CPU current?

[Kol12]

My 8700K running stock (no OC) reads in HWiNFO with a current draw of 50A max under load. Is this reading correct? 50A is almost the bulk of the 63A on my PSU's 12v rail and I have a 1080 Ti also... 

 

[Martin]

Yes, that should be the current read from VRM. I think your PSU should have multiple rails.

 

[Kol12]

Yes, that should be the current read from VRM. I think your PSU should have multiple rails.

Are you saying that reading is like a total current draw from the PSU? I have just one 12V rail, Seasonic X-750 - http://www2.seasonic.com/product/x-750/

 

[Martin]

No, this reading is from the Voltage regulator on the mainboard for the Vcore rail. So the max output power of the VR based on the screenshot is 57.6W (Iout * Vcore), then input current (pulled from the PSU) should be approx 57W / 12V = 4.75A. Note that I used a very simplified calc, which doesn't account for loss, etc.. But the point is that input voltage is 12V, output voltage ~1V (Vcore). The current reported you see is on the output side of VR.
Not sure if I explained it well

 

[Kol12]

If that's how amperage is calculated then that makes sense but 4.75A is a stark difference to 50A. Oh ok so the 50A is the current at the wall before it hits the VRM is that correct?

 

[Martin]

50A is the current the PSU can deliver at 12V. The current at wall is again different, that's the input side of the PSU at 110/230V.

 

[Kol12]

I'm kind of confused. Surely 50A is not going to just the VRM/Vcore rail.. Can you explain this any differently? I'd like to understand.  :-/

 

[Kol12]

Are you willing to have another crack at explaining this? New example: CPU now overclocked to 4.9Ghz. Load reading is 98.4W, Current 81A.

 

[Dalai]

I'll try to explain it. Your PSU can deliver up to 62 (or 63) Amps at 12 Volts, according to its specs. This equals around 744 to 756 Watts. Note that this value can be higher than the maximum rated output of your PSU; this is normal.

The reading you're getting in HWiNFO (50 Amps or 81 Amps) is at the CPU voltage, which is somewhere around 1.0 to 1.3 Volts. This equals the 57 (or 98) Watts HWiNFO shows. This value is not directly related to the output power of your PSU, because it's at a much lower voltage. Between the PSU and CPU there are voltage regulator modules (VRM) that convert the PSU's 12 V to a lower voltage that the CPU needs; IIRC it's higher than Vcore, something around 3.x volts or something, but I digress.

The power drawn from the wall is completely different from both of the above, and it has nothing to do with the power rating of your PSU. It has something to do with the efficiency of your PSU at a given AC voltage (110 vs. 230 Volts). That's why the efficiency rating can be slightly different for the different AC voltages.

Regards
Dalai

 

[Kol12]

To be really honest I do not understand. The CPU alone drawing 50-80 Amps is the whole of my PSU's 750W and more. An 8700K uses around 90-100W overclocked. 80 Amps doesn't correlate to the amount of power I ever draw from this machine. I know there is something I'm missing I just haven't got it yet. :/

 

[Dalai]

The CPU alone drawing 50-80 Amps is the whole of my PSU's 750W and more.

No, it's not. The 750 Watt of your PSU is the maximum limit of your PSU on all rails combined. Each rail has its own power limit, which is 62 Amps in case of the 12 V rail. The other rails can be ignored in regards to the CPU (12 V is the only one that matters).

A CPU can draw 100 Amps (just a random number, not necessarily related to reality), and it doesn't go over the 62 Amps limit of your PSU. Why? Because 100 Amps at 1.2 Volts is MUCH LESS power drawn than your PSU can deliver: 62 Amps at 12 Volts. Let's calculate the example in full: 100 Amps at 1.2 Volts equals 120 Watts. 62 Amps at 12 Volts equals 744 Watts. 120 is much smaller than 744, less than 20 percent.

I know there is something I'm missing I just haven't got it yet. :/

Power (Watt) is the product of current and voltage, or expressed mathematically, Power = Current * Voltage. You always have to take both voltage and current into account when talking/thinking about power.

Regards
Dalai

 

[Kol12]

@Dalai I'm sure what you say is correct it is just very confusing.  :-/ Obviously I need to do some electrical study to better understand this.

 

[fufsgfen]

@Dalai I'm sure what you say is correct it is just very confusing.  :-/ Obviously I need to do some electrical study to better understand this.

Try this one, input 1.3V and 60A, then click calculate:
https://www.rapidtables.com/calc/electric/watt-volt-amp-calculator.html

Next, clear all boxes with reset button and input wattage you got from previous calculation, then input 12V and click calculate

Next you can then continue to do same with whatever is your wall voltage, that probably gives you better picture of what is relation of amps and volts.

 

[Dalai]

OK, I'll try again. It's an equation:

Power = Voltage * Amperage

If you change one of the values, you need to change the other values, too. Let's say, you have these numbers:

744 Watts = 12 Volts * 62 Amps

If you change the Voltage, you must change the Amperage as well, so that the equation is still correct, and the Power stays the same. Let's change the Voltage to 1.2 Volts (VCore), which is what the VRM's on any motherboard do. What needs to be done to keep the equation valid? The Amperage needs to be multiplied by 10 because the Voltage was divided by 10. The result is this:

744 Watts = 1.2 Volts * 620 Amps

That's the theoretical Current you CPU could draw before the PSU's limit is reached. Theoretical because there are other factors that make is impossible to draw that much Current. But I digress.

It's also possible to change the Wattage on the left side of the equation:

74.4 Watts = 1.2 Volts * 62 Amps

These values are in the same ballpark of the ones shown by HWiNFO, aren't they?

In short: If one value is raised another value must be lowered to keep the equation valid. That's why 100 Amps at 1.2 Volts are a completely different thing than 100 Amps at 12 Volts - the resulting power is 10 times higher in case of the latter.

Regards
Dalai