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Do Psu only draws that amount of power which is needed?

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May 3, 2010 12:03:29 PM

Psu only draws that amount of power which is needed or always draws the amount of power at which it is rated at?So will an 850W psu draw 380 W(needed to run my system)ideally or will always draw 850W from main line and put unnecessary electricity bills to me?
I am about to buy Gamextream OCZ 850W.How is this psu for my future upgrade ie phenom x6 and existing HD4850?
a b ) Power supply
May 3, 2010 12:28:17 PM

Yes PSU electrical load draw what is the actual required from the PSU. The first one is actual power demand in Watts while your PSU rating is the Capacity in Watts.

But not all PSU have the same efficiency at lower load conditions. Some high capacity power supplies don't do well at low-load condition so its not very efficient to run. Below is an example of a good PSU with excellent efficiency even a low load condition.

http://www.jonnyguru.com/modules.php?name=NDReviews&op=...

Unless you are sure of your future demand it is still recommended to purchase close to the actual load of the computer(Save Money)

As to Gamextream OCZ 850W look it out on jonnyguru.com. Compare it to HX850 or XFX -850 and you will have an idea.
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a c 144 ) Power supply
May 3, 2010 1:43:59 PM

It's not good to run any PSU at close to is maximum ratings.

I think a good compromise is around 70 - 75% of rated power.
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a b ) Power supply
May 3, 2010 1:49:00 PM

+1 to what leon2006 said.

These are the official recommended power requirements based on your graphics card:

450 Watt or greater power supply with 75 Watt 6-pin PCI Express® power connector recommended (550 Watt and two 6-pin connectors for ATI CrossFireX™ technology in dual mode)

This applies to the entire system not just your graphics card.

As you can see from the above, an 850W PSU would be overkill for your needs. You could get a good 600-650W PSU and that will also give you enough power to Crossfire if you ever wish to do so in the future plus some headroom.

It is true that some of the lower quality PSUs have poor efficiency at low-load conditions, however the better quality units don't suffer from this problem. I too would advise that you purchase a PSU to meet your needs. If you can reasonably see yourself Crossfiring your cards in the future, or just want the headroom then get something around the 600W mark. If not, a solid 500W will do you fine.

Recommended brands are Corsair, Seasonic and the newer Antec models.
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May 4, 2010 11:36:36 AM

Ok ,but i am looking for a future proof psu so that i need not to upgrade in coming 10 years.will an OCZ gamerxtreame 700W is OK?
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May 4, 2010 11:51:16 AM

If you can predict:

1) That any PSU that is regularly used will last 10 years, and
2) What power requirements will be in 10 years (perhaps really high or really low or the same), and
3) That power supply and cabling standards will not change in 10 years...

then I suppose you could find a future proof PSU that will last you that long. However, realistically, if you plan to upgrade the other components and keep the same PSU, and you plan to use the system quite a bit, then you'll probably be pushing it to get past 5 years. You might strike it lucky though.
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May 4, 2010 12:11:06 PM

guys i am confuse.. so if i use a 600watt psu, the psu will consume 600 watt irregardless of my system watt consumption???
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May 4, 2010 12:15:27 PM

The PSU will draw form the wall your total system power consumption plus whatever is wasted as heat due to the AC to DC conversion. The higher the PSU's efficiency, the less power wasted as heat and the less drawn from the wall. Note that efficiency is not constant but varies with the load on the PSU.
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May 4, 2010 12:54:56 PM

randomizer said:
The PSU will draw form the wall your total system power consumption plus whatever is wasted as heat due to the AC to DC conversion. The higher the PSU's efficiency, the less power wasted as heat and the less drawn from the wall. Note that efficiency is not constant but varies with the load on the PSU.


thanks a lot.. cos i just want to make sure i do not waste unnecessary power..
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a b ) Power supply
May 4, 2010 3:47:58 PM

The rule of thumb is load your PSU to ~ 85% +/- of its capacity. Now you need to be careful with your calculation. The actual power capacity of a PSU is


EXAMPLE 12V Rail:

Read the PSU Voltage / Current Labels

For 12V Rail
Current (Amperes) X Voltage (DC Volts) = Capacity in Watts
70A X 12VDC = 840 Watts

Others would recommend lower %load of capacity then it becomes an issue of money.


85% of 840 Watts is 714 Watts.... If your load on the 12V rail/s is ~ 650 to close
700 watts you should be OK..

Additional Information:( SURGE CURRENT RATING)
Power Supplies are required to sustain load upto 150% of capacity during power up (fraction of a second) CMOS circuits(CPUS, GPUS, Memory, Chipsets, Drive Controllers, etc) are dead short at time-zero during power-up. There is a brief in-rush of current for a fraction of a second. You need a time-interval scope to see this in the lab.
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a b ) Power supply
May 4, 2010 4:37:02 PM

I prefer jcs recommendation of selecting the PSU wattage at 65->75% of anticipated load, ie if planning to xfire/SLI factor that in. I also try to allow 20 % at idle.

Ex I have a system that draws approx 100 W @ Idle and approx 300 Watts at max load (Use furmark/prime 95). I settled for the corsair 400 W. 25 % at Idle and 75% at max load. If I was planning on adding a 2nd GPU (not), that would have boosted idle by 20 to 50 Watts and Max by approx 100 W and I would have purchased a 550 ->600 Watt (or the closest match above 550.

PSU caluclators tend to report a higher wattage rating than need to offset the fact that many skimp (NOT GOOD IDEA) on the PSU. The low end PSU do NOT provide the wattage as rated when temp is above 30 C (Good PSUs are rated at 40/50C.

I normally use a power consumption chart and make to baseline and adjust as needed from the "Reference system"
http://www.techspot.com/review/209-ati-radeon-hd-5770/p...

leon2006 - the O'Scope you refer to is a Triggered" scope. When making inruch current measurements I set to trigger on voltage (Ch 1) and have a current probe on channel 2 to display the Inrush Current. Not sure of the spec for "Computer" PSUs, but it is not uncommon for Inruch current to be 2 - > 3 times stady state Current. High inrush current is normally attributed to The large filter capacitors and the duration (Millsec) is a function of the RC time constant. While the capacitance is Large the "R" is very small.
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a b ) Power supply
May 4, 2010 6:23:54 PM

CMOS circuits (CPUS, Memories, Chipset, GPUS, All modern electronics today) are base on cmos design. Its capacitive in nature specially the DRAMS. The time-zero state of a capacitor is a short circuit. This apply to all electronic circuits that use CMOS design. All electronic circuits will have in -rush current.

A typical example is DRAM... A single BIT (CELL) memory is one transistor and one capacitor(basic design). Now a byte is 8 bits. Then if you have 4 G of memory... Thats a lot of short circuit at time-zero.

There are methods to temper its effects like managing the power up sequence of the hardware. Its the law of physics and we can't do much about.

These are included in Transient electronics. These things happens in a very brief period. But for that period the power supply should have enough capacity to sustain it. Its a real load.

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a b ) Power supply
May 4, 2010 6:25:13 PM

Measurement of high current circuits require parameter translation. Its not a straight forward connection to the OSC scope that you are talking about.
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a b ) Power supply
May 4, 2010 6:27:22 PM

If you plan to crossfire plan ahead and calculate the load with crossfire video card.
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a b ) Power supply
May 4, 2010 6:42:09 PM

leon2006
(1) capacitance in microcircuits is very small when compared to say 100 uF filter caps. While that capacitance does cause increased curient at start-up, it is negable compared to external "real" capacitors.

(2) And Yes it is straight forward. I do it on 3 28 V PSUs connected to a satellite instrument. I connect a 10:1 voltage probe to channel 1, and a current probe to Ch2 ( it measures the intensity of the magnetic field).
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a b ) Power supply
May 4, 2010 7:32:35 PM

RetiredChief said:
leon2006
(1) capacitance in microcircuits is very small when compared to say 100 uF filter caps. While that capacitance does cause increased curient at start-up, it is negable compared to external "real" capacitors.


Consult with your nearby engineering professor if you want to determine or quantifiy how much current or energy is involved between a microcircuit dram cell (Q=CVx 8Gig)for example and a certain capacitor(Q=CV).



(2) And Yes it is straight forward. I do it on 3 28 V PSUs connected to a satellite instrument. I connect a 10:1 voltage probe to channel 1, and a current probe to Ch2 ( it measures the intensity of the magnetic field).


If you said so.
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May 5, 2010 12:16:57 AM

Also, capacitor aging reduces maximum output over time, so don't run it too near the limit.
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May 15, 2010 12:40:40 AM

Best answer selected by darksmart.
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