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For a fuller understanding of ATX power supplies, please read our article Power Supply Fundamentals & Recommended Units. Those who seek source materials can find Intel's various PSU design guides, closely followed by PSU manufacturers, at Form Factors.
For a complete rundown of testing equipment and procedures, please refer to the
SPCR's Revised PSU Testing System. It is a close simulation of a
moderate airflow mid-tower PC optimized for low noise.
In the test rig, the ambient temperature of the PSU varies proportionately with
its actual output load, which is exactly the way it is in a real PC
environment. But there is the added benefit of a precise high power load tester
which allows incremental load testing all the way to full power for any
non-industrial PC power supply. Both fan noise and voltage are measured at
various loads. It is, in general, a very demanding test, as the operating
ambient temperature of the PSU often reaches 40°C or more at full power.
This is impossible to achieve with an open test bench setup.
Great effort has been made to devise as realistic an operating environment for the PSU as possible, but the thermal and noise results obtained here still cannot be considered absolute. There are far to many variables in PCs and far too many possible combinations of components for any single test environment to provide infallible results. And there is always the bugaboo of sample variance. These results are akin to a resume, a few detailed photographs, and some short sound bites of someone you've never met. You'll probably get a reasonably good overall impression of that person, but it is not quite the same as an extended meeting in person.
One very important point is that the while our testing loads the PSU to full output (even >600W!) in order to verify the manufacturer's claims, real desktop PCs simply do not require anywhere near this level of power. The most pertinent range of DC output power is between about 65W and 250W, because it is the power range where most systems will be working most of the time. To illustrate this point, consider the power consumption of the systems described below. They were tested with the same equipment used to test power supplies.
REAL SYSTEM POWER REQUIREMENTS
A: Midrange P4
- Intel Pentium 4-2.8C
- AOpen MX4SGI-4DL2 motherboard
- 2 x 512 mb OCZ PC3700 DDRAM
- Seagate 7200.7 120G HDD
- Seagate Barracuda IV 40G HDD
- Matrox P650 VGA (dual head mode)
- Seasonic Super Tornado 350W PSU
- Asus QuieTrack CDRW
- 6-in-1 card reader / floppy drive
- 3 low speed fans
Maximum Power -- AC Input: 126W = DC Output: 102W
B: High A64
- Athlon A64-3800+ (130nm core)
- Soltek SL-K8TPro-939 motherboard
- 4 x 512 mb OCZ PC4000 DDRAM
- ATI 9800-256 Pro VGA
- Hitachi 7K400 HDD (400gb)
- Samsung P160 HDD
- Silverstone ST30NF PSU (fanless)
- M-Audio Firewire 410 external firewire-driven sound card
- low speed 80mm fan
Maximum Power -- AC Input: 184W = DC Output: 147W
C: High P4
- Intel Pentium 4-3.2 (Northwood)
- Intel D875PBZLK motherboard
- 2 x 256MB HyperX DDR400 PC3200
- ATI Radeon 9800XT 256MB DDR
- 16x Sony DVD-RW
- Zalman 400W PSU
- Creative SB Audigy-2 ZS Platinum
- 2 x 120mm fans and 1 80mm fan
Maximum Power -- AC Input: 236W = DC Output: 180W
- All the systems have two hard drives.
- The max AC input power was measured directly; it is what the system / PSU draws from the AC line. The DC output was calculated based on the efficiency of the PSU used in each system. It is the DC power delivered to the components by the PSU. We have measured the efficiency of the actual PSU used in the above systems. The results have been posted in previous PSU reviews.
- The highest power consumption was achieved while running PCMark04, a system benchmark which brings the VGA card into play. The recorded wattage is the highest peak seen during this benchmark. Sustained maximum was about 5~10% lower.
- The C: High P4 system could draw as much as 50~70W more (AC) with a P4-3.8 (the most power hungry desktop CPU). This would put max AC system power draw up to ~300W; the DC power delivery of the PSU at that point would be ~225W. Increasing memory by another 512mb might add 10W. A second VGA for SLI could add ~70W.
- Note that modern systems (after 2003) tend to draw most of their power off the 12V line. ATX12V PSU Design Guideline v1.3 and beyond specify much higher current for the 12V line than in the past. As a result, older PSUs that deliver high enough total power may fail to deliver enough 12V current to satisfy new systems.
- The measure power numbers are quite accurate and based on empirical testing, but for argument's sake, you could say they're as much as 10% too low. The max power draw of even the most extreme system discussed above would still be less than 400W DC.
SPCR's high fidelity sound recording system was used to create MP3 sound files of this PSU. As with the setup for recording fans, the position of the mic was 3" from the exhaust vent at a 45° angle, outside the airflow turbulence area. The photo below shows the setup. All other noise sources in the room were turned off while making the sound recordings.
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