AC2 Fundamentals Unit 3 – Parallel Resonance
The inductor and capacitor effectively exchange energy. The current exchange between the
capacitor and inductor is referred to as the circulating current. A parallel circuit consisting of
inductors and capacitors that exchange circulating current is referred to as a tank circuit.
Circulating current flows inside the tank as the inductor and capacitor exchange energy, but no
current is drawn from the generator. At resonance, the circulating current is maximum while the
total line current (IT) is 0.
In a practical parallel resonant LC circuit, the small series resistance in the inductor (RL) causes
the inductor current (IL) to be slightly lower than the capacitor current (IC). Because of the coil
resistance, inductor current (IL) does not equal capacitor current (IC), and a small amount of line
current flows. Circuit impedance (Z) is still very high and resistive.
This response curve shows that as frequency increases, the circuit impedance (Z) increases until
it reaches a maximum point, then it decreases again.