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Lei Mao

Machine Learning, Artificial Intelligence. On the Move.

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Introduction

I assembled a PC for gaming and deep learning earlier this year, so far I would say it has been working well in general. However, there is a problem that has been bothering me. Sometimes, after turning off my PC for several hours, I could not boot my PC by pressing the power button on the PC case. The lights for the CMOS button and Power button on the motherboard at the back of the PC case are on, and I could press the power button on the motherboard to boot the PC. After the PC started successfully, I turned off PC, immediately pressed the power button on the PC case again, the PC would boot normally. It did not happen all the time, but I would say it happened frequently, which is annoying.


In this blog post, I am going to describe what are the possible causes for this phenomenon, how I did experiment to find out where the most likely problem was, and how I fixed the problem.

Possible Causes

Most of the people suggested that it might be due to a mechanical problem on the power button. It is not convincing to me because I believe the mechanical problem would persists whenever I turned off the PC. In this scenario, pressing the power button on the PC case should never work immediately after I turned off PC. Some other people suggested that it might be a bad wire problem, or even a motherboard problem (which would be unfortunate).


One of my friends who studied electric engineering, however, considered it as a capacitor problem on the power button PCB board on the PC case. He told me that it is likely that after the PC was started using the power button on the motherboard, the “bad” capacitors on the power button PCB board got recharged somehow. That was why I could boot using the power button on the PC case normally immediately after I turned off the PC. After several hours, the electric stored in the capacitor was somehow gone, so the power button on the PC case would not be useful. I found this suggestion at least reasonable because it explained the inconsistency in the phenomenon.

Experiments

I did a small experiment to loosely confirm the capacitor hypothesis, although I never formally studied electric engineering. In one morning, I found I could not boot the PC using the power button on the PC case, I started my experiment.


I took off the power switch cable connected to the power button on the PC case from the motherboard. Then, instead of booting the computer using the power button on the motherboard, I tried to use a screwdriver to boot the PC by contacting the +/- pins the power switch cable used to connect. The PC started normally, which is a stronger evidence of intact mother board than booting the computer using the power button on the motherboard.


Once the PC got started normally, I turned it off, connected the power switch cable back to the motherboard. Then I tried to boot the PC by pressing the power button on the PC case. No response at all. Because in the last boot, the power button PCB has no contact to the motherboard, I think it loosely confirmed the capacitor hypothesis.

Fixes

Fixing the problem is trivial. I requested the replacement for the power button PCB from the PC case vendor, replaced the old PCB, and the computer seems to work fine now.

Acknowledgement

I would like to thank my electric engineering friend. Because I did not study electric engineering, I never knew even a power button could be so sophisticated. Actually I thought it was only one simple wire or something, that is why I could not understand why there will be such problem until he proposed the capacitor hypothesis to me.


I would also like to thank the warm hearted people from TechPowerUp who provided a lot of good suggestions.

References