nanit155102dc
New member
- Joined
- Nov 29, 2024
- Messages
- 13
Hi, while diagnosing another problem with my motherboard's CPU+DRAM LEDs lighting sometimes (see my other forum post), I noticed something highly reproducible:
After a hard shutdown --- PSU turned off while computer is on --- thereafter when the PSU is turned on then the motherboard backfeeds power from the 5V standby rail onto the 3.3V rail.
Fortunately it only drives about 1.5 V so it shouldn't break anything, but it is concerning and I worry it could get worse. Most importantly, I observed this happening perfectly reproducibly with two different motherboards of the same model and with two different power supplies:
Motherboard 1: PRO B650M-A WIFI, serial 601-7D77-030B2405001xxx
Motherboard 2: PRO B650M-A WIFI, serial same as before + 49
PSU 1: Corsair RM850x
PSU 2: MSI MAG A750GL
I am using a Ryzen 7600x with no discrete GPU, though I don't think these are relevant factors.
I am curious whether someone else can reproduce the same issue.
Steps to check:
1. Boot up the PC into BIOS or something else where a hard shutdown won't corrupt your files.
2. Measure the 3.3V rail. That's pin 1 or 2 of the ATX power connector (the pins on bottom left side, see photo below) or pin 12 (top left). You can measure relative to case or to pin 3 (third from bottom left). You will read around 3.3V, actually 3.135 - 3.465 V per the ATX standard.
3. Now turn off the PSU switch, while PC is still running (hard shutdown).
4. Turn back on the PSU. At this point I now see ~1.5 V on the 3.3V rail. In contrast the 12V and 5V rails are at 0, besides pin 9 (5V standby power).
5. Turn off the PSU. Voltage will drop from 1.5V to 0V.
6. Turn on PSU again. Voltage rises to ~1.5 V again.
7. Finally boot up: push the power button. Voltage rises to ~3.3 V. (At this point my PC sometimes gets stuck with CPU+DRAM LEDs, but it likely won't happen to you.)
8. Perform a soft shutdown: hold power button down or do 'shutdown PC' from your OS.
9. Voltage now should drop to 0 (in my case, slowly).
10. Turn off PSU.
11. Turn on PSU. Now the voltage should remain at 0 on the 3.3V rail, because we did soft shutdown and then PSU off.
12. Repeat hard/soft shutdowns as much as you want. You should reliably see 1.5V if last shutdown was hard, and 0V if last shutdown was soft. Beware, if trying another fully modular PSU you must not mix power cables even if they seem to fit.
After a hard shutdown --- PSU turned off while computer is on --- thereafter when the PSU is turned on then the motherboard backfeeds power from the 5V standby rail onto the 3.3V rail.
Fortunately it only drives about 1.5 V so it shouldn't break anything, but it is concerning and I worry it could get worse. Most importantly, I observed this happening perfectly reproducibly with two different motherboards of the same model and with two different power supplies:
Motherboard 1: PRO B650M-A WIFI, serial 601-7D77-030B2405001xxx
Motherboard 2: PRO B650M-A WIFI, serial same as before + 49
PSU 1: Corsair RM850x
PSU 2: MSI MAG A750GL
I am using a Ryzen 7600x with no discrete GPU, though I don't think these are relevant factors.
I am curious whether someone else can reproduce the same issue.
Steps to check:
1. Boot up the PC into BIOS or something else where a hard shutdown won't corrupt your files.
2. Measure the 3.3V rail. That's pin 1 or 2 of the ATX power connector (the pins on bottom left side, see photo below) or pin 12 (top left). You can measure relative to case or to pin 3 (third from bottom left). You will read around 3.3V, actually 3.135 - 3.465 V per the ATX standard.
3. Now turn off the PSU switch, while PC is still running (hard shutdown).
4. Turn back on the PSU. At this point I now see ~1.5 V on the 3.3V rail. In contrast the 12V and 5V rails are at 0, besides pin 9 (5V standby power).
5. Turn off the PSU. Voltage will drop from 1.5V to 0V.
6. Turn on PSU again. Voltage rises to ~1.5 V again.
7. Finally boot up: push the power button. Voltage rises to ~3.3 V. (At this point my PC sometimes gets stuck with CPU+DRAM LEDs, but it likely won't happen to you.)
8. Perform a soft shutdown: hold power button down or do 'shutdown PC' from your OS.
9. Voltage now should drop to 0 (in my case, slowly).
10. Turn off PSU.
11. Turn on PSU. Now the voltage should remain at 0 on the 3.3V rail, because we did soft shutdown and then PSU off.
12. Repeat hard/soft shutdowns as much as you want. You should reliably see 1.5V if last shutdown was hard, and 0V if last shutdown was soft. Beware, if trying another fully modular PSU you must not mix power cables even if they seem to fit.