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#21
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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
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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
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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. |
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