In a parallel LC circuit at resonance, what happens to the impedance and the source current?

In a parallel LC circuit at resonance, what happens to the impedance and the source current?

• A. Z is minimum; source current maximized
• B. Z is unchanged; source current unchanged
• C. Z is zero; source current unchanged
• D. Z is maximum (ideally infinite); source current minimized

Answer: (D)

When a parallel LC tank resonates, the currents from the inductor and capacitor oppose each other so their reactive effects cancel. In terms of impedance, the total admittance becomes zero at resonance, which makes the impedance of the network grow very large—ideally infinite in a lossless case. Since the source current is I = V/Z, a very large Z means the source current is minimized, approaching zero. In real circuits with some resistance or losses, the impedance is still very high and the source current remains very small, but not exactly zero. The essential idea is that at resonance the parallel LC tank looks almost like an open circuit to the source because energy sloshes between L and C without demanding current from the source.

When a parallel LC tank resonates, the currents from the inductor and capacitor oppose each other so their reactive effects cancel. In terms of impedance, the total admittance becomes zero at resonance, which makes the impedance of the network grow very large—ideally infinite in a lossless case. Since the source current is I = V/Z, a very large Z means the source current is minimized, approaching zero. In real circuits with some resistance or losses, the impedance is still very high and the source current remains very small, but not exactly zero. The essential idea is that at resonance the parallel LC tank looks almost like an open circuit to the source because energy sloshes between L and C without demanding current from the source.