Basic Electronics (PASSIVE COMPONENTS)

INTRODUCTION

We are living in an age of information Technology. Electronics is at the very foundation of the Information and Computer Age. Communications and Computers have made these tremendous successes that we see today because of the great achievements that have been recorded in the field of Electronics.

The impact of electronics have been so deep in almost all areas of our life such as health care, medical diagnosis and treatment, Air and space travels, Automobiles, etc. The basis of all the technological innovation and stride made by several countries of the globe can be directly traceable to their strengths in electronic design, manufacture, products and services.

We want to approach the teaching of the knowledge of electronics in the Laboratory Course by building and testing circuit for practical knowledge of the characteristics of different devices and in constructing the various circuits.  In these Laboratory experience we will categorize our study of the basic principles of electronics in study about; (i.) The different components, (ii) Circuits and (iii) measuring instruments used in electronics.

Components: There are two types of components that we come across namely Active and Passive components.

Resistors, capacitors, etc., are known as passive components because they can only attenuate the electrical voltage and signal and cannot amplify. Whereas devices like transistors, operational amplifier (Op Amp) can amplify or increase the amplitude and energy associated with the signals. Hence the transistors and OpAmp come under active devices.

Circuits: These components can be combined in different configuration by interconnecting them with conducting wires to build different electronic modules that provides a predetermined signal type and amplitude given a specific input signal type and amplitude. This module is called an electronic circuits. We would study about rectifiers, amplifiers, Oscillators, etc., under the category of circuits.

Measuring Instruments: Aside the components and circuits we must also have familiarity on the principle of operations and usefulness of some of the essential electronic measuring instruments such as digital multimeters, regulated power supplies, function generators, oscilloscopes, etc., These aid trouble shooting of circuits and identification of faulty components, whenever the circuits that we build do not work as expected.

PASSIVE ELECTRONICS COMPONENTS

RESISTORS

Resistors are components that oppose the flow of electrons (current). The symbols are shown in figure 1.0 below:

Figure 1: Symbols of resistor

Resistance is measured in units called “Ohm” (Symbol; Ω). Multiples of the units are represented as “kilo-ohms” (kΩ) = 1000 ohms and “mega-ohms” MΩ = 1000kΩ, that is 10⁶ ohms.

Broadly, we can categorise resistor in to Fixed Resistors and Variable Resistors.

Fixed Resistors: These are Carbon Film (5%, 10% tolerance) and Metal Film Resistors (1%, 2% tolerance) and wire wound resistors. A fixed resistor is one for which the value of its resistance is specified and cannot be varied in general.

Resistance Value

The resistance value is displayed using the color code (the colored bars/the colored stripes). This is because the average resistor is too small to have the value printed on it with numbers.

Color Coding:

Color	Value	Multiplier	Tolerance (%)
Black	0	0	-
Brown	1	1	±1
Red	2	2	±2
Orange	3	3	±0.05
Yellow	4	4	-
Green	5	5	±0.5
Blue	6	6	±0.25
Violet	7	7	±0.1
Gray	8	8	-
White	9	9	-
Gold	-	-1	±5
Silver	-	-2	±10
None	-	-	±20

Example 1:

(Red=2), (Green=5), (Orange=3)

           25×10³ = 25kΩ; Tolerance (Gold)= ±5%

Example 2:

(Brown=1), (Black=0), (Orange=3)

10×10³ = 10kΩ ohm; Tolerance (Gold) = ±5%

Example 3:

(Yellow=4), (Violet=7), (Black=0), (Red = 2)

 470×10²=47kΩ; Tolerance (Brown) = ±1%.

Tolerance: Tolerance of the resistor is an important property to be considered, a 100Ω resistor with 10% tolerance, means that its value can be any fixed value between 90 to 110 ohms.

Assignment:

Write a short note on the following types of resistors

-          Carbon film resistors

-          Metal film resistors

-          Ceramic resistor

-          Single-in line network resistors

-          Variable resistors

-          Light Dependent Resistance (LDR)

-          Thermistor

Resistor Connections

Individual resistors can be connected together in either a series connection, a parallel connection or combination of both series and parallel, to produce more complex resistor network whose equivalent resistance is the mathematical combination of the individual resistors connected together.

Resistor in Series

Resistors are said to be connected in “Series”, when they are daisy chained together in a single line. Since all the current flowing through the first resistor has not other way to go it must also pass through the second resistor and the third and so on. Then, resistors in series have a Common Current flowing through them as the current that flows through one resistor must also flow through the others as it can only take one path.

Then the amount of current that flows through a set of resistors in series will be the same at all points in a series resistor network.

For example; in figure 2.1 :

            I_R1= I_R2= I_R3= I_AB = 1mA

In the figure 2.1, R₁, R₂ and R₃ are all Connected together in series between points A and B with a common current, I flowing through them. 

Figure 2.1: Resistors in series

As the resistors are connected together in series the same current passes through each resistor in the chain and the total resistance, R_T of the circuit must be equal to the sum of all the individual resistors added together.  That is;

           

            R_T = R₁ +R₂ + R₃

There for in our example; R_T = (1 + 2 + 6)Ω

                                                = 9Ω.

We can see that the circuit in figure 2.1 can be replaced with figure 2.2 below:

Figure 2.2

R_EQ  in figure 2.2 is known as equivalent resistance, which can be defined as a single value of resistance that can replace any number of resistors in series without altering the values of the current or the voltage in the circuit.

Resistors in Parallel

Resistors are said to be connected together in “Parallel” when both of their terminals are respectively connected to each terminal of the other resistor or resistors. Unlike the previous series connection, in a parallel resistor network the circuit current can take more than one path as there are multiple nodes. 

Figure 2.3: Resistors in Parallel

Parallel Resistor Equation:

            

Then the inverse of the equivalent resistance of two or more resistors connected in parallel is the algebraic sum of the inverses of the individual resistance. The equivalent resistance is always less than the smallest resistor in the parallel network so the total resistance, R_T will always decrease as additional parallel resistors are added.

OHM’S LAW

This is an important and useful law in electronics. It proves that the current (I) flowing through a conductor is proportional to the voltage (V) applied across its ends. Mathematically:  V α I  or  V = IR where R is the constant of proportionality. R is called Resistance.

Figure 3.0: Useful relationship between Voltage, Current, Power and Resistance.

Ohm’s Law Circuit

Figure 3.1: Ohm’s Law Circuit

Ideal Voltage Source

Every voltage source (power supply, battery, etc.) has its own resistance due to its internal construction (this is known as internal resistance). Normally, this will be generally small of the order of few Ohms only, but it can still be of significance and cause difficulty in circuits as it will be discussed bellow.

Assume a 6V battery connected to a resistance of (a) 6Ω and (b) 1Ω. What would be the current according to Ohm’s law?

For (a):  I = [6V/6Ω] = 1A

For (b):  I  =  [6V/1Ω] = 6A

However, if you actually measured the current you would find the currents would not be these values.

Figure 3.2: Voltage source showing its internal resistance.

In a typical case, where the internal resistance of the battery is 1Ω, these currents were 0.86 and 3A only instead of 1A and 6A. Hence the currents in the two cases are [6V/(6+1)Ω] = 0.86A and [6V/(1+1)Ω]=3A. The voltages across the internal resistance are 0.86 and 3V and the voltages across the load resistance are 5.14V and 3V in the two cases respectively.

Ideal Voltage Source

An ideal voltage source is one that has a zero internal resistance. The process of regulating a power supply, what we really want to do is reducing its effective internal resistance to as small a value as possible. In reality, there are not ideal voltage sources, but one can get sources which are very close to ideal behaviour.

Applying Resistor as Voltage Dividers

Resistors can also be used to determine voltage, in a configuration called a voltage divider.

Figure 3.3: Voltage divider

The Voltage Division Rule, allows us to use the effects of resistance proportionality to calculate the potential difference across each resistance regardless of the current flowing through the series circuit. A typical “voltage divider circuit” is shown figure 3.3.

The voltage Divider Circuit is the simplest way of producing a lower voltage from a higher voltage, and is the basic operating mechanism of the potentiometer.

As well as being used to calculate a lower supply voltage, the voltage divider formula can also be used in the analysis of more complex resistive circuits containing both series and parallel branches. The voltage or potential divide formula can be used to determine the various drops around a closed DC network or as part of a various circuit analysis laws such as Kirchoffs of Thevenin’s theorems.

CAPACITORS

A capacitor is basically a device constructed using two parallel plate separated by an insulator or a dielectric. It is used for storing electrical energy and capacitor is characterised by very large resistance to Direct Current (DC) and Smaller resistance for alternating current. It is very useful as a filter, and for passing AC and blocking DC. The symbol is shown in figure 4.1 bellow;

Figure 4.1: Symbol of a capacitor

Figure 4.2: The Construction of a typical parallel plate capacitor.

The capacitance C of a capacitor is given by;

Where A is the area of the plates, d is the spacing between them, ε₀ is called the permittivity in free space and ε_r is the dielectric constant (relative permittivity). With these we can see that the capacitance of a capacitor is proportional to the area A of the plates and inversely proportional to the distance between the plates.

When a DC voltage is applied to a capacitor, it gets charged, just as the charges get accumulated on the plates there is a current flowing in the circuit. But as the capacitor gets charged the current starts to reduce and when it is fully charged, the current becomes zero. When we measure the resistance between the leads of a capacitor it will show infinite resistance, hence a capacitor will block DC current.

However, when AC voltage or current is applied the capacitance will offer what we called reactance, it is not resistance, it is reactance because it is only responding to time varying signals of voltage and current.

The value of reactance Xc offered by a capacitor is given by;

The unit of capacitance is Farad (F) , practically, value of capacitance is very small and thus we use sub-units such as microfarad, μF  (10^-6F), nanofarad, nF (10^-9F), and picofarad, ρF (10^-12F).

Capacitor Connections

Capacitor in Series:

When capacitors are connected in series the resultant capacitance is much more similar to that of resistors in parallel and is given by;

or 

Figure 4.3:  Capacitors in series

Capacitors in parallel:

When you connect your capacitors in parallel, the total capacitance can be calculated as you are doing in the case of resistors in series.

Figure 4.4:  Capacitors in Parallel

Breakdown voltage

This is an important characteristic of a capacitor; it is the maximum voltage that you can apply across a capacitor without breaking down the capacitor. If you keep on applying voltage to the capacitor, when the voltage exceeds breakdown voltage, the dielectric will breakdown making very large current to flow through them. The voltage depends on the kind of capacitor being used.

Types of Capacitors

 Electrolytic Capacitors: The unique characteristic of this kind of capacitor is that they have polarity i.e.  They have positive and a negative electrode. They range in value from about 1μf to thousands of μf. They a commonly used a ripple filter in a power supply circuit, or as a filter to bypass low frequency signals, etc.

Figure 4.5:  Electrolytic Capacitors

-  Tantalum Capacitors

Tantalum Capacitors are also a type of electrolytic capacitors that used materials known as tantalum for the electrodes (electrodes are conductors through which electricity enters or leaves an object, substance, or region). They are superior to aluminium electrolytic capacitors in temperature and frequency characteristics, these features make them more expensive.

They a useful in circuit that requires high stability in the capacitance value, often employed in analogue signal systems.

Figure 4.6: Tantalum Capacitors

- Ceramic Capacitors

Here they use materials such as titanium and barium oxide as their dielectric. These type of capacitors are employed in high frequency applications.

Figure 4.7: Ceramic Capacitors

- Polystyrene Film Capacitors

As the name obviates, polystyrene film is used as the dielectric in these devices. They find their applications in filter circuits or timing circuits with frequency of several hundred KHz or less.

Figure 4.8:  Polystyrene film capacitor

 - Electric double layer capacitors

This is a “Super Capacitor”, it has a capacitance of 0.47F (470, 000μF). One should be careful using this type of capacitor in a circuit, because it has polarity.

Figure: 4.9:  Electric double layer capacitors

-Polyester film capacitors

They use thin polyester film as the dielectric. They are cheap and handy, having tolerance about ±5% to ±10%.

Figure 4.10:  Polyester film capacitors

-                      Polypropylene capacitors

-                      Mica Capacitors

-                      Dipped mica capacitors

-                      Metallised polyester film capacitors

-                      Variable capacitors

These capacitors are used in adjusting frequency; here we can vary the value of capacitance using a special screwdriver.

 COILS or INDUCTORS

A coil is a copper wire wound in a spiral. 

Figure 4.11: Symbol of a coil/inductor

The unit of inductance imposed by an inductor is called Henry (H), the strength of the characteristics of an inductor is directly proportional to the number of turns it contains. The inductance of a coil can be greatly increased if it is wound around an iron rod, or ferrite core.

Two importance characteristics of an inductor in a circuit are:

i.                    An inductor can store energy in its magnetic field and

ii.                  It tends to resist any change in the amount of current flowing through it.

“Lenz’s law illustrated that the direction of induced current in a coil is such that it opposes the change in the magnetic field that produced it.

To further illustrate the operation of an inductor see figure 4.12:

Figure 4.12: Inductor in a circuit (Source: How Stuff Works 2003).

If the circuit in figure 4.12 is without the inductor, it normal operation will be that the bulb is ON when the switch is closed and OFF when the switch is open.

With the inductor L in the circuit, when the switch is closed, the inductor will want to build up a magnetic field, while doing this is inhibits the flow of current, this makes the bulb burn brightly at the onset of closing the key and once the magnetic field has been built the bulb gets dimmer as current is can now flow normally through the wire.

When the switch is opened after being closed, the magnetic field in around the coil keeps current flowing in the coil until the filed collapses. This causes the bulb to burn very brightly for a period of time even though the switch is open and then quickly goes out. When a current attempts to change in a conductor, there is a reactance unlike the case of a capacitor where reactance is imposed when the voltage attempt to change.

An application of the characteristics of an inductor can be found in the case of a transformer. In a transformer, the change in current of one coil affects the current and voltage in the second coil, a phenomenon known as mutual inductance.

This mutual inductance is also measured in units of the Henry.

Figure 4.13: Symbol of a transformer

When AC voltage to the primary coil, due to their mutual inductance between the primary and secondary coil, there is a voltage also induced in the secondary and therefore this can be used to get smaller AC voltage from a larger voltage or a larger voltage from a smaller one.

Relay

Another application of an inductor or coil is in relay. When current flows through a coil a magnetic filed is produced in a direction of the axis of the coil. The coil becomes a temporary magnet. In its application in relays this magnet attracts an armature that has been constructed to make or break an electrical connection due to the attraction. When the current to the coil is switched off, the armature is returned by a force, approximately half as strong as magnetic force, usually produced by a spring, to its relaxed position.

Figure 4.14: Image of a typical Relay

Resonance

Inductors and Capacitors combined in a circuit (tank circuit) produce a special characteristic. The impedance (i.e. resistance to the flow of current) of the circuit changes with the frequency of the voltage. This characteristic is employed in tuning circuit that select a particular radio station.