Power Supply Basic Facts.

[Zka17]

As I’m getting older learning new things is getting harder... Man, how many times I clarified for myself that the 5V and 12V are in the same rail! Well, I really appreciate this thread – by the end will get all this things correctly... hopefully...

Regarding my post #30, xeromist didn’t answer my question: How I’m getting different wattage value using the same PSU calculator with the same components? I was asking you to check if I didn’t miss anything filling up that form.

You’ve got 981W versus 648W I’ve got as result. That’s serious difference, DaemonCantor! How that happened?! Since then every day, multiple times checking it but every time gives me the same result: 648W. You’ve mentioned the Newegg’s PSU calculator, with the same hardware – that’s much simplier (asking for less info, offering less choises), I choosed CPU: AMD Phenom X4, high end desktop motherboard, one HD 5870, two sticks of 4GB DDR3, two blurays and three 5400rpm 3.5” HDDs. Guess what? “Our recommended PSU Wattage: 624W”! It seems pretty consistent with the Thermaltake’s one...(considering that is asking less things, so giving lower value). That’s far less than 1192W you were talking about...

I’m really trying to understand how you, DaemonCantor, came up with those high wattages! I could accept that different calculators may show different results (that’s why you should rely on the values obtained on a wattage meter at the wall), but the same calculator with the same parameters is giving the SAME results...

“Now remember the link I posted on the power of the 5870 that same system would need a 350watt + 354watts = 704watts minimum with out any room for possible extra power but you still have to add that into it and like I was saying you need a minimum of 30% for reserves which in this case is 212watts so now were at 704w + 212w = 916watts.” – are you reffering to post #12 (and this link: http://www.tomshardware.com/reviews/...2422-20.html)? Just because the wattage value of 354W given there for a single HD5870 is the whole system’s consumption, not the graphic card’s only! So from where did you get the extra 350W to end up with 704W?!

“Like Kipper stated basically 'Do the Math!'.” – yeap, man, but if you’re using a formula, then explain each value you put there!

As I said, I’m really trying to understand these things...

[XZEMi]

Thinking i'll invest in another corsair cx430. You know back in 1999 the 436watt enermax whisper power supply was $120 and people said it was really more than you'd ever need. I tested that theory and used one to run an amd x4 550BE converted it to sata really it had no trouble running the system but it wasnt nearly as efficient as these newer power supplies. The CX 430 is excellent and can be had for under $25 at newegg on a good sale. Oh and just a little something I looked up for rewiring my power supply connectors. connector_atx_pinout.gifpciexpress-pinout.gifSerial-ata-connector.gif

[xeromist]

I just wanted to comment on the single vs multi-rail bit. Some of the justifications given for multiple rails is that they provide some degree of isolation between components and that by splitting closer to the transformer you have less current traveling through many of the components so they can potentially last longer (given equal quality of components). Being multi-rail is not a direct indicator of quality, just a different approach to things. In the early days of the multi-rail fad the people utilizing it were doing it to produce cleaner and more stable power supplies. At some point the el cheapo manufacturers jumped on the band wagon because they realized they could use smaller & cheaper components while drawing on the good name of multi-rail established by their betters. Likewise there have been manufacturers using cheap components to produce single rail that might stand up to the current but with a nasty ripple and voltage swings barely within spec.

The drawback of multi-rail is that you have to properly allocate those rails whereas single rail is just plug & go. And if you have a device that draws less than the rail allocation this could mean some amount of power overhead being "trapped" in one of the rails and thus unusable elsewhere. Even with perfect allocation on your current build a side effect of this is should you move the PSU to a dramatically different build you might find that the rail division is no longer convenient.

All of that said, single or multi rail I am most concerned about the quality of the power produced. It doesn't matter how many rails it has, if it won't deliver clean power I won't buy it.

[Zka17]

Wow, xeromist, I never heard about "the drawback of multi-rail is that you have to properly allocate those rails whereas single rail is just plug & go" - it sounds serious though!

What exactly means to "properly allocate those rails"? Taking as example my Silverstone ST1500 with it's eight 12V rails...

[xeromist]

In the classic definition of multi-rail each rail is allocated a fixed division of power which cannot be used on other rails. Say a rail is allocated 15A but whatever is connected to it is only drawing 5A. There is another 10A of head room you aren't using. If another 15A rail is connected to something trying to draw 20A then you will hit overcurrent protection even though you still had power to spare elsewhere. In the case of an ST1500 this really shouldn't be a problem because each rail is rated to 25A and even high end cards don't pull more than that from a single connector. That's why such cards have 2 or more connectors.

Also, some "multi-rail" PSU's are a bit tricky because they are essentially combining rails as needed or otherwise trying to work around the issue of unequal distribution.

[Zka17]

So, based on these, every connector on the back of a PSU is meaning a separate rail?

In case of the ST1500 the 24 pin, 8 pin and 6 pin connectors are all meaning a rail? Or this one is "tricky" too?

Also, generally speaking, if you have a PSU with one PCI-E cable but the graphic card would require two - what is best to do? Split that PCI-E cable/connector - risking that you can ran into the over current protection. Or use some adapter cable from an other connector - which may not be designed for graphic cards (like other rail with lower amperage).

Thanks xeromist, I really feel that I'm getting closer to understand better this component of a pc!

[xeromist]

So, based on these, every connector on the back of a PSU is meaning a separate rail?

Not necessarily. I have had multi-rail power supplies with two connectors on the same rail but each of those rails was separate from each other.

In case of the ST1500 the 24 pin, 8 pin and 6 pin connectors are all meaning a rail? Or this one is "tricky" too?

To learn more about your PSU I would recommend Jonny's review. He generally does a pretty good job and he also discusses rail division on the ST1500
http://www.jonnyguru.com/modules.php...Story&reid=175

Also, generally speaking, if you have a PSU with one PCI-E cable but the graphic card would require two - what is best to do? Split that PCI-E cable/connector - risking that you can ran into the over current protection. Or use some adapter cable from an other connector - which may not be designed for graphic cards (like other rail with lower amperage).

Ideally it's best to use a PSU with the correct connectors but if you have an older PSU with adequate power and a shortage of connectors you can use adapters. It's important to keep the specs of the PSU in mind however and not just throw some adapters on any old way. I've used splitters on 25A rails with cards that weren't overclocked and 4pin molex adapters on drive/accessory rails with only a few other devices. In both cases I knew I had the power available so I felt safe doing it.

[Zka17]

Thanks very much, xeromist - just finished reading Jonny's review I found since my last post. It really is a nice review!

[FR3D1]

Hi XZEMi since ya like an expert at psus can u help me choose a good psu?
I want a good, cheap, sli certifiled psu, save electric so which is the best value to choose? What brand and model?

[Zka17]

FR3D1, I bet you would be more satisfied with XZEMi's answer if you would mention what you need that "good psu" for... Man, how you expect anybody to give you a correct hint if you don't post your system's specs?! As you can see in this thread, there are a lot of discussions even in the case we know all components of a pc. So, go ahead and specify your hardware!