Abstract :
It is shown that at low frequencies (e.g. 50c/s) the principal loss in tantalum capacitors is in the dielectric. Raising the frequency does not appreciably change the dielectric loss, but increases the series resistance loss, which begins to predominate at between 500 c/s and 5 kc/s. At higher frequencies, the very large resistance loss can cause the impedance to be several times greater than the calculated reactance and the phase angle to shift from ¿/2 towards zero. The series resistance of the capacitor also has a large negative temperature coefficient between 20 and ¿40° C. Hence at low temperatures the effects of series loss appear at even lower frequencies. Further, the impedance at a given frequency departs yet more from the calculated reactance (often by a factor of over 100 at 100 kc/s and ¿40° C). Most tantalum capacitors become series resonant between 100 kc/s and 10Mc/s, when the impedance reaches a minimum very nearly equal to the series resistance. Capacitors employing a sintered tantalum pellet, especially the `solid¿ type, are much less affected by low temperatures than the foil type. For increasing temperatures (e.g. 20¿100° C) all types show a slight reduction of losses.
