From Fundamentals of Industrial Instrumentation and Process Control

Chapter Objectives

This chapter deals with the basic passive components in ac circuits, and will help you become familiar with these devices and understand their application in instrumentation.

This chapter discusses the following:

  • Use of impedance as opposed to resistance in ac circuits

  • Equivalent circuit of a combination of components in ac circuits

  • Effective impedance to current flow of components in ac circuits

  • Time delays and time constants

  • Concept of phase angle

  • Resonant frequency

  • Concept of filters

  • AC Wheatstone bridge

  • Magnetic fields, meters, and motors

  • Transformers

3.1 Introduction

Three basic components, resistors, capacitors, and inductors, are very important elements in electrical circuits as individual devices, or together. Resistors are used as loads, delays, and current limiting devices. Capacitors are used as dc blocking devices, in level shifting, integrating, differentiating, frequency determination, selection, and delay circuits. Inductors are used for frequency selection and ac blocking, in analog meter movements and relays, and are the basis for transformers and motors.

3.2 Circuits with R, L, and C

3.2.1 Voltage Step

When a dc voltage is applied to a capacitor through a resistor, a current flows charging the capacitor (see Fig. 3.1 a). Initially, all the voltage drops across the resistor; although current is flowing into the capacitor, there is no voltage drop across the capacitor. As the capacitor charges, the voltage across the capacitor builds up on an exponential, and the voltage across the resistor starts to decline, until eventually the capacitor is fully charged and current ceases to...

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Passive Filters
Passive filters are implemented using only passive components such as resistors, capacitors and inductors. These filters do not produce any amplification of the input signal.
Signal Conditioners
Signal conditioners provide amplification, filtering, converting, and other processes required to make sensor output suitable for reading by computer boards.
Power Supplies
Power supplies are electrical devices that deliver electric power to one or several loads. They generate the output power by converting an input signal into an output signal.
Switching Power Supplies
Switching power supplies are DC sources that use a switching element (normally a power transistor) to generate the desired voltage. Switching power supplies are also called switch-mode products or switching mode power supplies (SMPS).
Frequency-to-Voltage Converters
Frequency-to-voltage converters accept a signal and convert its frequency to a corresponding analog voltage level.

Topics of Interest

Chapter Objectives This chapter will help you understand active devices, how they are used in instrument applications, and the difference between analog and digital circuits. In this chapter the...

3.1 Introduction Resistors, capacitors, and inductors these are the three basic passive elements used in electrical circuits, either as individual devices or in combination. These elements are used...

SIMPLE RC FILTERS A filter allows ac of some frequencies to pass through it with the amplitude (voltage or current) essentially unchanged, but the amplitudes of other frequencies are decreased. (An...

Capacitors are not useful in DC circuits since they contain insulating gaps, which are open circuits for DC. However, for voltages that change with time, a simple series circuit with a capacitor and a...

This chapter presents applications of the Laplace transform. Several examples are given to illustrate how the Laplace transformation is applied to circuit analysis. Complex impedance, complex...

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