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
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.
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.
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.
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
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 January 18, 2016