A center-tapped full-wave rectifier is an electrical circuit used to convert alternating current (AC) into direct current (DC). It is commonly used in power supplies to ensure efficient DC voltage conversion.

What is Center Tapped Full Wave Rectifier?

A center-tapped full-wave rectifier is a type of rectifier circuit that uses a center-tapped transformer and two diodes to convert both halves of the AC input waveform into a pulsating DC output. The center tap of the transformer acts as a reference point, allowing both positive and negative halves of the AC waveform to be utilized.

Figure 1.

Circuit Diagram of Center Tapped Full Wave Rectifier

The circuit (see Figure 1(a)) consists of:

1. AC Supply: Provides alternating voltage.
2. Center-Tapped Transformer: Steps down or steps up the AC voltage and provides a center tap as a neutral reference.
3. Diodes (D₁ and D₂): Rectify the AC voltage by allowing only one direction of current flow.
4. Load Resistor (R_L): Converts rectified voltage into useful DC output.

Working Principle of Center Tapped Full Wave Rectifier

The working of a center-tapped full-wave rectifier is based on the principle of rectification, where diodes allow current flow in only one direction. The center-tapped transformer provides two equal and opposite AC voltages with respect to the center tap, enabling the use of two diodes for rectification.

Operation of Center Tapped Full Wave Rectifier

Positive Half-Cycle

During the positive half-cycle of the AC input, terminal A is positive, and terminal B is negative. Diode D₁ becomes forward-biased, allowing current to pass through the load R_L. Diode D₂ is reverse-biased and blocks current flow.

Negative Half-Cycle

During the negative half-cycle of the AC input, terminal B becomes positive, and terminal A becomes negative. Diode D₂ is forward-biased, allowing current to pass through the load R_L in the same direction. Diode D₁ is reverse-biased and blocks current flow.

Thus, both half-cycles contribute to current flow in the same direction across the load resistor, resulting in a full-wave rectified output.

Waveforms of Center Tapped Full Wave Rectifier

The input and output waveforms (see Figure 1(b)) are as follows:

Input Voltage (V_in): A sinusoidal waveform with alternating positive and negative cycles.

Output Voltage (V_out): A pulsating DC waveform with only positive cycles due to rectification by D₁ and D₂.

Advantages of Center Tapped Full Wave Rectifier

1. Higher efficiency compared to half-wave rectifiers.
2. Utilizes both halves of the AC waveform, reducing ripple.
3. Provides higher DC output voltage.

Disadvantages of Center Tapped Full Wave Rectifier

1. Requires a center-tapped transformer, increasing cost and size.
2. Diodes must withstand full secondary voltage, requiring higher ratings.

Applications of Center Tapped Full Wave Rectifier

1. DC Power Supplies: Used in household electronic devices, chargers, and adapters.
2. Battery Charging Circuits: Provides regulated DC voltage for charging batteries.
3. Audio and Radio Circuits: Used in audio amplifiers and radio receivers.
4. Industrial Electronics: Used in motor controllers and DC-powered control systems.

Conclusion

The center-tapped full-wave rectifier is an efficient rectification technique, widely used in power electronics. It provides a smooth DC output with reduced ripple compared to half-wave rectifiers. Despite the requirement of a center-tapped transformer, its advantages make it a preferred choice for many applications.

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What is a Relay?

A relay is an electromechanical device that uses an electromagnetic coil to operate a switch. It allows low-power circuits to control high-power devices by opening or closing circuits using an energized or de-energized coil. Relays are fundamental in automation, control systems, and electrical circuits.

Construction of a Relay

Figure 1: The Relay.

A relay is an electrically operated switch that allows one circuit to control another, often with different voltage or current levels. It is used to isolate and protect low-power circuits or to control high-power loads. Its construction as shown in Figure 1,

1. Solenoid (Coil): A wire wound around a magnetic core that creates a magnetic field when current flows through it.
2. Spring: Maintains the armature’s position when no current flows through the coil.
3. Iron Core: Increases the magnetic effect of the solenoid.
4. Contacts:
   • NC (Normally Closed): Connected to the pole when the relay is not energized.
   • NO (Normally Open): Disconnected from the pole when the relay is not energized.
5. Pole: The movable arm that toggles between NC and NO contacts.
6. Insulators: Prevent electrical conduction between different parts.
7. Copper Alloy Contacts: Ensure efficient conduction when connected.

Working of a Relay

1. Energized State: When current flows through the coil, the solenoid generates a magnetic field. The magnetic force pulls the armature, moving the pole from NC to NO.
2. De-energized State: When current is removed, the spring pushes the armature back to its original position, reconnecting NC.

Working Principle of a Relay

Types of Relays

Relays can be categorized based on different criteria, including construction, operating principles, and application. Below are common types:

 Advantages of Relays

1. Isolation: Electrically isolates the control and load circuits.
2. Low Power Consumption: Operates with minimal control signal power.
3. High Reliability: Durable and can handle frequent switching.
4. Versatility: Suitable for a wide range of applications.
5. Cost-Effective: Affordable compared to other switching devices.
6. Customizability: Available in various configurations for specific requirements.

Disadvantages of Relays

1. Wear and Tear: Mechanical components are prone to failure over time.
2. Slower Operation: Electromechanical relays have slower switching speeds compared to solid-state devices.
3. Contact Arcing: Can occur in high-power circuits, reducing lifespan.
4. Size: Bulkier compared to modern semiconductor-based alternatives.

Applications of Relays

Relays find extensive use in various fields and applications:

1. Automotive: Controlling headlights, horns, and fuel pumps.
2. Industrial Automation: Used in programmable logic controllers (PLCs) and motor control systems.
3. Home Appliances: Found in washing machines, air conditioners, and microwave ovens.
4. Telecommunication: Switching circuits in telecommunication networks.
5. Power Systems: Protective relays safeguard equipment from overcurrent and voltage surges.
6. Medical Equipment: Used in diagnostic machines and life-support systems.
7. Aerospace: Relays control circuits in aircraft and space systems.
8. Lighting Control: Used in automated lighting systems for efficient power management.

Conclusion

Relays are indispensable components in electrical and electronic systems. Their ability to control high-power circuits with low-power signals makes them a key element in automation and safety systems. While newer technologies like solid-state relays are gaining popularity, traditional electromechanical relays continue to serve as reliable, cost-effective solutions for various applications.

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Construction of Variable Capacitor

Figure 1.

The construction (see figure 1) of a variable capacitor consists of the following major parts:

1. Rotor: A set of movable conductive plates. These plates rotate to adjust the capacitance by changing the overlapping area with the fixed plates.
2. Stator: A set of fixed conductive plates. These plates remain stationary and interact with the rotor plates to form the capacitor.
3. Shaft: A mechanical control element connected to the rotor. By rotating the shaft, the rotor’s position relative to the stator changes, altering the capacitance.
4. Dielectric Material: Air is commonly used as the dielectric in variable capacitors, though other materials may also be used depending on the application.
5. Housing: Encloses the assembly to provide mechanical support and protection from external elements.

Working Principle of Variable Capacitor

The variable capacitor operates on the same fundamental principle as a fixed capacitor, where capacitance C is given by:

\begin{equation}
C = \frac{\varepsilon A}{d}
\end{equation}

Where:
C = Capacitance (Farads)
ε = Permittivity of the dielectric material (Farads per meter)
A = Overlapping area of the plates (square meters)
d = Distance between the plates (meters)

In a variable capacitor, adjusting the rotor changes the overlapping area A, thereby altering the capacitance.  In certain designs, the distance d can also be varied mechanically. This principle allows the capacitance to be fine-tuned for specific applications.

Types of Variable Capacitors

Advantages of Variable Capacitors

1. Adjustable Capacitance: Provides flexibility for tuning and calibration.
2. Versatile Applications: Suitable for use in various circuits, from RF to audio frequencies.
3. Compact Design: Trimmer capacitors are especially compact and fit into small circuits.
4. Cost-Effective: Widely available and relatively inexpensive for most designs.

Disadvantages of Variable Capacitors

1. Mechanical Wear: Moving parts may degrade over time.
2. Limited Stability: Prone to drift due to temperature or mechanical vibrations.
3. Lower Capacitance Range: Typically offer smaller capacitance values compared to fixed capacitors.
4. Manual Adjustment: In most cases, manual tuning is required, which can be less convenient.

Applications of Variable Capacitors

Variable capacitors are essential in numerous applications, including:

1. Tuning Circuits: Used in radios, televisions, and communication systems for frequency tuning.
2. Oscillators: Employed in LC oscillators to adjust resonant frequencies.
3. Filters: Help in creating tunable filters for signal processing.
4. Matching Networks: Optimize the impedance matching in RF circuits.
5. Measuring Instruments: Used in capacitance bridges and other test equipment.

The post What is Variable Capacitor? Definition, Construction, Working Principle, Diagram, Types, Advantages, Disadvantages & Applications appeared first on Electrical and Electronics Blog.

It is widely used in industries such as food processing, textile drying, and material curing, where uniform heating of insulating materials is required.

Working Principle of Dielectric Heating

Figure 1: Polarization.

Dielectric heating, also known as capacitive heating, occurs when a non-conductive (dielectric) material is subjected to a high-frequency alternating electric field. This phenomenon involves the following principles:

1. Polarization: In the absence of an external electric field, a neutral atom has a balanced distribution of charges (Fig. 1a). When exposed to an alternating electric field, the positive and negative charges within the dielectric material are displaced, creating polarized atoms (Fig. 1b). This polarization generates a dipole moment (Fig. 1c), which represents the separation of charges $+q$ and $-q$ over a distance $d$.

2. High-Frequency Electric Field: A high-frequency alternating voltage is applied across electrodes containing the dielectric material. This results in the continuous realignment of dipoles in the direction of the rapidly changing field.

3. Energy Loss Due to Molecular Friction: The continuous reorientation of dipoles generates friction at the molecular level, leading to heat energy dissipation. This frictional energy loss is called dielectric loss, which is the primary source of heat in dielectric heating.

4. Dielectric Loss and Power Dissipation: The heat generated depends on the loss tangent ($\tan \delta$), angular frequency \(\omega\), and the applied electric field strength. The power dissipation is given by:

\[
P = V^2 \cdot \omega \cdot C \cdot \tan\delta
\]
where:
\(V\): RMS voltage applied across the dielectric
\(\omega\): Angular frequency of the AC signal (\(\omega = 2\pi f\))
\(C\): Capacitance of the dielectric
\(\tan\delta\): Loss tangent, representing the ratio of resistive power loss to reactive power

$$P = V^2 \cdot \omega C \cdot \tan \delta$$

5. Heat Distribution: Dielectric heating ensures uniform heating throughout the material as the electric field penetrates uniformly, unlike conventional heating methods where heat is conducted from the surface to the interior.

Working Steps in Dielectric Heating

1. Application of AC Voltage: An alternating voltage is applied across a dielectric material placed between two electrodes.
2. Molecular Polarization: In the presence of an electric field, molecules experience polarization, creating a dipole moment. The electric field continuously reverses its direction due to the alternating nature of the AC signal, causing dipoles to realign.
3. Energy Dissipation: The repeated polarization and relaxation of dipoles result in molecular friction, leading to heat generation. This energy dissipation is proportional to the frequency of the AC signal and the dielectric properties of the material.

Circuit Components & Operation in Dielectric Heating

1. Electrodes: Conductive plates that create an electric field across the dielectric material.
2. Dielectric Material: The insulating substance that undergoes heating.
3. AC Voltage Source: Supplies high-frequency alternating current to create the electric field.

The dielectric material is placed between the two electrodes connected to an AC power source. When AC voltage is applied, the electric field alternates at a high frequency. Polarization and depolarization of the dielectric occur as the field oscillates, generating heat within the material.

Circuit Diagram & Phasor Diagram of Dielectric Heating

Figure 2: Circuit diagram.

Figure 3: Phasor diagram.

The phasor diagram (see figure 3) illustrates the relationship between voltage, current, and their components in dielectric heating:

1. Voltage (V): The applied AC voltage is taken as the reference phasor.
2. Current (I): The total current ($I$) lags the voltage by an angle $\varphi$, which depends on the dielectric properties.
3. Resistive Component ( \( I_R \)​): In phase with the voltage and responsible for heat generation.
4. Capacitive Component ( \( I_C \)): Leads the voltage by 90°, representing the reactive power in the circuit.
5. Loss Angle ($\delta$): The small angle between the current ( \( I )\) and its capacitive component ( \( I_C )\).

Phasor Relations:

The following phasor relations are used in dielectric heating:

1. Total Current (\(I\)) 

The total current \(I\) is the vector sum of the resistive current \(I_R\) and the capacitive current \(I_C\):
\[
I = \sqrt{I_R^2 + I_C^2}
\]

2. Resistive Current (\(I_R\))

The resistive current \(I_R\) is in phase with the applied voltage \(V\) and is given by:
\[
I_R = \frac{V}{R}
\]
where \(R\) is the equivalent resistance of the dielectric.

3. Capacitive Current (\(I_C\))

The capacitive current \(I_C\) leads the applied voltage \(V\) by 90° and is expressed as:
\[
I_C = \omega C V
\]
where:
\(\omega = 2\pi f\): Angular frequency of the applied voltage.
\(C\): Capacitance of the dielectric.
\(V\): RMS voltage across the dielectric.

4. Phase Angle (\(\phi\)) Between \(I\) and \(V\)

The phase angle \(\phi\) is the angle between the total current \(I\) and the applied voltage \(V\). It is related to \(I_R\) and \(I_C\) as:
\[
\cos\phi = \frac{I_R}{I}
\]
\[
\tan\phi = \frac{I_C}{I_R}
\]

5. Dielectric Loss Angle (\(\delta\))

The dielectric loss angle \(\delta\) represents the phase difference between the resistive and capacitive currents. It is related to the loss tangent (\(\tan\delta\)) as:
\[
\tan\delta = \frac{I_R}{I_C}
\]

6. Final Relation Between \(\phi\) and \(\delta\)

The phase angle \(\phi\) and the loss angle \(\delta\) are related as:
\[
\phi = 90^\circ – \delta
\]

Derivation of Power Dissipation in Dielectric Heating

The power dissipation (\(P\)) in a dielectric material is derived as follows:

Step 1: Capacitance of the Dielectric Material
The capacitance (\(C\)) of the dielectric material is given by:
\[
C = \frac{\varepsilon \cdot A}{d}
\]
where:

\(\varepsilon\): Permittivity of the dielectric material
\(A\): Area of the electrodes
\(d\): Distance between the electrodes

Step 2: Power Dissipation Formula
The power dissipated in the dielectric material due to the applied voltage \(V\) and the dielectric loss factor is:
\[
P = V^2 \cdot \omega \cdot C \cdot \tan\delta
\]
where:
\(V\): RMS voltage applied across the dielectric
\(\omega\): Angular frequency of the AC signal (\(\omega = 2\pi f\))
\(C\): Capacitance of the dielectric
\(\tan\delta\): Loss tangent, representing the ratio of resistive power loss to reactive power

Step 3: Substitution of Capacitance
Substitute \(C = \frac{\varepsilon \cdot A}{d}\) into the power formula:
\[
P = V^2 \cdot \omega \cdot \frac{\varepsilon \cdot A}{d} \cdot \tan\delta
\]

Final Expression
The final expression for the power dissipation in dielectric heating is:
\[
P = V^2 \cdot \varepsilon \cdot \frac{A}{d} \cdot \omega \cdot \tan\delta
\]

Conclusion
The power dissipation (\(P\)) depends on:
The applied voltage (\(V\))
The frequency of the AC signal (\(f\)) The material properties (\(\varepsilon\) and \(\tan\delta\))
The geometry of the electrodes (\(A\) and \(d\))

Advantages of Dielectric Heating

1. Uniform Heating: Ideal for materials with poor thermal conductivity.
2. Non-Contact Process: No direct heating elements, reducing contamination risk.
3. Selective Heating: Only dielectric materials are heated, leaving other components unaffected.
4. Efficiency: High-frequency heating reduces processing time.
5. Versatility: Applicable to a wide range of insulating materials.

Disadvantages of Dielectric Heating

1. High Initial Cost: Equipment for dielectric heating can be expensive.
2. Frequency Sensitivity: The process is effective only at specific frequencies.
3. Material Limitation: Only suitable for materials with high dielectric loss factors.
4. Energy Loss: High-frequency fields may result in energy dissipation in unintended areas.

Applications of Dielectric Heating

1. Food Processing: Pasteurization and drying of food products. Baking and roasting without direct contact with heating elements.
2. Textile Industry: Drying of fabrics and yarns. Removing moisture from synthetic materials.
3. Plastic and Rubber Curing: Vulcanization of rubber. Preheating thermosetting plastics before molding.
4. Wood Processing: Drying and bonding of wood in furniture manufacturing.
5. Medical Applications: Sterilization of medical equipment.

Difference Between Induction Heating and Dielectric Heating

Conclusion

Dielectric heating is a highly efficient and specialized heating technique for insulating materials. Its ability to provide uniform heating with minimal contamination makes it indispensable in industries like food processing, textiles, and material curing. By understanding the principles, circuit operations, and phasor relationships, engineers can optimize dielectric heating systems for various applications.

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Figure 1: Block diagram of a function generator.

What is Function Generator?

A function generator is a signal source that produces various waveforms, such as sine, square, triangular, sawtooth, and pulse signals. The frequency, amplitude, and waveform type can often be controlled, making it versatile for different applications. These waveforms can be adjusted to specific requirements, depending on the use case.

Block Diagram of a Function Generator

The block diagram (like the one provided in your image) typically consists of the following components:

1. Frequency Control Network: This stage determines the frequency of the output waveform, which can be controlled manually or externally. External frequency control enables precise adjustment for synchronization.
2. Upper and Lower Constant Current Sources: These sources are responsible for generating a current that charges or discharges a capacitor to produce ramp-like signals for triangular and sawtooth waveforms.
3. Integrator: Converts the current into a voltage signal. By controlling the charging and discharging rates, different waveform slopes can be achieved.
4. Voltage Comparator and Multivibrator: This block compares the voltage from the integrator with a reference voltage, converting it into square waveforms or pulse signals.
5. Resistance-Diode Shaping Circuit: Used to shape the triangular or square waves into sine waves through nonlinear distortion.
6. Output Amplifiers: Amplify the generated waveforms and provide multiple outputs for driving different devices.

Types of Function Generators

Function generators can be classified into various types based on technology and usage:

1. Analog Function Generators: These use analog circuitry to generate signals and include components like operational amplifiers, resistors, capacitors, and diodes. Analog generators are cost-effective and reliable for basic applications.
2. Digital Function Generators: Digital function generators use digital signal processing (DSP) techniques to create waveforms. They offer greater precision, programmability, and flexibility in generating arbitrary waveforms.
3. Arbitrary Waveform Generators (AWGs): AWGs are advanced versions of digital function generators. They allow users to generate custom waveforms stored as data points in memory, making them highly versatile for specialized applications.
4. RF Function Generators: These are designed to operate at high frequencies, typically in the radio frequency (RF) range, for applications like communication testing.

Advantages of Function Generators

1. Versatility: Capable of producing multiple waveform types and frequencies.
2. Ease of Use: User-friendly interfaces with precise control over frequency, amplitude, and phase.
3. Cost-Effective: Basic analog models are affordable for educational and general-purpose use.
4. Integration: Many modern function generators include features like frequency counters and arbitrary waveform generation.
5. Wide Range of Applications: Suitable for R&D, production testing, educational labs, and more.

Disadvantages of Function Generators

1. Accuracy Limitations: Analog function generators have limited accuracy compared to digital models.
2. Complexity in Arbitrary Waveform Generation: Designing custom waveforms on arbitrary waveform generators may require specialized knowledge.
3. Frequency Range Limitations: Certain function generators may not cover very high or very low frequencies.
4. Distortion: At extreme ranges, waveforms may become distorted, impacting accuracy.

Applications of Function Generators

Function generators are essential tools in many fields. Some key applications include:

1. Testing and Debugging: Used to test circuits by simulating input signals. Debugging faults in communication, audio, and control systems.
2. Signal Simulation: Mimics real-world signals in systems like sensors and transducers.
3. Education and Training: Found in physics and electronics labs to demonstrate waveform characteristics.
4. Calibration: Serves as a reference signal source for calibration of other equipment.
5. Research and Development: Generates complex waveforms for experimental analysis and prototyping.
6. Communications: Produces modulated signals for testing communication systems.

Working of a Function Generator

1. Signal Generation: The frequency control network sets the base frequency. Current sources generate charging and discharging cycles for capacitors, which are processed to produce triangular, sine, and square waves.
2. Shaping Circuits: The resistance-diode shaping circuit converts triangular signals to approximate sine waves by manipulating voltage curves.
3. Output Delivery: Amplified waveforms are sent to the output terminals for use in external circuits.

Difference between Analog and Digital Function Generators

Conclusion

Function generators play a critical role in modern electronics, providing the flexibility to create, modify, and analyze signals in various domains. Their versatility, coupled with continuous advancements in technology, ensures their relevance in scientific research, industrial applications, and education. Understanding their design and operation allows engineers and students to harness their full potential effectively.

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Working Principle of a Toggle Switch

The working of a toggle switch is simple and involves mechanical movement:

1. The lever or handle of the switch is moved manually to change its position.
2. Depending on the switch type (e.g., SPST, SPDT), the movement of the lever either connects or disconnects the electrical circuit.
3. Internally, the movement aligns contacts to allow or interrupt the flow of current, completing or breaking the circuit.

Types of Toggle Switches

Advantages of Toggle Switches

1. Ease of Use: Simple design allows for easy operation.
2. Durability: Built to withstand frequent switching.
3. Versatility: Available in multiple configurations for varied applications.
4. Compact Size: Saves space in electronic devices.
5. Reliability: Provides consistent performance over time.

Disadvantages of Toggle Switches

1. Mechanical Wear: Frequent use may cause wear and tear over time.
2. Limited Current Capacity: Not suitable for high-current applications without proper design.
3. Manual Operation: Requires physical intervention for switching.
4. Design Constraints: Limited customization in aesthetic applications.

Applications of Toggle Switches

1. Home Appliances: Used in lighting systems, fans, and other household devices.
2. Automotive Industry: Controls lights, horns, and other functions in vehicles.
3. Industrial Equipment: Manages power and operational modes in machinery.
4. Aerospace: Found in control panels for aircraft systems.
5. Consumer Electronics: Incorporated in audio systems, computers, and gaming consoles.
6. Marine Applications: Used in boat control panels for navigation and lighting.
7. Testing and Prototyping: Commonly used in laboratories and test setups for circuit control.

Conclusion

Toggle switches are versatile, reliable, and widely used electrical components that provide a straightforward way to control circuits. With their various configurations and types, they cater to a broad range of applications, from household devices to industrial machinery. Proper maintenance and usage ensure their longevity and effectiveness.

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Series Connection of Batteries

Connection diagram :

The series connection of batteries is shown in Fig. 1(a). N number of identical batteries with terminal voltage of V volts and current capacity of I ampere each are connected in series. The load is connected directly across the series combination of N batteries as shown in Fig. 1(a). The load voltage is given by,

\[{{V}_{L}}=(V+V+……+V)\text{    }…..\text{N terms}\]

\[{{V}_{L}}=NV\text{ Volts}\]

However the series connection does not improve the current sourcing capacity. The current sourcing capacity of the series string is same as that of a single battery connected in the string, i.e. I amperes.

Figure 2. Series connection of batteries with different terminal.

It is not always necessary to connect all the batteries of same terminal voltages in series with each other. The batteries of different terminal voltages can be connected in series as shown in Fig. 2.

\[{{V}_{L}}={{V}_{1}}+{{V}_{2}}+{{V}_{3}}+{{V}_{4}}\]

Parallel Connection of Batteries

Connection diagram :

Figure 3.

The parallel connection of batteries is shown in Fig. 3. Batteries are connected in parallel in order to increase the current supplying capacity. If the load current is higher than the current rating of individual batteries, then the parallel connection of batteries is used. The terminal voltage of all the batteries connected in parallel must be the same. The load current is equal to the sum of currents drawn from the individual batteries.

\[{{I}_{L}}={{I}_{1}}+{{I}_{2}}+{{I}_{3}}+{{I}_{4}}\]

If all the batteries are of same current rating then they supply equal amount of current. But, if they are of different current ratings, then they share current in proportion with their current ratings.

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Working Principle of Thermistor

Thermistor have a Negative Temperature Coefficient (NTC) i.e. resistance decreases as the temperature increases. The materials used in the manufacture of thermistors include oxides of cobalt, nickel, copper, iron, uranium and manganese. The thermistor has very high temperature coefficient of resistance of the order of 3 to 5% per ºC. The resistance at any temperature T is given by,

\[{{R}_{T}}={{R}_{0}}\text{ exp }\beta \text{ }\left( \frac{1}{T}-\frac{1}{{{T}_{0}}} \right)\]

Where,

R_T – Thermistor resistance at temperature T (K)

R₀ – Thermistor resistance at temperature T₀ (K)

β – A constant determined by calibration

At high temperature, equation (1) reduces to,

\[{{R}_{T}}={{R}_{0}}\text{ exp }\left( \frac{\beta }{T} \right)\]

Working & Symbol of Thermistor

Figure 1.

The resistance-temperature characteristics is shown in Fig. 1 (b) and symbol in Fig. 1 (a). The curve is non-linear and the drop in resistance from 500Ω to 100Ω occurs for an increase in temperatures from 20 to 100ºC. The temperature of the device can be changed internally or externally. An increase in current through the device will raise its temperature carrying a drop in its terminal resistance. Any externally applied heat source will result in an increase in its body temperature and drop in resistance. This action tends itself well to control mechanisms.

Types of Thermistor

Figure 2: Various configurations of thermistor.

The thermistors are available in various configurations such as beads, disc, rod, washer as shown in Fig. 2. The smallest thermistors are made in the form of beads. Some are as small as 0.15 mm in diameter. And where greater power dissipation is required, thermistors obtained are in disc, washer or rod forms.

Advantages of Thermistor

1. Small size and low cost.
2. Fast response over narrow temperature range.
3. Good sensitivity in the NTC region.

Disadvantages of Thermistor

1. Non-linearity in resistance versus temperature characteristics.
2. Unsuitable for wide temperature range.
3. Very low excitation current to avoid self-heating.
4. Need of shielded power lines, filters etc. due to high resistance.

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Figure 1: Basic of Switch Mode Power supply (SMPS).

A switch mode power supply (SMPS) is a dc-to-dc series regulated power supply, in which the series pass transistor is operated as a switch. The output voltage of a SMPS is regulated by varying its duty cycle.

In SMPS, the series pass transistor (electronic switch in Fig. 1) does not operate in its active region. Instead it operates as a switch. This is how it is different from the conventional or linear power supply.

Block diagram of Switch Mode Power supply (SMPS)

The block diagram of a basic switching regulator is shown in Fig. 1. The block diagram shows that the SMPS is also a series regulator. The basic switch mode power supply consists of four components namely the unregulated dc voltage source V_in, an electronic switch S (a transistor or MOSFET), a pulse generator and a filter, as shown in Fig. 1.

Working of Switch Mode Power supply (SMPS)

The pulse generator generates rectangular pulses which are applied to the control terminal of an electronic switch. This switch is turned on and off with the help of these rectangular pulses. The switch is an electronic switch which is typically a transistor or MOSFET. It is used in its saturation and cut off regions and not in the active region. When the switch is on, it connects the unregulated dc input V_in as it is to the input of the filter and the filter input is disconnected from the dc input voltage V_in when the switch is open circuited.

Filter input voltage =  V_in        …..when switch is on

And, filter input voltage = 0       …..when switch is off

Figure 2: Waveforms of basic Switch Mode Power supply (SMPS).

This is shown in the waveforms of Fig. 2. Therefore at the input of the filter we get a rectangular waveform. The average value of this waveform can be adjusted by changing either the duty cycle or frequency of the rectangular pulses produced by the pulse generator. The duty cycle is defined as,

\[\text{Duty cycle (D) = }\frac{{{t}_{on}}}{{{t}_{on}}+{{t}_{off}}}=\frac{{{t}_{on}}}{T}\]

\[T={{t}_{on}}+{{t}_{off}}=\frac{1}{\text{Frequency}}\]

\[or\text{ T = }\frac{\text{1}}{f}\]

Typically, the operating frequency of the switching regulator will be in the range of 10 to 50 kHz. That means the total time T is of the order of 100 µs to 20 µs. The filter then converts the rectangular waveform at its input into a smooth dc voltage by removing the ripple contents. The expression for dc output voltage of a switching regulator is given by:

\[{{V}_{o}}=\frac{{{t}_{on}}}{T}\times {{V}_{in}}\]

\[or\text{   }{{V}_{o}}=D\times {{V}_{in}}\]

Thus the average output voltage is dependent on the duty cycle D. The average output voltage will increase with increase in the value of duty cycle as shown in Fig. 2.

Advantages of Switch Mode Power supply (SMPS)

The advantages of SMPS are as follows :

1. Low power dissipation in the series pass transistor as it operates as a switch and not in the active region.
2. High efficiency (upto 95%) due to reduced power dissipation in the transistor.
3. Small size : This is due to the smaller size of L and C at high operating frequencies and need of smaller heat sink for the series pass transistor.
4. Higher power handling capacity.

Disadvantages of Switch Mode Power supply (SMPS)

The disadvantages of SMPS are as follows :

1. Increased switching loss in the series pass transistor due to high frequency switching.
2. Radio Frequency Interference (RFD to the neighboring electronic circuits.
3. There is no isolation between input and output.
4. The load requires separate protection circuitry.
5. The transient response is slow as compared to the linear power supplies.
6. Ripple content in the output is higher than that for a linear power supply.
7. Load regulation is poor as compared to the linear regulators.

Difference between Linear regulator and Switch Mode Power Supply (SMPS) 

Types of Switch Mode Power supply (SMPS)

Figure 3: Classification of Switch Mode Power supply (SMPS).

The classification of SMPS is shown in Fig. 3. The SMPS are classified broadly into two categories namely :

1. Non-isolated type and
2. Isolated type.

No electrical isolation is provided between the load and source in the non-isolated type SMPS. Whereas, a transformer is included for providing the electrical isolation in case of the isolated type SMPS.

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When the anode terminal of a diode is connected to the negative terminal of voltage source and cathode to the positive terminal then the diode is reverse biased as shown in Fig. 1 (a) and its symbolic representation in Fig. 1 (b).

Operation of Reverse Biased PN Junction Diode

Figure 2: Reverse biased effect on layer and barrier voltage.

When an external bias is applied to a diode, positive to cathode and negative to anode, electrons from N-side are attracted to the positive terminal and holes from P-side are attracted to the negative terminal. As shown in Fig. 2, holes on P-side of junction are moved away from the junction and electrons are also moved away from the junction on N-side. This results in the depletion region to be widened and the barrier voltage to be increased.

As the barrier voltage at the junction increases, the holes and electrons are not able to cross the junction, hence the majority charge carrier current will be zero and the junction is said to be reverse biased. There will be very small reverse current due to minority charge carrier. Because of this very small reverse current flow, a reverse biased PN junction diode offers very high resistance. As there is no possibility of majority charge carriers the current flowing across a reverse biased junction, minority carriers i.e. holes on N-side and electrons on P-side, generated on each side can cross the reverse junction. The electrons on the P-side are attracted to the positive voltage crossing the junction on the N-side. The holes from N-side flow across to the negative voltage on P-side. Due to this, small current flows through the junction. Only a very small reverse bias voltage is necessary to move all available minority carriers across the junction and if voltage bias is increased further, current will not increase. And hence this current is referred as reverse saturation current. The reverse saturation current is very small ranging from nanoamperes to microamperes. It depends on the junction area, temperature and semiconductor material.

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