RLC circuit analysis

Chapter RLC circuit analysis

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • 16CHAPTERRLC circuit analysisWHENEVER YOU SEE AC CIRCUITS WITH INDUCTANCE AND/OR CAPACITANCE ASwell as resistance, you should switch your mind into “2D” mode. You must be ready todeal with two-dimensional quantities.While you can sometimes talk and think about impedances as simple ohmic values,there are times you can’t. If you’re sure that there is no reactance in an ac circuit, thenit’s all right to say “Z = 600 ohms, “ or “This speaker is 8 ohms,” or “The input imped-ance to this amplifier is 1,000 ohms.”As soon as you see coils and/or capacitors, you should envision the complex-num-ber plane, either RX (resistance-reactance) or GB (conductance-admittance). The RXplane applies to series-circuit analysis. The GB plane applies to parallel-circuit analysis.Complex impedances in seriesWhen you see resistors, coils, and capacitors in series, you should envision the RXplane.Each component, whether it is a resistor, an inductor, or a capacitor, has an imped-ance that can be represented as a vector in the RX plane. The vectors for resistors areconstant regardless of the frequency. But the vectors for coils and capacitors vary withfrequency, as you have learned.Pure reactancesPure inductive reactances (XL) and capacitive reactances (XC) simply add togetherwhen coils and capacitors are in series. Thus, XXLXC. In the RX plane, their vec-tors add, but because these vectors point in exactly opposite directions— inductive re-actance upwards and capacitive reactance downwards—the resultant sum vector willalso inevitably point either straight up or down (Fig. 16-1).284Copyright © 2002, 1997, 1993 by The McGraw-Hill Companies. Click here for terms of use.