How do PSU power rails work *Calling all Tech Heads**

Reedweaver · 17 May 2010 at 16:04

My PSU is 700W Jeantech Storm

12V1 is rated at 16A and powers the motherboard, CPU1 and first PCI-E plug
12V2 is rated at 25A and powers CPU2, the second PCI-E and S-ATA plugs
12V3 is rated at 17A and powers the Molex plugs.

12V1 is a 16A rail - Motherboard 4-6A – half of the i3 processor 3.25A, 4870 11A = 18.25A MAXED OUT
12V2 is a 25A rail - Other half of i3 processor 3.25A, 4870 11A, 3xRaptor HDDs 10A = 24.25 MAXED OUT
12V3 is a 17A rail -DVD Burner, 5xfans, fan controller = 8A so 9A to play with.

I need to take some pressure off the 12V1 and 12V2 rails…if I connect the 2nd pci-e power connector off both the 4870’s up to this rail how does the card know what power to pull from each source?

Can you even do this…

Akempster · 19 May 2010 at 11:49

they "draw" the correct number of amps they need to power it assuming its got the correct voltage. The rail just provides all the power andn the cards just suck up as much as they need of it.

Reedweaver · 20 May 2010 at 11:09

Having done some research its not a good idea to connect to two different rails..

By the way, in case you're ever tempted to hook the independent 12 volt rails together (I've seen people on the Internet who think this is a good idea), don't do it. Your 12 volt rails may have different ideas about what voltage they should set their rails to. One may be quite a bit different than another. They're separate rails, after all, and they have their own circuitry which controls the voltage. They're bound to vary a bit. And if they're just a little different then you can draw lots of current when you connect them together because each of the output circuits try to force the voltage on the same wires to a different value. That causes either a nice orderly shutdown from the over-current protection or smoke and sparks. There are some power supplies which have switches which allow you to gang the rails together. Once you've set the switch properly it's okay to connect them.

JonJ678 · 20 May 2010 at 19:53

It's very unlikely for an atx psu to have independent 12V rails. Certainly on the nicer side of 200 quid it's just not going to happen. Supplies which claim to are nearly always a single 12V rail which is artificially limited as a multiple rail system in the interests of safety. The 20A limit can be quite liberally interpreted by the manufacturer, and it's unlikely to switch off unless you go significantly over it.

I don't know where you're getting your values for the cpu current draw from. Some measurements from a 920 are available here which might be of use. It's very unlikely to be split equally between the 24 pin and 4/8 pin cable, and much of the cpu current draw isn't at 12V so you can't just divide the TDP by 12 to get the current it draws.

Nevertheless, assuming the information in the opening post is accurate, all you'd need to do is move some of the raptors onto 12V3. Molex -> sata connectors.

I strongly suspect that the "700W" figure is a work of fiction, and wouldn't expect the psu to be able to meet its ratings on all three rails simultaneously. Personally I wont use "multiple rail" power supplies, 60A at 12V doesn't feel great but it's hardly fatal so I don't see the point to the limiting.