A computer components & hardware forum. HardwareBanter

If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below.

Go Back   Home » HardwareBanter forum » General Hardware & Peripherals » Homebuilt PC's
Site Map Home Register Authors List Search Today's Posts Mark Forums Read Web Partners

motherboard 5v



 
 
Thread Tools Display Modes
  #21  
Old June 16th 19, 05:01 PM posted to alt.comp.hardware.pc-homebuilt
T. Ment
external usenet poster
 
Posts: 87
Default motherboard 5v

On Mon, 03 Jun 2019 14:48:43 -0400, Paul wrote:

This is just the first picture I could find, bigger than a thumbnail.

https://www.badcaps.net/forum/showthread.php?t=2265

The board doesn't have an ATX12V 2x2 power connector, with
the two yellow wires and two black wires.


I wondered if the power supply would do any better on a motherboard with
a 12V connector.


That means the *CPU* runs off +5V, as well as lots of
other logic. It's natural for such an unbalanced load
situation (only an amp or two load on +12V, but
15 amps off the +5V), for the +5V to be a little
on the low side.

This is called "cross-regulation". There is only one
feedback loop in the ATX power supply for regulation.
What you'd find is, the +5V resting at its low limit,
and the +12V "higher than normal". If an ATX supply is
"non-compliant to crossloading", then the +5V could
even be too low and out-of-spec.


So I swapped the K7S5A for a motherboard which has the 12v connector.
Now my multimeter reads 5.16v on the USB header pins. That's more like
it. To make sure, I swapped again for a third motherboard (12v). Same
result, 5.16v.

My $20 power supply is OK with a 12v motherboard. I'll match the K7S5A
with a better power supply.


  #22  
Old June 17th 19, 03:54 AM posted to alt.comp.hardware.pc-homebuilt
Paul[_28_]
external usenet poster
 
Posts: 1,467
Default motherboard 5v

T. Ment wrote:


So I swapped the K7S5A for a motherboard which has the 12v connector.
Now my multimeter reads 5.16v on the USB header pins. That's more like
it. To make sure, I swapped again for a third motherboard (12v). Same
result, 5.16v.

My $20 power supply is OK with a 12v motherboard. I'll match the K7S5A
with a better power supply.


And that's the cross-loading effect.

Balancing the loads leads to closer-to-correct
regulation.

If one rail of a three-rail supply is heavily loaded,
the fact there is only one feedback loop means
two rails will be high and one rail will be low.

Only a few supplies used separate circuits for
the rails. The concept never caught on. I think
the regulation on those could be in the 2% or so
ballpark.

There have also been a few supplies with four separate
12V rails. CPU1 and CPU2, motherboard, PCIe, some sort
of split like that. You had to make sure on those, that
the rails that were independent, didn't get joined
by accident. (That's because they're only nominally
12V, and if they are slightly different potentials,
a large current can flow between them. And also,
the ATX design is not push-pull, it's only push, and if
a "rail goes high", nothing actively works to bring
that voltage down.)

On the motherboard, there is only one push-pull
regulator, and that's the Vtt regulator for terminating
DIMM buses. Plus or minus two amps flows from that
one. It either has to sink 2 amps or source 2 amps,
on a nanosecond level basis.

You've done a good job by selecting a 12V Athlon board.
When I told people I had a board that ran the CPU
off 5V, not a lot of people believed it.

There are some other goofy practices. Biostar, on a
number of motherboards, joined ATX12V 2x2 12V to
the motherboard yellow 12V wire. One is 12V1 and the other
is 12V2. Now, because a lot of ATX supplies power
12V1 and 12V2 come from the same transformer, there is
no potential difference. It's only when you use
a supply with entirely separate 12V1 and 12V2, that
this would be a bad idea. I never did see an explanation
from Biostar, as to why this was a good idea.

I think the evidence for this came, when a Biostar owner
of a P4 board, accidentally forgot to plug in ATX12V
2x2, and the CPU and motherboard booted and ran without
it. That's the evidence they're joined on the motherboard.
I can't think of any other brands that do this.

If you need help on connectors, this site has a page on
ATX supplies.

http://www.playtool.com/pages/psucon...onnectors.html

Paul
  #23  
Old June 17th 19, 05:12 PM posted to alt.comp.hardware.pc-homebuilt
T. Ment
external usenet poster
 
Posts: 87
Default motherboard 5v

On Sun, 16 Jun 2019 22:54:28 -0400, Paul wrote:

My $20 power supply is OK with a 12v motherboard. I'll match the K7S5A
with a better power supply.


And that's the cross-loading effect.

Balancing the loads leads to closer-to-correct
regulation.


I understand volts and amps, but your EE talk is over my head. That's
OK, I never wanted to be an EE. I'm just a mechanic swapping parts. And
now my $20 power supply has a good home. So thanks for the ideas.


 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
motherboard help. Compaq Presario 5700T Motherboard marq122 via HWKB.com Homebuilt PC's 2 September 7th 06 10:05 PM
Use HDD from AMD Proc Motherboard in Intel Proc Motherboard Sekhar Asus Motherboards 1 August 15th 04 07:41 PM
Motherboard for AMD 64 CPU Tyrone Pitts Homebuilt PC's 3 April 19th 04 05:17 PM
new motherboard jay Overclocking AMD Processors 6 April 2nd 04 02:57 AM
New motherboard jay Overclocking AMD Processors 2 February 27th 04 09:55 PM


All times are GMT +1. The time now is 03:08 PM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 HardwareBanter.
The comments are property of their posters.