[zarcondg]

I repeat...
Quote:

PCI and PCI Express cards can use all three rails.

3.3V = PCI, and PCI express... YES!, if the computer is new enough to have them. (Remember ISA, and 16bit ISA) lol
3.3V on other parts, Depending on age of the computer.

The 3.3V used to power the older computer core stuff (CPU, RAM, possible chipset). digging deep into past memory, 200MHz Pentium I think era. Before that there was NO 3.3V rail. The CPU's either ran on 5V or sat behind a regulator. Before 3.3v (ATX and newer), 12V was the dirty power reserved for disk motors, and maybe fan motors. The 5V was the "clean rail" to drove digital stuff, in the old IBM AT, and "Baby AT" computers.
 
After the 200MHz Pemtium-ish era, CPU's and other stuff got faster, smaller, and required lower voltages. The 12V rail got the burden of all the stuff that had previously ran on 3.3V (or 5V way back).
 
The AT to ATX transition happened somewhere between the 486, and the Pentium days. Then the P4 required even more power, and we got the four pin yello/black plug to go with the other motherboard plug (And ATX12V was borne). Then they added more plugs for SATA, Graphics, etc.
 
The 16bit-ISA replacement, PCI remains to this day. "O" and some AGP stuff used 3.3V.

 

[zarcondg]

Fyi...
-12V was (8088 ish) for the graphics DAC output, and some PCI sound cards/onboard sound chips require -12V.

 

[Ntropic]

And let us not forget the glory that is ATX12V2.0, bringing us 24-pin ATX connectors, the sticky SOBs. Don't know how many minutes I've spent pulling them out slowly for fear of breaking something. And EPS12V: I don't have a PSU that has that.
 
Quote:

Rhe AT to ATX transition happened somewhere between the 486, and the Pentium days. Then the P4 required even more power, and we got the four pin yello/black plug to go with the other motherboard plug (And ATX12V was borne). Then they added more plugs for SATA, Graphics, etc.
 
 

 

[zarcondg]

yep, and the confusion the 24-pin ATX connectors bring to consumers thinking there old 20-pin power supply will work... lol
 
I don't remember what era the 20-pin became 24-pin, it was after the yellow/black four pin (Caution Colors) plug became.
 
well as for 3.3V. Some one made a processor that required 3.3V, they were NOT going to use the "Dirty 12V power"... So they demanded that the power supply would supply the required Voltages. Then PCIsig went from 5V 33MHz to 3.3V 33/66MHz. Then when the CPU maker went to a lower voltage, they had a mess on there hands... perhaps that was the birth of the "12V2" Caution Color plug.

 

[DaKi][er]

Quote:

Thanks for the answer. So the motherboard has an onboard SMPS to convert 12V to 3.3V? I figured it would be a simple linear regulator, in which case you'd be wasting a ton of heat. It does make sense not to run the extra 3.3V cables if it can be avoided, but why supply 3.3V in the first place then?

 
No, there is no regulator from 12V to 3.3V, Computers never use linear regulators for anything either (some soundcards and low power devices may, but they are exceptions). As has been said, 3.3V is used for some peripherals, but it's not used for high powered devices like the CPU and GPU which are the main power users in computers. 
 
The CPU in a PC can use typically 80 watts of power under full load, and with a core voltage of around 1.2V now days. Simply calculating for current, you'll find that is 66A which is a damn lot! To put that much current through a wire, it is going to have to be nearly a thick as your index finger. But say we use the 12v supply, that is only 6.6A which is much more manageable, you can add a bit onto that as the regulator that feeds the CPU isn't going to be 100% efficient but it is close enough. The regulators on your motherboard aren't simple switchmode regulators either, they are multi-phase, where instead of switching current on and off into one inductor, there are 6, 8, 12 and I think I've even heard of 16 phases where each phase is switched one after the other, which has the effect that the faster you switch something the more losses you have, so each phase only sees maybe 100khz switching frequency, but if you have 8 of them switch one after the other at 100khz then you effectively see 800khz of switching speed meaning you need smaller capacitors that don't have to remove the longer ripples that you would get at 100khz, but with the losses of only 100khz which means more efficiency and less heat
 
Circled in red is the circuitry that regulates the power just for the CPU in a modern desktop computer, that 8pin plug is just for supplying 12v to the board for it

 

[zarcondg]

more of a mod, then DIY build. trying images first time...
12v2 snubbers, and added 8AWG GND strap.

http://www.zarcondeegrissom.org/comps/LAB3stuff.html
further down on the page.
 
Diner-mints on the Motherboard. Will void the warranty.

also on that page.
 
This made most of the nasty stuff go away in my computers.
 
All I have left is PCI RFI bleeding through the Soundcard's chips.

 

[zarcondg]

[SNIP] there's my post, and in duplicate... Nice!

 

[zarcondg]

I just tried posting something here, and I got a "Since your new here..." pop-up message thing, and I didn't get the rest of what it said before it vanished. and now where's my post? lol
was it the link to a page on my site, or the pictures from my site? or is there a delay thing?
 
That 8-pin plug also feed power to the graphics slot on a lot of boards. I added 0.01uF and/or 0.047uF green diner-mints on the bottom of my boards, soldered to the leads of the electrolytic-battery-cans.

 

[zarcondg]

testing, 1, 2, 3...
here is a link to the page with some pictures of what I did to the power in "LAB3".
 
http://www.zarcondeegrissom.org/comps/LAB3stuff.html
near the bottom of the page.

 

[zarcondg]

They don't like the pictures from my site... O.K.
 
More of a "Mod", then a DIY build.
 
The ground strap thing, found an un-used "COM" spot on the PSU PCB. The blue caps are 1.0uF 250V, and the green ones (0.01uF & 0.047uF) 50V. All Poly.
 
That fixed most of the noise from the computer. All I have left is the PCI signal RFI bleeding through the soundcard chips.
[edit]
O.K. all fixed. they dumped the pics in my "Default thing", I moved them to "PSU MOD001" Gallery under my profile. I think y'all might like the Soundcard shield. There are other things I did inside the Power supplies. I have revisions about showing High-Voltage mods.

 

[Juaquin]

Quote:

The CPU in a PC can use typically 80 watts of power under full load, and with a core voltage of around 1.2V now days. Simply calculating for current, you'll find that is 66A which is a damn lot! To put that much current through a wire, it is going to have to be nearly a thick as your index finger. But say we use the 12v supply, that is only 6.6A which is much more manageable, you can add a bit onto that as the regulator that feeds the CPU isn't going to be 100% efficient but it is close enough. The regulators on your motherboard aren't simple switchmode regulators either, they are multi-phase, where instead of switching current on and off into one inductor, there are 6, 8, 12 and I think I've even heard of 16 phases where each phase is switched one after the other, which has the effect that the faster you switch something the more losses you have, so each phase only sees maybe 100khz switching frequency, but if you have 8 of them switch one after the other at 100khz then you effectively see 800khz of switching speed meaning you need smaller capacitors that don't have to remove the longer ripples that you would get at 100khz, but with the losses of only 100khz which means more efficiency and less heat

Awesome reply. Learn something new here every day. Thanks!