3.3V power rail

[gordanfreeman]

the last cpl days Asus Probe has popped up in the middle of a game telling me that my 3.3V rail is a little lower than normal (I have AP set to warn me if any of my rails deviates by more than 5%). both times the rail returned to a more normal number within a few seconds. i am using an Antec truepower 480 PSU on a system that should not push the limits of the unit. what does the 3.3V rail power? is this something i should be worried about?

 

[gordanfreeman]

anyone?

 

[JeremiahTheGreat]

i think it powers the CPU on old AXPs... *i think*

 

[KristopherKubicki]

Its pretty much unused these days.

Kristopher

 

[Zepper]

Yeah, the 3.3 doesn't get much use any more and that may be the problem. Switching power regulators don't like to be lightly or un-loaded. Hook about a 3 Ohm, 10 Watt resistor across the 3.3V rail and see if it doesn't smooth out for you.
.bh.

 

[gordanfreeman]

Originally posted by: Zepper
Yeah, the 3.3 doesn't get much use any more and that may be the problem. Swithching power regulators don't like to be lightly or un-loaded. Hook about a 3 Ohm, 10 Watt resistor across the 3.3V rail and see if it doesn't smooth out for you.
.bh.

2 ?'s on this:

1. is this something i could pick up at a place like radio shack?
2. how would i go about hooking it to my 3.3V rail. i am assuming i would need to test the connectors on the mobo plugin with a multimeter to find the right connectors that output 3.3V. if this isnt what i should do, i have no idea.

 

[MichaelZ]

3.3 doesnt matter at all now really... you can have it as low as 2.8 and not worry.

i overloaded a 300W codegen PSU on a overclocked 2.4B system with a ti4200, 2 hard disks and 2 optical drives. no problems even with the 3.3V sinking as low as 2.89V. i didn't see any stability issues but of course, it's probably not good for hardware.

 

[farmercal]

gordanfreeman, I wouldn't worry too much about Asus Probe telling you something is low. I had Asus Probe tell me all of my voltages were dropping out of tolerance for about 10 minutes and I did not have one glitch with my computer. I finally rebooted and everything was normal again.

What Happened? I think the program got a little flakey at that particular time. If you are real concerned about this, when it happens again, you can use a multi-meter with straight pins taped to the ends and measure the voltage of your 3.3volt rail right on the connector and see for yourself whether the program is reading the voltages correctly.

 

[Bassyhead]

what are the other power rails used for specifically? just curious.

 

[Zepper]

Yes you should be able to get a 2 or 3 Ohm power resistor at RS. Their web site will tell you if the item is stocked at most stores or special order (part no. in red). The 3.3V lines are ususlly colored orange, so you just connect the resistor between any orange and any black (ground) lead or the chassis.

Bassyhead et al.,
3.3V is mainly used for LED source and some support ICs. 5V used for power for add-on cards and is Vcc for some of the ICs on the board and storage drive electronics - I'm not sure but it also may be used for RAM power (thru a regulator of course), and 5V standby is obvious. 12V (in the ATX) used mainly for serial ports and maybe the RAM regulators, P4 12V powers the CPU via regulators and of course 12V powers your drive motors and fans directly. The Negative voltages (-5, -12) are seldom used any more and lots of new PSUs are leaving one or both out - I think mine has no negative Vs.
.bh.

 

[Jeff7]

I thought that PCI cards ran on 3.3V.

 

[imported_oralpain]

Athlons never used the 3.3 rail for the cpu power.

3.3v rail powers RAM, AGP, PCI, and very likely the chipset. It is FAR from unused.

 

[Bassyhead]

Originally posted by: oralpain
Athlons never used the 3.3 rail for the cpu power.

3.3v rail powers RAM, AGP, PCI, and very likely the chipset. It is FAR from unused.

tahts what i thought. powersupplies wouldnt be made to have the capability of delivering a high current of 3.3v if nothing used it.

 

[Zepper]

RAM used to run at 3.3 now most is 2.8 which is too close to 3.3 to be regulated down. I think 3.3, 5 and 12V are available to PCI cards - just depends on what the card mfr chooses to use.
.bh.

 

[imported_oralpain]

DDR still gets its power from the 3.3v rail. It's not "too close" to be regulated down. It can be regulated to what ever they want.

 

[mobiusman]

DDR memory is supplied by a low voltage stepdown controller whose minimum operating voltage 3.15V. This makes it ideal to be used with the 3.3V rail.
Full PCI compliancy requires a 3.3V, 5V, 12V and -12V rail. Prior to the PCI 2.2 standard there was no requirement for the 3.3V rail.
-Scott