MB draws CPU power from which: 12V or 5V?

[toronado455]

How does one determine which power rail (12V or 5V) a MB uses for CPU
power?
MBs are: MSI KT3 Ultra ARU and ASUS P4PE.

 

[Paul]

How does one determine which power rail (12V or 5V) a MB uses for CPU
power?
MBs are: MSI KT3 Ultra ARU and ASUS P4PE.

The P4PE is easy. It has the 2x2 ATX12V power connector, so it
definitely uses 12V.

The MSI board only has the main ATX power connector, and it is
hard to guess at which rail would be used. To find out, you
use a multimeter, and touch it to the common node between
the first group of filter capacitors and the input inductor
to the Vcore switcher. This is not something most people can
find on the first try.

See the lonely "donut" inductor next to the main power connector
here ? Use a multimeter, and check either lead of the two legs on
that coil, as that would tell you whether +12V is used or not.
Connect the ground of the meter to the computer case. The voltage
will either read +5V or +12V.

http://www.xbitlabs.com/images/mainboards/kt333-sis745/msi-kt3ultra-aru.jpg

If you have a motherboard, where you don't know which rail is
used, and cannot find some info on the web to clarify the
situation, then buy a power supply good enough to handle either
case. That means +5V @ 25A and +12V @ 15A as minimum figures.
You should be reasonably safe at those output limits, for a
minimal system configuration. If the computer has a lot of
disk drives, you'll need more of each.

For example, this power supply would easily cover either
requirement. All rails have a high limit, but the total
load allowed is still 350W. It means the supply is happy
if only one rail is heavily loaded, and you don't know which
rail that is going to be.

+3.3V@32A, +5V@32A, +12V@26A, -5V@1A, -12V@1A, [email protected]
http://www.newegg.com/Product/Product.asp?Item=N82E16817103455

Paul

 

[Guest]

How does one determine which power rail (12V or 5V) a MB uses for CPU
power?
MBs are: MSI KT3 Ultra ARU and ASUS P4PE.

Look at the power connectors on the motherboard. If there's a 4-pin
square connector in addition to the 20-24-pin connector, then the CPU
is powered by the +12V output. But if the motherboard has only one
large connector, most likely 20-pin, then almost always the CPU runs
from the +5V output, but there are a few (badly designed) exceptions.
Another method of determining the supply voltage is to check the
voltage ratings of the electrolytic capacitors (vertical cylinders) in
the CPU core voltage regulator circuit (near the CPU):

http://upload.wikimedia.org/wikipedia/en/4/4b/Badcaps-choyo.jpg

Notice the ring wrapped with thick solid wire; it's a choke, and there
are usually 2-3 of them. The chokes, the MOSFETs (black 1/2" plastic
squares), and the electrolytic capacitors are part of the CPU voltage
regulator. The particular electrolytics in this photo are rated for
6.3V (may be marked "6.3VW," the "VW" meaning "volts, working"),
indicating the CPU is powered by the +5V output, but on some boards
they're rated for 10V. If they're rated for 16 volts, the CPU is
powered by the +12V. If you have a voltage meter and are very careful,
you can verify the supply voltage from the pins of the MOSFETs, and the
supply voltage will be the highest voltage measured.

 

[toronado455]

Look at the power connectors on the motherboard. If there's a 4-pin
square connector in addition to the 20-24-pin connector, then the CPU
is powered by the +12V output. But if the motherboard has only one
large connector, most likely 20-pin, then almost always the CPU runs
from the +5V output, but there are a few (badly designed) exceptions.

This MB has a 20-pin and no 4-pin.

Another method of determining the supply voltage is to check the
voltage ratings of the electrolytic capacitors (vertical cylinders) in
the CPU core voltage regulator circuit (near the CPU):

http://upload.wikimedia.org/wikipedia/en/4/4b/Badcaps-choyo.jpg

Notice the ring wrapped with thick solid wire; it's a choke, and there
are usually 2-3 of them. The chokes, the MOSFETs (black 1/2" plastic
squares), and the electrolytic capacitors are part of the CPU voltage
regulator. The particular electrolytics in this photo are rated for
6.3V (may be marked "6.3VW," the "VW" meaning "volts, working"),
indicating the CPU is powered by the +5V output, but on some boards
they're rated for 10V. If they're rated for 16 volts, the CPU is
powered by the +12V. If you have a voltage meter and are very careful,
you can verify the supply voltage from the pins of the MOSFETs, and the
supply voltage will be the highest voltage measured.

It has both 16V and 6.3V caps.

Have a look at this pic:
http://www.xbitlabs.com/images/mainboards/kt333-sis745/msi-kt3ultra-aru.jpg

The group of 5 caps between the NB and the 2 chokes, and the 3 caps
between the 20-pin and the CPU are all 6.3V 2200uF caps.

Then, to the left of the 20-pin, there is 1 more choke, then 2 caps and
then a 3rd cap a little further to the left. All three of those are 16V
1500uF.

Does this info resolve anything or is it still a mystery?

(I've never used a voltage meter or multimeter so I'm not comfortable
with poking around in there with one.)

 

[Guest]

(e-mail address removed) wrote:

How does one determine which power rail (12V or 5V) a MB uses for CPU
power? MBs are: MSI KT3 Ultra ARU and ASUS P4PE.

It has both 16V and 6.3V caps.

Have a look at this pic:
http://www.xbitlabs.com/images/mainboards/kt333-sis745/msi-kt3ultra-aru.jpg

The group of 5 caps between the NB and the 2 chokes, and the 3 caps
between the 20-pin and the CPU are all 6.3V 2200uF caps.

Then, to the left of the 20-pin, there is 1 more choke, then 2 caps and
then a 3rd cap a little further to the left. All three of those are 16V
1500uF.

Does this info resolve anything or is it still a mystery?

The large 6.3V capacitors indicate the CPU is run from the +5V output
of the power supply.

(I've never used a voltage meter or multimeter so I'm not comfortable
with poking around in there with one.)

It's not difficult to let a meter probe slip and short something. I
use sharp stainless steel probes to reduce that possibility, but most
meters come with chrome plated brass probes that are relatively blunt.

 

[toronado455]

The large 6.3V capacitors indicate the CPU is run from the +5V output
of the power supply.


It's not difficult to let a meter probe slip and short something. I
use sharp stainless steel probes to reduce that possibility, but most
meters come with chrome plated brass probes that are relatively blunt.

Thanks for your help. Earlier today I ordered this power supply to use
with that system:

http://www.fsp-group.com.tw/english/1_product/2_detail.asp?mainid=1&fid=53&proid=87

It looks like it will be the perfect match.