Conformity Magazine- May 2008 - (Page 35) olymeric PTC current limiting devices have been used in a variety of applications. This article explores the possibilities of extending an AC-rated device to a DC application with a voltage value equal to the AC RMS value. Extensive testing data shows that there is no significant difference between the two types of voltage application. The results may be a basis for waiving lengthy life test when giving the device an equivalent AC or DC rating. Polymeric PTC current limiting devices are composed of conductive compounds of polymer stabilizers and carbon black which results in a positive temperature coefficient (PTC) resistor. This device undergoes an abrupt change of several orders of magnitude in resistance when an overcurrent heats it above a specific temperature range.[1] The high resistance in the switched state limits the current to a small value. This residual current produces a sustained self-heating, which latches the device into the protective high resistance state. The PTC will reset only when the circuit is interrupted and the device is allowed to cool below its switching temperature.[2] This resettability provides electrical safety, and has the benefit of not requiring replacement, as would be the case for a fuse. Due to the resettable advantage, PTC current limiters have been used to limit current in a variety of applications, such as audio and video equipment, telecommunications, battery protection, transformers and more. As a result, some AC-rated devices have had a new application in DC circuits and viceversa. This article explores the voltage polarity effects on PTC devices, and verifies if additional tests are required for an AC-rated device to be used in an equal, DC-rated application. Extensive testing data are used to validate the hypothesis. Performance and Reliability Test Program Underwriters Laboratories has developed a series of test programs to evaluate the long-term performance of PTC devices under UL 1434, Standard for Safety for ThermistorType Devices. In particular, the series includes four types of testing that are affected by applied voltage as described in the following sections. It is important to study the difference of the PTC device under different voltage polarities to ensure the reliable and safe performance of the device. In the meantime, due to the long testing time involved, it is also beneficial to discover if there are any performance concerns with the two types of voltage, and whether the tests can be waived from a conformance perspective. Aging Test Under this test, the samples of PTC current limiter are energized and conditioned for 1000 hours (approximately 42 days) while in the tripped state at maximum voltage (Vmax) and carrying steady-state current. This test measures the combined long-term effect of heating and voltage stress, and most times is the most severe test among all tests. P Overload and Endurance Tests During the overload test, samples of PTC current limiter are operated for 50 cycles while connected to Vmax and 120 percent of rated maximum current (Imax). Samples are then tested at Vmax and tripping current or greater for 6,000 cycles during the endurance test. Cold Operational Cycling Test This test is similar to the endurance test, except with 1000 cycles at an ambient of 0°C or lower. Thermal Runaway Test During the thermal runaway test, samples are subject to a voltage ramp from rated voltage (Vr) to 200 percent of Vmax. The test voltage is maintained at 200 percent of Vmax for 2 minutes. Possible Effects at Different Voltage Polarities This article explores the effects of voltage polarity on the polymeric PTC device, with DC voltage rating equal to the AC voltage RMS rating. mAy 2008 Conformity 35 http://www.ddbunlimited.com http://www.ddbunlimited.com
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