Application of Capacitors in Low Frequency and Long Time Constant Circuits

This paper discusses the low frequency (less than 1 cps) and long time constant applications and testing of capacitors as used in analog control systems. Emphasis is placed on tantalum electrolyric types, as they provide the necessary volume efficiency for airborne systems. The effective capacitance is always larger at the low frequencies than at normal capacitance bridge measuring frequencies. The amount and predictability of the change varies with the rate of charge, the dielectric material and, in the case of tantalums, with the electrolytic system. The polar characteristics of tantalum capacitors introduce other problems, such as doubling the effective capacitance during the first cycle of applied voltage, and hybrid charging curves as voltage levels change. The cause of the increase in charge or discharge time with low rates of change applied is not known. It is apparently allied with the characteristic of dielectric absorption (voltage recovery after shorting the electrodes) in that those capacitors exhibiting the least dielectric absorption also show the least low-frequency sensitivitye Polarization of molecules or molecular chains in the dielectric and/or electrolyte is the probable cause.