Resistances can be measured using dc bridges but can
you measure inductances and capacitances using dc bridges? Think.
The answer is NO because inductors and capacitors are frequency dependent elements and ∴ behave as short-circuits and open-circuits respectively, in dc circuits under steady-state condition.
That is why ac bridges are used to measure inductances, capacitances, mutual inductances etc.
AC Bridge vs DC Bridge
Ac bridges are different from dc bridges in two ways.
(1) Ac bridges use AC source while dc bridges use dc
source.
(2)
Simple galvanometer used in dc bridges as the null point indicator is replaced by vibration galvanometer
in ac bridges .
AC Bridges
Lets find the balance condition for AC bridge diagram shown.
The condition for balance is same as dc bridges but
instead of resistances, impedances are used i.e
But there is one important difference i.e not only should there be balance for the magnitudes of the impedances but also a phase balance. Writing the impedances in their polar form, the above condition becomes
But there is one important difference i.e not only should there be balance for the magnitudes of the impedances but also a phase balance. Writing the impedances in their polar form, the above condition becomes
Some commonly used ac bridges are :-
