On the Return-Voltage Buildup in Insulating Materials

Charged capacitors which have been temporarily short-circuited display a "return voltage" buildup which may cause severe shocks. This old phenomenon has been reinvestigated on various materials (polyethylene, polyester, Kapton®, etc.) poled by different techniques (HV, corona, plasma, etc.) after different poling times. Return voltage buildup is observed on all insulating (polar and non polar) materials. A simple model based on the drift of charges in the material and the sign reversal of the Coulomb force acting on them is presented. Expressions for the total charge density, the depth of the charge centroid and the mobility of charges in the sample are derived in terms of measured quantities such as the initial rate of buildup and the maximum return potential. A confirmation of the model is obtained for the return voltage due to electrons injected at known depths in polymer sheets by an electron gun. As a consequence, the same technique might be used with electrode-poled samples, to assess the injecting character of the electrodes, and possible modifications produced by contamination, surface treatments, etc.