I purchased and tested a Gigabyte GA-H110TN motherboard, which conveniently accepts 12V input.
Enclosed a picture of the tested layout :
I am using a Dell DA-2 12V 18A AC adapter to power my system (Intel Pentium G4600T + 8 Gb CMSX8GX4M2A2400C16).
Current wiring is the following :
The Dell DA-2 adapter is plugged into the DC-IN 5.5 barrel connector.
I am using the internal 4-Pin Mini Fit Jr to output 12V current from the Dell DA-2 to the powered riser.
A MSI GTX 750 Ti Low Profile is connected to the riser.
As I am using a powered riser, the PCIe x4 slot stays within the 25W limit.
The other 40W the GTX 750 Ti needs under load are provided by the 12V AC adapter.
It works fine, except for two issues I would like to submit to you.
Whenever I am using the 4-Pin ATX_12V internal connector, as 12V output or 12V input, the CPU fan is spinning even if the motherboard is powered OFF.
I am pretty sure a hard drive would be spinning too.
I did two tests to try to narrow down the possible reasons for this strange behaviour.
1.a. 4-Pin ATX_12V internal connector left unused :
I removed the 4-Pin to Molex cable and the riser. Nothing is connected to the 4-Pin internal connector.
The DC power jack is connected.
Result : the motherboard acts "normally" : the CPU fan does not spin until the motherboard is powered on.
1.b. 4-Pin ATX_12V internal connector used as 12V input :
I used the 4-Pin as 12V input, using a DC-DC board (Picobox X7-ATX-500) providing 12V current to the motherboard.
The DC power jack is not connected.
Result : the CPU fan is spinning even if the motherboard is powered OFF.
CONCLUSION : whether the 4-Pin ATX_12V internal connector is used as output or input, the motherboard seems to be continually fed with 12V current even if it is not powered ON.
What do you think could cause this strange behaviour ?
Do you think it could be dangerous for the motherboard ?
After several reboots, the system won't POST.
Both CPU and GPU fans keep on spinning at full speed but no display.
I have to wait few minutes or remove the GPU to be able to start the system.
Could it be VRM related ?
Here is the background story
, as updating to the latest BIOS also caused some issues.