1-14
How the Typical Series-LC Circuit Differs From the Ideal
As you learned much earlier in this series, resistance is always present in practical electrical circuits;
it is impossible to eliminate. A typical series-LC circuit, then, has R as well as L and C.
If our perfect (ideal) circuit has zero resistance, and a typical circuit has "some" resistance, then a
circuit with a very small resistance is closer to being perfect than one that has a large resistance. Let's list
what happens in a series-resonant circuit because resistance is present. This is not new to you - just a
review of what you have learned previously.
In a series-resonant circuit that is basically L and C, but that contains "some" R, the following
statements are true:
XL, XC, and R components are all present and can be shown on a vector diagram, each at right
angles with the resistance vector (baseline).
At resonance, the resultant reactance is zero ohms. Thus, at resonance, The circuit impedance
equals only the resistance (R). The circuit impedance can never be less than R because the
original resistance will always be present in the circuit.
At resonance, a practical series-RLC circuit ALWAYS has MINIMUM impedance. The actual
value of impedance is that of the resistance present in the circuit (Z = R).
Now, if the designers do their very best (and they do) to keep the value of resistance in a practical
series-RLC circuit LOW, then we can still get a fairly high current at resonance. The current is NOT
"infinitely" high as in our ideal circuit, but is still higher than at any other frequency. The curve and
vector relationships for the practical circuit are shown in figure 1-7.
