Purpose of 2 12v rails

[Herr Rogers]

What's the purpose of two 12v rails on a PSU?
What makes it worse for OCing?

[3DFlyer]

The pupose? I haven't quite figured that out yet.

As for OC'ing it does nothing except hurt OC's.

Which would you rather have? 2 split 18amp rails? Or a Big Strong 33amp rail?

I chose the latter, and I don't see any weakness at all.

Even OCZSteve has stated he likes the single rail units better.

If you have a 12v oriented system (especially an Intel system!) stick with a strong single rail unit.

There are a couple good dual rail units out there, but the single rail units still OC better. :)

[veryhumid]

i thought they did two separate rails to stabilize things. I think it's easier to make two small rails working together, than one big rail.

anyone know why it hurts overclocking?

here's a link i found over at silent pc: http://forums.silentpcreview.com/viewtopic.php?t=23916

[3DFlyer]

i thought they did two separate rails to stabilize things. I think it's easier to make two small rails working together, than one big rail.

anyone know why it hurts overclocking?

Becasue they are spliting the amperage (the current that the voltage (pressure) pushes through). In other words, one strong rail is better than to weak rails.

Also when you cross-load one (load the 5+ along with the 12+) the unit will become even more unbstable.

That's why guys are using the OCZ 520's instead of ther 600's. The 600 is rated stronger, but it's actually weaker.

volts*amps = watts

Edited: I had to change my Ohm's Law around. I had it backwards...arrrgh!

[larrymoencurly]

Does it have to do with some safety standard, where they the shutdown power or current limit has to be made too low for a single rail to power everything in a super-fast computer?

[speed bump]

Its easier to build a two rail meduim wattage PSU VS a one rail high wattage PSU espcially with your 12v line which is your highest voltage rail in your system.

[marker]

Becasue they are spliting the amperage (the pressure that pushes the voltage through). In other words, one strong rail is better than to weak rails.

Also when you cross-load one (load the 5+ along with the 12+) the unit will become even more unbstable.

That's why guys are using the OCZ 520's instead of ther 600's. The 600 is rated stronger, but it's actually weaker.

volts*amps = watts

Actually, the Voltage is the "Pressure" that drives the current (amps = volts / resistance).

I suspect that the dual rails add stability. It is probably more difficult to design a single rail that can source the fully rated amps than two rails that each provide half the fully rated amps. Parts (transistors, capacitors, inductors, wires, connectors) that can handle higher amps (i.e. 33) are generally larger, more difficult to manufacture, and more expensive to buy. In fact, from what I remember, most of the molex connectors are only good for around 10 - 15 amps. I imagine dual rails also help isolate the affects of power spikes on different circuits and if one 12v circuit is shorted out anything in its path will only be hit by half the amps.

[Know Nuttin]

I thought it was because Intel thought that the limit should be 240VA through a single +12v. That would equate to only 20A max, which is not enough, so they split it to 2. Problem is, internally, it's still one big rail but just feeding 2 outputs.

Other issue is if one rail needs the power that it cannot provide but the other rail is being underutilized. So you have a deficiency in one and an abundance in another. Either +12v cannot exceed 20A output regardless of how much excess there is available. So if you have a CPU that needs more than 20A, it's out of luck.

[veryhumid]

you would think that if some of your components used most of what a rail has to offer it would just pull some off the other rail?

this is so stupid, thank you very much latest ATX standard :(

The quest for a solid, but silent psu continues. I wonder how quiet those 520 are...

[dj_2004]

Quoted from davidhammock200: "Electrical isolation for noise and improved voltage regulation. Some devices, usually electromechanical ones having motors, produce noise spikes & voltage fluxuations, these can include pumps, compressors, fans, drives, lights (when turned on & off) and probably TEC’s & Peltiers. By putting these “noisy” devices, that often turn on & off, or at least often change their power (amperage) demands, on a separate rail we isolate them from the MoBo, CPU, Video Card(s) and the other electronic components that are sensitive to noise & sudden changes in voltage. This is a good thing! ATX12 V2.0 is the future."

[veryhumid]

here's some more good info from silentpc:

TECH TIP: ABOUT "INDEPENDENT" 12V LINES

Intel's ATX12V V2.2 PSU Design Guide, the industry bible for PSU makers, states:

"In cases where expected current requirements is greater than 18A a second 12 V rail should be made available.

"The 12V rail on the 2 x 2 power connector should be a separate current limited output to meet the requirements of UL and EN 60950.

"12V1DC and 12V2DC should have separate current limit circuits to meet 240VA safety requirements."

It's important to remember that when there are two 12V lines, they still draw from the same main source. It's not like there are two 120VAC:12VDC power conversion devices in a PSU, this would be way too costly and inefficient. There is only one, and the two rails draw from the same transformer. Each rail is coming from the same 12V source, but through its own "controlled gateway".

An analogy that may help: Think of 12V1 and 12V2 as two identical water taps fed off short pipes joined in a Y-junction to a single larger pipe. The total amount of water flow available through the two pipes is always the same, it's dictated by the pressure behind the big pipe. Each of the two taps have a maximum potential water flow potential that is lower than the maximum available through both taps together. How much water flows through each pipe depends on how much each is open. The "position of the tap" in the PSU is dictated by the power demand of the components connected to it.

PSU makers' specs are misleading in that they rate the current capacity of each 12V rail independently. What really matters is the total 12V current: Generally, up to 20A is available on any one 12V line, assuming the total 12V current capacity or the total current capacity on all the output lines is not exceeded.

What the above means is that you don't need to worry about imbalances in power draw on the 12V lines -- as long as no single rail is asked to deliver more than 20A. PSU makers seem to mark each line for max current on a purely arbitrary basis. A PSU like the Seasonic SS-400HT, rated for 29A max total on the 12V lines, can be labeled many different ways:

12V1: 11A, 12V2: 18A
12V1: 12A, 12V2: 17A
12V1: 13A, 12V2: 16A
12V1: 14A, 12V2: 15A
12V1: 15A, 12V2: 14A
12V1: 16A, 12V2: 13A
12V1: 17A, 12V2: 12A
12V1: 18A, 12V2: 11A

It could also be marked 19A + 10A or 20A + 9A, but being a cautious bunch, engineers will probably not specify more than 18A on any one line. This allows a 2A margin of error for the current limiting circuit.

[3DFlyer]

How quiet are the 520's? If you are referring to the OCZ520...VERY! :) If you have any other fans at all running you cannot hear it, and if you turn them all off, it is barely detectable. If you have a video card fan running, you will not hear that PSU at all. If you want a powerful unit, that's about as powerful as they come with spending upwards of 500 dollars or more.

[3DFlyer]

Actually, the Voltage is the "Pressure" that drives the current (amps = volts / resistance).



You are correct, I had my Ohm's Law backwards. Thankyou for correcting that. I'm now going to edit that in my post above. :)