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.
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 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 January 18, 2016