Can someone explain split rail 12v to me?

[Elstupido]

Question is, what connectors from the PSU are supplied from each rail? Is there a standard for this, or is it totally up to the manufacturer. Reason for asking is I am trying to troubleshoot a problem with my system crashing after installing a new video card, with no external connector. Power is supplied by the PCIe16 slot

I think my PSU is undersized at 10amps on 12v v1, and 15amps on v2. System seems to shut down after a short period of time indicating an overloaded PSU. Wish I had the ability to balance the load better.

 

[Modelworks]

What video card ?
The pcie slots cannot provide much current.

 

[MarcVenice]

One 12v rail is dedicated to the 4 pin connector that you plug into the motherboard, and which feeds the CPU. This is standard ATX 12v design as I learned not long ago. The other rail is probably dedicated to the PCI-e plug and some molex connectors. What PSU are we talking about ? Short period of time could also mean heat, on the CPU or GPU, or perhaps on the PSU too. Oh, and the PCI-e 16x slot can provide 75w of power. If this is coming from the same 12v rail as the CPU feeds of from, you might get intro trouble.

This is pure speculation, but if the 12v1 rail does supply the CPU/MOBO with power, then thats a whopping 10*12 = 120w for the CPU, Ram, NB, PCI-e 16x slot. Thats not nearly enough of course. It's more realistic to assume that the 15a 12v rail is supplying the CPU/MOBO and the PCI-e slot with power though, which would be 180w of power, which MIGHT still not be enough for the CPU and the GPU.

Another thing, usually, if the card isn't getting enough power, you get a message saying the card is being underclocked not performing at it's maximum operating levels. That's what happens with some nvidia cards. Once again, what PSU, it sounds AWFULLY underpowered tbh.

 

[Elstupido]

After doing some reseach, I am under the assumption that the 4 pin CPU connector supplies the PCI slots, and the CPU from v1. Yet another search said the 4 pin CPU connector, is supposed to supply only the CPU from v2.

Anyway here are my specs

350w enlight PSU, made by Delta
ATI 2600xt, vid card, passive cooling by Gigabyte...power draw around 75w, max for a PCI slot, no external connector
ATI 550 TV tuner, around 30 watts to my surprise
Athalon 3800x2 windsor processor 65 watts
320gb WD YS Hdd
2 liteon 20x DVD drives
2 1gb crucial, 2 2gb Gskill memory sticks. Total 6gb in 4 slots
500gb WD external USB HDD
Onboard sound and NIC

Anyway, I have a Corsair 550vx coming in tomorrow with 41 amps on one rail, fuck the split rails, never did understand the need for them.

 

[MarcVenice]

Sounds like a cheap PSU to me, and I bet it's not rated at 50c continous, so when the heat goes up, the output capacity goes DOWN. And I bet it can't even deliver the full 300w of power on the 12v rails. You've did well buying that 550vx.

 

[Elstupido]

MarcVenice, I agree that this PSU was inexpensive, but Delta is a not a slouch in the PSU arena, not the best, but certainly not in the realm of many other discount PSU's rated at a much higher wattage. Even at 300watts, this should be more than enough to power my system, on a single rail. That being said, I really think a lot of the problems result from the split rail. Heat rating, or lack thereof has always been the bain of PSU wattages. Playing games with numbers.

I wish there were a standard for rating a PSU ie, ambient temps, equipment temps, etc., but until then we are stuck with artificial wattages for PSU's.

I still have yet to find a definative answer to my original question re 12v rails and just what connectors they supply. Just saying a PSU is cheap tells me nothing, especially with a respected manufacturer such as Delta.

And yes I do believe this is an underpowered PSU, regaurdless of cost, due to split rail.

We will see if I have a bad card, which is possible, but unlikely, after I upgrade the PSU

 

[Modelworks]

The atx spec states that Rail 2 is for cpu power alone and rail 1 is everything else.
Rail 2 must supply 16.5A peak current for 10ms;minimum voltage during peak >11.0vdc

The only reason they are split is because its considered 'slightly' unsafe to have a 20A+ 12vdc single line that could be shorted. I say slightly because its not really unsafe as long as proper care is taken.

I'm glad manufacturers are getting away from that and going with single 12v outputs.

 

[Elstupido]

Modelworks, thank you for the reply, that is how I read the specs, but the specs are not really followed. We are at the mercy of the PSU manufacturers.

You are right on the money about the single output.

Thanks again

 

[imported_wonderwrench]

Here is some good info
http://www.playtool.com/pages/...tirail/multirails.html

 

[jonnyGURU]

Originally posted by: Elstupido
Question is, what connectors from the PSU are supplied from each rail? Is there a standard for this, or is it totally up to the manufacturer. Reason for asking is I am trying to troubleshoot a problem with my system crashing after installing a new video card, with no external connector. Power is supplied by the PCIe16 slot

I think my PSU is undersized at 10amps on 12v v1, and 15amps on v2. System seems to shut down after a short period of time indicating an overloaded PSU. Wish I had the ability to balance the load better.

Your problem is not related to the PSU having a split +12V rail.

The "split" of a +12V rail is nothing more than a "current limiter", called an "OCP" for Over Current Protection, on a connector or group of connectors prior to exiting the PSU housing.

Split +12V rails are typically split from the same source, so any problems with voltage regulation (voltage dropping out of spec under heavy loads) would be present if it's a problem whether the particular unit had one +12V rail or a dozen if all +12V rails come from the same source. OCP does not regulate voltage, it merely limits how much current travels through them.

The ONLY symptom of a power supply that's having a rail exceeded is a unit shutting down, since this is the duty of an OCP. An OCP is programmed at a particular value and once this value is exceeded it's told to shut the power supply off.

Unfortunately, the same marketing people that told you that split +12V rails was going to give you "stability" is now telling you that single +12V rails are more stable. Same marketing people, both statements are lines of bull crap. But they'll have you believe that if your voltages drop making your system unstable, etc. that it's because you have split +!2V rails. Fact of the matter is, unless your PC is completely shutting down during Crysis or FEAR or whatever, your problem is not related to the number of +12V rails you have and how much power is on each rail.

Voltage droop, excess ripple and noise, etc. are all symptoms that can happen regardless of +12V being split or not because these are all symptoms of a rail being too weak (failing regulation) or something failing on the secondary filtration stage (like failing caps).

As far as rail assignment goes on your particular unit: When a PSU has two +12V rails, it is typical for the CPU to be on it's own rail and all other connectors to be on a second +12V rail.

 

[Elstupido]

Thanks Johnny, that explains it perfectly! I think my card is bad and I just RMA'ed it after buying the Corsair VX550W, no regrets there, looks like a very nice unit, for many upgrades to come.