. Turn on power (PSU is running fine)
2. Set one motor pin (e.g. digital pin 9) to HIGH
3. Upload a new sketch to arduino (doesn't matter which one, e.g. the Blink example)

-> PSU turns itself off.

Why does this this happen and what can I do to prevent it? The PSU seems to perform well otherwise.

^{simulate this circuit – Schematic created using CircuitLab}

Edit: I just found these alternative steps to reproduce:

1. Power on
2. Set one motor pin to HIGH
3. Disconnect 5V rail of PSU from motor driver

-> PSU turns off

Further tests:

1. Disconnect motor from driver
2. Power on
3. Set one motor pin to HIGH
4. Upload new sketch

-> PSU stays on!

1. Connect motor to driver
2. Power on
3. Set one motor pin to HIGH
4. Disconnect motor from driver

-> PSU turns off

It seems to me that the PSU doesn't like losing load on 5V after it has seen load once.

Answer 1

As multiple comments have said, most ATX supplies require a minimum load on the 5V line for proper regulation, many tutorials use ~10 Ohm resistor or smaller on the 5V rail.

This is due to the type of SMPS topology, typically an LLC resonant converter, that requires a minimum load for tuning of the resonant circuit within a given frequency range.

Solution: Try putting a constant load on the 5V line, and your issue may go away.