Preparation Notes Chapter 2

109ES: BASIC ELECTRONICS AND INSTRUMENTS

PREPARATION NOTES

BY: PROF. SAURABH SHUKLA

CHAPTER 2 Basic Electronic Circuits and it’s applications

Topics to be covered:
Diode as a Switch, Diode as a Rectifier, Half Wave and Full Wave Rectifiers with and
without Filters; Design of un regulated DC power supply, Zener Diode – Operation and
Applications.

2.1 Diode as a Switch

A diode functions as a switch by leveraging its forward and reverse bias characteristics: it
acts as a closed switch (ON state) when forward-biased, allowing current to flow through it
with low resistance, and as an open switch (OFF state) when reverse-biased, blocking current
flow with high resistance. This one-way current flow makes the diode suitable for high-speed
switching applications in circuits like rectifiers, logic circuits, and voltage clamping.

How it Works

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Forward Bias (ON State):

When a voltage is applied in the forward direction (positive to anode, negative to cathode),
the diode's resistance decreases significantly, making it act like a closed circuit or switch,
allowing current to pass.

Reverse Bias (OFF State):

When a voltage is applied in the reverse direction (negative to anode, positive to cathode), the
diode's resistance increases dramatically, causing it to act like an open circuit or switch,
effectively blocking the flow of current.

Key Characteristics

One-Way Action:

The fundamental principle is the diode's ability to only permit current flow in one direction.

Bi-Stable States:

The diode can exist in two distinct states – ON or OFF – similar to a switch.

Switching Speed:

Switching diodes are specially designed for quick transitions between the ON and OFF states,
making them ideal for high-frequency applications.

Applications

High-Speed Rectifiers: Used in circuits that convert AC to DC power.

Logic Circuits: Employed in digital systems to control the flow of signals.

Voltage Clamping: Protects circuits from voltage spikes by blocking excessive voltage.

RF Switching: PIN diodes, a specialized type, are used as switches for radio frequencies.

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2.2 Diode as a Rectifier

A diode acts as a rectifier by using its inherent ability to conduct current in only one direction
(forward bias) while blocking it in the other (reverse bias). When connected to an alternating
current (AC) source, the diode allows current to pass during one half-cycle of the AC
waveform and blocks the other, converting the bidirectional AC into a unidirectional, or
pulsating, direct current (DC). This process is known as rectification, and the diode itself is
often called a rectifier diode or used in a rectifier circuit.

The main application of p-n junction diode is in rectification circuits. These circuits are used
to describe the conversion of a.c signals to d.c in power supplies. Diode rectifier gives an
alternating voltage which pulsates in accordance with time. The filter smoothes the pulsation
in the voltage and to produce d.c voltage, a regulator is used which removes the ripples.

There are two primary methods of diode rectification:

• Half Wave Rectifier

• Full Wave Rectifier

2.2.1 What Is Half Wave Rectifier?

In a half-wave rectifier, one half of each a.c input cycle is rectified. When
the p-n junction diode is forward biased, it gives little resistance and when
it is reversed biased it provides high resistance. During one-half cycles, the
diode is forward biased when the input voltage is applied and in the
opposite half cycle, it is reverse biased. During alternate half-cycles, the
optimum result can be obtained.

Working of Half Wave Rectifier

The half-wave rectifier has both positive and negative cycles. During the
positive half of the input, the current will flow from positive to negative
which will generate only a positive half cycle of the a.c supply. When a.c
supply is applied to the transformer, the voltage will be decreasing at the
secondary winding of the diode. All the variations in the a.c supply will
reduce, and we will get the pulsating d.c voltage to the load resistor.

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In the second half cycle, the current will flow from negative to positive and
the diode will be reverse biased. Thus, at the output side, there will be no
current generated, and we cannot get power at the load resistance. A
small amount of reverse current will flow during reverse bias due to
minority carriers.

Advantages of Half Wave Rectifier

• Affordable
• Simple connections
• Easy to use as the connections are simple
• Number of components used are less

Disadvantages of Half Wave Rectifier

• Ripple production is more

• Harmonics are generated
• Utilization of the transformer is very low
• The efficiency of rectification is low

Applications of Half Wave Rectifier

Following are the uses of half-wave rectification:

• Power rectification: Half wave rectifier is used along with a

transformer for power rectification as powering equipment.
• Signal demodulation: Half wave rectifiers are used for demodulating
the AM signals.
• Signal peak detector: Half wave rectifier is used for detecting the
peak of the incoming waveform.

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2.2.2 What Is Full Wave Rectifier?

Full-wave rectifier circuits are used for producing an output voltage or output current which
is purely DC. The main advantage of a full-wave rectifier over half-wave rectifier is that such
as the average output voltage is higher in full-wave rectifier, there is less ripple produced in
full-wave rectifier when compared to the half-wave rectifier.

Working of Full Wave Rectifier

The full-wave rectifier utilizes both halves of each a.c input. When the p-n junction is
forward biased, the diode offers low resistance and when it is reverse biased it gives high
resistance. The circuit is designed in such a manner that in the first half cycle if the diode is
forward biased then in the second half cycle it is reverse biased and so on.

Advantages of Full Wave Rectifier

• The rectifier efficiency of a full-wave rectifier is high

• The power loss is very low

• Number of ripples generated are less

Disadvantages of Full Wave Rectifier

• Very expensive

Applications of Full Wave Rectifier

Following are the uses of full-wave rectifier:

• Full-wave rectifiers are used for supplying polarized voltage in welding and for this
bridge rectifiers are used.

• Full-wave rectifiers are used for detecting the amplitude of modulated radio signals.

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Difference between Half Wave Rectifier and Full Wave Rectifier

Parameter Half Wave Rectifier Full Wave Rectifier

Definition The half-wave rectifier is a A full-wave rectifier is a

rectifier which is used for rectifier which is used for
converting the one-half cycle converting both the half
of AC input to DC output cycles of AC input into DC
output

No. of diodes used 1 2 or 4 depending on the type

of circuit

Form factor 1.57 1.11

Rectifier efficiency 40.6% 81.2%

Ripple factor Ripple factor of a half-wave Ripple factor of a full-wave

rectifier is more rectifier is less

2.2.3 Full wave bridge rectifier:

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Construction

The construction of a bridge rectifier is shown in the figure below. The bridge rectifier circuit
is made of four diodes D1, D2, D3, D4, and a load resistor RL. The four diodes are connected in
a closed-loop configuration to efficiently convert the alternating current (AC) into Direct
Current (DC). The main advantage of this configuration is the absence of the expensive
centre-tapped transformer. Therefore, the size and cost are reduced.

The input signal is applied across terminals A and B, and the output DC signal is obtained
across the load resistor RL connected between terminals C and D. The four diodes are
arranged in such a way that only two diodes conduct electricity during each half cycle.
D1 and D2 are pairs that conduct electric current during the positive half cycle/. Likewise,
diodes D3 and D4 conduct electric current during a negative half cycle.

Working

When an AC signal is applied across the bridge rectifier, terminal A becomes positive during
the positive half cycle while terminal B becomes negative. This results in diodes D1 and
D2 becoming forward biased while D3 and D4 becoming reverse biased.

The current flow during the positive half-cycle is shown in the figure below:

Current flow direction will be: A-D1-R-D2-B-A

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During the negative half-cycle, terminal B becomes positive while terminal A becomes
negative. This causes diodes D3 and D4 to become forward biased and diode D1 and D2 to be
reverse biased.
The current flow during the negative half cycle is shown in the figure below:

Current direction will be: B-D3-R-D4-A

From the figures given above, we notice that the current flow across load resistor RL is the
same during the positive and negative half-cycles. The output DC signal polarity may be
either completely positive or negative. In our case, it is completely positive. If the diodes’
direction is reversed, we get a complete negative DC voltage.

Thus, a bridge rectifier allows electric current during both positive and negative half cycles of
the input AC signal.

The output waveforms of the bridge rectifier are shown in the below figure.

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Characteristics of Bridge Rectifier

Ripple Factor

The smoothness of the output DC signal is measured by a factor known as the ripple factor.
The output DC signal with fewer ripples is considered a smooth DC signal while the output
with high ripples is considered a high pulsating DC signal.

Mathematically, the ripple factor is defined as the ratio of ripple voltage to pure DC voltage.

The ripple factor for a bridge rectifier is given by

For bridge rectifiers, the ripple factor is 0.48.

Peak Inverse Voltage

The maximum voltage that a diode can withstand in the reverse bias condition is known as a
peak inverse voltage. During the positive half cycle, the diodes D1 and D2 are in the
conducting state while D3 and D4 are in the non-conducting state. Similarly, during the
negative half cycle, diodes D3 and D4 are in the conducting state, and diodes D1 and D2 are in
the non-conducting state.

Efficiency

The rectifier efficiency determines how efficiently the rectifier converts Alternating Current
(AC) into Direct Current (DC). Rectifier efficiency is defined as the ratio of the DC output
power to the AC input power. The maximum efficiency of a bridge rectifier is 81.2%.

Advantages

• The efficiency of the bridge rectifier is higher than the efficiency of a half-wave
rectifier. However, the rectifier efficiency of the bridge rectifier and the centre-tapped
full-wave rectifier is the same.

• The DC output signal of the bridge rectifier is smoother than the output DC signal of a
half-wave rectifier.

• In a half-wave rectifier, only half of the input AC signal is used, and the other half is
blocked. Half of the input signal is wasted in a half-wave rectifier. However, in a
bridge rectifier, the electric current is allowed during both positive and negative half
cycles of the input AC signal. Hence, the output DC signal is almost equal to the input
AC signal.

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Disadvantages

• The circuit of a bridge rectifier is complex when compared to a half-wave rectifier

and centre-tapped full-wave rectifier. Bridge rectifiers use 4 diodes while half-wave
rectifiers and centre-tapped full wave rectifiers use only two diodes.

• When more diodes are used more power loss occurs. In a centre-tapped full-wave
rectifier, only one diode conducts during each half cycle. But in a bridge rectifier, two
diodes connected in series conduct during each half cycle. Hence, the voltage drop is
higher in a bridge rectifier.

2.3 Design of un regulated DC power supply

An unregulated DC power supply converts AC power into DC power using a transformer, a
rectifier (diode bridge), and a filter capacitor to smooth the output. While simple and low-
cost, the output voltage varies with changes in the input AC voltage or the load current. The
design requires specifying the desired DC output voltage and current, then selecting a step-
down transformer, a full-wave bridge rectifier, and a filter capacitor large enough to minimize
voltage ripple, and a bleeder resistor to discharge the capacitor when the load is disconnected

Components and their Functions

1. 1. Transformer:

Steps down the high-voltage AC mains to a lower, safer AC voltage.

2. 2. Rectifier:

Converts the AC voltage into pulsating DC using diodes. A full-wave bridge rectifier is
common, converting both halves of the AC cycle.

3. 3. Filter Capacitor:

Smooths the pulsating DC output from the rectifier into a more stable, though still
fluctuating, DC voltage.

4. 4. Bleeder Resistor:

Discharges the filter capacitor when the load is disconnected, a safety feature that also helps
the capacitor discharge more quickly.

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Steps to Design an Unregulated Power Supply:

1. Define Specifications: Determine the required output DC voltage and the maximum
load current.

2. Select the Transformer: Choose a transformer with a secondary AC voltage high

enough to compensate for the diode voltage drops and provide the desired DC output,
plus some allowance for ripple.

3. Choose the Rectifier: A full-wave bridge rectifier is typical for better efficiency.

4. Calculate Filter Capacitor Value: Select a capacitor with a capacitance large enough
to limit the output voltage ripple to an acceptable level for your application. The value
is calculated using the load current, the ripple voltage, and the time period of the AC
input.

5. Determine Bleeder Resistor Value: Choose a high-value resistor (e.g., 1kΩ) to

safely discharge the capacitor when the power supply is switched off.

6. Add Overcurrent Protection: Incorporate a fuse with a rating slightly higher than
the maximum load current to protect the circuit.

Considerations for Use

• Voltage Fluctuation:

The primary drawback is that the output voltage is not constant and will fluctuate with
changes in the input AC line voltage and the connected load.

• Ripple:

While the capacitor reduces ripple, it does not eliminate it entirely, and some AC ripple will
remain in the DC output.

• Applications:

Unregulated supplies are suitable for applications that can tolerate this voltage variation, such
as low-power, non-sensitive electronic devices or as a preliminary stage for a regulated power
supply.

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2.4 Zener Diode – Operation and Applications.

A Zener Diode, also referred to as a breakdown diode, is a specially doped semiconductor
device engineered to function in the reverse direction. When the voltage across a Zener
diode’s terminals is reversed and reaches the Zener Voltage (also known as the knee voltage),
the junction experiences a breakdown, allowing current to flow in the opposite direction. This
phenomenon, known as the Zener Effect, is a key characteristic of Zener diodes.

A Zener diode is a highly doped semiconductor device specifically designed to function in the
reverse direction. It is engineered with a wide range of Zener voltages (Vz), and certain types
are even adjustable to achieve variable voltage regulation.

History of Zener Diodes

Clarence Melvin Zener (1905- 1993)

Clarence Melvin Zener, a distinguished theoretical physicist at Bell Labs, made significant
contributions to the understanding of Zener Diode’s electrical properties. In 1934, he published a
groundbreaking paper postulating the phenomenon of breakdown effect, which was subsequently
named after him. Zener’s pioneering work led to the development and recognition of the Zener
diode as an essential electronic component.

How does a Zener Diode work in reverse bias?

A Zener diode functions similarly to a regular diode when forward-biased. However, in

reverse-biased mode, a small leakage current flows through the diode. As the reverse voltage
increases and reaches the predetermined breakdown voltage (Vz), current begins to flow
through the diode. This current reaches a maximum level determined by the series resistor,
after which it stabilizes and remains constant across a wide range of applied voltages.

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There are two types of breakdowns in a Zener Diode: Avalanche Breakdown and Zener
Breakdown.

Avalanche Breakdown in Zener Diode

Avalanche breakdown occurs in both normal diodes and Zener diodes when subjected to high
reverse voltage. When a significant reverse voltage is applied to the PN junction, the free
electrons gain enough energy to accelerate at high velocities. These high-velocity electrons
collide with other atoms, causing the ejection of additional electrons. This continuous
collision process generates a large number of free electrons, resulting in a rapid increase in
electric current through the diode. In the case of a normal diode, this sudden surge in current
could permanently damage it. However, a Zener diode is specifically designed to withstand
avalanche breakdown and can handle the sudden current spike. Avalanche breakdown
typically occurs in Zener diodes with a Zener voltage (Vz) greater than 6V.

Zener Breakdown in Zener Diode

When the reverse bias voltage applied to a Zener diode approaches its Zener voltage, the
electric field within the depletion region becomes strong enough to attract and remove
electrons from their valence band. These valence electrons, energized by the intense electric
field, break free from their parent atoms. This phenomenon takes place in the Zener
breakdown region, where even a slight increase in voltage leads to a rapid surge in electric
current.

Avalanche Breakdown vs. Zener Breakdown

The Zener effect is predominant in voltages up to 5.6 volts, while the avalanche effect
becomes more prominent beyond that threshold. Although both effects are similar, the
distinction lies in the fact that the Zener effect is a quantum phenomenon, whereas the
avalanche effect involves the movement of electrons in the valence band, similar to an
electric current. The avalanche effect allows a larger current through the diode compared to
what a Zener breakdown would permit.

Circuit Symbol of Zener Diode

Zener diodes come in various packaging options, depending on their power dissipation
requirements. Some are designed for high-power applications, while others are available in
surface mount formats. The most commonly used Zener diode is packaged in a small glass
enclosure, with a distinctive band indicating the cathode side of the diode.

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Zener diode symbol and package outlines

The symbol used to represent a Zener diode in circuit diagrams is similar to that of a regular
diode, but with a unique addition. It consists of a triangle or arrowhead pointing towards the
cathode side (the side with the band) of the diode. This triangle is accompanied by two
perpendicular lines at the cathode end, one extending upwards and the other extending
downwards. These lines indicate the specific behaviour of the Zener diode and help
distinguish it from other types of diodes in circuit diagrams. The symbol provides a visual
representation that allows engineers and technicians to easily identify and understand the
presence of a Zener diode in a circuit.

V-I Characteristics of Zener Diode

The diagram given below shows the V-I characteristics of the Zener diode.

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When reverse-biased voltage is applied to a Zener diode, it allows only a small amount of
leakage current until the voltage is less than Zener voltage.

The V-I characteristics of a Zener diode can be divided into two parts as follows:
(i) Forward Characteristics
(ii) Reverse Characteristics

Forward Characteristics of Zener Diode

The first quadrant in the graph represents the forward characteristics of a Zener diode. From
the graph, we understand that it is almost identical to the forward characteristics of P-N
junction diode.

Reverse Characteristics of Zener Diode

When a reverse voltage is applied to a Zener voltage, a small reverse saturation current Io
flows across the diode. This current is due to thermally generated minority carriers. As the
reverse voltage increases, at a certain value of reverse voltage, the reverse current increases
drastically and sharply. This is an indication that the breakdown has occurred. We call this
voltage breakdown voltage or Zener voltage, and Vz denotes it.

Zener Diode Specifications

Some commonly used specifications for Zener diodes are as follows:

• Zener/Breakdown Voltage – The Zener or the reverse breakdown voltage ranges

from 2.4 V to 200 V, sometimes it can go up to 1 kV while the maximum for the
surface-mounted device is 47 V.

• Current Iz (max) – It is the maximum current at the rated Zener Voltage (Vz –
200μA to 200 A)

• Current Iz (min) – It is the minimum value of current required for the diode to break
down.

• Power Rating – It denotes the maximum power the Zener diode can dissipate. It is
given by the product of the voltage of the diode and the current flowing through it.

• Temperature Stability – Diodes around 5 V have the best stability

• Voltage Tolerance – It is typically ±5%

• Zener Resistance (Rz) – It is the resistance to the Zener diode exhibits.

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Application of Zener Diode

Following are the applications of Zener diode:

Zener diode as a voltage regulator:

The zener diode is used as a Shunt voltage regulator for regulating voltage across small loads.
The Zener diode is connected parallel to the load to make it reverse bias, and once the Zener
diode exceeds knee voltage, the voltage across the load will become constant. The breakdown
voltage of Zener diodes will be constant for a wide range of currents.

Zener diode in over-voltage protection:

When the input voltage is higher than the Zener breakage voltage, the voltage across the
resistor drops resulting in a short circuit, this can be avoided by using the Zener diode.

Zener diode in clipping circuits:

Zener diode is used for modifying AC waveform clipping circuits by limiting the parts of
either one or both the half cycles of an AC waveform.

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