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|DC load on PSU
||DBS-2100 PSU load tester
|Any number of thermometers
DC voltage regulation
|Heath / Zenith SM-2320 multimeter
|Kill-A-Watt Power Meter
The core PSU test tool on SilentPCReview's test bench is the DBS-2100 load tester, made specifically for testing computer power supplies. The machine consists of a large bank of high power precision resistors along with an extensive selection of switches on the front panel calibrated in Amps (current) and grouped into the 5 voltage lines: +5, +12, -12V, +3.3, +5SR. Leads from the PSU connect into the front panel.
The DC output connector closest to the PSU on each set of leads is used for hookup to ensure that the current is distributed through as many short leads as possible. The wires get very hot when pushing a PSU to high output.
The Seasonic Super Silencer was tested at 5 DC output power levels:
65W: A very typical DC power draw by many system at low / modest load.
90W: Established previously as a typical max power draw of a midrange desktop PC.
150W: For higher power machines.
300W: Perhaps a typical peak demand on real systems
400W: Full rated power.
Care was taken to ensure that the load on each of the voltage lines does not exceed the ratings for the PSU. The PSU was left running 10 minutes at each power level before measurements are recorded.
The DBS-2100 is equipped with 4 exhaust fans on the back panel. A bypass switch toggles the fans on / off so that noise measurements can be made. The resistors get very hot under high loads.
Kill-A-Watt AC Power Meter is plugged into an AC outlet on the side of the DBS-2100 in the above picture. The AC power draw of the PSU is measured at each of the 4 power loads. The Kill-A-Watt is used to measure:
Efficiency (in AC-to-DC conversion) at each power level. This is the efficiency figure provided by PSU makers, obtained by dividing the DC power output (as set on DBS-2100 load) by the AC power consumption. Efficiency varies with load, and also temperature. The main advantage of high efficiency is that less power is wasted as heat. This means a cooler PSU that requires less airflow to maintain safe operating temps (read: quieter).
Power Factor (PF). Read directly off the Kill-A-Watt. In simple terms, it tell us how much AC power is lost to harmonics (unnecessary electromagnetic energy) while driving the PSU. In practical technical terms, it is the difference between the V(oltage) x A(mperes) and AC power in Watts measured by Kill-A-Watt.
PF varies somewhat with load. The ideal PF is 1.0, which means no AC power is lost. A PF of 0.5 means that to deliver 100W in AC to a PSU, your electric company actually uses 200W and this is often shown in your electric bill as savings (depends on your electric utility company and your account with them). 100W is lost or wasted. Active PF Correction (PFC) power supplies usually have a PF of >0.95. Passive PFC units usually run 0.6 - 0.8. Non-PFC units usually measure 0.5-0.7. PF is not significant in terms of noise, heat or performance for a PC, but it is relevant to electricity consumption and energy conservation.
The Heath / Zenith SM-2320 digital display multimeter, a fairly standard unit, is used to measure the fan voltages and the line voltages of the PSU output. The latter is done via the terminal pin on the front panel, above the connections for the DC outputs from the PSU.
The Test Lab is a spare kitchen measuring 12 by 10 feet, with an 8 foot ceiling and vinyl tile floors. The acoustics are very lively and allows even very soft noises to be heard easily. The PSU under test is placed on a piece of soft foam to prevent transfer of vibrations to the table top. Temperature in the lab was 24C. This is something of a problem as PSUs usually operate in environments that easily reach 45C. Sited next to or above the CPU, the PSU is always subject to external heat. This brings us to the next topic...
In-case Thermal Simulation
The solution is a AC bulb in an empty case with the PSU mounted normally. The distance between the bottom of the PSU and the top of the bulb is about 7 inches. A 60W bulb was used for all the load tests up to 150W, then swapped for a 100W bulb for the 300W and 400W load tests. The bottom front vent of the case and its bezel have been modified for unrestricted airflow; the approximate size of the intake hole is the equivalent area of a 92mm fan,
The PSU must cope with the heat generated by the light bulb plus whatever heat it generates within itself. In a real system, there would be other air exhausts paths and mostly likely at least one case fan. So a Panaflo 80mm L fan was mounted on the back panel of the test case and connected to a voltage controller. The PSU was run through its load range with the fan turned on to 7V, about the level at which most PC silencers would run their case fan.
A highly accurate calibrated B&K model 1613 sound level meter on temporary loan from the University of BC's acoustics lab was used for noise measurements.
This professional caliber SLM dates back to 1978, weighs over 10 pounds, and is completely analog in design. It has a dynamic range that spans over 140 dB. The microphone used has a 1" diaphragm that's very responsive to low sound levels and low frequencies. The unit's absolute sensitivity reaches below 0 dBA -- at one point in the midband (1kHz) it was reading -4 dBA for background noise in the UBC anechoic chamber.
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