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PSu confusion, plz help

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November 9, 2007 5:04:40 PM

I do apologize for the noobiness of my question, PSU has never been my strength. My question is on the +12V rails on a powersupply. I've tried to go through the sticky but it either confused me or did not answer my qusetion.

Ok now to my question: Lets say that I want to make a SLI system with the new 8800GT, C2D, 1x DVD burner, 1x WD Caviar 500GB HDD, 4x1GB crucial ballistic ram. On the modern PSU it will show the amperage on the +12v rails, usually there are four listed each 12v rail listad as 18A. Now My confusion is if a video card needs 12V 22A rail, (or two for sli, 2x12v 22A=44A?), but the PSU lists the 12v rails as 18A. SO would the two being used for a SLI setup combine the amperage of the two unused to create the needed amperage? I was told that the rails aren't additive and so then according to what I was told this could not be. Please help dispel this confusion of mine. Thank you.

More about : psu confusion plz

a c 243 ) Power supply
November 9, 2007 5:18:01 PM

Manufacturers power reccomendations are for the entire system not just the card.
Figure out the total combined amperage of your psu, it will sometimes be marked on the psu label as combined amperage or total wattage available to the +12v rails.
watts divided by volts = amps
November 9, 2007 5:46:02 PM

the PSU that I am looking at does not say the total amperage just the total watts. Now I understand the watts divided by volts=amps but if a Video card needs 22A on the 12V rail and the PSU has 4x 12v rails each 18A (4x 12v - 18A for ex.), then will the card receive only the 18A or does video card take the additional amperage that it lacks (4A) from the remaining three 12v rails amperage (54A)?
Related resources
November 9, 2007 5:59:21 PM

The video card does not need 22a

They recomend 22a because if you have a single rail PSU (which used to be standard) the other components use a lot of the available power.

18A is enough for any video card

Or you could get a PC Power and Cooling with a large single rail. I have the 610 silencer with 49a on one rail. Enough power for OC and 2 GTX video cards really...

It's on sale on newegg for $119 with free shipping...
a c 94 ) Power supply
November 9, 2007 6:09:54 PM

Neither. First, I can't think of a single video card that requires 22A. I can think of some that would need that much with the entire system attached, but video cards themselves don't need that much. Think of the x1950 series of cards, some of the most power hungry out there. A single card requires 30A on the 12V rail. Two of them in crossfire however don't require 60A, I believe its around 40-45A. This is because the x1950 doesn't need 30A for just the card, but the system as a whole would require that much. If you look at the jump in power needed for another card, the x1950 actually needs about 15A, the other 15A is spread across the CPU, motherboard, drives, etc.

Remember that multi rail PSUs spread the power out among the cables. Rail 1 might be attached only to the ATX bundle. Rail 2 might be attached only to the 4/8 pin CPU bundle. Rail 3 might be attached only to the 6pin(s) PCIe wires, while rail 4 might handle the other 6pin PCIe or the normal "molex" connectors It wouldn't take power from the other rails, as it doesn't need to. Lets pretend that you have a 4 rail PSU that does output 18A on each rail. Rail 1 is powering the motherboard through the ATX bundle, and rail 2 is powering the CPU. You don't need 30A on the 3rd rail, because you are already powering most of the system. If the power draw of the x1950 IS 15A, you easily have that on the 18A that the third rail provides.

Last, remember that there is a difference between what the rail can provide individually, and what the PSU can provide. Each rail separately might be able to output 18A, but that doesn't mean the PSU can output that much. For examply, you might think that 4*18A = 72A total, but the PSU might be able to only output 60A at any given moment. This means that the PSU can only output a max COMBINED power of 15A per rail. This is still plenty for most systems however.
November 9, 2007 6:12:37 PM

edited due to the lack of understanding at the time.
November 9, 2007 6:15:26 PM

4745454b said:
Neither. First, I can't think of a single video card that requires 22A. I can think of some that would need that much with the entire system attached, but video cards themselves don't need that much. Think of the x1950 series of cards, some of the most power hungry out there. A single card requires 30A on the 12V rail. Two of them in crossfire however don't require 60A, I believe its around 40-45A. This is because the x1950 doesn't need 30A for just the card, but the system as a whole would require that much. If you look at the jump in power needed for another card, the x1950 actually needs about 15A, the other 15A is spread across the CPU, motherboard, drives, etc.

Remember that multi rail PSUs spread the power out among the cables. Rail 1 might be attached only to the ATX bundle. Rail 2 might be attached only to the 4/8 pin CPU bundle. Rail 3 might be attached only to the 6pin(s) PCIe wires, while rail 4 might handle the other 6pin PCIe or the normal "molex" connectors It wouldn't take power from the other rails, as it doesn't need to. Lets pretend that you have a 4 rail PSU that does output 18A on each rail. Rail 1 is powering the motherboard through the ATX bundle, and rail 2 is powering the CPU. You don't need 30A on the 3rd rail, because you are already powering most of the system. If the power draw of the x1950 IS 15A, you easily have that on the 18A that the third rail provides.

Last, remember that there is a difference between what the rail can provide individually, and what the PSU can provide. Each rail separately might be able to output 18A, but that doesn't mean the PSU can output that much. For examply, you might think that 4*18A = 72A total, but the PSU might be able to only output 60A at any given moment. This means that the PSU can only output a max COMBINED power of 15A per rail. This is still plenty for most systems however.


I do apologize for my last post, I wrote it before I saw your post 4745454b. Thank you for the input.

And so now I understand about the Amps, what I still am lacking in understanding is how to correctly determine what PSU 'Y' will work in 'X' situation. For example my way of thinking in the past was that yes, wattage is inportant, but Amps is what carried the system to work. Now I know better, and I thank you all. Now my situation X is that I want a PSU that will correctly power a EE6750 along with 2x 8800GT in SLI (DVD and HHD are single, othing more). How would I know how much Wattage my system will now need? I thought that a 600Watt PSU could typically power a SLI configuration (the 8800GT I believe in SLI take around 480W unless I am mistaken). So why or what would the reason be that I would want (or need?) to get a 1KW PSU or a 700W PSU?
a c 139 ) Power supply
November 9, 2007 6:31:56 PM

A video card in the PCI-e x16 slot can get 75 watts of +12volts from the slot alone.
That's why many video cards do not need any extra power from a 6 pin PCIe adapter.
One PCIe 6-pin adapter is able to provide up to an additional 75 watts (8800GT).
Two PCIe 6-pin adapters is able to provide up to an additional 150 watts (8800GTX).
If you video card and PSU have the new 8 pin PCIe adapter it can provide up to 225 watts. (HD 2900XT)

Your 8800GT will not need 22Amps. And no, an 8800GT SLI system will not need 44Amps - it will need 26Amps.
The 22Amps +12V recommendation is for your entire system with a single 8800GT.
One +12V rail is usually dedicated to the CPU alone.
The best PSUs have multiple independent 12 volt rails.
Most PSUs use multiple current limited 12 volt rails derived from a single rail.



November 9, 2007 6:37:47 PM

so the 8800GT will need average about 150Watts then with a SLI system around 300-400 watts. Ok, So for my build will these be ok?

OCZ StealthXStream OCZ600SXS ATX12V / EPS12V 600W Power Supply - $89.99

or for 25$ more should I get,

OCZ GameXStream OCZ600GXSSLI ATX12V 600W Power Supply - $114.99
a c 139 ) Power supply
November 9, 2007 6:38:39 PM



Although the minimum recommended 450Watt and 24Amp power supply would probably meet your described system needs I think most everyone would recommended stepping up a size (or two if not from a respected PSU Mfgr) just to be safe and allow for future upgrading.
a c 139 ) Power supply
November 9, 2007 6:50:48 PM

It would be best if you googled up some reviews on the PSU's you're considering so you know what you're paying for and what type of quality you're getting.
StealthXStream OCZ600 review

How important is it to you to get a "SLI Certified" PSU?

November 9, 2007 7:12:59 PM

I would look at something along the lines of a 650-750watt PSU.OCZ,THERMALTAKE,or any other solid name brand will do.Just make sure the psu you get has at least 28 amps on the 12v rails each.Or if it is a single 12v rail,then 50 amps would be what you are looking for.Goodluck.

Dahak

M2N32-SLI DELUXE WE
X2 5600+ STOCK (2.8GHZ)
2X1GIG DDR2 800 IN DC MODE
TOUGHPOWER 850WATT PSU
EVGA 7950 GX2 550/1400
SMILIDON RAIDMAX GAMING CASE
ACER 22IN WS LCD 1680X1050
250GIG HD/320GIG HD
G5 GAMING MOUSE
WIN XP MCE SP2
November 10, 2007 4:00:00 PM

And the saga continues... (you like that? I thought it was clever)

First I want to thank everyone on TH Forums and this thread. For the last 4-5 days I have, literally, spent 2-3 hours on the forums and newegg and google to research and create a new gaming rig. I've learned a lot lately about PSUs, SLI, Case Mods, etc...

Right now I am still worrying about my decisions to go SLI or not, (I have another thread in the Video Card section: http://www.tomshardware.com/forum/246064-33-false-truth#t1760088). Not only that but 'IF' I were to go SLI, then my PSU question rises. Should I stay with the OCZ 700w which is certified SLI for 8800GT (www.slizone.com)? I would really like a modular PSU due to that fact that the P180 family are a little difficult to manage (built a pc with the silver P180B and it was a pain but was and is my favorite looking case). Now the Corsair HX520 is in my price range and is modular but is not listed in the slizone recommended PSU for the 8800Gt, would that be enough to power my future system (SLI 8800GT C2D e6750 OC to 3.0GHz)?
a c 243 ) Power supply
November 10, 2007 4:05:48 PM

According to EVGA you would need a minimum 450w\24amp psu for the 8800 GT in SLI, so the 520w\40amp Corsair HX520 should do.
November 10, 2007 5:12:46 PM

Overclocking can drive the required watts and amps much higher. As you do not have an understanding of power supplies I doubt that you understand requirements for a stable platform when overclocked.

Long term stable overclocking requires stable power and I suggest that you demand an 80 PLUS certified power supply. Seasonic makes outstanding power supplies, for example, and the power statements for them are continuous power, unlike many manufacturers who's power ratings are actually maximum peak power, not maximum continuous power.

As soon as you could afford one I would purchase an UPS [uninteruptable power supply] that would provide at least 550 watts, APC makes some good ones and Seasonic is not picky about sine or stepped sine wave, as some other power supplies are.

Here is a discussion about power supplies that you may be able to get a little more information from.
a b ) Power supply
November 10, 2007 6:34:11 PM

Stretch your budget and buy this. I have seen it run Dual 8800GTX cards with Raid 0 raptors, and a Qx6700 without even breathing hard. Problem solved. http://www.newegg.com/Product/Product.aspx?Item=N82E168...
What can I say? I have a friend with way more money than sense.
November 10, 2007 7:30:13 PM

I just got a great deal on a Cooler Master Real Power 750W 12V V2.2 SLI Active PFC 80PLUS power supply (well under $100 after MIR). Although I don't pretend to know what ALL that means, I checked the settings for the rails and it's pretty nice.

My suggestion would be to set your sights a little high. Find about 5 PSU's that you really like (and that are a little overkill) and search around different sites for a week. You're bound to find one that's on for a really good deal. Before I bought mine, I was gonna go with an OCZ 600W or 700W GameStream for over $100 (even more for the 700 of course) when I stumbled across the deal on the Cooler Master.
a c 139 ) Power supply
November 11, 2007 2:02:24 PM

tvh said:
Now the Corsair HX520 is in my price range and is modular but is not listed in the slizone recommended PSU for the 8800Gt, would that be enough to power my future system (SLI 8800GT C2D e6750 OC to 3.0GHz)?
The HX520 should easily handle your new system (including more OC'ing headroom for the CPU and GPUs).
Thermaltake Purepower RX 600W @ $105 after rebate is also modular & is listed as supporting SLI 8800GTs @ Slizone.
[:wr2:5] But over at ClubIT you can get that same Purepower RX 600W for $89 [:wr2] ($122 @ Zipzoomfly - go figure)
Purepower RX 600W review





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