Conformity Magazine- May 2008 - (Page 39) Overload/Endurance Test Results The percent shift of tripped temperature after the overload/ endurance test is shown in Table 3. The data also shows that the percent shift is larger for the AC test than for the DC test. Cold Operational Test Results The percent shift of tripped temperature after the cold operation test is shown in Table 4. The results for two models appear to be similar given the measurement accuracy and sample differences. Thermal Runway Test Results The percentage shift in the tripped temperature for the thermal runaway test is shown in Table 5. There is no discernable difference between the AC and DC test results, possibly due to the short duration of the test (less than 5 minutes). Conclusions Based on this study, we found that there is no significant difference in performance for the same voltage but different polarities. The AC test condition appears to be more severe than the DC test condition during the aging and overload/ endurance tests. These results may be used to waive long-term life tests when giving new DC ratings to a polymeric PTC current limiting device that has an equivalent AC rating. This will save the time in conformance testing and allow shorter time for the product to receive a compliance certification. If the DC performance is evaluated first and the device has a percentage shift more than 10% (above Class 2 per UL 1434 definition), it is prudent to perform the aging and overload/ endurance tests in order to extend the AC rating. Further study will be needed to examine whether manufacturing techniques will affect this conclusion, and to analyze the internal structure of test samples. Yanqing Du was a Senior Project Engineer at Underwriters Laboratories specializing in product safety and compliance issues, and is currently a Managing Engineer at Exponent Inc. She can be reached at du@alum.mit.edu. Albert Martin and Mario Gomez are with Tyco Electronics, and can be reached at amartin@tycoelectronics.com and mario@tycoelectronics.com. Acknowledgment The authors wish to thank Mr. Wayne Benns of Underwriters Laboratories Inc. for his support to the presented work. Special thanks to Professor Markus Zahn of MIT for helpful comments on this article. References 1. Narkia, M, A. Ram, and F. Flashmer. Electrical Properties of Carbon Black filled Polyethylene. Polymer Engineering Sci., vol. 18, No, 8, p. 649, June 1973 Doljack, Frank A. PolySwitch PTC Devices-A New LowResistance, Conductive Polymer-Based PTC Device for Overcurrent Protection. IEEE Trans. On Components, Hybrids and Manufacturing Technology, CHMT-4, No. 4, December 1981. Underwriters Laboratories Inc., UL 1434, Standard for Safety for Thermistor-Type Devices, 1st Edition, April 3, 1998. 2. 3. FAST Link www.conformity.com/1750 mAy 2008 Conformity 39 http://www.conformity.com/1750 http://www.pseinc.com http://www.pseinc.com
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