Page 60 - Energize April 2022

TECHNICAL

capacity of most linear AC sources. This is due to both the added reactive power being sometimes called “stiff sources” because
dissipated by the source and the current being drawn much closer to zero crossing of their outputs remain constant, even when
the voltage waveform. The graph shows a typical curve that you would use for derating a switching from no load to full load.
linear source’s output capacity. Typically, stiff sources have used
Switching AC power sources, on the other hand, need not be derated for power factor analog technology to provide tight
since they operate much differently with reactive loads. The power devices dissipate much regulation and low source impedance, but
less power and therefore run much cooler as a result. sources that use switching technologies
You do, however, have to derate the VA rating of both switching and linear sources now perform just as well as linear sources.
when performing tests at low voltage settings. Make sure to take this into account when An example of this type of AC source is
testing a device at its worst-case, low voltage input. For an example, on the CSW series, California Instruments’ CSW Series. It has
each phase amplifier is rated for 1,850 VA maximum. The amplifier has a maximum a voltage accuracy specification of ±0,1%,
current rating of 16 A from zero to115 V and a maximum voltage rating of 156 V. The a THD specification of 0,25%, and very fast
current derates in a linear manner above 115 V to maintain the maximum limit of 1850 load response times.
VA. (i.e., 1850 VA/146 V = 12,67 A).
When operating at voltages below 115 V, the current is the limiting parameter. At 5 V Slew rate
output you have 16 A available providing 80 VA. In other words, you can’t obtain 1850 A For many tests that require that you
at 1 V. simulate real-world conditions, such
as fluctuations in voltage, sags, surges,
dropouts, or spikes, you need a source
with a fast slew rate. An AC source’s slew
rate is the time it takes the source to
respond to a change in the programmed
voltage or frequency. High performance
AC sources typically have slew rates of
less than 50 μs.

User and test-system interfaces
AMETEK offers many different interfaces,
including RS-232, USB, GPIB and Ethernet,
for integrating an AC source with your
test system. In addition to the hardware,
AMETEK also supplies IVI drivers with
each AC source for use with National
Instruments LabVIEW and LabWindows/
CVI. Other user-developed systems are
readily supported due to the use of SCPI
Figure 1 VA programming syntax. This eases the
tasks of ATE system programming and
Regulation and distortion integration.
Load and line regulation should be tight, and distortion low. Poor regulation and distortion In addition, a Graphical User Interface
can lead to faulty test data that may not be discovered until units are in the field, or lead application is supplied to provide a means
to false failure in testing. Typically, quality AC sources will have a voltage accuracy of +/- of remote operation. Figure 2 shows how
0,1% and a maximum total harmonic distortion (THD) of no more than 0,25%. you use this application to easily create a
AC power sources with poor regulation are sometimes called “soft sources.” A soft test waveform with harmonic content.
source has a high output impedance and low peak current capability and cannot provide
the peak currents that may be required for stress testing components properly. This Facility requirements
leads to a higher failure rate. For example, if you used a soft source to perform the tests If the AC source must supply a lot of
specified by IES LM-41-1985, “IES approved method for photometric testing of indoor power, you may have to plan for a new
fluorescent luminaires” to test a fluorescent lamp and ballast, the source would produce a power distribution and disconnects at
distorted waveform and invalidate the test results. your facility. Physical and environmental
requirements must also be taken into
Response time account. Large AC sources can require
Another consideration is the load response time, or the time it takes an AC source to significant floor space and will require a
respond to a change in the load. AC sources with fast load response times generally have high volume of airflow within a specified
low source impedance and tight regulation. AC sources with these characteristics are temperature range.

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