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SLI and power suppliy



 
 
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  #1  
Old April 3rd 07, 02:51 PM posted to alt.comp.periphs.videocards.nvidia
Larry Stone
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Posts: 2
Default SLI and power suppliy

I have an 8800GTX and plan to add another. How critical is the power supply.
What happens if the PS is insufficient? The certified PS's at slizone all
cost at least $150.


  #2  
Old April 3rd 07, 03:15 PM posted to alt.comp.periphs.videocards.nvidia
peter
external usenet poster
 
Posts: 405
Default SLI and power suppliy

The PSU is the most important piece of hardware you can buy...at $150 its
cheap compared to what you paid for that 8800GTX...and you are adding
another $$$$$$ video card.
peter
"Larry Stone" wrote in message
newsZsQh.28585$oV.8460@attbi_s21...
I have an 8800GTX and plan to add another. How critical is the power
supply. What happens if the PS is insufficient? The certified PS's at
slizone all cost at least $150.



  #3  
Old April 3rd 07, 04:30 PM posted to alt.comp.periphs.videocards.nvidia
Andi Cole
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Posts: 23
Default SLI and power suppliy

My 8800GTX's requires two 6-pin PCI-e connectors each, that makes the
picture clear as to how much power is required. I'm using an Etassis 850
which has four 6-pin connectors so no need for the adapters. I really
wouldn't skimp on the PSU.

Andi.




"Larry Stone" wrote in message
newsZsQh.28585$oV.8460@attbi_s21...
I have an 8800GTX and plan to add another. How critical is the power
supply. What happens if the PS is insufficient? The certified PS's at
slizone all cost at least $150.



  #4  
Old April 3rd 07, 05:43 PM posted to alt.comp.periphs.videocards.nvidia
DRS
external usenet poster
 
Posts: 588
Default SLI and power suppliy

"Andi Cole" wrote in message

"Larry Stone" wrote in message
newsZsQh.28585$oV.8460@attbi_s21...
I have an 8800GTX and plan to add another. How critical is the power
supply. What happens if the PS is insufficient? The certified PS's at
slizone all cost at least $150.


My 8800GTX's requires two 6-pin PCI-e connectors each, that makes the
picture clear as to how much power is required. I'm using an Etassis
850 which has four 6-pin connectors so no need for the adapters. I
really wouldn't skimp on the PSU.


Nvidia told Xbitlabs the air-cooled 8800GTX peaks at 145W (12v air-cooled).
That's 12.1A in my book. That's roughly in line with the real-world tests
that show the 8800GTX is only about 25% more power hungry than the X1950
XTX. In any event, SLI 8800GTX has been shown to run quite happily on a
decent 500W PSU with 40A or so on the 12v rails, so when you factor in the
CPU @100% and what have you 20A per card is obviously an over-estimate.

The fact is you do NOT need a monster PSU to run SLI 8800GTX. You were
conned into buying a much more powerful PSU than you really needed.


  #5  
Old April 3rd 07, 06:54 PM posted to alt.comp.periphs.videocards.nvidia
Phil Weldon
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Posts: 276
Default SLI and power suppliy

'Andi Cole'
| My 8800GTX's requires two 6-pin PCI-e connectors each, that makes the
| picture clear as to how much power is required. I'm using an Etassis 850
| which has four 6-pin connectors so no need for the adapters. I really
| wouldn't skimp on the PSU.
_____

Your statement "I really wouldn't skimp on the PSU." is good advice.

However, like 'DRS' said.

AND

Molex connectors are specified for up to 4 Amps per pin. The 6-pin Molex
connector is limited to 12 Amps ( three + 12 VDC pins and three earth/ground
pins).
Display adapter + 12 VDC connectors are pretty much standardized for now at
6 pins. Since (according to the reference 'DRS' gave) the 8800GTX can draw
up to 12.1 Amps one 6-pin Molex connector is not quite enough. That leaves
two choices for the manufacturer; going to a non-standard connector OR
adding a second 6-pin Molex connector.

So you see, you can't just count pins and expect to come up with the power
requirements. Considering that the max current draw is less than 1% more
than the 6-pin Molex connector rated capacity you could easily get by with
just one connector attached. For extreme overclocking of the 8800 GTX using
the extra connector would be prudent, but not necessary.

(Using extra pins has a benefit in addition to accommodating the rated
current of a Molex connector. More pins means less voltage drop across the
connections, but this is a minor consideration when the contact resistance
is on the order of 0.01 Ohms, the Voltage ~ 12, and the current ~ 4 Amperes.
At lower voltages and higher currents the contact resistance becomes more
important. That's one reason for the change to + 12 VDC from +5 VDC for the
CPU voltage regulator/converters.

Phil Weldon


"Andi Cole" wrote in message
...
| My 8800GTX's requires two 6-pin PCI-e connectors each, that makes the
| picture clear as to how much power is required. I'm using an Etassis 850
| which has four 6-pin connectors so no need for the adapters. I really
| wouldn't skimp on the PSU.
|
| Andi.
|
|
|
|
| "Larry Stone" wrote in message
| newsZsQh.28585$oV.8460@attbi_s21...
| I have an 8800GTX and plan to add another. How critical is the power
| supply. What happens if the PS is insufficient? The certified PS's at
| slizone all cost at least $150.
|
|
|


  #6  
Old April 3rd 07, 07:15 PM posted to alt.comp.periphs.videocards.nvidia
Paul
external usenet poster
 
Posts: 13,364
Default SLI and power suppliy

Larry Stone wrote:
I have an 8800GTX and plan to add another. How critical is the power supply.
What happens if the PS is insufficient? The certified PS's at slizone all
cost at least $150.



Well, the question would be, how well behaved is your brand of power
supply, when it overheats ? Does it just cleanly shut off ?

The more you spend for a supply, the better the chances are that
it has a few safety features. There are some older prebuilt computers,
where when the provided supply failed, it virtually guaranteed killing the
motherboard it is connected to. With more modern power supplies,
that should be less of a concern. As long as we aren't dealing with a
$25 power supply.

Considering the amount of power that will be used by the two video cards
(24 amps for two cards, 6 amps for a cheaper Core2, from +12V), one
thing a more expensive power supply can give you, is more efficient
operation. Some of the new supplies are "80%+" efficient. Which means,
when you are gaming and drawing 12V @ 30A, the supply does not get
as hot internally, and the PSU fan doesn't have to spin as fast.
The efficiency means you save a bit of electricity. The expensive supply
may also have a working over-temperature, overcurrent, and overvoltage,
and perhaps in the event of a failure (as all power supplies can fail),
the rest of your system may be well protected. I don't see a downside
to spending $150+, seeing as you have $1000 tied up in two video cards.

HTH,
Paul
  #7  
Old April 3rd 07, 07:32 PM posted to alt.comp.periphs.videocards.nvidia
DRS
external usenet poster
 
Posts: 588
Default SLI and power suppliy

"Paul" wrote in message
Larry Stone wrote:
I have an 8800GTX and plan to add another. How critical is the power
supply. What happens if the PS is insufficient? The certified PS's
at slizone all cost at least $150.


Well, the question would be, how well behaved is your brand of power
supply, when it overheats ? Does it just cleanly shut off ?


That's a very good question. A couple of years ago THG tested thirty or
forty PSUs, including many big brand names, and about one third failed to
meet their own rated specs. Some just melted under the "torture" tests
(meaning they were expected to run at their own spec'd maximum rating).
PSUs are the most under-rated component of any PC.


  #8  
Old April 3rd 07, 07:35 PM posted to alt.comp.periphs.videocards.nvidia
Phil Weldon
external usenet poster
 
Posts: 276
Default SLI and power suppliy

'Larry Stone' wrote:
| I have an 8800GTX and plan to add another. How critical is the power
supply.
| What happens if the PS is insufficient? The certified PS's at slizone all
| cost at least $150.
_____

You ask "What happens if the PS is insufficient?"

Depends.

If you mean what happens if the PS capacity is insufficient, then
1. Everything boots up normally
2. Word processing and web surfing works normally
3. Playing a DVD movie works normally
4. While playing a game that requires intense 3D acceleration higher
currents are required by the CPU and the graphics adapter(s), PS output
voltages begin to sag, you begin to see artifacts in the images, and
eventually the system locks up and you have to reboot
5. If you are overclocking anything #4 happens sooner.

There should be no damage to the system from a power supply with
insufficient capacity, though the power supply itself may die an early
death.

The question you actually want to ask is 'will my present power supply be
sufficient when I add an additional 8800 GTX?'
In which case you need to post information on your power supply (total 12
VDC power, current capacity for each voltage and for each + 12 VDC rail,
specification level ATX +12VDC 2.2 is the newest specification), CPU, and
other devices ( drives, memory).

What is your time worth to you? Do you enjoy tinkering with hardware and
setup? Or do you consider all that a distraction from USING your high end
graphics?

My approach is likely different from yours since I don't play games that
require intense 3 D acceleration. But here's what I'd do.

I'll assume that the present power supply is of good quality and rated for
550 Watts and is ATX +12V rev. 2 or higher and that I have an Intel Core 2
Duo E6600 CPU, two 500 GByte serial ATA hard drives, a DVD R/W drive and 2
GBytes of memory, and one 8800 GTX. And that I am not overclocking.

Given the above, I'd add the second 8800 GTX and see what happens. If
everything goes smoothly, then great. If start up and low utilization works
fine, but problems occur during intense gaming, then I get a 650 Watt ATX
+12V rev. 2.2 power supply (and that will cost at LEAST $150.)

There is one little drawback to this approach. The new 8800 GTX may be
faulty. You are going to have a hard time getting an RMA if the
manufacturer blames the power supply (how is the manufacturer going to know?
well, during trouble shooting with technical support, you should be honest
about the actual operating conditions, right?). And suppose you do get the
RMA, and you ship back the 8800 GTX and wait around and get the replacement
and the same problem occurs? And THEN you get a new power supply, and that
fixes the problem.

So you have to have confidence in your own abilities in trouble shooting and
in your own understanding of the system OR be tolerant of highly frustrating
conditions.

Bottom line? In your case, a new power supply, ATX +12V rev 2.2 costing
more than $150 and a capacity of 650 Watts (more if significantly
overclocking CPU and two 8800 GTX) is a shortcut to success.

Hope this helps.

Phil Weldon





"Larry Stone" wrote in message
newsZsQh.28585$oV.8460@attbi_s21...
|I have an 8800GTX and plan to add another. How critical is the power
supply.
| What happens if the PS is insufficient? The certified PS's at slizone all
| cost at least $150.
|
|


  #9  
Old April 3rd 07, 08:11 PM posted to alt.comp.periphs.videocards.nvidia
Paul
external usenet poster
 
Posts: 13,364
Default SLI and power suppliy

Phil Weldon wrote:
'Andi Cole'
| My 8800GTX's requires two 6-pin PCI-e connectors each, that makes the
| picture clear as to how much power is required. I'm using an Etassis 850
| which has four 6-pin connectors so no need for the adapters. I really
| wouldn't skimp on the PSU.
_____

Your statement "I really wouldn't skimp on the PSU." is good advice.

However, like 'DRS' said.

AND

Molex connectors are specified for up to 4 Amps per pin. The 6-pin Molex
connector is limited to 12 Amps ( three + 12 VDC pins and three earth/ground
pins).
Display adapter + 12 VDC connectors are pretty much standardized for now at
6 pins. Since (according to the reference 'DRS' gave) the 8800GTX can draw
up to 12.1 Amps one 6-pin Molex connector is not quite enough. That leaves
two choices for the manufacturer; going to a non-standard connector OR
adding a second 6-pin Molex connector.

So you see, you can't just count pins and expect to come up with the power
requirements. Considering that the max current draw is less than 1% more
than the 6-pin Molex connector rated capacity you could easily get by with
just one connector attached. For extreme overclocking of the 8800 GTX using
the extra connector would be prudent, but not necessary.

(Using extra pins has a benefit in addition to accommodating the rated
current of a Molex connector. More pins means less voltage drop across the
connections, but this is a minor consideration when the contact resistance
is on the order of 0.01 Ohms, the Voltage ~ 12, and the current ~ 4 Amperes.
At lower voltages and higher currents the contact resistance becomes more
important. That's one reason for the change to + 12 VDC from +5 VDC for the
CPU voltage regulator/converters.

Phil Weldon


While I don't have a reference that lists the exact part number for the
PCI Express 2x3 power connector, pictures of it do suggest it is a member
of the Mini-Fit Jr. family. Mini-Fit Jr. is used for a lot of the ATX power
connectors. The pins on Mini-Fit Jr. are rated to carry more than 4 amps,
and the details are in a table here. Which raises the question as to why
the 8800GTX has two 2x3 power connectors. If it really does average around
12 amps total of consumption, then one 2x3 connector should be plenty. Now, if
current software and applications cannot drive 8800GTX to the wall, then
maybe we haven't seen the worst of it yet. It would almost seem two connectors
were used, to impress us (or annoy us), rather than solve a real problem.
I'm still waiting for Xbitlabs to get of their duff and do some measurements.
They are the only enthusiast web site to have a clue.

http://www.molex.com/catalog/pdf/5566VW.PDF

Paul



"Andi Cole" wrote in message
...
| My 8800GTX's requires two 6-pin PCI-e connectors each, that makes the
| picture clear as to how much power is required. I'm using an Etassis 850
| which has four 6-pin connectors so no need for the adapters. I really
| wouldn't skimp on the PSU.
|
| Andi.
|
|
|
|
| "Larry Stone" wrote in message
| newsZsQh.28585$oV.8460@attbi_s21...
| I have an 8800GTX and plan to add another. How critical is the power
| supply. What happens if the PS is insufficient? The certified PS's at
| slizone all cost at least $150.
|
|
|


  #10  
Old April 4th 07, 12:12 AM posted to alt.comp.periphs.videocards.nvidia
DaveW[_2_]
external usenet poster
 
Posts: 164
Default SLI and power suppliy

Running TWO 8800 GTX cards in a computer requires a massive, expensive PSU.
They are not kidding. If you choose to use an economical lower power rated
PSU you can plan on burning out your cards due to low voltage.

--

DaveW

___________
"Larry Stone" wrote in message
newsZsQh.28585$oV.8460@attbi_s21...
I have an 8800GTX and plan to add another. How critical is the power
supply. What happens if the PS is insufficient? The certified PS's at
slizone all cost at least $150.



 




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