Motherboard voltages

[Handruin]

I just swapped out my old power supply (antec 300 watt) with my new Antec Neo 480 and I noticed a change in the voltage monitoring utility.

Prior to the PS swap, the "-5" voltage was within range. Now the "-5" voltage is in the positive with a reading of "+3.55". This set off the alarm in the monitoring tool which I found quite odd. So I downloaded MBM to see if it would report the same thing, and sure enough, it does. Is this something I should be concern with? My machine is running stable, even under full load.

 

[Groltz]

The -5 V and -12 V signals usually are no better than +/- 5% even on very good units; there's no point bothering getting them better than that since they are low-current and mostly unused anyway.

http://www.pcguide.com/ref/power/sup/specElec-c.html (Load Regulation)

You will notice that the negative rails on your PSU are very week when compared to the positive rails. This is due to them being there more or less for legacy reasons. Although a 20 pin ATX motherboard connector has pin 12 -12v and pin 18 -5v, they're generally not used by the motherboard. They are there for:

1. ISA cards
2. Serial port or LAN
3. Older FDD

http://www.bleedinedge.com/guides/psu_select/psu_select_03.html (Negative Rails)

 

[Groltz]

If you look at a specification sheet for a typical PC power supply, you see that the supply generates not only +5v and +12v, but also -5v and -12v. Because it would seem that the +5v and +12v signals power everything in the system (logic and motors), what are the negative voltages used for? The answer is, not much! In fact, these additional negative voltages are not used at all in many modern systems, although they are still required for backwards compatibility.

Although -5v and -12v are supplied to the motherboard via the power supply connectors, the motherboard itself uses only the +5v. The -5v signal is simply routed to the ISA bus on pin B5 and is not used in any way by the mother-board. It was originally used by the analog data separator circuits found in older floppy controllers, which is why it was supplied to the bus. Because modern controllers do not need the -5v, it is no longer used but is still required because it is part of the ISA Bus standard.

http://cma.zdnet.com/book/upgraderepair/ch08/ch08.htm (Power Supply Function and Operation)

 

[Handruin]

Many thanks! I wonder if this is the result of using the 24 to 20 pin converter supplied with the PS. Thanks again for all the links!

 

[sechs]

One of those negative voltages is no longer offered in the latest ATX specification (which also requires the 24-pin connector).