Jerome H Silverman
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posted 01 December 2006 18:10
     
Very common. Especially as the capacitor ages. A lot of the literature blames dielectric adsorption. This occurs when dipoles in the dielectric line up with the applied electric field and get mechanically locked in position, which traps some of the electrode surface charge. When the external electric field is removed, dipoles free themselves randomly from thermal or other excitation, which frees the trapped charge resulting in a voltage reappearing on the electrodes. Another source of voltage recovery is charge that leaks from the electrode into the dielectric, and that slowly leaks back out when the electric field is removed.
But in my opinion the major reason for the voltage recovery in aluminum electrolytic capacitors is that some of the etched pores in the aluminum are vanishingly small so that the ohmic resistance of the electrolyte becomes a significant factor in connecting those areas to the capacitor proper. Therefore if you short the charged capacitor it takes a while for the charge in those areas to escape. As the capacitor ages and the electrolyte dries out, the ohmic resistance increases and so then does the voltage recovery. The frequency dependence of an electrolytic capacitor is also related to this phenomenon. Even when new, aluminum electrolytics can show a considerable capacitance value change between the test frequencies of 60 Hz and 1 KHz.
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posted 30 November 2006 03:49
