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RL-RC Circuits & Applications SVES Circuits Theory

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Introduction In this chapter, phasor algebra will be used to develop a quick, direct method for solving both the series and the parallel ac circuits. Describe the relationship between current and voltage in an RC & RL circuits Determine impedance and phase angle in RC and RL circuits

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Impedance and the Phasor Diagram Resistive Elements Use R =0° in the following polar format to ensure the proper phase relationship between the voltage and the current resistance: The boldface Roman quantity Z R, having both magnitude and an associate angle, is referred to as the impedance of a resistive element. Z R is not a phasor since it does not vary with time. Even though the format R 0° is very similar to the phasor notation for sinusoidal current and voltage, R and its associated angle of 0° are fixed, non-varying quantities.

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Resistive ac circuit.

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Resistive ac circuit Voltage is 100 volts Peak

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Waveforms for Last Example Resistive

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Phasor diagram of Example Resistive

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Analysis of Resistive Circuits The application of Ohm’s law to series circuits involves the use of the quantities Z, V, and I as: V = IZ I = V/Z Z = V/I R = Z

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Impedance and the Phasor Diagram Capacitive Reactance (X C ) Use C = – 90° in the following polar format for capacitive reactance to ensure the proper phase relationship between the voltage and current of an capacitor: The boldface roman quantity Z c, having both magnitude and an associated angle, is referred to as the impedance of a capacitive element.

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Impedance and the Phasor Diagram Z C is measured in ohms and is a measure of how much the capacitive element will “control or impede” the level of current through the network. This format like the one for the resistive element, will prove to be a useful “tool” in the analysis of ac networks. Be aware that Z C is not a phasor quantity for the same reason indicated for a resistive element.

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Analysis of Capacitive ac Circuit The current leads the voltage by 90 in a purely capacitive ac circuit

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Capacitive ac circuit.

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Capacitive ac circuit, Voltage is 15 volts peak

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Waveforms for Example current leads the voltage by 90 degrees

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Phasor diagrams for Example

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Impedance and the Phasor Diagram Inductive Reactance (X L ) Use L = 90° in the following polar format for inductive reactance to ensure the proper phase relationship between the voltage and the current of an inductor: The boldface roman quantity Z L, having both magnitude and an associated angle, in referred to as the impedance of an inductive element.

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Impedance and the Phasor Diagram Z L is measured in ohms and is a measure of how much the inductive element will “control or impede” the level of current through the network. This format like the one for the resistive element, will prove to be a useful “tool” in the analysis of ac networks. Be aware that Z L is not a phasor quantity for the same reason indicated for a resistive element.

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Inductive ac circuit.

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Inductive ac circuit Voltage is 24 volts Peak

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Inductor Waveforms for Example voltage leads the current by 90 degrees

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Phasor diagrams for Example V 8.0

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Three cases of impedance R – C series circuit

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Illustration of sinusoidal response with general phase relationships of V R, V C, and I relative to the source voltage. V R and I are in the phase; V R leads V S ; V C lags V S ; and V R and V C are 90º out of phase.

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Impedance of a series RC circuit.

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Development of the impedance triangle for a series RC circuit.

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Impedance of a series RC circuit.

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Phase relation of the voltages and current in a series RC circuit.

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Voltage and current phasor diagram for the waveforms

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Voltage diagram for the voltage in a R-C circuit

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An illustration of how Z and X C change with frequency.

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As the frequency increases, X C decreases, Z decreases, and decreases. Each value of frequency can be visualized as forming a different impedance triangle.

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Illustration of sinusoidal response with general phase relationships of V R, V L, and I relative to the source voltage. V R and I are in phase; V R lags V S ; and V L leads V S. V R and V L are 90º out of phase with each other.

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Impedance of a series RL circuit.

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Development of the Impedance triangle for a series RL circuit.

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Impedance of a series RL circuit.

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Phase relation of current and voltages in a series RL circuit.

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Voltage phasor diagram for the waveforms.

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Voltage and current phasor diagram for the waveforms

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Voltages of a series RL circuit. 61 V

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Voltages of a series RL circuit.

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Reviewing the frequency response of the basic elements.

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Frequency Selectivity of RC Circuits Frequency-selective circuits permit signals of certain frequencies to pass from the input to the output, while blocking all others A low-pass circuit is realized by taking the output across the capacitor, just as in a lag network A high-pass circuit is implemented by taking the output across the resistor, as in a lead network

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The RC lag network (V out = V C )

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FIGURE An illustration of how Z and X C change with frequency.

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Frequency Selectivity of RC Circuits The frequency at which the capacitive reactance equals the resistance in a low- pass or high-pass RC circuit is called the cutoff frequency: f c = 1/(2 RC)

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Normalized general response curve of a low-pass RC circuit showing the cutoff frequency and the bandwidth - 3 dB point Cutoff point normalized

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Example of low-pass filtering action. As frequency increases, V out decreases

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The RC lead network (V out = V R )

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Example of high-pass filtering action. As frequency increases, V out increases

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High-pass filter responses.

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High-pass filter responses, filters in series Each r-c combination - 20dB / decade

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Observing changes in Z and X L with frequency by watching the meters and recalling Ohm’s law

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RL Circuit as a Low-Pass Filter An inductor acts as a short to dc As the frequency is increased, so does the inductive reactance –As inductive reactance increases, the output voltage across the resistor decreases –A series RL circuit, where output is taken across the resistor, finds application as a low- pass filter

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Example of low-pass filtering action. Winding resistance has been neglected. As the input frequency increases, the output voltage decreases

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RL Circuit as a High-Pass Filter For the case when output voltage is measured across the inductor –At dc, the inductor acts a short, so the output voltage is zero –As frequency increases, so does inductive reactance, resulting in more voltage being dropped across the inductor –The result is a high-pass filter

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FIGURE Example of high-pass filtering action. Winding resistance has been neglected. As the input frequency increases, the output voltage increases.

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