In any practical parallel LC circuit, there is always some resistance in the branches, mainly in the inductive branch is

the resistance of the coil winding (distributive resistance). This means that the current through the inductive branch

will lag the applied voltage by slightly less than 90. The currents through the two branches do not cancel entirely, so

there is some total current. This line current furnished by the generator is very small. A circuit that draws only a small

amount of current has a high impedance.

When the frequency of the applied voltage in a parallel circuit is such that substituting the values of L, C, and f in the

formula results in an equality, a resonant condition exists. If L, C, or f is then changed, the circuit is off resonance, the

current is increased, the impedance is decreased, the reactances are unequal, and the frequency formula is no longer

usable.

The above facts apply to parallel resonant circuits where the resistance in series with the capacitor is insignificant, and

the Q of the coil is high.

calculated by

P = EI.

Whether the circuit is resonant or nonresonant, total power is equal to the sum of the power dissipated in each

resistance. Power dissipated in a circuit using a high Q coil and a condenser free of losses will be very small.

The energy lost in a parallel resonant circuit is mainly due to resistance in the windings of the coil, although, at very

high frequencies, losses also become apparent in the capacitor. Series resistance in the inductive or capacitive branch is

carefully kept to a minimum, since the smaller the series R, the smaller the I2R loss. Any resistance across the tuned

circuit (which is sometimes unavoidable) should be kept as large as possible; because the power lost is proportional to

E2/R. As with the series circuit, large losses make the resonance curve nonselective.

impedance versus frequency and is similar to the current versus frequency curve of the series resonant circuit

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