Evaluation Process

This content was originally featured on Amdmb.com and has been converted to PC Perspective’s website. Some color changes and flaws may appear.

We evaluated each power supply unit (PSU) on features and performance.  A full range of equipment was used to test each PSU under controlled load conditions.  In addition to measuring the power going in and coming out we looked at voltage regulation, electrical noise (AC ripple), airflow, sound level, efficiency and cost.  Here is a block diagram of the test bench setup and a list of the equipment I used during testing.

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  • FLUKE 87-III True RMS digital multi-meter (Accuracy +/- 0.05% of 3-digit reading)
  • WattsUp? Pro – digital wattmeter and power analyzer (Accuracy 3% of displayed value)
  • Hitachi V-650F 60 MHz dual trace oscilloscope (Accuracy +/- 3% of input range)
  • Powerstat Variable Autotransformer, 1.4 KVA, 0-140 VAC
  • FLUKE  52-II digital thermometer (Accuracy +/- 0.3ºC/0.5ºF)
  • Extech Model 407736 digital sound level meter (Accuracy +/- 1.5 dB)
  • AccuLab V1-10kg digital balance (Accuracy +/- 1g)
  • Homemade power supply load tester – 300 watt combined load

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To be ATX compliant, a power supply must be designed and built according to a set of standards created by Intel (ATX Specification – Version 2.1), which along with motherboard specifications also defines the size, form factor, connectors, voltage outputs, etc., that a power supply must incorporate.  More detailed information can be found in the ATX12V Power Supply Design Guide.  All of the power supplies in this roundup conform to the ATX 2.03 specification.

Power supply manufacturers generally include the maximum total output in the name of their products.  This is frequently a major selling point.  But just because a particular unit is advertised as being a 400 watt model does not mean it will operate reliably and for any length of time if fully loaded 24/7 at the maximum 400 watts.

A general rule-of-thumb is not to exceed 70% of the total rated output.  For a 400 watt PSU that means the total continuous load (not intermittent loads) being drawn by the PC should not exceed 280 watts.  Doing so may shorten PSU components life. The AC ripple present on all DC outputs also increases as the load goes up.

One of the biggest variables in determining a power supplies true output load potential is the temperature that the power supply components were designed to operate at.  Manufacturers don’t always divulge these operational temperature limitations.  To illustrate this point, consider two very different power supplies: each advertised as a “400 watt ATX PSU”.  One unit is designed to produce 400 watts as long as the power supply’s internal temperature does not exceed 25ºC (not realistic).  The second unit is designed to produce 400 watts as long as the temperature of internal components does not exceed 50ºC (realistic with a built-in safety margin).  Something else you will notice about these two power supplies is one weighs and costs considerably more than the other.

The switching-mode power supplies used in modern PCs are designed to convert alternating current (AC) into direct current (DC) used by the computer’s internal components.  A standard ATX power supply will produce three different voltages to power the motherboard, CPU, memory, hard drives, optical drives, etc.  These are: +3.3 volts, +5 volts, and +12 volts.  In addition, the PSU also generates several other voltages: -5 volts, -12 volts, and +5 volts STBY.

Establishing a controlled load is critical to testing and evaluating a PC power supply.  I built my own power supply load tester using 10 wire-wound, ceramic resistors of various sizes.  This unit places a ~300 watt combined load on the power supply being tested.

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Here is the Ohm’s Law key if you are trying to remember how the different variables relate… :)

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(Click to enlarge)

Most all ATX power supplies incorporate over-voltage and over-current shutdown circuits to help protect the PSU from these conditions.  What many people do not realize is that some ATX, switching-mode power supplies (usually older models) will self-destruct if turned on without any load connected!  To be safe, never turn on a switching-mode power supply without a minimal load connected to one or more of the outputs.  In general, placing a 5 ohm, 25 watt resistor across the +5 volt output is sufficient.  Be sure to check and see what your particular PSU manufacturer recommends.  Inexpensive power supply testers are available that incorporate a load resistor.

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PC Power & Cooling ATX PSU Tester ($10.00)

OK, before we get into the nuts-n-bolts (arcs-n-sparks) of testing lets take a quick look at each of our ten ATX power supply’s specifications and features.

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