Rectifier (using diode)

In electronics device rectifier are widely used in power section .It is used to rectified alternative current(ac) into direct current(dc) this process is known as rectification .In this device, diode capacitor ,inductor ,resister is widely used.Rectifier also make with the help of vacuum tube
diodes,mercury arc values and other components
If we wants to convert direct current(dc) into alternative current(ac) this device is known as inverter.
The type of rectifier are determine by the number of diode used in specific arrangement of diode There are following type of rectifiers which are only the uses of diode :

1. half- wave rectifier
2. full-wave rectifier
   
3. bridge rectifier

In the half-wave rectifier ,only one either positive part or negative part of AC wave is passed.It is depends upon the uses of diode in positive half of transformer or negative half of transformer in series .It is very inefficient ,because only one half signal is reaches in output section of it .Half-wave rectification is achieved with the help of three diode in three-phase system and with the help of one diode in one phase system.
The output of DC voltage of half of rectifier can be calculated by followings theoretical equations.

1. Vpeak=Vrms*1.1414
   
2. Vdc=Vpeak/3.14

In the Full-wave rectifier,full waveform of AC wave is passed to output section.In this rectifier whole part of wave form are achieved in out put so this is more efficient than half wave rectifier.Four diode are used in full wave rectifier circuit if there is non-center trapped transformer is includes.Two diode are used in full-wave rectifier circuit if there is center-trap transformer is includes.

Half-Wave Rectifier:

According to the diode action ,the rectifier conducts signal (i.e current) only at the time of positive half cycle of input a.c signal where as the negative half-cycle of a.c signal are cutoff or suppressed. At the time of negative half-cycle ,no current conduct in the circuits.Therefore no voltage drop across the lad resistor.That's why,signal (i.e current ) always flows in only one direction (i.e d.c signal) via the load resistor after every half-cycles.

Circuits Explains:

It consists of one diode ,transformer and load with complete close circuits.So this single S/C diode acts as a half-wave rectifier.The a.c signal which is going to rectified is applied in series with the signal diode and load resistance as shown in above.Usually ,a.c supply is applied via a transformer .By the transformer action ,It has two advantages ,one is it allows at step up or step down the a.c input signal as our desire and other is it isolates the rectifier circuits form power line and therefore it decrease the risk of electric shuck in the circuits or equipments.

Working Explain:

The applied a.c signal across the secondary winding change polarities after each half -cycle of supply a.c voltage.At the time of the positive half-cycle of supply a.c voltage,upper end of secondary windings becomes positive w.r.t the lower end of its.In this condition diode is in forward biased and it conduct current in the circuits.At the time of negative half-cycle of in put a.c voltage ,upper end of secondary windings is negative w.r.t the lower end of its.In this condition ,diode becomes reverse biased and it does not conduct current in the circuits.In this way, current flows via the load at the time of positive half-cycle of input a.c voltage only where as no conduct current through the load at the time of negative half-cycle of supplied a.c voltage in the same direction is obtain across load for other next cycle.But this output is pulsating d.c .Hence to reduce it an extra filter circuits is used .

Limitations:

• The output is low because a.c supply delivers power only half-cycle time
• To remove pulsating d.c it required extra filter circuits .

Full-Wave Rectifier:

In this full-wave rectification,two diode are used in such way that they work alternately.Current flows via the same direction at the time of both half-cycle of input a.c voltage .During the positive half-cycle of input a.c voltage one diode conduct current through the load and during the negative half-cycle of the input a.c voltage ,next one diode conducts current via load in same direction.In this way full-wave rectifier circuits utilises the both half-cycle of input a.c voltage to produce the d.c output .Therefore its efficiency is higher than half-wave rectifier.

Center-Tap Full-wave Rectifier:

Circuits details:

It consists of two diode ,one center-tapped transformer and load with the power supply in closed circuits as shown in figure below.Secondary winding of center-tap transformer is used with those two diode s connected in such way that each diode use s one half-cycle of input a.c voltage. That means upper diode use to conduct the upper half-cycle of secondary winding whereas lower diode is use the lower half-cycle of secondary windings.

Working Explains:

At the time of positive half-cycle of secondary winding voltage ,the upper end of secondary winding becomes positive w.r.t the lower end of its.Under this condition ,upper diode is in forward biased whereas lower diode is in reverse biased.Hence upper diode conduct positive half-cycle of the input of the a.c voltage where as lower diode does not conducts.At the time of negative half-cycle of secondary winding ,upper end of secondary winding become negative w.r.t the lower end of its.Under this condition ,lower diode is in forward biased where as upper diode is in reverse biased.Hence the lower diode conduct current via load of the negative half-cycle of input a.c voltage and upper diode does not conducts current .The current in the load is in same direction for both half-cycle of input a.c voltage.In this way d.c output is obtain across the load .

Limitations:

• It is difficult to locate exact the center tap on the secondary winding of transformer.
• The diode used must have large peak inverse voltage.
• Each diode utilises only one half-cycle of secondary voltage,so the d.c output is small.

Full-Wave Bridge Rectifier:

It consists of four diode ,transformer and load resistor connected as shown in figure below .The a.c signal is applied to the diagonally opposite ends of the two diode whereas another two ends of the diode is connected to the load resistor as shown in figure.

Operation details:


At the time of positive half-cycle of secondary winding voltage the upper end of the secondary winding becomes positive w.r.t the lower end of its.Under this condition diode D1 and D4 are in forward biased where as other diode are in reverse biased .Hence the diode D1 and D4 conduct current through the load resistor.

At the time of negative half -cycle of the secondary winding voltage ,the lower end of the secondary winding is positive w.r.t the upper end of the secondary winding .Under this condition , D2 and D3 are in forward biased and where as other diode are in reverse biased .Hence the diode D2 and D3 conduct current through the load resistor .This flows of current is in same direction as for positive half -cycle of the secondary winding voltage .Hence d.c output is obtain across load resistor.

Advantages:

• No required of center-tap transformer
• For the same secondary winding voltage the output is twice that of center-tap full wave rectifier circuit.
• For same d.c output ,the PIV is one-half of that of center tap full wave rectifier.

Limitations:

• Four diode are need for this circuit.
• The voltage drop in the internal resistance is twice as in the center tap full wave rectifier circuits because of four diode.
  

S/C diode as Rectifier

S/C diode has a wide range of application ,one of them is rectification.The diode is used in rectifier circuits to rectified a.c input signal .Here diode and load resistor (RL) are connected in series as given below.The rectified signal (i.e d.c output) is obtained across the load resistor.

Circuit details:

Diode conduct current in forward biased or one direction.So at the time of positive half-cycle of a.c input signal ,the S/C diode conducts this signal in the circuits because of forward bias .As a result the positive half-cycle of input signal appears across the load resistor as shown in figure below.But at the time of negative half-cycle of input a.c signal ,diode does not conduct current because of reverse biased.As a result no voltage appears across the load resistor .In this type of circuits the negative half-cycle are suppressed

but output consist of only positive half-cycle of in put a.c signal.In this way ,S/C diode act as rectifier (i.e a.c input signal changes into d.c out put).

amp circuit

Multi-Stage Transistor amp circuit:

In the field of amp circuit ,one stage in not capable to amplifying weak signal to required strength or level .Therefore multi-stage amp circuit is necessary to amplifying up to desire level signal stage amp circuit is generally insufficient to drive or run at output of electronics equipments .Therefore an extra amplification is necessary .For this other level or stage is needed .In order to accomplished this ,the output of one stage amp circuit stage is coupled to another or next input of amp circuit .The whole construction of this amp circuit is Multi-stage amp circuit.It has wide range of application in this field .For example ,Radio communication system , number of amplification stage may be greater than five stages.

According to coupling devices amp circuit are classified as followings;

• Direct-coupled amp circuit :In the case of direct coupling (i.e d.c coupling the one stage of the amp circuit are directly connected to another stage of amp circuit without d.c isolation.This type of amplifier widely used in the low frequency (i.e less than 10Hz) signal to be amplified .For example ,amplifying thermo-couple current ,photo-electric current etc.There is no coupling devices (such as capacitor ,transformer etc) because at low frequency the electrical size of these elements become large .Therefor output of one stage is directly connected to the input of the next stage of an amp circuits with out coupling devices.

Circuits Details:

There are two stage direct-coupled amp circuit.It consists of two NPN transistor and next so on It is very simple amp circuit .The first transistor output from collector is connected to the input of the next stage of the transistor base .And similar way for the other additional stage .This circuits is stable even temperature is change .

The weak a.c signal is given to the base of the first transistor .The amplified a.c signal appears across the output or collector of the same transistor .This amplified a.c signal is also give to the base of the next transistor of further amplification and amplified a.c signal is appears across the collector load or out put of the same transistor . For other additional stage similar way ,this type of amp circuit amplified the weak signal which is less than 10Hz.

Advantage :

• The construction of this type of amp circuit is very simple.
• It contains minimum number of resistor.
• This amplifier circuits does not contains coupling devices .So it has low cost .

Limitations:

• This amp circuit can not be used at high frequency (i.e greater than 10Hz) for amplification.
• Because of temperature change ,the operating point of amp circuit is shifted.
  
  

1. RC-coupled amp circuit : A capacitor is used as the coupling devices ,in the RC -coupled amp circuit of electronics equipment.Here capacitor connects the output of the first stage of amp circui to the input of the an next stage of amp circuit.By the same way for other required stage.Here the capacitor is used allow to pass the a.c signal where as block the d.c biasing voltages.This type of circuits is most useful circuits.It is simple in construction.and it has low cast .It give well audio fidelity over wide rage of frequency.Capacitor is used as coupling device which connect the output of the first stage of transistor to the next stage input base of next stage of other transistor in amp circuit and soon for other stage.This type of amp circuit is generally employed for voltage amplification.

Circuits details:

Here weak a.c signal is given to the input base of the first transistor .Amplified a.c signal is appears across its collector resistor or load (Rc).This amplified a.c signal is also given to the input base of the next transistor for further amplification with the help of coupling capacitor .This amplified a.c signal is appears across the collector load (Rc).This stage does further amplification of the applied a.c signal.In this same way ,cascaded (i.e one stage after another stage) stage s amplify the a.c signal and as a result overall gain of amp circuits is increase .But here total gain of the amp circuit is less than the gain of the individual stages because output of first stage drives the second stage and soon .Due to the shouting effect of the input resistance of second stage of the amp circuit ,the effective load resistance of first stage of amp circuit is reduced. But in the case of 3-stage amplifier circuits total gain of the amp circuit is equal to the product of the individual stage of amp circuit .

Advantages:

• It contains the resistors and capacitors as coupling devices .So it has lower cost .
• It provided excellent audio fidelity over wide range of frequency.So it has excellent frequency response.
• This amp circuits is easy to make and compact.

Limitations:

• This type of amplifier has low voltages gain and as well as power gain.
• It has noise in the circuits with age.
• It has poor impedance matching.
  

Transformer-coupled amp circuit :A transformer is used as coupling devices in transformer coupled amp circuit .It also allow to pass the a.c signal whereas block the d.c signal .In this circuits additional impedance matching circuits is necessary.The main disadvantage of RC-coupled amplifier is low voltage or power gain .This is over come by this transformer-coupled amplifier circuits.In RC-coupled amp circuit ,effective load resistance (Rc) of each individual stage is decrease because of low resistance presented by input of each individual amplifier stage and previous stage .That means impedance of input of an amplifier is low whereas its output impedance is large.In the case of multi-stage amp circuit ,the large output impedance of one stage of an amplifier circuits become in parallel with the low input impedance of next other stage.Therefore effective load resistance is decreased.Transformer has a properties of impedance change with the frequency ,the low impedance of a one stage of amplifier circuits load can be reflected as high load impedance to the preceding stage of an amplifier circuits.This type of amplifier is generally employed when the load resistance or impedance is small.RC coupled amplifier is used for voltage amplification whereas Transformer-coupled amplifier is used for power amplification.

Circuits details:

This figure shows two amplification stage of transformer coupled amp circuits .Here transformer is used as coupling devices which connect the output of one stage to the input of next amplification stage of amp circuit. In this type of amplifier circuits primary coil of this transformer is made the collector load where as secondary coil is connects as input to the next stage.

In this amp circuit an weak a.c signal is given to the input base of first transistor ,amplified a.c signal is appears across primary coil of the coupling transformer .By transformer action, the voltage is developed across primary coil is transferred to the input of the next of next transistor by the secondary coil of transformer,amplified a.c signal is shown by similar way across the next collector load or resistor.

Advantages:

• In this amplifier circuits ,no a.c signal power is lost in base resistor or collector.
• It provides an excellent impedance matching due to transformer-coupled.
• It provides higher voltage and power gain.

Limitations:

• It is bulky and fairly expansive at audio frequency.
• The gain is change considerably with frequency .So it has a poor frequency response.
• It has high frequency distortion.
• This amp circuit tends to introduce hum in the output stage.

amp circuit

Introduction :
In almost all electronics equipments contains amp circuit. In this type of equipments ,there is necessary to rise or increase strength or level of signal which is weak.The importance of amp circuit is very very much in this field .So we can say that it has wide range of application .Amp circuit are used in order to amplifying weak electrical signals.
The transistor are widely used in many places such as scientific ,medical ,communication field etc to produced signal in the micro volt and Milli volt range which is weak level signal .So it should be necessary to increase its level or strength using amp circuit drive required equipments .Amp circuit are classified according to their uses or ranges like below:

• Pre-amp circuit ,
• Voltage-amp circuit and
• Power-amp circuit.

Before the invention of transistor ,vacuum tube ,Mercury arc values etc are used in amp circuit .But now transistor is mostly used in amp circuit.According to the number or stage of transistor used in amp circuit ,it is classified as below:

• Single-stage transistor amp circuit and
• Multi-stage transistor amp circuit .

How amp circuit amplifies?

Now we are discussed about transistor amplifying or analysis of amp circuit.This type of configuration is called common emitter configuration.

This type of amp circuit consists of transistor and its biasing resistors ,which help to amplified the weak a.c signal in sufficient level or strength at the out put of the amp circuit. When weak base a.c signal is given to the base of the transistor as above figure ,then the very large voltage(i.e Bita times or 100 times the base currents) appears across the collector resistor( Ro) ,because of high value of collector resister .So we get very large output voltage .This way the transistor amplifies weak signal.

Practical amp circuit:

This amp circuit is a practical circuits .For the required amplification of weak signal ,various circuits elements and their working principle in the amp circuit .Some important terms of practical amp circuit which are described as following:

Input capacitor(Cin):The practical amp circuit consist of an electrolytic capacitor at the input of the each stage which is connected in base of the each transistor in the case of transistor amp circuit .This is used to allows to pass the a.c signal through it whereas block the d.c biasing signal .

Biasing Resistance circuits :It consists of three resistor (i.e Rb1,Rb2 and RE) which are used to proper biasing and stabilisation circuits .This circuits maintain the proper operating point .If the biasing circuits of amp circuit should not be proper arrangement some portion of the negative-cycle of input signal may be cut off in the collector load or out put .

Coupling capacitor(Cc) :This capacitor is used to couple one stage of amp circuit to the next stage of amp circuit .According to the capacitor function ,it isolates the d.c biasing signal of one stage of amp circuit of the next of amp circuit , But only allows to pass the a.c signal which is applied for amplification . If it is not used in amp circuit whole biasing function is unbalanced due to shouting effect of collector resistor.

Emitter by pass capacitor(CE):This capacitor is used in emitter side for grounded it .It is used in parallel with emitter resistor and provided a low resistance path or way, to the high frequency signal or amplified a.c signal .The amplified signal is flowing via collector resistor (Rc) as a result voltage drop across it which reduce the output voltage ,i f we not used it in amp circuit.

Requirements of faithful amplification

The system of increasing the strength or level of weak signal in a circuit without any changes into its waveform is called faithful amplification.
The principles of transistor reveals that it will work correctly if its emitter-base junction (i.e input circuit) connected in forward biased and the collector-base junction(i.e output circuit ) connected in reverse biased at the all times.To achieving faithful amplification , this is main key factor. To achieve this ,the followings general conditions must be satisfied:

1. Proper amount of zero signal collector current
2. Minimum amount of proper base-emitter voltage at any times
3. Minimum amount of proper collector-base voltage at any times

The first and second conditions shows that base-emitter junction shall connected correctly in forward biased in the circuit at the all times and the third condition shows that base-collector shall connected correctly in reverse biased in the circuit at the all times.

To fulfil of these condition transistor work between saturation to cutoff region i.e active region.

Classification of transistor amp circuit

There are many applications of transistor in the electronics and communication field.Manly it is used in amplification purpose in the circuits.They are classified according to their uses ,frequency capabilities, coupling devices and mode of operations which are as following:

1. According to use; transistor amplifier are classified as voltage amplifiers and power amplifiers.At first it is use to increases the voltage level or strength of the signal while the after it increase the power level or strength of the signal in the circuit.
2. According to the frequency capabilities; these are classified as audio amplifiers ,RF amplifiers etc which are use to amplify the lying in the audio range (i.e 20Hz -20kHz) and radio frequency range etc respectively in the circuit.
3. According to coupling devices; if there is a single stage amplifier, it is unable to amplify the signal to fulfill the practical requirements.So other additional amplifier i s necessary to amplify the signal.But to preform this of the one stage is required to coupling to the other next stage.So depending upon the coupling devices used in the circuit,the transistor amplifier are classified as direct-couple amplifier,R-C couple amplifiers, transformer couple amplifiers etc.
4. According to the mode of operation; the transistor amplifiers are classified as class-A amplifiers,class-B amplifiers, class-C amplifiers .This classification in depends upon the portions of the input waveform which reached into output stage. For more

Electro-mechanical switch or Relay

Introduction:

It is a mechanical switch but it preform electrically switching operation (i.e turn ON or OF current ) in an electronics or electrical circuit.




This type of switch is devanced or improved form of mechanical switch .In general type of Relay it consist of a solenoid and leaver carring with armature .Those are suspended on the constant body with the help of the spring .When solenoid enerasised it pulls lever downwards and vice-versa .Solenoid consist of extra power supply with the switch .When the switch of the solenoid is closed ,the armature is closed the relay contacts (i.e turn On or closed circuit). When the switch of the solenoid is open ,then the armature contacts of the relay is open (i.e turn OFF or open circuits).In this way this type of the switch is preform the switching operation .

Advantages:
Following are the advantages of the electro-mecanical switch over mecahnical switch;
This type of switch need a small power to perform its switching peration .This type switch handel large power.

1. It has no risk of sparkings durings switching operation.
2. This can perform switching operation from a distance .So it can handeled high voltages.
3. In this type of switch relay current is smaller than load current.

Limitations:

* The speed of switching operation is very small.
* Littel wear and tear is possible because of moving parts.

• 
  

Electronics Switch

This type of switch preform switching operation (i.e ON or OFF current) in an electronics or electrical circuit with the help of electronics components such as transistor ,tubes etc. It has many advantages and popular because of their high speed of switching operation and no sparking in the circuit..Transistor ,SCR, Tubes etc are used as a switch.In the case of transistor switch, it operates at saturation region or at cut off region.

• The transistor is cut off region or no cur5rent flows in collector load , when input base voltage is proper amount of negative potential .At this condition ,there is no voltages drop across the collector resistor or load and the out put voltage is theoretically Vcc.

Therefore,

Ic=0 and

Vce=Vcc

This condition shows that ,it is open circuit (i.e OFF state).

• The transistor is in saturation region or current flows in collector resister or load, when base input voltage is proper amount of positive potential. In this condition, almost whole voltage(Vcc) will dropped across collector resister or load and out put voltage is theoretically zero.

Therefore,

Ic=Isat and VCE=0

This condition shows that ,it is a closed circuit (i.e ON state).

Advantages:

This type of switch have following advantage than other type of switch:

1. It preform noiseless switching operation because of no moving parts in this type of switch.
2. The size and weight of its is smaller than other.
3. It has easy to maintenance .
4. It has lower cost than other.
5. It has very fast speed of switching operation than other type of switch.
6. It give well services (i.e trouble free) than other.

Mechanical switch

This type of switch operated mechanically to preform turn ON or OFF current in an electronics or electrical circuits is called a mechanical switch .This type of switch perform switching function with some sparking. So it has low efficiency than other .
The tumbler switch is c one of the example of a mechanical switch which is used in many home appliances to perform turn ON or OFF power supply such as bulbs , heater , fans etc .In mechanical switch ,load is connected in series with a mechanical switch and the power supply (i.e battery).When the switch is close ,there is current flows in the circuits (i.e through load).When switch is open ,then there is no current flows in the circuits (i.e via load).

Disadvantages Of Mechanical Switch.

1. In the close circuits ,the switch carries whole load current.When if there is high load currents,the contacts of switch enable to carry it without overheating.So the size of the switch is increase accordions to amounts of load current.
2. The speed of switching operation is very low ,because of its high inertia.
3. When the load current is high in the circuit ,then the sparking is possible at the contacts of the switch at the time of switching operations(i.e ON or OFF).

Solid-State switching Circuits

Introduction:
In many operations,there is required to switch on (i.e close circuit ) or switch off (i.e open circuit) in electrical circuits or electronics circuits .In some high sensitives applications it is necessary that to switch ON or switch OFF should be very fast and but not sparking in the circuits.Because of these two major reasons , the mechanical switches can not be used in the circuits for the switching purpose .This type of switch has high inertia.Therefore its speed of operations is low.This type of switches has sparking properties at the time of conduction of switching operations.
In this fast and movable world ,very high speed of switching operations without any sparking switching devices is needed.For this purpose tube and transistor are used in the switching circuits to accomplished switching operations .Such type of switches are called electronics switches .The electronics switches are also widely used to generates non-sinusoidal wave (i.e square wave , triangular wave or saw-tooth wave ,rectangular wave).
A circuits which ,preform turn ON or turn OFF current in an electronics or electrical circuits is called a switching circuits.Actually this circuit consists of followings two parts:

1. A switch and
2. Electrical or electronics circuits.

Switch:

A switch is a devices which preform turn ON or OFF circuits in an electronics or electrical circuits.The switches can be mainly classified as the following types:

1. Mechanical switch,
2. Electronics switch and
3. Electro-mechanical switch (or relay).

Transistor Oscillators

Introduction:
The transistor are used in many electronics equipments.One of the applications of transistor is used in an oscillator to generates continuous undamped oscillations of desired frequency which the tanks circuit and feedback circuits are connected to it properly.All oscillators have similar functions,although they have different name.This functions is that ,generating continuous undamped oscillation of any desired frequencies.The main difference between these oscillator is the method or ways by which energy is feeded to the transistor circuits.There are followings type of transistor oscillators:

1. Tuned collector oscillator,
2. Hartley oscillator,
3. Wine Bridge oscillator,
4. Copit's oscillator,
5. Phase shift oscillator and
6. Crystal oscillator.

Oscillator

Introduction:

There are so many electronics devices which is widely used in specific purpose in this field.Those many electronics devices need a sources of an energy at a specific frequency which may ranging from few Hz to the several high MHz or GHz. An electronics devices which are used for this purpose is called oscillators . It is widely used in electronics equipments such as radio , TV receivers etc.It is used to generates high frequencies wave (i.e carrier wave ) audio frequency and Radio frequency wave.For the repair radio and television and other electronics equipments, audio frequency and radio frequency is used.Oscillator are extremely used in radar ,computers (electronics) and other devices.

Oscillator are used to produced sinusoidal or non-sinusoidal (i.e square wave, rectangular wave ) waves.It is most useful electronics devices which has widely range of applications in this field .

Sinusoidal Oscillator:

An electronics devices that produces sinusoidal oscillations in desired range of frequencies is known as sinusoidal Oscillators.
Actually , it dose not create energy but only acts as an energy converter, such as d.c energy convert into a.c energy of required frequency .The frequency of oscillations is depends upon the elements used in the its circuits.Alternator is not oscillator although it produces sinusoidal frequency .The main differences between these are one is alternator is mechanical devices having ratings parts whereas oscillator is non-ratings devices.Second is it is used to converts mechanical energy into a.c energy whereas oscillator is used to converts d.c into a.c energy . Third difference is it can not generates high frequencies whereas an oscillator can generates from a few Hz to several MHz.

Advantages:

Electronics oscillator have the followings advantages:

1. It has very high efficiency.
2. It can produces waves from few Hz to very high frequencies MHz.
3. It is non-ratings electronics devices .As a results it has little tear and wear .Hence it has more longer life.
4. The operation of an oscillator is very silent because of absence of moving parts on it .
5. It has good frequencies stability.
6. The frequency of oscillations can be changed when needed.
   

Improtant Terms of SCR

To get more knowledge about SCR ,there are following very improtant terms as belows:

1. Break over voltage : It is minimum forward voltage when gate terminal being open , at which SCR starts to conducts heavily (i.e turned ON).
2. Peak reverse voltage: It is minimum reverse voltage (i.e cathode terminal positive w.r.t anode terminal),that can be applied to the SCR without conducting in the reverse direction.
3. Holding current: It is the maximum anode current (IA) ,when the gate being open , at which SCR is turn off from ON conditions.
4. Forward current rating : It is the maximum anode current that the SCR is capable to pass without destruction of the SCR.
5. Ckt fusing rating: It is the product of square of forward surge current and the time of it.

Working of SCR

In solid state S/C silicon controlled rectifier , anode terminal is always kept at positive potential w.r.t cathode terminal. The load is connected in the series with the anode .The working of SCR circuit can be grouped as followings:

1. When gate is open:In the SCR circuit with no voltage is applied to the gate i.e open gate,junction J2 is reverse biased whereas J1 and J3 is forward biased connection . Therefore , the condition in the junction J1 and J3 is as similar as npn transistor with base open .As a result no current flows via the load resister RL.,at that time the SCR is cut off state.If the applied voltage in the circuit is slightly increased ,a stage is reached when junction J2 breakdown because of reverse biased . Now the SCR conducts rapidely and at that time SCR side to be ON state .The amount of the applied voltage at which SCR conducts rapidely with open gate is called break over voltage.
2. When gate voltage is applied : When the gate terminal is positive w.r.t cathode , here junction J1 and J3 is forward biased where as the junction J2 is reverse biased.When small gate voltage is applied the SCR conduct heavily. In proper biasing condition ,the electron starts to move from n-type material to cross junction J3 towards left and holes from p-type material towards right .As a result ,electrons attracted across the junction J2 and gate current flows ,the anode current heavily increase .Consequently more electron available at junction J2 .This process run continues ,So junction J2 breaks down at an extremely small time and the SCR starts conducts heavily in the circuit .Once the SCR starts to conducts ,the gate loses its control properties .If gate voltage is removed ,even the anode current conducts heavily in the circuit. To stop current conduction , the applied voltage is reduced to zero voltage.

Conclusion:The following conclusions are drawn out from the working principles of SCR:

1. An SCR has two state ,one is ON state and other is OF state(i.e either is conducts heavily or does not conducts ). So SCR behaves like electronics switch.
2. On the SCR ,there are two ways to run the SCR as electronics switch.The first ways is to keep the gate open and make applied voltage equal to the break over voltage .The second ways is to apply the gate voltage ,and supply voltage is less than break over voltage.
3. When the gate voltage is applied then the break over voltage is always much greater than supply voltage .
4. To makes SCR non-conducting (ie open the SCR),reduce the supply voltage to zero.