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Basic Electronics Ninth Edition Basic Electronics Ninth Edition ©2002 The McGraw-Hill Companies Grob Schultz

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Basic Electronics Ninth Edition Basic Electronics Ninth Edition ©2003 The McGraw-Hill Companies 19 CHAPTER Capacitive Circuits

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Topics Covered in Chapter 19 X C and R in Series Sine-Wave V C Lags I C by 90 RC Phase-Shifter Circuit X C and R in Parallel

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Topics Covered in Chapter 19 (continued) Sine-Wave I C Leads V C by 90 RF and AF Coupling Capacitors Capacitive Voltage Dividers The General Case of Capacitive Current I C

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RC Voltage and Current Series Circuit The sine-wave ac voltage across a capacitor lags the capacitor’s charge and discharge currents by 90°. The sine-wave ac voltage across a resistor is always in phase with its current. The total sine-wave ac voltage for a series RC circuit always lags the total current by an angle between 0° and 90°.

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VRVR I Waveforms and Phasors for a Series RC Circuit I I VCVC Note: since current is constant in a series circuit, the current waveforms and current phasors are shown in the reference positions.

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Source Voltage and Current Phasors I Note: the source voltage lags the current by an amount proportional to the ratio of capacitive reactance to resistance. VSVS I VSVS X C < R I VSVS X C = R I VSVS X C > R

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Phasors for Series RC Circuits VRVR VCVC VTVT Voltage Phasors R XCXC ZTZT Impedance Phasor

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I = 2 A The Impedance of a Series RC Circuit V S = 100 R = 30 X C = 40 C X R Z R XCXC The impedance is the total opposition to current flow. It’s the phasor sum of resistance and reactance in a series circuit A Z V I S Z

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The Tangent Function opposite adjacent negative angle opposite adjacent positive angle

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I = 2 A The Phase Angle of a Series RC Circuit V S = 100 R = 30 X C = 40 30 40 50 Tan R X C V S lags I by 53° I VCVC VSVS -53°

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KVL in a Series RC Circuit I = 2 A V S = 100 R = 30 X C = 40 60 V 80 V 100 V V R = IR = 2 x 30 = 60 V V C = IX C = 2 x 40 = 80 V VV S

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RC Voltage and Current Parallel Circuit The sine-wave ac charge and discharge currents for a capacitor lead the capacitor voltage by 90°. The sine-wave ac voltage across a resistor is always in phase with its current. The total sine-wave ac current for a parallel RC circuit always leads the applied voltage by an angle between 0° and 90°.

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Current Phasors for Parallel RC Circuits IRIR ICIC ITIT Current Phasors

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Currents in a Parallel RC Circuit V S = 120 R = 30 X C = 40 IRIR ICIC I T = 5 A ITIT A R V I S R A X V I C S C AIII CRT

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Phase Angle in a Parallel RC Circuit V S = 120 R = 30 X C = 40 I T = 5 A 4 A 3 A5 A Tan I I R C The total current leads the source voltage by 37°.

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Impedance in a Parallel RC Circuit V S = 120 R = 30 X C = 40 I T = 5 A 4 A 3 A5 A T S EQ I V Z

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Summary of R, X C and Z Resistance (R) in Ohms, Voltage in phase with current. Capacitive Reactance (X C ) in Ohms, Voltage lags current by 90°.

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Summary of R, X C and Z (continued) Series circuit impedance (Z T ) in Ohms, Voltage lags current. Becomes more resistive with increasing f. Becomes more capacitive with decreasing f.

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Summary of R, X C and Z (continued) Parallel circuit impedance (Z EQ ) in Ohms, Voltage lags current. Becomes more capacitive with increasing f. Becomes more resistive with decreasing f.

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Summary of Formulas Series RCParallel RC 22 CRT III T S EQ I V Z R C I I Tan fC X C 2 1 22 CRT VVV 2 2 CT XRZ R X Tan C fC X C 2 1

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