PSU voltage query

CurlyWhirly · 19 Jul 2009 at 13:29

When you go to buy a power supply, the output voltage is usually shown for 3 voltages: +3.3v, +5v and +12v.

Most CPU's seem to use +1v or above so where is it derived from and why is it not mentioned in PSU specs ? (perhaps it is derived from the +3.3v rail and so isn't needed to be displayed as it's already covered) ?

Also generally speaking what sort of devices would these voltages power ?
I know that +12v is used for graphics cards but what about the +3.3v and +5v outputs ?

Finally why are negative voltages used on the 5v and 12v rails ?

Using the Hardware Monitor I get a weird reading from the -5v and -12v rails and am concerned about the PSU although it's been showing those kind of readings for ages :confused:

(below)

RJC · 19 Jul 2009 at 13:59

Ignor software to read voltages, HW Monitor shows my 12v line as 3v, -12 reads same as yours.

gareth170 · 19 Jul 2009 at 14:01

ignore any Hardware Monitoring softwares, 95% of the time they are wrong.

the +3.3v, +5v and +12v are the more important ones (I think).

Mattus · 19 Jul 2009 at 16:11

The CPU takes power from the 12v, as do graphics cards. The 3.3 and 5 aren't used very much in modern systems, though USB power is 5v and the memory can run off 3.3.

CurlyWhirly · 19 Jul 2009 at 19:29

Thanks for the feedback.

I'll ignore the voltage readings as it seems that it must be a bug of some sort as other people are also affected so I have nothing to worry about!

CurlyWhirly · 19 Jul 2009 at 19:33

Mattus said:
The CPU takes power from the 12v, as do graphics cards.

Crikey, I'm amazed as I would have thought that a CPU would get it's power off the 3.3v rail as the core voltage is usually only just over 1v :eek:

This 12v rail must have some sort of mechanism to get the voltage down to ~ 1v for the CPU core!

Mattus said:
The 3.3 and 5 aren't used very much in modern systems, though USB power is 5v and the memory can run off 3.3

Cheers.

I assume that the motherboard runs off 5v along with hard drives and CD/DVD drives ?

p.s. I assume that VBAT is the voltage of the BIOS battery and when this drops from it's 3v rating, it means that the battery needs replacing?

I never ran Hardware Monitor on my old PC (which is scrap now)

gareth170 · 19 Jul 2009 at 20:31

CurlyWhirly said:
I assume that the motherboard runs off 5v along with hard drives and CD/DVD drives ?

p.s. I assume that VBAT is the voltage of the BIOS battery and when this drops from it's 3v rating, it means that the battery needs replacing?

I never ran Hardware Monitor on my old PC (which is scrap now)

the motherboard runs off the 12v rail also the hard drives and CD/DVD drives ..

yes the VBAT is the voltage of the BIOS battery

CurlyWhirly · 22 Jul 2009 at 10:38

gareth170 said:
the motherboard runs off the 12v rail also the hard drives and CD/DVD drives ..

Thanks, I didn't know that.

Now I can see why the +12v rails are a lot more important in power supplies as they have quite a lot of load on them powering the mobo, hard drive(s), CD/DVD drive(s) and the power hungry graphic card(s) as well.

JonJ678 · 22 Jul 2009 at 12:18

12V rail does almost everything. However it isn't the psu which drops it down to 1.3V or so for the cpu, it's the circuitry around the cpu socket. The mosfets and so forth which more expensive boards use heatsinks on and you occasionally see water cooled

CurlyWhirly · 25 Jul 2009 at 22:04

JonJ678 said:
However it isn't the psu which drops it down to 1.3V or so for the cpu, it's the circuitry around the cpu socket. The mosfets and so forth which more expensive boards use heatsinks on and you occasionally see water cooled

I did not know that, thanks for the info