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The ramifications of a 12V rail that sits at 13V?

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Bullion

Registered
Joined
Apr 17, 2002
Hi all,
Just a quick question i have a 400watt codegen psu, that runs a volt modded mobo feeding a 2100tbredb around 2V.

The PSU rails are pretty odd,
5V -> 4.8V
12V -> 12.77 under load (sometimes it goes up to 13V)
-12V -> -2.86 (yes -2.86)

SO i ask what are the likely ramifications of a psu that pushes 13V at the mlex connectors???
 

Rich_L

Registered
Joined
Jul 5, 2003
Some power supplies set up so that if you load the 5V rail it has to raise the 12V rail. In other words the 12V rail in the PS feeds the 5V rail. It is common on older AMD boards that use 5V as the source for processor core voltage regulator circuit to have 12V this high. I would not worry about it.

Oh, the -12V is not used on most motherboards so it is most likely not even connected. Not a problem!
 

L337 M33P

Member
Joined
Jun 5, 2003
Location
TEH INTERNETS
Rich_L said:
Some power supplies set up so that if you load the 5V rail it has to raise the 12V rail. In other words the 12V rail in the PS feeds the 5V rail. It is common on older AMD boards that use 5V as the source for processor core voltage regulator circuit to have 12V this high. I would not worry about it.

Oh, the -12V is not used on most motherboards so it is most likely not even connected. Not a problem!

Close but no cigar.

The see-saw effect of the 5V and 12V rails is due to the fact that both rails are derived from secondaries on one transformer. Increasing the load on one of these rails will increase the total flux linkage in the transformer core and result in a larger voltage across the other, unloaded rail. And my A7N8X gets CPU Vcore from 5V, the 12V rail sits at 12.54 ;). 10% is the limit for voltage variation, so you would need 13.2V before it starts going out of whack.

Antecs use 3 separate transformers, and I would assume PCP&C do too. Almost all other PSU manufacturers use a 2-transformer design.

The -5V rail isn't used, the -12V is. -12V is used for RS-232 and ethernet? signalling, whereas the -5V was only used on some ISA cards and really old floppy drives.
 

Rich_L

Registered
Joined
Jul 5, 2003
While –12V is used for RS-232 on the serial ports, for the past 6-8 years all most all RS232 driver/receiver chips are single supply VCC of 3.3V or 5V, with charge pump circuits for the RS-232 signal levels. Ethernet using –12V, that goes back to the AUI and Coax cable days of what the 1980s and most of those card had there own -12v source. Like I said most motherboards being used now do not use –12V from the PS.
 

L337 M33P

Member
Joined
Jun 5, 2003
Location
TEH INTERNETS
Rich_L said:
While –12V is used for RS-232 on the serial ports, for the past 6-8 years all most all RS232 driver/receiver chips are single supply VCC of 3.3V or 5V, with charge pump circuits for the RS-232 signal levels. Ethernet using –12V, that goes back to the AUI and Coax cable days of what the 1980s and most of those card had there own -12v source. Like I said most motherboards being used now do not use –12V from the PS.

Touché

:D