True requirements for PSU?

vbimport

#1

Hi,

I’m updating my so-called “recording” computer (I was thinking about maybe FX-6000-series or early i5 CPU:s) and long story short - I got drunk and now I have i7-4770K. :doh:

The setup would now then be:

  • i7-4770K (using IGP, so no discrete GPU card)
  • (Some motherboard, don’t know exactly yet)
  • 8GB RAM
  • M-Audio Delta 1010LT sound card
  • DVD-drive (not absolutely necessary…)
  • One or two SATA-hard drives (7200rpm)

Now when looking at power consumption figures in different tests (for example this or this) the full CPU load consumption figures are well below 200 watts. Even if utilizing the IGP 100% at the same time it would raise the wattage, about what, 20-30 watts?

So, first of all - the figures lead me to believe that the i7 setup would consume about as much power as my trusty old Athlon 4400+ x2 setup (with 4GB RAM, 2x DVD-RW, 2 TV-cards, case fan, 5xHDD and 8400GS GPU)?

I have run the 4400+ setup with the 350W PSU so the question is that would it be enough to run i7?

When looking different motherboard manuals, some state that “500W or greater PSU is recommended”. Why in earth, if in tests system under full CPU load is under half of that?

Yes, I know, it’s stupid to use old PSUs with (too) high-end CPUs but the 350W is a spare, so if it can do it, why not to put it in use. The PSU is single-rail model and apparently puts out max of 20A in +12V rail. Correct me if I’m wrong but the main concern using old PSU:s is the amperage from the +12V rail? Is the 20A enough?

Does anyone have somekind of experience about “how low can you go” - or maybe has someone measured their power consumption with 4770K? Also if someone has a prebuilt brand-name machine with i7 - could you get me some specs about the PSU (wattage, +12V amperage), probably the brand name machines have as small PSU:s that can be.

And are these 700W plus PSUs just meant for people using big, hungry gaming GPU’s and without them they are really not needed? (for argument’s sake let’s leave the servers and their PSU:s out).


#2

[QUOTE=Mastus;2709243]Hi,

I’m updating my so-called “recording” computer (I was thinking about maybe FX-6000-series or early i5 CPU:s) and long story short - I got drunk and now I have i7-4770K. :doh:

The setup would now then be:

  • i7-4770K (using IGP, so no discrete GPU card)
  • (Some motherboard, don’t know exactly yet)
  • 8GB RAM
  • M-Audio Delta 1010LT sound card
  • DVD-drive (not absolutely necessary…)
  • One or two SATA-hard drives (7200rpm)

Now when looking at power consumption figures in different tests (for example this or this) the full CPU load consumption figures are well below 200 watts. Even if utilizing the IGP 100% at the same time it would raise the wattage, about what, 20-30 watts?

So, first of all - the figures lead me to believe that the i7 setup would consume about as much power as my trusty old Athlon 4400+ x2 setup (with 4GB RAM, 2x DVD-RW, 2 TV-cards, case fan, 5xHDD and 8400GS GPU)?
[/QUOTE]

The amperage thing is somewhat of a myth as explained here:
http://www.overclock.net/t/880633/psu-amps-myth
The more important part is how much of the total power is getting to the 12V rail. A good PSU will deliver 80% or more.

You need to think about the total draw on the PSU which is the sum of your components. Also you are forgetting about case fans, external hard drives, etc. In addition, if you are going to use the PSU longer than a year you will see degradation of the capacitors over time. There are a couple of different power calculators out there that will tell you how much wattage you need. This one is pretty useful:

Plugging in your components into it and using Soundblaster as a substitute for your audio card plus guesstimating how many fans are on your case, I get 213W, 263W recommended. So your 350W PSU should be able to handle the load allright.

What you may have not known is that there can be a special sleep/idle state for Haswell CPU’s that require a PSU that can deliver very low levels of power to the Haswell and stably. It is supposedly possible to disable this idle/sleep state in the BIOS of Haswell boards, so you could potentially use your PSU. Or you could just shut off your PSU when you are finished with your computer.

All that being said, you might want to consider upgrading to at least a 80Plus Bronze or Gold PSU to save on power over the long run and avoid having to deal with workarounds for the idle/sleep state.


#3

[QUOTE=yojimbo197;2709258]The amperage thing is somewhat of a myth as explained here:
http://www.overclock.net/t/880633/psu-amps-myth
The more important part is how much of the total power is getting to the 12V rail. A good PSU will deliver 80% or more.[/QUOTE]

Umm, I’m little confused. Power equals voltage multiplied by amperage. The link explains that if you have for example, a 500W power supply - and if it doesn’t deliver ~75% of that power in the +12V rail - it should be considered as lower wattage PSU.

And in that case you might miscalculate the power supply in question to be adequate when in reality it’s not.

So staring at the PSU’s rated wattage doesn’t cut it. Voltage is constant, can’t rely on wattage - so the only thing that I can look is the rated amperage.

Example: Let’s take a 500W PSU which delivers 35A on the +12V rail (12V*35A = 420W), which equals 84% of the total power delivered in +12V rail. For argument’s sake let’s take a 800W PSU, and it too delivers 35A on the +12V rail. So the wattage in the +12V rail is the same 420W, but in this case it is only 52,5% of the rated wattage. And in reality (if you don’t have anything that draws huge amounts of power from +5V or +3,3V lines) these two PSU:s are equal in practice.

[QUOTE=yojimbo197;2709258]
You need to think about the total draw on the PSU which is the sum of your components. Also you are forgetting about case fans, external hard drives, etc. In addition, if you are going to use the PSU longer than a year you will see degradation of the capacitors over time. There are a couple of different power calculators out there that will tell you how much wattage you need. This one is pretty useful:

Plugging in your components into it and using Soundblaster as a substitute for your audio card plus guesstimating how many fans are on your case, I get 213W, 263W recommended. So your 350W PSU should be able to handle the load allright.
[/QUOTE]

Oh yeah, forgot the case fan. 2-3 watts on the 12V line should be enough for a case fan? Only possible USB device is a memory stick. And yes, I must check the caps before even thinking about using the PSU.

[QUOTE=yojimbo197;2709258]
What you may have not known is that there can be a special sleep/idle state for Haswell CPU’s that require a PSU that can deliver very low levels of power to the Haswell and stably. It is supposedly possible to disable this idle/sleep state in the BIOS of Haswell boards, so you could potentially use your PSU. Or you could just shut off your PSU when you are finished with your computer.

All that being said, you might want to consider upgrading to at least a 80Plus Bronze or Gold PSU to save on power over the long run and avoid having to deal with workarounds for the idle/sleep state.

[/QUOTE]

Yes, I read about this. It is not a problem - I don’t use sleep state and at least the HDD’s are whirring away so that should be enough load to keep the PSU up and running.

I’m still wondering about how much power (amps) is needed from the +12V line to keep things running… Manufacturers should really put power consumption figures in their products, not just vague “1 megawatt PSU is recommended” when it seems that it is grossly overrated.

If memory serves me correct, too big a PSU also hinders efficiency and targeting the 50% load mark would be good.


#4

Well, I found the data needed so I’ll post it here if someone wonders the same thing

When looking the Intel Haswell CPU datasheet, the answer is right there on page 98.

So when considering i7-4770K (84W TDP) we see that the true power requirement for the CPU is 153 watts (That is peak power, and CPU may draw this power for maximum of 10 milliseconds before throttling).

Pulling 153 watts from the +12V line would take current of (I = P/U), 153 watts divided by 12 volts = 12,75 amps peak current.

When looking at the old (2007) PSU design guide, page 18 which states that requirements from +12V line (with a 84W TDP CPU) is 13 amps continuous current.

I think that the PSU design guide information most likely is outdated, but the 13A requirement isn’t too far off and at least I’m comfortable calculating PSU requirements with that figure.

I cannot comprehend why CPU manufacturers “advertise” their CPUs with the quite useless TDP values. As far as I know, proper use of TDP is when designing or applying different cooling systems (for knowing how much thermal power the cooler has to be able to dissipate). But when buying a boxed CPU, it comes with a cooler - so I’d expect that it is capable to handle the CPU it came with under normal conditions.

Think about it - you go to auto store and ask how many mpg do you get with a - let’s say a brand new Honda Civic. The answer is “2 gallons per mile” and you are like :eek: … and the clerk continues that “Yes, after a mile the car has warmed two gallons of cooling fluid to a certain temperature”. :a

…oh, and yes I’ll be buying a little more beefier PSU :iagree:


#5

How beefier? Will you pass the 500w range?

I prefer smaller-than-larger simply because I have to pay for these things’ utilities usages. We use 650-750s for home RAID systems for years now, but I have strong resistence to anything beyond that. For me, the 750s are hosting two RAIDs (two 4’s or two 5’s) at that point, and the 650s are hosting single RAIDs (4-5-6 disks).

Now… when we can get a half-dozen 10Tb SSDs for my magical $139 price, maybe I can consider going back to 450s.


#6

[QUOTE=ChristineBCW;2710336]How beefier? Will you pass the 500w range?

I prefer smaller-than-larger simply because I have to pay for these things’ utilities usages. We use 650-750s for home RAID systems for years now, but I have strong resistence to anything beyond that. For me, the 750s are hosting two RAIDs (two 4’s or two 5’s) at that point, and the 650s are hosting single RAIDs (4-5-6 disks).

Now… when we can get a half-dozen 10Tb SSDs for my magical $139 price, maybe I can consider going back to 450s.[/QUOTE]

Well, it happened so that some local guy was selling his Corsair VX550, so I bought that (30€). Did not buy it because of the wattage, but for the cabling, which seemed nice.

And by beefier - I must add that I mean such a PSU that complies with the newer ATX specifications. My old 350W PSU was following the old ATXv.1.x standard, so a lot of the power was available in the +5V and +3,3V rails. So in practice old 350W PSU is equivalent of a less than 200W modern PSU.

When I did check the stores for a brand new one, I was aiming at the 350-400 watt range. There was absolutely no need for a bigger one (no oc, no discrete 1,21 jigowatt graphics card and only a few HDD’s).

It seems that the Haswell chips are quite good in the power department (see Review 1 or Review 2)


#7

[QUOTE=Mastus;2710355]…no discrete 1,21 jigowatt graphics card…[/QUOTE]
Wait. I don’t understand. How are you going back to the future? How can you be driving in your car, hit the PSU’s cabling and go ZAP, back to the future?!! You mean - you’re stuck HERE?!! Jeepers.

Well… this IS what all those gamers with 4 cards and 1200w PSUs are thinking of, right? “Uh - that’s 4 cards AND a DeLorean…”

Ah yes… I always forget THAT part. (Boy - they shoulda asked for a DB6 or a XKE instead…)


#8

This system without discrete graphics should stay far away from 200W @12V.

Your old PSU should handle it without big problems as long you don´t use Standby on this sytem. Maybe also Standby is not a prob with old Single-rail-design.

What PSU you have exactly?