195W Graphics on 300W Power?

[Aegean BM]

I put new life in an older hand me down HP Pavilion Elite m9040n by switching from the integrated GPU of Intel Core 2 Quad Q6600 to discrete AMD Radeon HD 7750. I chose the 7750 because it was cheap, had minimum oomph, didn't need additional power connectors, and would probably not overdraw on the 300W PSU. I also bought a UPS (saved me twice in the last 6 months). The UPS display says PSU is only drawing 120 W as I type this. I can get to 180 W if I play 3D game like Borderlands @ 1920x1200. I can artificially stress it with benchmarks up to 220 W, but that's the limit.

Minimum oomph is far from happy oomph. Seems to me that I could replace the 7750 with something much beefier. If the 7750's TDP of 75W leaves about 120W untouched, is there any reason I shouldn't get a better card as long as the TDP is under 195W (75W + 120W)? I know this assumes I'm not going to put anything else in there like more memory, or plug in another USB device, or ... anything. I also know that power from the wall does not equal motherboard power, does not equal TDP. I'm depending on it. If I'm drawing 300W from wall, I shouldn't be maxing out a no name 300W PSU that probably would not even get an 80+ rating.

I'm no noob, but I'm far from expert. I'm more noob++, you know, the classic "knows enough to be dangerous". Thanks for the sanity check. And a double thanks if you stop me from doing something stupid.

 

[alvine]

if you are thinking about upgrading from 7750 you need to get a better power supply

 

[geofelt]

Any card stronger than a 7750 will need an auxiliary 6 pin pci-e power connector.
A very strong card will need two or more.
I would not trust any 300w psu to handle that load.

The newer 28nm cards(GTX6xx/amd7xxx) cards do not need lots of power, and a 500w psu need not be expensive.
Here is a handy chart of psu requirements:
http://www.realhardtechx.com/index_archivos/Page362.htm
A GTX660ti or 7850 will need only a quality 450w psu.

 

[Derza10]

Does that PSU even have the 6 pin connectors that you would need if you added a better GPU?

 

[blader15sk8]

I wouldn't push it. Power supplies rarely live up to their rating, especially after a few years. Also, more importantly than that wattage of the PSU is the Amperage on the 12v rail(s). Just because the power supply itself is capable of a certain wattage doesn't mean it puts enough of that out to the rail(s) the GPU is on.

 

[popatim]

Exactly as Blader says, the 300w your psu can make is not all on the 12v rail.

Face it, if you want a better GPU you'll need a better PSU to go with it.

 

[Aegean BM]

Does that PSU even have the 6 pin connectors that you would need if you added a better GPU?

Yes, it has some 6 pin connectors. HP had a discrete card in there with a 6 pin. The card went bad, so the family threw the "broken" computer to me for scrap. It's been an awesome learning experience changing out parts, good prep for a new build later this year.

 

[Aegean BM]

I wouldn't push it. Power supplies rarely live up to their rating, especially after a few years. Also, more importantly than that wattage of the PSU is the Amperage on the 12v rail(s). Just because the power supply itself is capable of a certain wattage doesn't mean it puts enough of that out to the rail(s) the GPU is on.

Degradation over time? good point

Amps on 12v rails trumps watts on PSU? D'oh! I oversimplified it. That's exactly why I posted the question. Most people jump to the end conclusion of get a bigger better power supply without really explaining WHY my current conclusion is faulty other than to refer to some GPU/PSU recommendation chart.

Double thanks for not letting me do something stupid.

 

[Aegean BM]

Exactly as Blader says, the 300w your psu can make is not all on the 12v rail.

Face it, if you want a better GPU you'll need a better PSU to go with it.

Yes, I'm coming to that conclusion that if I really want to do it right, a better GPU means getting a better PSU. Because this machine is upgraded about as far as I can take it, a really decent GPU will be wasted on it. I'm mostly looking at this as a learning exercise, you know, like let the kids learn to drive on the beater car.

Since I drew my false confidence from the total watts out of wall, if I wanted to regain it, I would measure the 12v rail(s) amps. To be safe I'd need to know total possible amps - current usage without discrete GPU - 10% fudge factor (margin of error, future PSU degradation). That would tell me the total amps a new card could take. So I borrow an ampmeter. Can I get total possible amps? Can I get current usage? Basic electronics is deep water for me. But hey, I want to learn to swim.

 

[geofelt]

Yes, I'm coming to that conclusion that if I really want to do it right, a better GPU means getting a better PSU. Because this machine is upgraded about as far as I can take it, a really decent GPU will be wasted on it. I'm mostly looking at this as a learning exercise, you know, like let the kids learn to drive on the beater car.

Since I drew my false confidence from the total watts out of wall, if I wanted to regain it, I would measure the 12v rail(s) amps. To be safe I'd need to know total possible amps - current usage without discrete GPU - 10% fudge factor (margin of error, future PSU degradation). That would tell me the total amps a new card could take. So I borrow an ampmeter. Can I get total possible amps? Can I get current usage? Basic electronics is deep water for me. But hey, I want to learn to swim.

If a PSU is a quality unit, and has a 6 pin pcie connector, then it will be able to power that connector. Usually, a 300w unit will not.
If a psu does not have the requisite 6 pin connectors, I would not try to use a molex to 6 pin adapter. Either the psu is very old, or it is cheap.

Look at the data plate on the psu. Every UL certified psu must have one. Look for the amps on the +12v rail(sometimes two). If the power is expressed in watts, then divide by 12v. to get amps. For example, here is a Seasonic 300w unit that can deliver 288W(24a) on the two +12v rails. Note that the 24a total is less than the sum of the two 17a rails.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817151086
Here is a cheap diablotek 300w psu. It delivers only 17a:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817822014