PSU Questions

[BrainStorm]

Q1: If Athlons draw power from the 12V rail, then why does my 5V rail dip when the CPU is at 100% load (from 4.83 at idle all the way to 4.63 when loaded with Toast, Prime95 and CPU Stability all at once) rather than the 12V rail dipping (actually increases a bit from 12.31 to 12.43)?

Q2: Given that the Athlon is running off the 12V rail, why does the dip in the 5V rail cause system instability? Is it because the mobo is getting starved for power from the 5V rail?

[Tismedt]

I thought the athlons ran off the 5v rail. I'm no expert by far. Somebody correct me if I'm wrong. If it does run on the 12v rail I have no idea about your problem. Are you running the 1600 at default vcore?

Quote from Hoots Sticky
"Lastly, there is the resistance on the motherboard of the copper voltage trace from the ATX connector to the MosFets used for regulating the Core Voltage of the CPU. Though the +5V goes to places all over the motherboard, for the various circuits, the bulk of the current is delivered to that regulator. This resistance, though typically much less than that seen in the ATX connector, does contribute to some of the voltage drop. Like the conductors coming from the PSU to the ATX connector, the longer the path to the core regulator, the greater the resistance. That's why you see some motherboard manufacturers moving their ATX connector as close as physically possible to the core voltage regulation circuit lately. Even they learn from experience"

[CChaos]

AFAIK, not all PSUs have this feature:

Individual voltage outputs and regulation for stable +5v, +3.3v, +12v outputs.

In other words don't think of the 12v and 5v as seperate entities unless you are sure your PSU really works that way. This is from Antec's page for thier Truepower series.

[CChaos]

Found just what I was looking for on another thread. Thanks Redstone!

Originally posted by Redstone
Truepower 430. Very happy...

Big difference between the two is the dedicated output circuits.

This is also from their website:
"TruePower has dedicated output circuitry for each voltage line, which means that there is no combined output limit other than, of course, that of the entire power supply. Each voltage line can perform up to the specification on the label, without regard to or interaction with the loads on the other lines, until the full capacity of the power supply itself is reached."

It also has low noise technology, which means the fans run at lower RPMs unless called upon to aggressively cool the supply.

Hope this helps...

[BrainStorm]

I understand what you guys are saying, but I thought that really only applied to the 5V and 3.3V rails; i.e., the 12V rail ~is~ on separate "output circuitry" in ATX power supplies.

Can anyone confirm or deny?

[BrainStorm]

From the following URL at Antec:

http://www.antec-inc.com/pdf/truepower_%20inclusion.pdf

Traditional power supply designs typically feature circuit sharing for the 3.3V and 5V lines off the output transformer. That means that as you increase the load on one output circuit, the available capacity for the other decreases – for example as you load up on devices that use 5 volts (like drives and PCI cards), you have
less and less ability to power devices that need 3.3 volt output (like your CPU). In addition, by connecting the two systems at this point you increase the chance of having interactions between the output circuits – for example, a sudden 5V draw could potentially cause the voltage on the 3.3V line to drop, harming system stability.

Seems to confirm what I said above re sharing of 5V and 3.3V, but still leaves me puzzled as to my original question.

[mariner1911]

It was my understanding that Athlons run on the 5v and P4s run on the 12v, hence the extra 12v connector on P4 motherboards

[BrainStorm]

there does seem to be some confusion/disagreement on this point. Some Websites say one thing, some say the other.

Wish someone could say definitively. *sigh*

[larrymoencurly]

How about measuring the pins of the transistors that supply the CPU core voltage to see what the highest voltage on them is?

[BrainStorm]

wow, you're completely past me with that suggestion...I'm not that technically electrically astute...*lol*

[warnerwh]

On my MSI KT7 Turbo 2 motherboard for my Athlon 1800+ I have a four pin connector on the motherboard that the manual says if for 12v to the cpu. I believe this is there to supply a more stable voltage to the cpu although it's not an Intel chip. I'd say it's the 12v line that supplies the cpu. They (Amd and intel) probably did this to make sure the current to the chip is more stable. But like the above post I've read both 5 and 12 at various places.

[larrymoencurly]

I think 12V is used because it lets them transmit the same amount of power at less than half the current, meaning lower losses through the wires and connectors. But maybe it's also more efficient to convert 12V instead of 5V to the 1.x CPU core voltage.

None of my mobos is new enough to have the 12V connector. :(