Topic: Estimating heat output - 5850 (Read 2191 times)

OK then - honest question from a point made earlier.

Calculating power used by a graphics board should be easy, with my limited understanding. It's a DC circuit, running with 12V feeds, so assuming you can measure the current flowing on each of the feeds into the GPU board (two direct feeds from PSU and the feed from the PCIe slot itself), and assuming your 12V feed is a clean 12V (if not, I suppose you could measure the exact '12V' potential at all three points), prep school physics says that the power used (converted / dissipated as heat / available to do work / etc.) is simply the voltage multiplied by the total current. If the current is measured in amps then multiplying this by the voltage gets you power in watts.

OK, so getting a representative overclocked 5850 mining at full load, then measuring the input current and voltages (the different inputs could have different potentials... after all, the two PCIe power feeds are usually fat cables straight to the PSU, with minimal resistance and hence minimal voltage drop... but the PCIe *slot* feed of up to 75W has to come from the ATX / logic board connector, and then across a bunch of PCB traces... my guess is that this route (and the thinner-gauge PCIe pins) will have a higher resistance and hence a larger voltage drop), multiplying them together will give you a total power draw from *one* typical 5850 card.

Let's say the measurement and calculation gives 175W. I've got 5 slots, so if I filled them all with 'typical' 5850s, I'll need 875W from the PSU purely on the 12V supply, ignoring the CPU and other loads.


My question is - at what point do I have to take the nature of AC into account? Is it only an issue when comparing 'power-from-wall' readings (in the USA, from 'kill-a-watt' type devices - in the UK I've not seen specific brand name power analysers, I just use Maplin power meters and extension multiplugs with power analysers built in)??

Residential AC is typically single phase, isn't it? Are the mains-electricity power meters measuring power the same way (volts x amps)?? If so, then the RMS issue comes into play, and you can't assume (in the UK) 240V times the current to be the power. Or can you?

Sine-wave alternating current and RMS voltage makes things annoyingly confusing when switching to DC output. Is this a complete non-issue, and when a PSU says their 12V rail will handle, say, 850W, and my mains power meter says I'm pulling 650W from the wall, I've got a nice fat buffer and am safe and efficient? Or is the rated 850W based off 'peak' power supplied in AC form, whereas in reality only RMS is continuously available to convert to DC, and my 'buffer' may not be anywhere near as large as I think?


I'd like someone who really knows the detail of AC conversion to DC, and how mains power meters measure, to educate me here... Haploid23 - you said that reading 1000W from the wall is a measurement of AC power... is that peak (non-continuous), needing an RMS calc, or are 'watts' always 'watts' regardless of AC, DC, single or three phase, etc??

I buy UPS backups for my servers, and have an emergency generator, and notice that *these* appliances often mess about with 'VA' ratings instead of good old *watts* - this suggests that 'watts are watts', and the 'VA' is just a scam to make the power supply seem more powerful (since it's going to be peak, not RMS, voltage - surely)??

But WTFDIK. 

PSUs are most efficient when operated somewhat below their rating, this also gives some margin for unforeseen circumstances. If you ignore the CPU,
 you'll need  a ~1200 watt PSU (875W / 0.75). 0.75 is a derating factor which keeps the PSU from running close to the limit.

http://www.anandtech.com/show/2624/3


In theory, all AC power meters will report the same number that you are being billed for. Check with Maplin to see how well their meters compare to the utilities.


[/quote]

This is where things fall off the rails. Since current and voltage are sinusoidal you can't simply multiply them unless it is a pure resistive load (it isn't.)

Maybe http://en.wikipedia.org/wiki/Power_factor will help clear things up. If not, try http://en.wikipedia.org/wiki/Volt-ampere

In general, your PSU manufacture will quote an efficiency number, typically the absolute highest measurement they can coerce out of a golden unit at the exact right temperature and age. For the sake of argument, we'll pick 80% efficiency. Your 875W to power the GPUs will draw ~1100W from the outlet.

Any one have an answer to this?

Mustn't feed the trolls.

Transistor gating/switching for the 2.15 billion transistors in a 5850.  There is 'work' being done still.

Like I said... it's less than 1%.  You cannot apply 'high school' level of physics in entropy with semiconductors.

It 'work' efficiency wasn't an issue, then we wouldn't need to continue to shrink dies sizes.

Cheers,
Kermee

I'm puzzled by your comment. What actual work is being performed by the 1% here? Even the air that is moved isn't being stored at a higher pressure, so after you run your 1KW rig for 1 hour you have increased the heat in your room exactly the same as a 1KW heater.

I updated the post there's 5 5850's I mentioned it a little earlier in the thread and I assumed it was known already. Plus I wrote that when I was drunk lol

I have a CORSAIR Professional Series Gold AX1200 (http://www.newegg.com/Product/Product.aspx?Item=N82E16817139014)

How would I go about calculating watts used by each card then?

it actually generates less. 5850's TDP is 150w, that means it's designed to dissipate 150w of heat. the actual usage, even at 100% load, will draw less than 150w. overclocking the core (however underclocking the memory) might put you close to the TDP, but it shouldn't go over unless there's heavy overvolting.

and also note that AC and DC power are a little different. the rated 150w TDP is based on DC pulled from the PSU, when you read 1000w your meter, that's AC power, which is pulled from the wall. in this case, you need to take into account the PSU efficiency before claiming it's 170w of real DC power draw

Don't forget that the cooler your card is, the same will be for your room.

I have my 5870's mining continuously in my basement, both barely hit 40°C on a hot day.  But if I were to add a few MSI 5770 Hawks' @ 85% fan speed, my basement begins to warm up a bit, and so do my cards.

I tried setting up a simple build in my room and couldn't bare the heat after a few nights.  97°F inside, while it's 71°F outside just ain't right. 

Catfish, that was a far lengthier reply than I was requesting - a simple "no" would have sufficed.    That said, thanks for writing all of that!  Makes sense that transistors simply increase resistance to force the electricity to another path though.  No physical movement involved.  So, if that's the case, then Kermee is wrong - it's not "basically" 100% of electricity is direct heat loss - it IS 100% electricity is direct heat loss.  Unless you happen to have glowing-hot semiconductors, which, probably won't last long anyway. 

That system cannot possibly be drawing 1kw of power, unless you have multiple GPU's.  I calculate it would use about 230w, PSU efficiency included.

With how many GPUs?

I'll just post exact specs.

MSI 890FXA-GD70
corsair 1200watt gold series
5xsapphire hd 5850
2x1gb ram
this hd (http://www.newegg.com/Product/Product.aspx?Item=N82E16822136770&Tpk=22-136-770)
Sempron 130

At 920/325 I get around 1kw. I'm too lazy to find exact numbers feel free to figure it out if you want.

In a sense, the physical 'gates' do for the switching. =)

Cheers,
Kermee

More like 50w if your sempron is just idling while mining...

Total, 200w would be my guess.

Then again, it also depends how efficient your PSU is... add 20% or more to those figures for the heat loss from the transformers in the PSU.

Yes I agree that's an estimate on my part but I have a semperon so 150w for mobo/ram/cpu/hd is a pretty safe bet.

Do transistors actually move?  I'm not familiar enough with how they work to know the answer...

This is true.  I am assuming a stock card when I say 150w, and even then, that's not really an exactly accurate number.

Overclocked 5850's generate more than this My rig is set up for 920/325 and its 5 cards. According to my kw reader its about 1kw so closer to 170w per card.

CPUs, GPUs, uC's, and most other semiconductors are less than 1% efficient.

So yes, basically 100% of TDP comes off as heat.

Cheers,
Daniel

Thanks SgtSpike!

Yes.  All wattage eventually turns into heat.  Even fans blowing air... the air creates friction until it slows to a stop, and all of that friction turns into heat.  Same with light - as it is absorbed by the various surfaces it touches, it turns into heat.  The reflected light is what we see, but it all keeps bouncing around until it is all absorbed.  But the majority of wattage in a computer is used by transistors switching, which creates heat as well.