Chapter 3 – Electronics
Practically all mechatronic and measurement systems contain electrical circuits and components.
To understand how to design and analyze these systems, a firm grasp of the fundamentals of basic
electrical components and circuit analysis techniques is a necessity. When electrons move, they
produce an electrical current, and we can do useful things with the energized electrons.
Electronics is the science and technology of passage of charged particles in gases, vacuum, or in
semiconductors. The following are the salient features of an electronic device.
1. It can respond to very small control signal.
2. It can respond at a speed far beyond the speed at which mechanical or electrical devices
can.
3. It is photosensitive.
4. It can rectify alternate current into direct current.
5. Some electronic devices can produce x-ray radiations.
Voltage or electric potential is like potential energy and current is like kinetic energy. Direct
current (DC) is a unidirectional current which does not change its value appreciably. The DC
voltage is a unidirectional voltage like that of a battery. In alternating current or voltage, the circuit
direction of current or voltage reverses at regular intervals of time. The sine wave is most
frequently used in alternating current or voltage. Electric energy spent per second is called power.
Electric power for a DC supply is given by VI and measured in terms of a unit called watt W. For
alternating current (AC), power is equal to VI cos(Φ), where cos(Φ) is called the power factor and
Φ is the phase shift between voltage and current.
Figure 3.1 Schematic symbols for basic electrical elements.
3.1 Passive Electrical Components: Resistor, Capacitor and Inductance
3.1.1 Resistor: Resistance is defined as the property of a material due to which it opposes the flow
of electricity through it. Metals are good conductors of electricity. They offer very low resistance
to the flow of current. The resistance offered by a conductor depends upon the length, cross-
sectional area, and nature of materials and the temperature of the conductor. If the effect of
temperature can be neglected, then resistance R = rl/a, where l is the length of the conductor, a is
the cross-sectional area, and r is called the specific resistance, which depends on the nature of the
material of the conductor.
A resistor is a dissipative element that converts electrical energy into heat. Resistors are used for
many purposes in a variety of applications. The most common uses are to limit current through a
device and to drop down or set a voltage value. Most resistors are coded using colour bands. The
first band gives the resistance value of the resistor in ohms. The fourth band, which is far away
from the usual band, indicates the accuracy of the value.
Figure 3.2 Examples of basic circuit elements.
R= ρ L/A
Figure 3.3 Resistor
Actual resistors used in assembling circuits are packaged in various forms including axial-lead
components, surface mount components, and the dual in-line package (DIP) and the single in-
line package (SIP), which contain multiple resistors in a package that conveniently fits into
circuit boards. These four types are illustrated in Figures 3.4.
Figure 3.4 Resistor packaging.
Purpose : - Voltage division, Biasing, current limiting, power dissipation, bleeding of charge etc.
Table 3.1 Colour and resistance value
3.1.2 Capacitor: A capacitor is a passive element that stores energy in the form of an electric
field. This field is the result of a separation of electric charge. The simplest capacitor consists of a
pair of parallel conducting plates separated by a dielectric material. The dielectric material is an
insulator that increases the capacitance as a result of permanent or induced electric dipoles in the
material. The property of the capacitor by which it stores charge on its plates is called capacitance.
The charge of the capacitor is proportional to applied voltage. The proportionality constant is the
capacitance of the capacitor. The capacitance of a capacitor is defined as the amount of charge
required to create a unit potential difference between its plates. The unit of capacitance is farad.
When a DC voltage is applied to a capacitor, it momentarily acts like a short circuit and the
capacitance acts like an open circuit. When a capacitor is connected to a source of alternating
voltage, the continuous charge and periodical reversal of the applied voltage causes a continuous
change in state of the capacitor with a continuously changing current. A capacitor acts like a short
circuit for an AC voltage. Since the capacitor blocks DC, this property is used to block the DC
components from an AC input signal which is superimposed with a DC signal, so the capacitor
transmits only the AC signal to the output. Since the capacitor acts like a short circuit for AC
signals they are used to filter AC voltage from DC voltage. It is used as a reservoir to smoothen
pulsating DC.
Capacitance of a capacitor depends on the dielectric constant k, area of one side of the plate A,
number of plates N, and separation of the plate surfaces d: C = kA(N-1)/d
Capacitance can be varied by changing any of the variables on the right-hand side of the above
equation. Figure 3.5 shows the symbol and construction of capacitor. A capacitor is also called a
condenser.
Figure 3.5
Capacitors are used for many purposes in a variety of applications such as filtering and smoothing
voltage signals, creating delays in timing circuits, and storing charge and energy for later use (like
a battery). It can also be used for timing purposes in electrical circuits using a simple RC circuit,
which is a resistor and capacitor in series. The primary types of commercial capacitors are
electrolytic capacitors, tantalum capacitors, ceramic disk capacitors, and mylar capacitors.
Capacitors come in many sizes and shapes. Often the capacitance is printed directly on the
component, typically in μF or pF, but sometimes a three-digit code is used. (e.g., 102 implies 10 ×
102 pF = 1 ×10−9 F = 1 nF). If there are only two digits, the value reported is in picofarad (e.g., 22
imply 22 pF).
3.1.3 Inductor: A wire wound in the form of a coil makes an inductor. The property of an
inductor is that it always tries to maintain a steady flow of current and opposes any fluctuation in
it. An inductor is a passive energy storage element that stores energy in the form of a magnetic
field. The simplest form of an inductor is a wire coil, which has a tendency to maintain a magnetic
field once established. They are important in some filter, radio tuning, and power circuits.
Regardless, understanding inductance is very important because many mechatronic system
components, including relays, solenoids, transformers, and motors, contain coils which have
inductance.
The property of the inductor due to which it opposes any increase or decrease in current by the
production of a counter emf is known as self-inductance. Inductance is sometimes called electrical
inertia. The emf developed is proportional to the rate of current through the inductor.
Mathematically, e = LdI/dt, where the proportionality constant L is called the self-inductance of
the coil, I is the current, and e is the voltage developed.Mutual inductance arrangements use two
coils—one acting as the power coil and the other as the supply output. A mutual inductance profile
is used as a sensor for measuring small displacements. A linear variable differential transformer
(LVDT) works under the principle of mutual inductance. The unit of measure of inductance is the
henry (H = Wb/A).
Typical inductor components range in value from 1 μH (1 × 10 −6 H) to 100 mH (100 × 10−3 H =
0.1 H). Inductance is important to consider in motors, relays, solenoids, some power supplies, and
high-frequency circuits. Although some manufacturers have coding systems for inductors, there is
no standard method. Often, the value is printed on the device directly, typically in μH or mH.
Transformer: A transformer is a device by means of which electric power in one circuit is
transferred to another circuit without a change in frequency. It essentially consists of two or more
inductive windings wound on the same core. A transformer can raise or lower the voltage in a
circuit with corresponding decrease or increase in current. Transformers are effectively used to
step up and step down voltages in power stations and substations, respectively, so that power
losses can be minimized.
3.2 Active Elements: Active elements have an auxiliary source of power which supplies a major
part of the output power while input signals supply only an insignificant portion. There may or
may not be a conversion of energy from one form to another. Diodes, transistors, Solenoid drives,
electronic amplifiers, servosystems, and digital systems are some examples of active devices.
Semiconductor Devices: Graphite, silicon, and germanium are semiconductor materials. At very
low temperatures, say 0°C, the crystal behaves like an insulator as there are no free electrons
available in it. However, as the temperature increases and reaches room temperature, the
conductivity of the crystal improves. In order to improve the conductivity of these materials, some
impurities are intentionally added. This addition of impurities in a semiconductor is known as
doping. Usually either pentavalent impurities such as phosphorous and antimony or trivalent
impurities such as iridium, gallium, or boron are added to pure semiconductors.
Whenever a pentavalent impurity is added to a pure silicon crystal, a free electron is donated.
Hence pentavalent impurities are called donors. Such semiconductors are called N-type
semiconductors. On the other hand, if a trivalent impurity is added it creates a vacancy at one of its
covalent bonds. In a semiconductor formed by doping with a trivalent impurity, holes are in the
majority. Such semiconductor is called P-type semiconductors and also acceptors since holes can
accept electrons.
3.2.1 Diode: A point contact diode has a metal base on which a germanium semiconductor wafer
is mounted and a tungsten alloy wire is allowed to press against the semiconductor. This device
allows current to flow in one direction and is called a diode. The disadvantage of the contact type
diode is that the contact area is very small. Junction diodes can be made with a P-type and an N-
type semiconductor material. This type of diode is called a P-N junction diode. The P region is
called the anode and the N region the cathode. If a P-N junction diode is connected in a circuit
with the anode connected to the positive terminal and the cathode connected to the negative
terminal of a battery, it is said to be forward biased. On the other hand, when the polarity of the
battery is reversed, the diode is said to be reverse biased.
Figure 3.6
The forward and reverse characteristics of a diode are illustrated in Fig. 3.6. It shows the diode
current for various applied voltages plotted against the applied voltage for both forward and
reverse bias. For the forward-biased diode, current increases steeply for every increase in applied
voltage after an initial small voltage. This small voltage is called the cut-in voltage and depends on
the semiconducting materials. For germanium, it is of the order of 0.2 V and for silicon it is
around 0.6 V. For reverse bias, the current is very small till the breakdown voltage. After the
initial voltage, a large reverse current flows through the diode. It may get damaged if the current is
not limited through an external resistor.
Since the diode conducts in one direction only, it can be used as a rectifier to convert AC into DC.
If AC input is given to a diode during the positive half-cycle, the diode is forward biased. If AC
input is given to a diode during the negative half-cycle, it is reverse biased. As the diode conducts
when it is forward biased, the positive half-cycle is transmitted to the output and it appears across
the load. To improve the average value of DC voltage, centretap rectifier with two diodes or
bridge rectifiers with four diode circuits are used.
Zener diode: In a normal diode the reverse breakdown occurs at a very high voltage, beyond
which the current increases steeply for every increase in applied voltage. By varying the doping
level, diodes with lower voltages from 2 to 200 V can be manufactured. Diodes which operate in
the breakdown region by reverse biasing are called zener diodes. The voltage at which breakdown
occurs is known as zener voltage. The zener diode is of great importance for voltage regulators
since it can be used as a constant-voltage source by applying the reverse voltage that exceeds the
zener breakdown voltage.
Tunnel diode: By increasing the doping level one can obtain breakdown voltage at 0 V. This will
happen when the concentration of impurities is increased by more than 1000 times. The diode
starts conducting at zero volt in both positive and negative directions. Such a diode is known as
tunnel diode or Esaki diode.
Light emitting diode (LED): In a forward-biased diode, free electrons cross the junction and
recombine with holes. Whenever electrons combine with holes, energy is radiated. In a rectifier
diode this energy is liberated as heat. But in light-emitting diode, this energy is radiated as light—
red, green, yellow, orange, or infrared. LEDs producing the visible radiation are often used in
instrument displays, digital readouts, digital clocks, calculators, etc. LEDs producing infrared
radiation are used in burglaralarm systems.
Photodiodes: When a P-N junction of the diode is housed in a glass package, strong light hitting
the junction increases the reverse current. A photodiode is a normal diode optimized for its
sensitivity to light and is housed in a glass package. The glass window lets the increasing light
pass through the housing and hit the P-N junction. This light produces additional holes and
electrons, giving rise to higher reverse current. Photodiodes are always used in reverse-biased
conductors.
3.2.2 Transistor: A junction transistor consists of either an N-type semiconductor sandwiched
between two P-type semiconductors or a P-type semiconductor sandwiched between two N-type
semiconductors. In the former case the transistor is referred to as a PNP transistor and in the latter
case as an NPN transistor. The semiconductor junctions thus formed are housed in a hermetically
sealed case of either plastic or metal. Three leads are brought out from the three semiconductor
regions. The sandwiched semiconductor region is called the base. The other two regions are called
the emitter and the collector. The collector region is large in size and more heavily doped.
Figure 3.7 (a) NPN transistor circuit and (b) PNP transistor circuit.
Common-emitter circuit is the basis for many amplifiers, from audio frequencies through ultra-
high radio frequencies. The common-emitter configuration produces the largest gain of any
arrangement.
The common-base circuit provides somewhat less gain than a common-emitter circuit. But it is
more stable than the common-emitter configuration in some applications, especially in radio-
frequency power amplifiers.
The common-collector circuit can be used to match high impedances to low impedances.
Integrated Circuits: Circuits which use separate circuit elements—resistors, capacitors,
transistors, diodes, etc.—connected together are called discrete circuits. Printed circuit (PC)
boards provide ways to interconnect hundreds of discrete devices in a single board. Integrated
circuits (IC) permit hundreds of transistors, diodes, and resistors to be formed and connected
together to realize a complex circuit within the size of a small pill. All these circuit elements are
made on tiny wafers of silicon. The main advantages of ICs are that they (a) save space, (b)
provide improved reliability, (c) improve performance, (d) match devices, and (e) have a low cost.
Depending on the scale of integration, integrated circuits are classified into medium-scale
integrators (MSIs), large-scale integrators (LSIs), and very large-scale integrators (VLSIs). The
basic structure of an IC consists of four distinct layers. The bottom layer is of P-type silicon,
called substrate, over which the IC is built. The second layer is a thin N-type layer grown on the
substrate. The third layer is a thin layer of silicon oxide. It acts as a barrier to protect portions of
the wafer against impurity penetration. The fourth layer is an aluminium layer which provides
necessary interconnection between components. All the components are built in the second layer
by a series of diffusion steps.
3.3 Digital Electronic Components: The devices used in digital systems function in a binary
manner. These devices exist in two possible states ‘on’ and ‘off’ or ‘1’ and ‘0’. The manipulation
of digital signals is known as digital logic. Digital logic consists of many sometimes innumerable
pulses racing around.
Logic Gates: The building blocks of a digital system that control its output based on the
conditions of the input are called logic gates. A gate is a logic circuit with one or more input
signals but has only one output signal. The input and output signals can consist of either high or
low voltages corresponding to ‘0’ or ‘1’, respectively. There are basically three types of gates—
OR gate, AND gate, and NOR gate. All other gates are combinations of these three.
Flip-flops: In the digital world, normally binary bits are retained or stored in a group that
represents either a number or a coded information. The group bits are called digital number system
(DNS) data and are to be stored electronically. An electronic circuit that retains a single bit of
DNS data is called flip-flop. Generally, flip-flops are used as memory devices for storing previous
input values.
Shift Register: A register is a combination of flip-flops that can delay digital signals, or store
them for a short time. A logical 1 or 0 is called a bit. A flip-flop can store, remember, or register a
single bit. A flip-flop is therefore referred to as a one-bit register. If N bits are remembered, N
registered flip-flops are required. When an array of flip-flops has a number of bits in storage, it
becomes necessary on occasions to shift bits from one flip-flop to another. An array of flip-flops
that permits such shifting is called a shift register.
Multiplexer: Multiplexing is using an analog switching circuit which allows the connection of a
number of analog signals at a time to a common load.
Counter: A counter is an electronic device that counts. It generates a sequence of count values
determined by selected encoding and the status input.
