Will this PSU run a GTX 750 ti?

spoe71

Distinguished
Jan 9, 2011
14
0
18,510
Hello, friends. May I please ask for your guidance here? I am quite uneducated on these matters.

I am looking to upgrade my graphics card from a GT 720 1gb ddr3 card to the PNY GeForce GTX 750 Ti OC - Performance Edition - graphics card - GF GTX 750 Ti - 2 GB, which seems a nice buy at 150.00 and a substantial upgrade over my current card. Like the GTX 750 ti, my current card works on just PCI power (there is no 6-pin connector required, from what I understand). I also understand from reading that the 750 ti needs 20 amps, though I do not know if my PSU has that. It seems one would simply add all the 12v rails together, but clearly I should not assume that given my lack of knowledge on the matter.

Here is the information on my PSU:

Dell switching power supply (from a Dell XPS 8700)
DC Output: Max output power: 460 watts
+12VA--18A
+12VB--16A
+12VC--8A
+3.3V--17A
+5V--25A
-12V--0.3A
+5Vaux--3.0A

+5 and +3.3V shall not exceed 142 watts
+12VA, +12VB, and +12VC shall not exceed 385 watts.

Thank you for your time and effort!
Scott

In case it matters, I have a Core i7-4790 processor (3.6 GHZ), 8GB ram, a dvd drive, and a 1tb hard drive. I won't be running but a single monitor (21" flat screen from Dell).
 

spoe71

Distinguished
Jan 9, 2011
14
0
18,510
Thanks, Nuckles_56. Can you tell me how one determines exactly how to compute amp requirements? I ask for my own educational purposes so that should this issue arise again, I can do the math myself. For example, if the GTX 750 requires 20 amps to work properly, what figures from the information I posted above allow me to know if a psu is good enough?
 
I will try to make it quickish and easy to understand.

First

Wattage(W) = voltage(V) x current(A)
Current(A) = Wattage(W) / Voltage(V)

Now with that out of the way.

Modern power supplies provide several key voltages computers use to run the hardware within. We will only be looking at 12, 5 and 3.3 today. The others are not used nearly as much and +5Vaux or 5VSB are used when the system is OFF to keep parts of the board running. You may notice the systems usb ports can still charge your phone with the system off. This uses the AUX or Standby power.

Moving along.

Once upon a time computer hardware was slow and did not use nearly as much power at this time you could get by with using the lower rails like 5 and 3.3. You may have even had some regulators that did not even use switching mode power because the drop from 3.3v to 2.5v or 5 to 3.9 was not that big. Everything was good. Even as cpus started to take more power these 3.3 and 5 volt rails could handle the load without much issue. The 12 volt rail was still used, but not nearly as much.

Fast forward many years and cpu's are taking 100+ watts to run. Now if you remember the math from above wattage = voltage * amps and amperage = wattage / voltage.

Lets use that math to see how many AMPS a 100 watt cpu takes from each rail.

3.3 volt rail
100/3=33.3333333333333

5 volt rail
100/5=20

12 volt rail
100/12=8.333333333

Now, why are you making me read this? Well higher amperage requires larger wires. A house in the USA or Canada generally uses 14 gauge wire for 15 amps. So imagine how big you need for 33 amps. Well that would need 8 gauge wire(good upto 45 amps. This is like Electric range/stove wire we are talking about now). This is large and un-usable in a computer.

So they came up with a better idea. Run everything off of 12 volts. So now even a 300 watt video card will only draw 25 amps @ 12 volts. This lets them use smaller wire(but also the reason why PCI-E connectors have multiple wires and you need multiple plugs to run a 300 watt video card).

At this point all modern power supplies tend to use a design that makes almost all of the power into 12 volts on one large rail. For multi rail power supplies, they just use current limiters(most new ceiling cans have these to prevent use of 100 watt bulbs as well) to prevent you from over drawing on any single set of cables. These all connect back to the main 12 volt rail.

For this reason Dell was kind enough to have proper specs listing that the combined 12 volt rating for ALL the 12 volt rails is 385 watts or 32.08333amps

But wait a second, I see 385 for the 12 volt rails and 142 for the 3.3+5 that is 527 watts from a 460 watt power supply? Not quite. The 3.3 and 5 volt rails actually get power from the 12 volt rail as well by something called a DC-DC converter. They have so left over 12 volt power, but in the case that you use heavy 3.3/5 volt devices, you WILL loose some of the power from the 285 watt combined rail.

For this reason the power supply as a whole is rated at 460 watt. As long as no single rail is overloaded AND the total does not pass 460 watts(18.6 watts of that may be on the standby + negative 12 volt rail), the power supply will run just fine.

This was done to allow compatibility for older hardware while maximizing efficiency. Some older systems had a full power supply for each voltage and this simply added to power supply losses(more wasted heat).

The 750 ti is rated to use 60 watts. This is so low that it does not even need an extra power cable and get get what it needs from the boards pci-e slot(the slot is good for upto 75 watts).

Ok, I lies about keeping it short.

Sorry.
 

spoe71

Distinguished
Jan 9, 2011
14
0
18,510


Dear Nukemaster,

Thank you so much for taking the time to explain this to me! From your detailed explanation--did I say thank you yet?---the 385 combined watts from the 12-volt rails churns out smidge over 30 amps, which is plenty to meet the amp requirements for the 750 ti !! That's correct, yes?

That said, and since I have more than enough for what I need, I wonder if you might suggest an even better card? I have a spare 6-pin connector in the case; however, I don't want to run into heat troubles. I like the low-power idea of the 750 ti, and I have read good things about its ability to bridge performance and low-power.

 

spoe71

Distinguished
Jan 9, 2011
14
0
18,510
Well, I wanted to keep it at 200.00 or fewer dollars, but if you think I'll run into power issues, I'll just stick with the 750 ti. I'm sure it is more than enough for what I need anyway.

About the most advanced game I'll play on it will be Dragon Age Inquisition. I've looked at a website that clearly indicated I should be fine with my system (minus the terrible graphics card I have now, of course).
 
The GTX 760 is a 170 watt part. It needs 2 pci-e 6 pin connectors.

While I have no doubt with proper part selection(no heavy overclocks) one can get a 760 to run on that power supply. It may not be the best option at the end of the day.

I am almost sure my system has not reported much over 350(if that, it has been a while since I checked) watts at the wall with its GTX 670(the 760 is like a 670 with some shaders disabled, but a higher clock speed to make up for it).

My media center has a hard time pulling much over 150 from the wall with its i5 750(under volted slightly) + GTX 650ti.

Only real high end systems(my 2600K + 670 is not a high end system compared to the parts on the market) consume 500+ watts in most cases. This is in part due to the more efficient parts on the market now.
 

spoe71

Distinguished
Jan 9, 2011
14
0
18,510
Thanks for all of this superb information, fellas. I sincerely appreciate your time and effort. I'll go with the GTX 750 ti, as per your advice. You've both been real gents for helping me out tonight. Internet communities can use more folks like those who frequent the forums here at Tom's Hardware because you all help technologically deficient people like me make informed decisions.

Scott