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 The cover of this month's Radio & Television
News magazine is part of the issue's story on performance testing of resistors.
The author was an engineer for International Resistance Company (IRC), which is
still in business as part of
TT Electronics. The massive ovens were used for load-life testing
to certify resistor products for both military and commercial uses. When required,
humidity enclosures subjected resistors to increased levels to test for insulation
breakdown at high voltage. As the article observes, since a 10-cent resistor can
take down a multi-thousand system, it is important to guarantee every component's
integrity.
Resistor Trial by Test
 By Guy B. Entrekin
Chief Product Eng., International Resistance Company
Extensive laboratory tests insure reliable service from the components used in
electronic equipment.
We have become so used to taking the reliability of electronic components for
granted that we often forget "how they got that way." Since the performance of even
the most elaborate equipment depends on many small and relatively inexpensive parts,
the reliability of a 10-cent resistor is just as important as the most complicated
tube.
One example of the lengths to which manufacturers go to insure the reliability
of even small components is typified by the resistor testing program at International
Resistance Company, Philadelphia. Its laboratory checks approximately a half-million
resistors a year - putting them through a wide variety of tests to insure uniformity,
quality, and performance.
For example, MIL specifications for all types of resistors require tests under
load. These tests are made under conditions of various applied voltages and ambient
temperatures. IRC utilizes several massive load-life ovens, like that pictured on
this month's cover and on this page, to perform these load-life tests. Each of these
precision-engineered ovens can test approximately 1200 resistors at a time and is
uniquely designed to meet the variety of conditions necessary to fulfill military
and customer specifications for load-life testing.
The ovens are designed so that various voltages can be applied to produce whatever
wattages are specified in the test of a particular resistor. The resistors are supported
in such a way that the heat dissipated by one resistor does not affect another unit
being tested. "Still air," rather than circulating air, is maintained within the
ovens as an added safeguard to guarantee conformity.
Another interesting aspect of MIL specifications for load-life testing is the
requirement that ovens be equipped to feed loads intermittently to the resistors.
During test, loads are normally applied for 1 1/2 hours, then cut off for 1/2 hour,
then applied for 1 1/2 hours, alternating on and off for the full test period.
This intermittent application of the load produces a temperature cycling effect
- important because continuing heating and cooling of any resistor introduces stresses
similar to those obtained when equipment is turned on and off.
These massive load-life ovens in IRC's resistor test section
operate 24 hours a day, testing hundreds of resistors of every kind. Resistors hang
in trays which are individually controlled at varying loads depending on the test
being made.
The actual voltages applied to resistors during test depend on their resistance
range and ambient temperature of the test. These load-life ovens have a test voltage
range of 0 to 750 volts-more than adequate for standard testing since the majority
of all resistors test at 500 volts or less. For tests requiring the application
of higher voltages, a specially designed oven in the test section is used.
Moisture cycling box which can duplicate any humidity conditions called for by
MIL specifications for testing resistors.
Since the load-life ovens operate 24 hours a day, 7 days a week, automatic recording
equipment and built-in safety devices of many types are used to give tight control
over testing. In every test run, intermittent readings are taken to trace the pattern
of resistance change with time. MIL specifications usually require at least four
readings per thousand hours of test. More frequent readings are taken if tests require
them.
Testing a single group of resistors may take six weeks or longer depending on
the number and type of tests they must undergo. Where a shelf-life test is indicated,
resistors are stored under the same or more severe conditions than they would encounter
in a distributor's stock or in a local service shop, and are periodically tested
for as long as three years. This test insures that resistors will not deteriorate
in stock while awaiting use.
Resistor testing is an involved, lengthy, often laborious undertaking, but it
has made an invaluable contribution to the development of new and better electrical
equipment and the amazing reliability of such gear under all conditions.
This is just one example of the infinite pains manufacturers take to insure uniform,
inexpensive, and reliable components.
It is because of such painstaking procedures that U. S.-built electronic equipment
has earned a reputation for excellent performance that is second to none in the
world.
Posted October 13, 2020
(updated from original post on 1/18/2016)
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