8:33 PM

Classification of Relays

Types of Relays

There are different types of relays including electromagnetic relays, latching relays, electronic relays, non-latching relays, multi-dimensional relays and thermal relays which are classified based on the function, application type, configuration or structural features, etc. Now we look at various types of relays which are more popularly used in many applications.

Latching Relays

A latching relay is a relay which maintains its state after being actuated, that’s how this type of relays are also called as impulse relays or keep relays or stay relays. In some applications, it is needed to limit power consumption and dissipation, for that kind of applications a latching relay is best suitable. A latching relay consists of internal magnets such that when the current is supplied to the coil, it (internal magnet) holds the contact position and hence it requires no power to maintain its position. So even after being actuated, removal of drive current to the coil cannot move the contact position but remains in its last position. Thus, considerable energy is saved by these relays.

Latching relays can be made with one or two coils and these coils are responsible for the position of the armature of the relay, hence the latching relays don’t have any default position as shown in above figure. In one coil type relay, the armature position is determined by the direction of current flow in the coil whereas in case of two coil type, position of the armature is depending on the coil in which current flows. These relays can maintain their position once they are actuated but their reset position depends on the control circuitry.

Reed Relay

Similar to the electromechanical relays, reed relays also produce the mechanical actuation of physical contacts to open or close a circuit path. However, compared with electromagnetic relays these relay contacts are much smaller and have low mass. These relays are designed by coils wounded around the reed switch. The reed switch of the relay acts as armature and it is a glass tube or capsule filled with an inert gas within which two overlapping reeds (or ferromagnetic blades) are hermetically sealed.

The overlapping ends of a reed consist of contacts so that input and output terminals can be connected to them. When the power is supplied to the coils, a magnetic field is produced. These fields cause reeds to drawn together, thereby their contacts makes a closed path through the relay. Also during de-energizing process of the coil, reeds are separated apart by the pulling force of spring attached to it.

The switching speed of the reed relay is 10 times faster than an electromechanical relay due to the less massive, different actuating medium and smaller contacts. However, these are suffers from electrical arcing due to smaller contacts. In the event of switching arc jumps across the contacts which results to melting of the contact surface over a small section. Further, this leads to the welding of the contacts if both contacts are still closed.

Thus after the de-magnetization of the coil spring force may not sufficient to separate them. It is an undesirable condition of the relay. This problem is overcome by placing series impedance like a resistor or ferrite between the relay and system capacitance so that inrush currents are reduced thereby arcing is avoided in the relay. Many switching applications use the reed relay due to the small size and high speed.

Polarized Relay

As the name indicates, these relays are very sensitive to the direction of current by which it is energized. It is a type of DC electromagnetic relay provided with an additional source of permanent magnetic field to move the armature of the relay. In these relays magnetic circuit is built with permanent magnets, electromagnets and an armature.

Instead of spring force, these relays use magnetic forces to attract or repel the armature. In this, the armature is a permanent magnet, pivoted between the pole faces formed by an electromagnet. When the current flows through the electromagnet, it produces a magnetic flux. Whenever the force exerted by the electromagnet exceeds the force exerted by permanent magnet, the armature changes its position. Similarly when the current is interrupted, the electromagnetic force is reduced to less than that of permanent magnet and hence armature returns to its original position.

The magnetic flux Φm produced by the permanent magnet passes through the armature branches into two parts namely Φ1 and Φ2. The flux Φ1 passes through the left working gap of the magnet while Φ2 passes through the right working gap of the magnet. If there is no current in the coil, due to these two fluxes armature will stay either at left or right of the neutral position since in such magnetic system neutral is not stable.

Whenever the current supplied to the coils of the relay, an additional working magnetic flux Φ passes through the working gap of the magnet. Due to these magnetic field interactions, a force effect on the armature which is depends on the magnitude of the current, initial position of the armature, polarity of the current, power of the magnet and the value of the working gap. Depends on these parameters combination, the armature of the relay turns to a new stable state thereby closes the right contact and hence the relay picks up.

There are different types of polarized relays depends on the magnetic circuit configuration. The two most popular types of these relays include differential and bridge type relays. In differential magnetic system, the difference of two fluxes of permanent magnet acts on the armature. In bridge type magnetic system, the field created by the coils divided into two fluxes which have opposite signs in the working gap area but the magnetic flux of the permanent magnet is not divided into two fluxes. For normal size relays differential type of magnetic system is widely used.

Buchholz Relays

These relays are gas operated or actuated relays. These relays are used to detect incipient faults (or internal faults which are minor faults initially but on the time course these are turned into major faults). These are most widely used for transformer protection and are accommodated in the chamber in between the transformer tank and conservator. These are used only for oil immersed relays that are majorly employed for power transmission and distribution systems.

The figure below shows the operating principle of Buchholz relay. When incipient faults (or slow developing faults) occur inside the transformer, the oil level falls because of gas accumulation. This causes the hollow float to tilt and hence the mercury contacts are closed. These mercury contacts complete the path of the alarm circuit so that operator knows that there is some incipient fault occurred in the transformer.

Whenever a severe fault takes place in the transformer such as short circuit of phases or earth fault, etc., the pressure inside the tank abruptly increased due to the fast reduce level of the oil. Thus the oil rushes towards the conductor and due to this, lower side flap valve gets deflected. So it closes mercury switch contacts thereby trip circuit is enabled. Thus the transformer is disconnected from the supply source.

Overload Protection Relays

Overload protection relays are specially designed to provide the overcurrent protection of electrical motors and circuits. These overload relays can be different types such as fixed bimetallic strip type, electronic or interchangeable heater bimetallic, etc. If the electric motors are overloaded, then the motors are needed to be protected from overcurrent. For this purpose, overload sensing equipment such as heat operated relay is used. Heat operated relay consists of a coil that heats up bimetallic strip or solder pot melts and thus, releases the spring for operating auxiliary contacts which are in series with the coil. The coil gets de-energized by sensing excess current in the load due to overload. The temperature of the motor winding can be estimated using the motor armature thermal model, electronic overload protection relay by measuring motor current. Thus, motor can be accurately protected using overload protection relay.

Solid State Relays (SSRs)

Solid state relays use solid state components such as BJTs, thyristors, IGBTs, MOSFETs and TRIACs to perform the switching operation. The power gain of these relays is much higher than the electromechanical relays because the control energy required (to power the control circuit) is much lower compared with power to be controlled (switching output) by these relays. These relays can be designed to work for both AC and DC supply.

Due to the absence of mechanical contacts, these relays have high switching speeds. A SSR consist of a sensor which is also an electronic device and this sensor responds to a control signal in order to switch on or off the power to the load.

SSRs are classified into different types, however major types of these relays include photo-coupled SSRs and transformer coupled SSRs. In transformer coupled SSR, a small DC current is supplied to the primary of the transformer through a DC to AC converter. This current is then converted to AC and stepped-up to operate the solid state device (TRIAC in this case) as well as triggering circuit. The degree of the isolation between the input and output depends on the design of the transformer.

In case of photo coupled SSRs, photosensitive semiconductor device is used for performing the switching operation. The control signal is applied to the LED so that photosensitive device turns into conduction mode by detecting the light emitted from LED. The isolation provided by this type of SSR is relatively high compared with transformer coupled SSR due to the photo detection principle.

Solid state relays have faster switching speeds as compared with electromechanical relays. Also due to no moving parts, its life expectancy is higher and they tend to produce very less noise

Inverse Definite Minimum Time Relays (IDMT Relays)

This type of relay gives a definite-time current characteristic at higher values of the fault current and an inverse time current characteristic at lower values of the fault current. These are widely used for protection of distribution lines and they offer to set the limits for current and time settings. In this type of relay, the operating time of the relay is approximately inversely proportional to the fault current near the pickup value and becomes constant slightly above the pickup value of the relay. This can be achieved by using the core of the magnet which gets saturated for the current slightly greater than the pickup current.

The pickup value the point where the actuating quantity or fault current initiates the relay to operate is called the pickup value. The relay is called IDMT because of its characteristic that when the actuating quantity reaches its infinity value the time doesn’t approach zero. At lower values of the fault current it gives inverse time characteristics while at higher values it gives definite time characteristics as shown in figure. The operating time becomes constant from a particular value till the actuating quantity becomes infinity which is shown in the graph (a curve is obtained, which becomes constant).

Differential Relays

Differential relays operate when the phasor difference of two or more similar electrical quantities exceeds a predetermined value. A current differential relay operates when there exists a result of comparison between the magnitude and phase difference of the currents entering in and leaving out of the system to be protected.

Under normal operating condition, the currents entering and leaving are equal in magnitude and phase so the relay is inoperative. But if a fault takes place in the system, these currents are no longer equal in magnitude an phase. This type of relay is connected in such that the difference between the current entering and current leaving flows through the operating coil of the relay. Hence the relay coil is energized under fault condition due to the difference quantity of the current. Thus the relay operates and opens the circuit breaker so as to trip the circuit.

The above figure shows the principle of differential relays in which there are two CTs connected either side of the power transformer i.e., one CT on the primary side and the other at the secondary side of the power transformer. The relay compares the currents on both sides and if there is any unbalance then relay tends to operate. The differential relays can be current differential relays, voltage balance differential relays and biased differential relays.

6:29 PM

Types of Relays and Relay Driver Circuit

Types of Relays and Relay Driver Circuit:

We have already studied about what is Relay and Relay Operation in the earlier post. Now let us see different types of relays and how to drive the relay. This article describes you the different types of relays like latching relay, reed relay, polarized relay and so on.

Types of Relays:

Here is the list of various types of relays, their working and applications.

1. Latching Relay:

These relays are also called as impulse relay, stay relays or keep relays as they continue the process what they are doing in the last state when the power is switched off. This mechanism can be achieved with the solenoid which operates in ratchet and cam method. In this method power is consumed only for a particular time. Hence these relays are more preferable than other relays. These relays operate in bistable mode. Hence they have two stable states.

2. Reed Relay:

This type of relays is more preferable in contacts. These relays have low switching voltage and current ratings. These are famous because of their switching speeds. You need to keep these relays inside a vacuum or inert gas to protect from atmosphere.

3. Polarized Relay:

Polarized relays work only when the current flows through the coil in one direction. This relay has a diode in series with coil. This diode blocks the current in other direction. The importance is given to this relays based on its sensitivity. They played major roles in telephone exchanges. This relays are also used in telegraphic distortion.

4. Biased Relays:

This relays have a permanent magnet above the armature. This relay is operates when the current through the coil establishes a force which opposes the magnet. If the established force is in the same direction with magnet then relay will not work, even if you apply a large current through the coil.

5. Buchholz Relay:

Generally these relays are used for safety purpose. These relays are used to know the amount of gas present in the large oil filled transformers. These relays produce warning if it senses fast production or slow production of gas in transformer oil.

6. Overload Protection Relay:

As the name indicates, these relays are used to prevent the electrical loads from damages by short circuits and over currents and abnormal voltages. In this relays a heating element is placed in series with the motors. Thus when over heat occurs the heating element which is connected to the motor heats up and in turn releases the spring to operate the contacts of the relay.

7. Solid State Relay (SSR):

As the name implies, these are designed with solid state components. These are high reliable as they do not have any moving objects in their design. The basic element in solid state relay is output switch or a triac but more often back to back SCRs. As there is no moving parts these are used in the applications where sparking is major issue.

8. Solid State Contactor Relay:

These relays provide the features of both SSR relays and contactor relays. These relays designed for the correct on – off cycles and they have good heat sink. As a result these relays have number of advantages.

9. Inverse Definite Minimum Time Relay (IDMT Relay):

This relay works based on induction principle. This relay has copper or aluminium disc which rotates between damping magnet and electro magnet. Fluxes induce the currents in the disc which produce rotational torque. This disc rotates to a point where it operates the contacts that break the circuit and remove the fault connection.

10. Differential Relay:

Differential relays are used to check the Dissimilarity between input and currents. The distinction between currents may be in magnitude or in phase or in both. For good operation magnitude and phase angle variations should be zero. If there is any unbalance between input and output currents then relay will actuate.

Relay Driver Circuit:

Relay Driver Circuit Diagram

Here is the simple NPN Transistor circuit to drive the relay. We can use less power to drive the relay. We have to make sure that base gets enough current to turn on the transistor. If the transistor gets enough base current then the transistor conducts and relay energizes. When there is no base voltage, the transistor is in open thus blocking the current through coil of the relay.

Here the diode 1n4001 or 1n4007 is connected across the relay, used to protect the transistor from damage due to the back emf generated in the relay coils when transistor is in off. Base resistor is used to set current to the base of the transistor.

Now let us see how to choose the value of R1. Say relay needs 50mA current to the coil to pull the armature and assume coil has resistance of 240 ohm. To turn on the transistor the base current should be greater than the collector current divided by current gain hfe. As the BC337 or BC338 has a minimum current gain of 100 at 100mA. So we need to provide the base current of at least 50mA/100 = 0.5mA.

In practice it is better to double the value, say 1mA of base current. If the input voltage Vin is switching between 0V and 12V. Ib = 12/1mA =12K. So it is better to use 10k resistor.

6:20 PM

What is Relay and How it Works?

What is a Relay?

A relay can be defined as a switch. Switches are generally used to close or open the circuit manually .Relay is also a switch that connects or disconnects two circuits. But instead of manual operation a relay is applied with electrical signal, which in turn connects or disconnects another circuit.

Relays can be of different types like electromechanical, solid state. Electromechanical relays are frequently used. Let us see the internal parts of this relay before knowing about it working. Although many different types of relay were present, their working is same.

Every electromechanical relay consists of an consists of an

Electromagnet
Mechanically movable contact
Switching points and
Spring

Electromagnet is constructed by wounding a copper coil on a metal core. The two ends of the coil are connected to two pins of the relay as shown. These two are used as DC supply pins.

Generally two more contacts will be present, called as switching points to connect high ampere load. Another contact called common contact is present in order to connect the switching points.

These contacts are named as normally open (NO),normally closed(NC) and common(COM) contacts.

Relay can be operated using either AC or DC.

In case of AC relays, for every current zero position, the relay coil gets demagnetized and hence there would be a chance of continues breaking of the circuit.

So, AC relays are constructed with special mechanism such that continues magnetism is provided in order to avoid above problem. Such mechanisms include electronic circuit arrangement or shaded coil mechanism.

Working:

Relay works on the principle of electromagnetic induction.
When the electromagnet is applied with some current it induces a magnetic field around it.
Above image shows working of the relay .A switch is used to apply DC current to the load.
In the relay Copper coil and the iron core acts as electromagnet.
When the coil is applied with DC current it starts attracting the contact as shown. This is called energizing of relay.
When the supply is removed it retrieves back to the original position. This is called De energizing of relay.

There are also such relays, whose contacts are initially closed and opened when there is supply i.e. exactly to opposite to the above shown relay.

Solid state relays will have sensing element to sense the input voltage and switches the output using opto-coupling.

Relay Contact Types:

As we have seen that relay is a switch. The terminology “Poles and throws” is also applicable for relay. Depending on the number of contacts and number of circuits it switches relays can be classified.

Before we know about this classification of contacts we have to know the poles and throws of a relay switch.

Poles and Throws:

Relays can switch one or more circuits. Each switch in relay is referred as pole. Number of circuits a relay connects is indicated by throws.

Depending on the poles and throws, relays are classified into

Single pole single throw
Single pole double throw
Double pole single throw
Double pole double throw

Single Pole Single Throw:

A single pole single throw relay can control one circuit and can be connected to one output. It is used for the applications which require only ON or OFF state.

Single Pole Double Throw:

A single pole double throw relay connects one input circuit to one of the two outputs. This relay is also called as changeover relay.

Though the SPDT has two output positions, it may consist of more than two throws depends on the configuration and requirement of the application.

Double pole single throw:

A double pole single throw relay has two poles and single throw and it can be used to connect two terminals of a single circuit at a time. For example, this relay is used for connecting both phase and neutral terminals to the load at a time.

Double pole double throw:

A DPDT (double pole double throw) relay has two poles and two throws for each pole. In motor direction control, these are used for phase or polarity reversal.

The switching action between contacts for all these relays is performed when the coil get energized as shown in figure below.

Relays can be classified into different types depending on their functionality, structure, application etc. Learn about different types of relays .

Relay Applications:

Relays are used to protect the electrical system and to minimize the damage to the equipment connected in the system due to over currents/voltages. The relay is used for the purpose of protection of the equipment connected with it.

These are used to control the high voltage circuit with low voltage signal in applications audio amplifiers and some types of modems.

These are used to control a high current circuit by a low current signal in the applications like starter solenoid in automobile. These can detect and isolate the faults that occurred in power transmission and distribution system. Typical application areas of the relays include

Lighting control systems
Telecommunication
Industrial process controllers
Traffic control
Motor drives control
Protection systems of electrical power system
Computer interfaces
Automotive
Home appliances

8:32 AM

Electrical Symbols & Electronic Symbols

Electrical and Electronic Schematic Symbols:

Electrical symbols or electronic circuits are virtually represented by circuit diagrams. There are some standard symbols to represent the components in a circuits. This article gives some of the frequently used symbols for drawing the circuits. There are many electrical and electronic schematic symbols are used to signify basic electronic or electrical device. These are mostly we used for draw circuit diagrams.

Below are different kind of symbols we mentioned category wise. Hope this information will helps to understand clearly.

Electrical Symbols & Electronic Symbols

	SYMBOL	COMPONENT	MEANING
		WIRES
		Wires	Represents a conductor that conducts electrical current. Also called a power line or electric line or wire.
		Connected Wires	Represents the connection of two conductors. Dot shows the junction point.
		Unconnected Wires	Represents two unconnected wires/conductors. Any of the two methods can be followed. Drawing two lines perpendicularly is an older method of representing unconnected wires.
		Input Bus Line	Represents a bus for input or incoming data.
		Output Bus Line	Represents a bus for output or outgoing data.
		Terminal	Represents start or end point.
		Bus Line	Represents a number of conductors joined together to form a bus wire.
		SWITCHES
		Push Button (Normally Open)	This switch is in ON state when the button is pressed otherwise it is in OFF state.
		Push Button (Normally Closed)	This switch is initially in ON state. This goes to OFF state when it is released.
		Spst Switch	Single pole single throw is abbreviated as SPST. This allows the flow of current when the switch is closed. This acts as ON/OFF switch. Poles define the number of circuits it can be connected to and throws defines the number of positions that a pole connects.
		Spdt Switch	Single pole double throw is abbreviated as SPDT. This switch allows the current to flow in any one of the two directions by adjusting its position.
		Dpst Switch	Double pole single throw is abbreviated as DPST. This switch can drive two circuits at a time.
		Dpdt Switch	Double pole double throw is the full form of DPDT. This can be defined as two independent SPDT switches. This can connect the four circuits by changing the position.
		3 Position Rotary or Multi Switch	This switch is operated by rotation. This has 3 positions. Often used in fan control, citizen band radio and tuning.
		5 Position Rotary or Multi Switch	This switch is operated by rotation. This has 5 positions.
		Relay Switch	This represents the relay switch. This can control the AC Loads using the DC voltage applied to the coil.
		SOURCES
		AC Supply	This represents AC supply in the circuit.
		DC Supply	This represents the DC power supply. It applies DC supply to the circuit.
		Constant Current Source	Current source is used to deliver or absorb electric current. The symbol represents an independent current source which delivers constant current.
		Controlled current Source	It is a dependent current source. Usually depends on other sources (voltage or current).
		Controlled Voltage Source	It is a dependent voltage source. Usually depends on other sources (voltage or current).
		Single Cell Battery	This provides supply to the circuit.
		Multi Cell Battery	Combination of multiple single cell batteries or a single large cell battery. The voltage is usually higher.
		Wave Generators
		Sinusoidal Generator	Represents sine wave generator.
		Pulse Generator	Represents pulse or square wave generator.
		Triangular Wave	Represents triangular wave generator.
		GROUND SYMBOLS
		Ground	It is equivalent to theoretical 0V and is used as zero potential reference. It is the potential of perfectly conducting earth.
		Signal Ground	It is a reference point from which the signal is measured. There may be several signal grounds in a circuit due to the voltage drops in a circuit.
		Chassis Ground	Connected to the chassis of the circuit. It is the frame or box in which the circuit is built. It acts as a barrier between user and the circuit and prevents electric shock.
		RESISTOR SYMBOLS
		Fixed Resistor	It is a device that opposes the flow of current in a circuit. These two symbols are used to represent fixed resistor.
		VARIABLE RESISTOR
		Potentiometer	It is a three terminal variable resistor which acts as an adjustable voltage divider. It is used in power supplies that produce varying DC voltages like volume controls or joysticks.
		Rheostat	It is a two terminal variable resistor. They are generally used to control the current in the circuit. Generally used in tuning circuits and power control applications like heaters, ovens etc
		Preset	It is a mini variable resistor. It is also called Trimmer Resistor or Trim Pot. The resistance is adjusted with rotary control present on top of it with the help of a screw driver. They are used to adjust the sensitivity of the circuit like temperature or light.
		Thermistor	It is a temperature sensitive resistor. They are used in temperature sensing, current limiting circuits, over-current protection circuits etc.
		Varistor	It is a Voltage Dependent Resistor. It has non-linear current-voltage characteristics. Generally used in circuit protection from voltage surges and excessive transient voltages.
		Magneto Resistor	They are also called as Magnetic Dependent Resistors (MDR). The resistance of magneto resistor varies according to the external magnetic field strength. They are used in electronic compass, ferrous material detection, position sensors etc.
		LDR	They are also called as Photo Resistors. The resistance of LDR varies with the intensity of the light incident on it. They are generally used in light sensing applications.
		Tapped resistor	A wire-wound type fixed resistor with one or more terminals along its length. Generally used in voltage divider applications.
		Attenuator	It is a device used to lower the power of a signal. They are made from simple voltage dividers and hence can be classified in the family of the resistors.
		Memristor	Memristor is a device derived from the word memory resistor. The resistance of memristor is varied according to the direction of flow of charge. Memristors can be used in signal processing, logic/computation, non-volatile memory etc.
		CAPACITOR SYMBOLS
		Non Polarized Capacitor	Capacitor stores the charge in the form of electrical energy. These two symbols are used for non-polarized capacitor. Non-polarized capacitors are big in size with small capacitance. They can be used in both AC and DC circuits.
		Polarized Capacitor	Polarized capacitors are small in size but have high capacitance. They are used in DC circuits. They can be used as filters, for bypassing or passing low frequency signals.
		Electrolytic Capacitor	Almost all electrolytic capacitors are polarized and hence used in DC circuits
		Feed through Capacitor	They provide low impedance path to ground for high frequency signals
		Variable Capacitor	The capacitance of the variable capacitor can be adjusted by turning the knob. They are widely used to adjust the frequency , that is for tuning.
		INDUCTORS
		Inductor	Inductor stores the charge in the form of magnetic field. This contains a conductor wound in the form of a coil. The current flowing in the coil induces voltage by phenomena of faradays law of electromagnetic induction. Here the coil is wound on non-magnetic forms like plastic or ceramic.
		Iron Core Inductor	These are used as substitutes to ferrite core inductors. Ferrite core or Ferromagnetic inductors have high permeability and require air gap to reduce it. Iron powdered core inductors have this air gap integrated.
		Ferrite Core Inductors	Core material, in this type of inductors is made of ferrite material. These are mostly used to suppress the interference of electromagnetic waves.
		Center Tapped Inductors	These are used in coupling of signals,
		Variable Inductors	Movable ferrite magnetic core variable inductors are most common. The inductance is varied by sliding the core in or out of the coil.
		DIODES
		Pn Junction Diode	A PN junction diode allows the current to flow only in forward bias condition. These diodes can be used in clipping and clamping circuits , as rectifiers in dc circuits etc.
		Zener Diode	In forward bias condition, it acts as normal diode and allows current. It also allows current to flow in reverse bias condition when the voltage reaches a certain break-down point. Generally used in voltage regulator and over voltage protection circuits.
		Photodiode	Photodiode detects the light energy and converts it into current or voltage by a mechanism called photoelectric effect. These are used in CD players , Cameras etc.
		Led	Light emitting diode is similar to PN junction diode but they emit energy in the form of light instead of heat. These are mostly used in indication , lightening applications.
		Varactor Diode	Varactor diode is called varicap or variable capacitance diode. The capacitance of this diode varies according to the applied input voltage. This is used in frequency controlled oscillators , frequency multipliers etc.
		Schottky Diode	The symbol represents Schottky diode. It has low forward voltage drop and it can switch rapidly. Used in voltage clamping, rectifiers, reverse current and discharge protection
		Shockley Diode	This is a four layer diode. This had fast switching operation and hence is used in switching applications.
		Tunnel Diode	This is also known as Esaki diode.It can switch very fastly and can perform well in micro wave frequency range. This is used in oscillator circuits and micro wave circuits.
		Thyristor	It consists of four layers of alternating P and N materials. They act as bistable switches and are used in circuits where high voltages and currents are involved.
		Constant Current Diode	Also called as Current Limiting Diode or Current Regulating Diode. It limits the current to a specified maximum value.
		Laser Diode	The laser diode is similar to light emitting diode. The active region is formed in intrinsic region in PIN structure. Laser diodes find its applications in laser printing, laser scanning etc.
		TRANSISTOR SYMBOLS
		NPN	It is made of combination of P-type semiconductor between two N-type semiconductors. It is switched ON when the base-emitter junction is forward biased. They are commonly used for amplifying and switching applications.
		PNP	It is made of combination of N-type semiconductor between two P-type semiconductors. It is switched ON when the base-emitter junction is reverse biased. These are used for amplifying and switching applications.
		JFET	Junction field effect transistor is a unipolar device and is voltage controlled device. The gate junction is reverse biased for its operation.It is used as buffer amplifier, analog switch, oscillators etc.
		N- Channel JFET	N-channel JFET is made by n-type silicon bars which form two PN junctions at the side. Majority charge carriers here are electrons.
		P-Channel JFET	P-Channel JFET is made by p-type silicon bar which forms two PN junctions at the side. Majority charge carriers here are holes.
		MOSFET	MOSFET is similar to JFET but it is operated in enhancement mode also that is, it increases the width of depletion layer. This can be operated in both enhancement and depletion mode. They are used in SMPS, amplifiers , oscillators etc.
		Enhancement MOSFET	The enhancement mode MOSFET has positive gate operation. It induces negative charges into the n-channel and thus number of negative charges increases, enhancing the channel conductivity.
		Depletion MOSFET	The depletion mode has negative gate operation. This decreases the width of the depletion layer.
		Phototransistor	The photo transistor converts the light energy falling on it to its corresponding electrical energy. This can be used in light sensing applications.Base is left disconnected as light is used to enable the flow of current.
		Photo Darlington	Photo Darlington Transistor is similar to phototransistor with very high gain and sensitivity
		Darlington Transistor	This is a configuration of pair of transistors, which produces high current gain. They are used in power regulators, output stages of audio amplifiers, display drivers etc.
		LOGIC GATES
		And Gate	This is the basic gate and it implements logical conjunction. The output of the AND gate is high, only if both the inputs are high otherwise both are low.
		Or Gate	The OR gate implements logical disjunction.The output is high if any one of the inputs is high.
		Nand Gate	It is complement of AND gate. The output is low only when both the inputs are high, otherwise it is high. Any Boolean function can be implemented using combination of NAND gates
		Nor Gate	NOR gate is a not OR gate. Output of this gate is high, if both the inputs are Low, otherwise it is High.
		Not Gate	Inverter or NOT gate implements logical negation. This gate inverts the input.
		Exor	This gate implements exclusive OR logic. The output of this gate is high ,if both the inputs are different.
		Exnor	This gate implements negation of EXOR logic. The output of this gate is high , only if the two inputs are identical.
		Buffer	It is an audio signaling device. Generally used in alarms, timers and for confirmation messages.
		Tri-State Buffer	Similar to a normal buffer but with a control signal. In case of active high buffer, it operates normally only when control signal is 1. In case of active low buffer, it operates normally only when control signal is 0.
		Flip Flop	Flip flop is the also a memory element but this is a synchronous device. The figure below shows the basic D-flip-flop.
		AMPLIFIERS
		Basic Amplifier	An amplifier is a device that amplifies a relatively small input signal i.e. it increases the power of the signal. They are used in communication systems, audio devices etc
		Operational Amplifier	Operational Amplifier (Op Amp) is a voltage amplifier with very high gain. The input is differential. They are used in instrumentation devices, signal processing, control systems etc
		ANTENNA
		Antenna	This symbol belongs to Aerial or Antenna. It converts electrical power into radio waves. It is used in wireless communication to transmit or receive the signals.
		Loop Antenna	Loop antenna is named after its loop like shape of wire or other electrical conductor. They are used as receiving antennas in low frequency range.
		Dipole Antenna	It is most widely used antenna. They consist of two bilaterally symmetrical conductive elements. Generally used in set-top TV, shortwave transmission and FM receivers.
		TRANSFORMER
		Transformer	Transformer is the basic element that transfers energy in one circuit to the other circuit through electromagnetic induction. They are generally used in electric power applications to increase or decrease the voltage of AC current.
		Iron Core	Uses a piece of magnetic material as core. Generally Ferro magnetic metals like iron are used. The core has high permeability and is used to confine the magnetic field.
		Center Tapped	The center tapped transformer is similar to a normal transformer except that it has its secondary winding divided into two parts with same number of turns in each part. This results in two individual output voltages across two line ends. Used in rectifier circuits.
		Step Up Transformer	The no. of turns in secondary winding is more than that of primary winding. The output voltage is higher than input voltage. Significantly used in inverters.
		Step Down Transformer	The no. of turns in secondary winding is less than that of primary winding. The output voltage is lesser than input voltage. It is widely used in low power applications.
		MISCELLANEOUS
		Buzzer	This is sound producing device. This produces buzz sound when the voltage is applied.
		Loud Speaker	This is also an audio device. The electrical signal is converted into sound signal here.
		Light Bulb	The symbol represents the light bulb. The bulb glows when required voltage is applied.
		Motor	This converts the electric energy to mechanical energy.
		Fuse	Symbol represents the fuse that protects the circuit from over current.
		Microphone	It converts sound signals to electrical signals.
		Crystal Oscillator	Used to generate clock signal of very precise frequency.
		ADC	Analog to Digital converter is used convert analog signals (usually voltage) to digital values.
		DAC	Digital to Analog converter is used to convert digital code to analog signals.
		Thermocouple	It is used to measure temperature.