Question on the power supply's 3.3v rail

Golbeze · Feb 10, 2012, 03:56 PM

I was just wondering, what components are powered through the 3.3 volt rail? Reason I ask is because I recently bought a power supply tester, and in testing it out on my psu I discovered its 3.3v rail was at 2.5 volts (ouch). So, I bought a new Corsair 750W psu, which is working great, tested it before installing it and all voltages are where they should be. But it got me curious, as with my old power supply I experienced a few crashes, and thus I'm wondering if maybe the undervolted 3.3v rail was the cause. I figure, being a CIT major, learning what components are powered through that rail may help me in the future.

mkk · Feb 10, 2012, 04:26 PM

Not so much nowadays but it's up to the motherboard design. A good number of years ago even the CPU could draw power from there but that's all regulated down from 12V now of course. Memory slots are a possibility since memory doesn't draw that much power. Expect today mostly miscellaneous minor add-in components and signaling to draw from the 3.3V level, things that don't need much power to run and might be using a voltage not too far from the level of the source.

Good old PCI cards could use 3.3V or 5V, and 3.3V is fed through PCIe card slots though mostly for signaling. 12V is available in abundance to anything that draws any real amounts of power on a PCIe card. It's possible that the use of 3.3V on the PCIe bus could rise in the future when everything might trickle down towards lower voltages, like if some future version of USB goes down to 3.3V or so. (the dependence of 5V is the most common reason for why some PCIe cards have molex connectors, as PCIe lacks 5V in the slot) But motherboard designers shouldn't be keen on increasing the 3.3V lines throughout todays motherboard design. In the end it's in large part a legacy thing today.

Golbeze · Feb 10, 2012, 04:44 PM

Hmm, now it's making me more curious. I got my current motherboard back in December 2009; a Foxconn ELA LGA 775 Intel P45 ATX Intel Motherboard, which is now discontinued. No idea how it uses the 3.3v rail. Guess I'll have to see if the crashes return, maybe test my old psu again to see what the exact values of the other voltages were.

Feb 10, 2012, 03:56 PM	#1
Golbeze HardwareHeaven Addict Join Date: Feb 2006 Posts: 276 Rep Power: 0 System Specs	Question on the power supply's 3.3v rail I was just wondering, what components are powered through the 3.3 volt rail? Reason I ask is because I recently bought a power supply tester, and in testing it out on my psu I discovered its 3.3v rail was at 2.5 volts (ouch). So, I bought a new Corsair 750W psu, which is working great, tested it before installing it and all voltages are where they should be. But it got me curious, as with my old power supply I experienced a few crashes, and thus I'm wondering if maybe the undervolted 3.3v rail was the cause. I figure, being a CIT major, learning what components are powered through that rail may help me in the future.

Feb 10, 2012, 04:26 PM	#2
mkk Cthulhu/Dagon 2012 Join Date: Oct 2003 Location: Gefle, Sweden Posts: 4,490 Rep Power: 132 System Specs	Re: Question on the power supply's 3.3v rail Not so much nowadays but it's up to the motherboard design. A good number of years ago even the CPU could draw power from there but that's all regulated down from 12V now of course. Memory slots are a possibility since memory doesn't draw that much power. Expect today mostly miscellaneous minor add-in components and signaling to draw from the 3.3V level, things that don't need much power to run and might be using a voltage not too far from the level of the source. Good old PCI cards could use 3.3V or 5V, and 3.3V is fed through PCIe card slots though mostly for signaling. 12V is available in abundance to anything that draws any real amounts of power on a PCIe card. It's possible that the use of 3.3V on the PCIe bus could rise in the future when everything might trickle down towards lower voltages, like if some future version of USB goes down to 3.3V or so. (the dependence of 5V is the most common reason for why some PCIe cards have molex connectors, as PCIe lacks 5V in the slot) But motherboard designers shouldn't be keen on increasing the 3.3V lines throughout todays motherboard design. In the end it's in large part a legacy thing today.

Feb 10, 2012, 04:44 PM	Thread Starter #3
Golbeze HardwareHeaven Addict Join Date: Feb 2006 Posts: 276 Rep Power: 0 System Specs	Re: Question on the power supply's 3.3v rail Hmm, now it's making me more curious. I got my current motherboard back in December 2009; a Foxconn ELA LGA 775 Intel P45 ATX Intel Motherboard, which is now discontinued. No idea how it uses the 3.3v rail. Guess I'll have to see if the crashes return, maybe test my old psu again to see what the exact values of the other voltages were.