Bitcoin Forum

Hey guys, I'm working on a mining rig and I have a question.

I bought a 650W XFX power supply (80 plus bronze), and I want to run
two 5850's on it.
The rest of the rig: Phenom II x4, 2 HD's, 4 GB's RAM.

Any tips are appreciated, thanks.

Should be fine.  I had a 5850 running on a cheap-ish 400w, along with an X4, and it ran fine unless I tried overclocking.  A quality 400w would have done fine though.  An additional 5850 will only draw 150w more, so even a quality 550w would be fine.

Be careful with overclocking but you should be fine otherwise.

650W should be no problem.  Even a small overvolt is a possibility but you will find that efficiency of the PSU will further amplify the diminshing returns of overvolting.  If you are running the cards at stock voltage then I'd say 650W is ideal.

Do be careful with overvolting though.  My dual 5850 system is currently drawing 304W from the wall for 724.3 MH/s from a 0.1V undervolt.  A 0.1V overvolt is going to send the power consumption far in the other direction and more might push the system over 650W.  I would not overvolt without a power meter so that I can be sure that the PSU is not being overworked.

You didn't say if the rig is dedicated to mining or dual purpose. If it is dedicated you could certainly replace the the CPU with a Sempron 140, lose the HDDs (boot from 4GB USB stick), and get down to 1GB ram.

teukon knows more about maximizing MH/j as he has extensively tested low power setups.

An overvolt isn't going to get anywhere close to 650w...  You're talking about a single 5850 drawing 300w to get an entire mining system up to 650w of draw.  That's more than double its stock wattage draw.  Not going to happen, especially with a pathetic 0.1v overvolt.

I'd say you MIGHT hit 200w/card at the max overvolt you can give it.  Even that would surprise me though.

My apologies, I didn't do any calculations and was just impressed with the large power reduction I experienced with my 0.1V undervolt.  I didn't express myself well and just wanted to get across my impression that I wouldn't be overvolting to any great degree if I wasn't fully confident that the PSU could take it.

Just as an example of an overclock I performed myself: Overclocking from 1.0875V@725MHz to 1.25V@1110MHz should approximately double your power consumption.

Without some testing I couldn't say for sure if a 650W PSU could handle any dual 5850 mining rig and I would guess that some dual 5850 rigs will require more than 650W.  However, it seems very unlikely than a 0.1V overvolt could worry such a PSU even with a heavy base system power draw.

dude... iam running 2 HD5850 on 950mhz core since weeks on a 400W PSU :)

Your calculations regarding power consumption are wrong.  I'd love to see you come up with proof of those assumptions.

measuring with kill-a-watt i saw my power consumption go up about 90 watts at full load when I over clocked and increased the voltage to my video card. It was hardly worth the MH increase for my power costs.

of course that was just me and one experience.

However if you look arround (http://www.xbitlabs.com/articles/cpu/display/power-consumption-overclocking_4.html)



that a 50% increase is not uncommon(yes i know the article is cpu but their isnt that much difference between that and a gpu)


newegg recommends 750 watts for his system (http://educations.newegg.com/tool/psucalc/index.html)

however if you look here, toms hardware did some tests and found dual 5850s under load to hit 469 watts in a crossfire config at standard clocks and voltages (http://www.tomshardware.com/reviews/radeon-hd-5850,2433-13.html)

looking under that you see the power for a single card, which is 164 watts below that of 2(in crossfire, i believe it is slightly higher Independently but am not going to google the point as I think it is within 10 watts.

now with overclock and increasing the voltage you can add 50% more per card, in line with the examples i have so far. Brings us up to 633 , which is pushing the limits of that PSU especially if it is a bit older. And thats using their cards and their system, it can be easily different with different brands.


Personally I think a lot of you kill your potential with raising the voltages. That PSU sounds fine if you just overclock and not increase the voltages, but it can be iffy if you want to increase the voltage. If you are going to do it, I would spent the 20 bucks and get a kil-a-watt or something simular so you can look at your own power consumption increase and work out if the added electricity costs(got to error on the high side due to added cooling from added heat) are worth the added MHs

Good analysis joules, and I wouldn't doubt a 50% increase in power consumption of the video cards themselves.  I AM doubting the double-your-power-usage claim though.

Also, one much consider than tomshardware is using a system that is stressed on both CPU and GPU (and RAM, for that matter), not just the GPU's like what bitcoin mining does.

This rig has multiple purposes, but it will be mining 24/7, so losing the HDs and processor is not an option.

But besides that, thanks for the tip guys, I won't overclock the cards =)
Appreciate it.

I'm not in a position to test power consumption increases right now (otherwise I'd have done that from the beginning).  I'm just calculating based on my limited knowledge of electricity and how cards work.

For the doubling, I'm noting that an increases in voltage from 1.0875V to 1.25V is a 15% increase in voltage which implies a 32% increase in power.  Overclocking from 725 MHz to 1110 MHz is a 53% increase.  Overall this is more than a 100% increase.

I have some possibly faulty assumptions and if you have knowledge to allow me to dodge these then let me know.

1) Increasing temperature increases electrical resistance which decreases current and therefore decreases power.  I'm assuming that the temperature of the GPU is kept constant as voltage and clock rate are increased but this assumption isn't too bad because many people ramp up the cooling when they overclock/overvolt.

2) I'm assuming that the core clock affects the power consumption linearly but this could easily be way off.

While Tom's Hardware gives some good stats it would be unwise to look at the max performance power consumption and use that to tell if you are safe for overclocked mining.  It's quite possible that the 'max performance' concerns core clocks which are significantly less than 1110 MHz.  Mining is inherently more stable than gaming or desktop graphics so you can push the clock rate quite a lot higher.

Overall, there are many factors to consider and, as my earlier comments show, it is very hard to make accurate predictions of power consumption using theory alone.  If you want to be safe then I can only recommend getting a power meter and ensuring that you are not stressing your PSU.  Honestly though, I wouldn't be worried about overclocking and would wager good money that a typical non-dedicated miner with 2x5850s at stock voltage, no matter what the clock rate, could possibly worry a 650W PSU.  If you overvolt too then you might run into problems.  I stress 'might' this time because people didn't pick up on it in my original post.

I run a similar setup with a Phenom II X3, 3 HDD, overclocked 5870 and 5850 on a 500W standard 80+, drawing 470VA at the wall. At 80% efficiency, the actual usage is about 370W (380VA given my PSU is supposed to have at least 0.97PF). So I doubt even with overclocking, you're going to take overload the 650W. Using VA because people in some places have to pay for low PF.

Just for reference, shutting down the 5870 from mining, leaving the 5850 for testing.

725/305 10.88V
Load: 305~315VA

915/305 10.88V
Load: 335 ~ 345VA

935/305 10.88V
Load: 335 ~ 345VA

915/305 11.00V
Load: 350  ~ 356VA

935/305 11.00V
Load: 350 ~ 360VA

915/305 11.12V
Load: 350W~360VA

935/305 11.12V
Load: 355 ~ 365VA

725/305 11.18V
Load: 315~325VA

915/305 11.18V
Load: 355 ~ 365VA

935/305 11.18V
Load: 355 ~ 365VA

945/305 11.18V
Load: 365 ~ 375VA

So at my extreme end, 725 -> 945 30.3%, 10.88V -> 11.18V 2.75%, idle is generally around 180~190VA regardless of settings.

This are rough numbers though since I only waited about 40~60 seconds to get an idea of the draw at each settings :D

Thanks for this, very helpful!  I'm assuming that you are not putting 10.88V through your cards but, rather, 1.088V.  Also, are you really able to put 1.118V through the card?  I've been working on the assumption (I make a lot of those) that the cards operate on multiples of 0.0125V so I would expect 1.1125V or 1.125V.  Given that 1.118V is closer to 1.1125V than 1.125V I'd guess that the driver is rounding your figure down and the card is really on 1.1125V.  Any thoughts on this?

If I find the time I'll post some values for higher overclocks and overvolts (I can only go up to about 1140MHz and am limited to 1.3V but it could be helpful).  I've not done this because it would mean installing Catalyst 11.6 and rebooting the system (11.4 doesn't allow overvolting or overclocking past 900MHz on my cards as far as I know).

LOL, I didn't check and just assumed the numbers (no decimal places) in the Sapphire tweaking program was 10.88V. On hindsight that was stupid :D

And yeah, the card works on a multiple, I was going to go for 1.090 1.095, 1.100 etc when I discovered it would auto jump to 1100, which does seem like a 0.0125 from 1.088V. 1.118V was a surprise because I did a rough slide to 1.115 expecting the software to do the jump to 1.124 but it went to 1.118 instead. Maybe that's the internal limit on my card, I'll check again when I'm back home.



Yup, Catalyst doesn't do past 900 on mine, which was why I installed the Sapphire program. 945 seems to be upper limit for my 5850 at least at 1.118V, I didn't push it further because I haven't checked the maximum allowed voltage and didn't want to burn the card :)

Guys what sort of power supply would you need for a rig with 4 5850's, and another one with 4 6990's?

With my mildly overclocked pair, it's already 370W so for 4x5850, that's at least 750W worth. At least 1KW~1.2KW for that to be safe as it's not ideal to run a PSU at close to 100%, they tend to be most efficient at 50~70% and last longer that way too.

Each 6990 takes about 400W to run, without overclocking so that's at least 1.6KW but I think 1.5KW is the highest being sold.

Personally, I'd rather go with 2 smaller 600W (for the 5850) or 900W (6990) PSU if you don't have any issue with unconventional arrangements (i.e. PSU not in the same casing) since they tend to cost less per W than a single monolithic PSU. Just need a bit of basic wiring work to ensure they both start and stop at the same time.

That's interesting, I may well investigate this now that I have my power meter.  I have no GUI or sliders and am only changing my voltage by modifying the profile file with a text editor.


Catalyst 11.6 does allow me to go past 900 but this might be peculiar to my card.  I'm using Catalyst 11.4 because it seems to be quite a bit faster for me than Catalyst 11.6.

You are wise to show such restraint.  I took my good card up to 1.3V on air cooling in an attempt to find it's maximum and learnt 3 interesting things.
1) Catalyst 11.6 doesn't allow you to go beyond 1.3V.
2) My card is actually able to reach higher clocks at 1.25V than at 1.3V (this is because the cards are less stable at higher temperatures).
3) My Sapphire HD5850 Xtreme will take 1.3V without exploding.  If only I had my power meter back then.

Just for your information, I ran at 1.3V for only 20 mins or so and half of that time I wasn't running mining software.  I did run at 1.25V for 3 hours continuously mining at 1110MHz (461.1 MH/s) and maintained a temperature of 56*C with aftermarket air cooling.  The card showed no signs of distress; there were no funny smells.  Since then I've bought the voltage down and it runs just as it did before testing.

If you find it easier since you're also using a Sapphire card, you can download Trixx from Sapphire's website to overclock.



I'm also using the 5850 xTreme, looks like 1.3v is the official max if you can't go above that with Catalyst. Most people seem to indicate it's safe up to 1.35v with somebody pushing it up all the way to 1.6v before. But personally, I think I will put 1.3v as upper limit, especially given the temperatures around here. I'm already at 80+ with just 940Mhz. But the card scales pretty decently based on your results. I was getting about 5MH/s more per 10Mhz and our difference (80Mhs for 170Mhz) is still about there.

I believe Trixx requires both a GUI and MS Windows so it's not useful to me unfortunately, thanks for the tip anyway.


WOW! 1.6V!  That's a 47% overvolt!  I'll bet whoever did that would be able to beat my 461.1 MH/s personal best 5850 rate (stability unknown.  I ran it for 3 hours and it crashed exactly when I started up the other card).

Man... 1.6V.

Ah damn me for not noticing that, sorry for the dumb suggestion. :)



Have you tried starting them both at lower clocks, then slowly tweaking up? Sometimes the start up surge causes momentary voltage sags as the PSU suddenly has to deal with another 150W of demand so kills the stability.

As far as I know, that was with liquid nitrogen cooling... :)
http://www.xtremesystems.org/forums/showthread.php?235693-***-Official-Radeon-HD5800-Series-Overclocking-Thread-*** (http://www.xtremesystems.org/forums/showthread.php?235693-***-Official-Radeon-HD5800-Series-Overclocking-Thread-***)

I'd love to know more about this.  For me I've found I do better if I start from cold and dive straight into a high clock rate.  It may be a good idea to make my idle voltage the same as my performance voltage to ease the transition.  I've found that changing clocks while mining can cause the drivers to freeze if the clocks are high enough.

I once left my good card running at 1025 MHz at stock volts for 12 hours and it was fine.  Then I change the clock rate to 1020 while mining and it crashed instantly.  I reset and started it at 1020 MHz and it was fine.  Indeed, I can often get 1030 MHz like this and it runs fine once it's there.  I can sometimes get 1035 MHz like this but even if I do the card will crash after a few hours.  I get more and more kernel warnings the higher I take the clock but even at 1030 MHz the extra 5 MHz is a greater boost to profit than the hardware errors are a drain.

Perhaps this has something to do with temperature.  When the cards are at idle they are at much lower temperatures and can probably take the jump more easily.  I've found that if I turn my fans up I can reach higher clock rates.

I wrote a script to change my cards clock rates on the fly if the temperature left a certain range but all too often it would crash the cards so I stopped using it (I wish I could control my fans in software).

Anyway, I'm certainly going to increase my idle voltage from 0.95V to 0.9875V so that at least the voltage isn't changed when I start/stop mining.  This could be a good stability tip for people who do pool mining and occasionally have no work to do due to server issues.