In this thesis I was focused on the energy varistor, which is mostly integrated in surge protection devices, and its functioning exposed to higher temperature and humidity. Such conditions are more an exception rather than a common environment, but the standard declares such conditions. The thesis consists of the description of the varistor and equipment used in the tests. For the tests I chose two different types of insulation and a reference varistor, without insulation. Insulation material is epoxy, the most widespread insulation among varistors and other electronic components and polyurethane potting, which allows encapsulation in the housing and is increasingly used in electronic assemblies. I monitored intermediate results with the help of special custom designed device AMTTV V1, which allows monitoring of the leakage current. Final measurements were carried out on the Keithley 2410, which is used to measure the nominal voltage. First I tested varistors only at elevated temperatures, which is declared by producer of varistors, as the maximum operating temperature. The results according to standard IEC 61051-2 were positive. In experiment 2, I refered to the standard EN 50539-11 (paragraph 7.6.1). The experiment was conducted at a relative humidity of 93 % and a temperature of 40 °C. Reference varistors and those with epoxy coating showed a stable leakage current through the whole period of test. Much worse behaved varistors with polyurethane potting. Despite the apparently alarming results, all products passed the test. Standard requires that the varistors need to be measured after the test and nominal voltage must not raise for more than 10 %. Tests gave me directions for further research and testing of different insulation materials.
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