Spurious Signal Generation in Plastic Film Capacitors

Plastic film capacitors with dielectric materials such as Mylar or polycarbonate have been observed to generate spurious signals which appear as small spikes in capacitor voltage in the range of 10-300 µV. Such transients repeat at intervals varying from fractions of a second to years, becoming more frequent during periods of temperature change. The tendency to produce this phenomenon can persist for the normal life of the capacitor? depending on thermal and electrical stresses encountered. The effect under study is not related to dielectric breakdown or corona discharge which are due to high electric stress. The observed transients can appear at the capacitor electrodes with either polarity during application of voltage as low as zero. The source of charge and energy for this phenomenon is dielectric absorption (DA) which is history dependant relating directly to the nature of the dielectric material. Materials with high DA such as Mylar are much more susceptible than "nonpolar" dielectrics such as polystyrene. The following considerations in minimizing generation of spurious signals are discussed: 1) selection of materials; 2) limitations on use conditions; 3) preconditioning or annealing; 4) capacitor screening techniques.