Categorization is an important aspect of technical and scientific fields, and electrical engineering is no exception. Categories help us to organize our thoughts, perform systematic observations, and form conceptual connections.
Quite a few types of components are regularly used in electronic circuits and systems, but if we thoughtfully consider certain defining characteristics of these components, we can place them in three broad categories. Let’s start with the following brief definitions, and then we’ll take a closer look at the three categories.
Passive components work with electrical energy that is already present. In other words, they don’t inject electrical energy into a circuit. The most common passive electronic components are resistors, capacitors, and inductors. Resistors convert electrical energy into heat, capacitors store electrical energy in an electric field, and inductors store electrical energy in a magnetic field.
Thus, they all handle electrical energy in a way that is passive—they can influence electrical signals by converting energy from one form to another, but they cannot increase the power of a signal by injecting energy supplied by a source (such as a battery or power-supply circuit).
Less-common passive components include ferrite beads, which are similar to inductors and are used for noise suppression; antennas, which efficiently generate or receive electromagnetic radiation; and transformers, which use closely spaced inductors to increase or decrease the amplitude of an AC voltage.
Sensors generate or modify electrical signals in response to physical conditions. Passive sensors include thermistors, which measure temperature, photoresistors, which measure light intensity, and resistive strain gauges. All of these are simply specialized applications of resistance.
An active component can contribute to the power of an electrical signal by injecting energy supplied by a source component. We can think of this in terms of amplification or as switching action. An amplifying component uses a power source to increase the power of an input signal, and an electronic switch allows one signal to control—i.e., to pass or block—a second higher-power signal.
The most emblematic active components are called transistors. A bipolar-junction transistor (BJT) functions like a current-controlled current source, and a metal oxide semiconductor field-effect transistor (MOSFET) functions like a voltage-controlled current source. Both types of transistors can be used as a switch or as an amplifier.
Diodes are common components that allow current to flow only in one direction. Are they active or passive? Actually, this question is difficult to answer. Diodes resemble passive components in that they have only two terminals and cannot increase the power of a signal, but they rely upon semiconductor junctions (like BJTs) and their electrical characteristics are very different from those of resistors, capacitors, and inductors. If we say that an active component must allow an applied electrical signal to control the current flowing through another portion of the component, then we would classify diodes as passive.
The most familiar electromechanical component is the electric motor. Though the functional details of motors vary widely, almost all of them have the same fundamental purpose: to convert electrical energy into mechanical energy in the form of rotational motion.
A generator reverses this relationship: mechanical motion produced by, for example, moving water or wind is converted into electric power. Solenoids and speakers are electromechanical components that convert electrical energy into linear motion.
Some electromechanical components don’t seem very mechanical, because we can’t see them moving. These piezoelectric devices, such as quartz crystals and ceramic resonators, experience mechanical changes in response to an applied electric field. Quartz crystals are widely used for producing high-precision timing signals.
Some other electromechanical components are rather mundane, but that doesn’t mean that they’re unimportant. Mechanical switches are still an effective way to control a circuit, and connectors help us to conveniently and securely transfer power-supply voltages and electrical signals from one portion of a system to another.
In this video, we examined the difference between active and passive components, and we also mentioned some examples of electromechanical components. The next tutorial provides an introduction to semiconductors.
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