1 or 2 12v rails in a PSU?

I was thinking of getting a geforce 8800 gtx but it is a power sucking monster that wants 30 amps on the 12v rail. My enermax 620W psu only has 22 amps on each of 2 12v rails. Most psu's seem to have multiple 12v rails with lower amperages compared to those with high amp single rail versions. It has been a long time since I studied psu spec issues. Is problematic to by a psu with only a single 12v rail? Does anyone suggest a good one?

 

The purpose of the multi-rail PSU is to provide constant, stable power to the demanding controllers such as the gForce 8800 you mentioned. If your PSU doesn't have the amperage for the GPU, you should either get one that the PSU will support or get a PSU to support the GPU. As I understand it, getting a single rail PSU would be a step down in technology and performance.

This one would do the job.

 

An alt. route might be to spend another $20 or so for a more efficient gfx card like the HD6770, if this 8800GTX/5770 comparison is anywhere near accurate.

 

Well, I bought generic HD 4850 card and a thermaltake tr2 600w psu (32A and 24A on the two 12v rails) but the card produces nothing but scrambled video on startup, and black screen in windows xp.

Is it possible I am still underpowering the hd4850 (it would be easier to find a new psu than to fix incompatability problems)? I even tried using a secondary enermax psu 620w to no avail, so I doubt it is underjuiced.

The power requirements are vague for the Hd 4850: 450 Watt or greater power supply with 75 Watt 6-pin PCI Express power connector recommended. Are there really any psu's that don't have 75w pcie connectors? How would one tell, as it is not listed in any psu spec that I have ever seen?

Unfortunately, I suspect I just have an incompatability with my old asus A8ae LE motherboard pcie x16 1.0 slot with the 2.0 card, even though others have used the hd 4xxx series successfully on that mobo (but the hd 5xxx series was universally unsuccessful).

 

Are you sure your monitor can support the resolution and refresh rate the video card is sending it?

Whats the make and model of the monitor?

 

Hmm, from what I recall, the 4850 needed a fair bit of juice to power.

The LE designation on your motherboard name makes me think: "just like the real one but with bits left out and de-tuned", similar to the SE often added to the names of cheaper graphics cards.

I'm leaning more towards a 'board compatibility problem now, esp. after your detail about 5xxx not working with it.


Check shnerdly's comments too.

 

The computer is a hp a1210n with a microATX board that was used in about 40 other hp/compaq computers; the OEM boards I run accross always seem to have compatability problems, but I am limited in funds. What is de-tuned?

The monitor is a small Hp vs17 LCD monitor I am using mainly because it is easy to move about when I am testing the computer. I have a larger crt monitor (viewsonic G790) and an hd TV which I have not tried. I thought when a card is initially installed it would default to the lowest refresh rate and lowest resolution until the drivers loaded in windows; is that wrong? I think windows crashes, though I can't tell with the screen blacked out. The appearance of the screen when the computer first is turned on is lots of artifact with lots of random blocks of color and random characters accross the screen, then it goes black after the windows xp startup screen. I can at least see enough to set the bios to pci-e.

If the the res and refresh is too high, is there a way to lower it prior to startup?

The computer runs fine when I set to onboard video.

 

Yes, many of the 'boards fitted by HP/Compaq would slot into the reduced functions/fewer memory slots/hobbled by BIOS categories detuned = crippled, compared to the original 'board it is based on.

Another problem that might fit the description of the output at boot is a damaged VGA cable or connector, check the pins, wriggle the cable. Or possibly a bad RAM stick - many chipsets of about this age used the RAM in the closest slot to the CPU to power the graphics, try switching sticks or cleaning the RAM contacts with a pencil eraser.

You should be able to change the refresh rate from Windows Safe Mode.

 

http://techreport.com/articles.x/14967/10.

The HD 4850 draws 'actual' 240W under full load which is 20 amps,if your right and your PSU has two rails you really need to use the 32amp rail,if your PCI-E connector is on the 24amp rail you can buy a molex to PCI-E converter to use the other rail, my PSU actually came with them standard,the only worry then though is overloading the 32 amp rail but as your limited in funds it may be an option.

http://www.play.com/PC/PCs/4-/35161....html?_$ja=tsid:11518|cat:3516123|prd:3516123

As to multiple rails they are a hindrance as this thread proves,my current 950W PSU has a single 80amp 12V rail so there is no way one rail can be over loaded and the headroom on the other rail wasted,the single rail doesn't affect stability at all either.

I agree I don't think under powering is your problem the card will hardly draw any current loading windows and you say its corrupted in bios also? That's bad and not a symptom of an under powered card,under powered cards lock up and corrupt when you run a game or benchmark.

I have little advice really other than try a different monitor,try the card in a another computer to confirm it works and try a bios flash,I can't find any info on the ASUS site regarding your motherboard so I don't know if there's one available.

 

It took some time, but I borrowed a second, much newer computer with an i3 and a pcie x16 2.0 slot and the the hd 4850 produced absolutely no video in it. Unfortunately I was still having to use the thermaltake psu, but it makes me think the card is defective. I am going to RMA it. Time was running out on the return and could not get a molex adapter quick enough.

Is there a way to tell which cables on a psu have the high amps on the 12v, and which the low? Will it always be the pcie or molex or does some portion af the 12v rail go into the mobo connectors?

 

The easiest way is the look at the diagram on the side of the PSU to see if they are labelled,they sometimes have a colour code and key relating to the the yell cable which is the +12 line,all the lines with the 12V V1 will be yellow and all the yellow lines with +12V V2 will have a stripe on so it will be labelled +12 V2 Y/W to signify a yellow and white stripe.The other way is to see how many connectors is on each set of cables the higher amperage obviously usually has the most. A lot of the time especially in cheap PSU's they don't even have seperate rails they're all connected internally and the rails just signify how much power can go through each set of wires.

Usually everything on the motherboard is connected to V1 and everything else is connected to V2.

If you don't have many other devices connected I would connect it the way you already have and see how it performs,when you've got a new card install it and run a benchmark such as 3d mark vantage,leave it running for an hour and intermittently feel your PSU with your hand,it shouldn't get much warmer than warm/low hot tap water.

 

I got the replacement HD 4850 and so far it works great! It even seems (I have not yet pushed it for hours) to run NWN2, a game which is increadibly picky with hardware.

I don't own any of the 3dmarks, but I have the freeware of 05, 06 (my single core cpu could barely handle 06). I there a way to make them run continuosly?