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How do amps come into play with a PSU? - Page 2

post #11 of 17
GPUs draw more power when they're working harder. Pretty much, the more instructions, calculations, and pushing around data they're doing, the more '0's and '1's (which are represented by voltages) need to switch places, hence more current flows and waste heat is produced. Also, the higher the voltage, the more power is used.

The listed TDP is somewhere around the expected max realistic power draw if not overclocked, as a kind of guideline as to how much heat the cooling solution needs to handle. This corresponds to some kind of realistic worst-case scenario in terms of workload and thus power draw. In practice the coolers can of course dissipate more than the listed TDP, so there's some leeway. If you overclock, power draw will increase some, since there may be more calculations per second. If you increase voltage, power draw will increase by even more, so you can go well over listed TDP.

There's no way a GTX 560 Ti by itself should be drawing 20A (from the +12V rail) unless it's heavily overclocked, with some voltage increases.

The computer power supply takes in AC voltage from the wall (something like 115V or 230V AC depending on which country you live in and where), filters it, transforms it, and filters it some more so you get +12V DC, +5V DC, +3.3V DC, and -12V DC. There's a fast-acting feedback loop that essentially adjusts how much power is sent over the transformer to the output side in real time, in response to changes in the load drawn by the computer. That is how it can handle a wide range of power draws by a computer, without wasting a whole lot of power if it's not pulling much.

On almost all computer power supplies, there's not much real distinction between +12V rails. They're all generated by more or less the same circuit. It's just that certain wires corresponding to certain plugs, are grouped together for safety purposes, into different rails. There's a separate over-current protection trip point set on each +12V rail, if in fact the power supply has this feature. If the limit is reached--which should only happen if there's an unintentional short--then the power supply should shut off. So it doesn't really matter which rails you're pulling from. For most modern units, the trip points are set such that there's pretty much no way to hook up a computer such that it will exceed the trip point, unless there's a short (in which case you'd want it to shut down).
post #12 of 17
Quote:
Originally Posted by mikeaj View Post

GPUs draw more power when they're working harder. Pretty much, the more instructions, calculations, and pushing around data they're doing, the more '0's and '1's (which are represented by voltages) need to switch places, hence more current flows and waste heat is produced. Also, the higher the voltage, the more power is used.
The listed TDP is somewhere around the expected max realistic power draw if not overclocked, as a kind of guideline as to how much heat the cooling solution needs to handle. This corresponds to some kind of realistic worst-case scenario in terms of workload and thus power draw. In practice the coolers can of course dissipate more than the listed TDP, so there's some leeway. If you overclock, power draw will increase some, since there may be more calculations per second. If you increase voltage, power draw will increase by even more, so you can go well over listed TDP.
There's no way a GTX 560 Ti by itself should be drawing 20A (from the +12V rail) unless it's heavily overclocked, with some voltage increases.
The computer power supply takes in AC voltage from the wall (something like 115V or 230V AC depending on which country you live in and where), filters it, transforms it, and filters it some more so you get +12V DC, +5V DC, +3.3V DC, and -12V DC. There's a fast-acting feedback loop that essentially adjusts how much power is sent over the transformer to the output side in real time, in response to changes in the load drawn by the computer. That is how it can handle a wide range of power draws by a computer, without wasting a whole lot of power if it's not pulling much.
On almost all computer power supplies, there's not much real distinction between +12V rails. They're all generated by more or less the same circuit. It's just that certain wires corresponding to certain plugs, are grouped together for safety purposes, into different rails. There's a separate over-current protection trip point set on each +12V rail, if in fact the power supply has this feature. If the limit is reached--which should only happen if there's an unintentional short--then the power supply should shut off. So it doesn't really matter which rails you're pulling from. For most modern units, the trip points are set such that there's pretty much no way to hook up a computer such that it will exceed the trip point, unless there's a short (in which case you'd want it to shut down).

no... a 560 ti Flat out cannot draw 20A unless the PCB starts burning tongue.gif
    
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post #13 of 17
To the OP: Amps is very important on a power supply. It is actually more important that the rated wattage because power supplies can be rated differently by different manufacturers. Amps is the amount of current that your power supply can give at a certain voltage provided by the cables. +3.3, +5, and +12V for example.

The recommendation for 20A on +12V is not used by the GTX 560 Ti by itself, that also includes your CPU and other peripherals. 20A is a minimum recommendation for an entire system with one GTX 560 Ti.
Quote:
Originally Posted by Pentium4 531 overclocker View Post

20A is a LOT of power, thats more than my entire sig rig running full throttle
no you take the amp draw times 120V from the wall, and multiply to get wattage
you take the 120V from the wall and divide by wattage to get amperage
you take amperage and multiply by wattage and you should get 120V
You are confused or at least made some errors in your explanation.
Second line, you take the WATTAGE and divide by 120V from the wall to get AMPERAGE.
Third line, you take AMPERAGE and multiply by 120V from the wall to get WATTAGE.
And this is only giving your overall current and wattage draw, not at specified voltages on the rails of your PSU.

Also, you need to take into account the efficiency of the power supply at different loads. Typically, it is 80-88% @ 40% and 60% loads. That means that the wattage you see drawn from the wall is not the actual wattage that your power supply is giving to your computer components. You have power that is lost to heat in the transformation process from AC to DC voltage. We are talking about 12-20% of power. The power rating on power supplies is only concerned with how much it can supply, not how much it can draw.

I'm an EE student.
post #14 of 17
Quote:
Originally Posted by Doomtomb View Post

To the OP: Amps is very important on a power supply. It is actually more important that the rated wattage because power supplies can be rated differently by different manufacturers. Amps is the amount of current that your power supply can give at a certain voltage provided by the cables. +3.3, +5, and +12V for example.
The recommendation for 20A on +12V is not used by the GTX 560 Ti by itself, that also includes your CPU and other peripherals. 20A is a minimum recommendation for an entire system with one GTX 560 Ti.
You are confused or at least made some errors in your explanation.
Second line, you take the WATTAGE and divide by 120V from the wall to get AMPERAGE.
Third line, you take AMPERAGE and multiply by 120V from the wall to get WATTAGE.
And this is only giving your overall current and wattage draw, not at specified voltages on the rails of your PSU.
Also, you need to take into account the efficiency of the power supply at different loads. Typically, it is 80-88% @ 40% and 60% loads. That means that the wattage you see drawn from the wall is not the actual wattage that your power supply is giving to your computer components. You have power that is lost to heat in the transformation process from AC to DC voltage. We are talking about 12-20% of power. The power rating on power supplies is only concerned with how much it can supply, not how much it can draw.
I'm an EE student.

Yep my mistake for the second line, and to be honest.. he doesnt even need to look at efficiency here... hes just gotta look at the label that tells him... YOUR PSU CAN OUTPUT SUCH AND SUCH WATTS ON THE +12V RAIL, and he should know =P
    
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post #15 of 17
Doom, I very much doubt that any 560ti comes from the mfg with a stamp that says you only need a PSU w/20A on the 12V rail to run the entire system. That would mean a PSU w/240 watts on the 12V rail, and given the layout of most current units, that would translate to about a 275-300W unit.

My guess is that wouldn't provide enough headroom, and would suspect all mfg's would recommend something like a 400W unit for the entire system, which would in turn likely be capable of providing around 26-30 DC Amps on the 12V circuit, total.

However, given a TDP of 170W (or around 15A at 12V+), I'd imagine that 20A is probably on the very high end of what you could ever get a 560ti alone to draw, unless it's OV/OC'd pretty extremely.

I also agree w/Pentium insofar as the AC/DC conversion 'efficiency' is 100% irrelevant to what's being asked about here. The numbers provided on a PSU label, be they Amps or Watts (which are just Amps X Voltage), are always referring to what comes OUT of the PSU, i.e. DC power, which is also what type of current that the part (i.e. GPU/CPU etc) will be consuming.

Bottom-line here, look at what the mfg states as a minimum in terms your PSU's ability to provide 12V+ amperage, and buy a PSU that can provide *at least that much*, and you'll be fine. That number always refers to the amperage needed for a whole system, with their gfx card in it.

And make sure you look at the PSU's rating for the TOTAL amount of amperage available on the 12V+ circuit, do not just add up the individual rails, because that will make you think the PSU is far more powerful than it really is. There's an Antec out right now that says it has 3 x 30A 12V rails ... yet the actual total 12V amperage it can provide is like 56A. The total is what matters, the 'rails' are irrelevant.

Note also that if the PSU maker does NOT list the TOTAL 12V+ amperage on the label, DO NOT BUY that PSU, because only shady companies do that kinda crap nowadays.
Edited by brettjv - 2/1/12 at 8:04pm
    
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post #16 of 17
Quote:
Originally Posted by brettjv View Post

Doom, I very much doubt that any 560ti comes from the mfg with a stamp that says you only need a PSU w/20A on the 12V rail to run the entire system. That would mean a PSU w/240 watts on the 12V rail, and given the layout of most current units, that would translate to about a 275-300W unit.

My guess is that wouldn't provide enough headroom, and would suspect all mfg's would recommend something like a 400W unit for the entire system, which would in turn likely be capable of providing around 26-30 DC Amps on the 12V circuit, total.

However, given a TDP of 170W (or around 15A at 12V+), I'd imagine that 20A is probably on the very high end of what you could ever get a 560ti alone to draw, unless it's OV/OC'd pretty extremely.

I also agree w/Pentium insofar as the AC/DC conversion 'efficiency' is 100% irrelevant to what's being asked about here. The numbers provided on a PSU label, be they Amps or Watts (which are just Amps X Voltage), are always referring to what comes OUT of the PSU, i.e. DC power, which is also what type of current that the part (i.e. GPU/CPU etc) will be consuming.

Bottom-line here, look at what the mfg states as a minimum in terms your PSU's ability to provide 12V+ amperage, and buy a PSU that can provide *at least that much*, and you'll be fine. That number always refers to the amperage needed for a whole system, with their gfx card in it.

And make sure you look at the PSU's rating for the TOTAL amount of amperage available on the 12V+ circuit, do not just add up the individual rails, because that will make you think the PSU is far more powerful than it really is. There's an Antec out right now that says it has 3 x 30A 12V rails ... yet the actual total 12V amperage it can provide is like 56A. The total is what matters, the 'rails' are irrelevant.

Note also that if the PSU maker does NOT list the TOTAL 12V+ amperage on the label, DO NOT BUY that PSU, because only shady companies do that kinda crap nowadays..

According to EVGA spec sheet they say 30A on +12V rail which is definitely for the entire system so I'd don't know what the 20A requirement is referring to. It seems like it would make sense to me to make the requirement for the entire system as most people cannot determine their CPU draw and other components that easily so this is just an estimate.

And you're right, the PSU efficiency and draw from the wall don't really matter. I only wanted to correct it because it was brought up earlier.
post #17 of 17
Quote:
Originally Posted by Doomtomb View Post

According to EVGA spec sheet they say 30A on +12V rail which is definitely for the entire system so I'd don't know what the 20A requirement is referring to. It seems like it would make sense to me to make the requirement for the entire system as most people cannot determine their CPU draw and other components that easily so this is just an estimate.
And you're right, the PSU efficiency and draw from the wall don't really matter. I only wanted to correct it because it was brought up earlier.

Now we're in agreement thumb.gif

As an aside, because gfx card makers adhere to PCI-Ex specs, the number of plugs that connect to a given card can give you a reasonable estimate of how much MAX amperage a given card (at STOCK volts) might pull when placed under full load (although this estimate will be high in nearly all cases).

Board Only: 75W (6.25A) maximum draw
1 x 6-pin : 75W (6.25A) + 75W (6.25A) = 150W (12.5A) max draw
2 x 6-pin : 75W (6.25A) + 75W (6.25A) + 75W (6.25A) = 225W (20A) max draw (I think this is where the 20A number arose in this discussion)
1 x 6-pin + 1 x 8-pin: 75W (6.25A) + 75W (6.25A) + 150W (12.5A) = 300W (25A) max draw
2 x 8-pin: 75W (6.25A) + 150W(12.5A) + 150W (12.5A) = 375W (31.25A) max draw

Those are MAX draws at stock voltages. The plugs themselves can provide more power than that if the card requests it due to over-volting or the like, as long as the OCP on the PCI-Ex cable (i.e. the 'rail') isn't tripped by the draw (and the wires don't, you know, burst into flames).
Edited by brettjv - 2/1/12 at 10:39pm
    
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