Second, we use an ordinary shunt on powerful wire resistors with a total resistance of about 0.61 Ohms to make oscillograms of the power consumed by the PSU and of the supplied AC voltage.
When testing a PSU, the probes of a digital two-channel oscilloscope are attached to this board. One channel is drawing the oscillogram of the AC voltage and another – the oscillogram of the power consumption of the PSU. Then, these oscillograms are processed with a special utility which calculates the parameters we’re interested in: the active, reactive and full powers consumed and the power and efficiency factors.
To take oscillograms we use a digital dual-channel “virtual” oscilloscope M221 from the Slovak company ETC (we call it “virtual” because this oscilloscope is a board installed into the computer and, unlike ordinary oscilloscopes, cannot work without a computer since it doesn’t have its own hardware tools for controlling and displaying information). The analog part of the oscilloscope has a pass band of 100MHz; its maximum speed of digitization of a random signal is 20 million samples per second and it has a sensitivity range from 50 mV/del to 10 V/del. Besides measuring the efficiency and power factors of the tested PSU, the oscilloscope is employed to estimate the amplitude, shape and frequency content of the pulsation of the PSU’s output voltages.
For a quick evaluation of the currents and voltages during the tests, and also for periodical tests of other measurement equipment, we use a Uni-Trend UT70D multimeter which accurately measures currents and voltages, also of non-sinusoidal shape, which is very important when testing PSUs without power factor correction. Many measurement devices, without the “TrueRMS” label, cannot produce adequate measurements of alternating currents and voltages whose shape is other than sinusoid.
A digital thermometer Fluke 54 Series II with thermocouples 80PK-1 and 80PK-3A (the model names are quoted from the Fluke catalogue) measures the temperature inside the PSU in our tests. Unfortunately, our remote infrared digital thermometer was inaccurate on shiny metal surfaces (like the aluminum heatsinks of computer power supplies), so we had to use a thermocouple thermometer instead.
An optical tachometer Velleman DTO2234 measures the speeds of PSU fans. The PSU remains closed at that (i.e. we don’t interfere with its regular thermal conditions) – we only stick a thin stripe of reflective material on one of the fan’s blades.
And the last touch: to feed the same AC voltage to all PSUs, irrespective of its daytime fluctuations, and to test PSUs at a higher or smaller AC voltage, they are attached to the power grid through a laboratory auto-transformer Wesley TDGC2-2000 with an acceptable load power up to 2,000 watts and an adjustable voltage range of 0-250 volts.