Computer only boots when my phone is plugged in?

Question

I'm aware it's common for computers to have booting trouble because of USB devices. I have the opposite problem. My computer will not turn on unless my phone is plugged in.

If I try to turn my computer (desktop, self built, a bit old) on without the phone (Galaxy S5 with a fast-charging USB 3.0 cord) it does nothing, not so much as a whimper. If the phone is plugged in to the motherboard (haven't tested the case USB ports), it works just fine. In fact, if I press the power button and THEN plug my phone in, the computer springs to life without my having to press the power button again.

I've had a few power supplies die before, but I don't think that's the issue this time. I have a lot of stuff plugged into one outlet (remedying this soon) so maybe it's electrical. I've tried unplugging a bunch of stuff to get it to start again, which didn't have any effect.

Is my phone and it's cord acting like a car battery and pair of jumper cables, bringing to life a machine that has enough power to run but not enough to start? Is it holding some sort of charge that says to boot up that it only responds to as soon as it can? Or could it be a simpler, less concerning issue? Like the presence of the USB fast-charge device messing up some baseline power value somewhere because I turned on my current power supply for the first time with my phone plugged in and it thinks that's what normal looks like?

Thanks for any and all help.

Answer 1

Did this started happening after your replaced the Power supply? Boot to the BIOS and check the power settings. What is the make and model of your motherboard?

The nuke way to try would be to reset the BIOS settings by removing the CMOS battery while the workstation power cable is unplugged (The MOBO vendor should have instructions on how to reset it).

Answer 2

From the description, it sounds like you have an issue with +5VSB (Stand-By) rail in your new Power Supply. It looks like this primary power rail can't work without minimal load. Normally this rail must be stable with zero load, but it looks like you hit an exception. PC mainboards usually have the USB VBUS fed directly from the +5VSB, such that your phone provides this minimal load. You need to check if the +5VSB is there when you attach the PC cord and flip the PS power switch (if present) on. Without +5VSB the computer won't start.