What is SCR ( Silicon Control Rectifier ) ? | Construction of SCR | Working of SCR | Characteristics of SCR | SCR applications.
Almost all electronic controls used in modern industries consist of electronic circuits with Thyristors.The Silicon Controlled Rectifier (SCR) and the Triac are examples of Thyristor.
Thyristors are four layer device which can be switched ‘ON’ or ‘OFF’ electronically to control relatively large amounts of current for motors and other electrical equipment.
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Content :
- 👉 Introduction
- 👉 Construction of SCR
- 👉 Working of SCR
- 👉 Characteristics of SCR
- 👉 Some terms regarding SCR
- 👉 SCR Rating
- 👉 Application of SCR
👉 Introduction:
Alternating current can be converted in to direct current using PN junction diodes.
In order to control the voltage or current, series dropping resistor has to be used, there is loss of power in the dropping resistor.
So it results in inefficient, inconvenient and uneconomical operation. By using SCR the load control can be done with very small control power.
👉 Construction of SCR:
The cross-sectional view of a typical SCR and the symbol are shown in Fig 1.
Basically, the SCR consists of a four-layer pellet of P and N type semiconductor materials.
Silicon is used as the intrinsic semiconductor to which the proper impurities are added.
👉Arrangement of leads :
 |
| Fig 2 |
The actual arrangement of the leads of a large capacity SCR is shown in fig 2.
The lower P type material is connected to the stud and it marks the anode connection. It is fitted to the heat sink.
Cathode and gate leads are taken out as shown in fig 2.
👉Working of SCR:
Working of SCR can be understood by two transistor analogy as shown in fig 3 (a) and (b)
 |
| Fig 3 |
If we divide the middle two layers in two parts. the construction becomes as shown in Fig 3 (a).
If we look at it carefully it is seen one is PNP type transistor and the other is the NPN type.
In this collector of first transistor is connected to the base of the second transistor and the collector of second transistor is connected to the base of the first transistor.
When anode is connected to the positive supply and the cathode is connected to the negative supply through load. and gate is not connected to any supply, both the transistors are in OFF state, they offer very high resistance and whole applied voltage is dropped in the device (SCR) and the load does not get any voltage.
When a short duration positive pulse is given to the gate with respect to the cathode. base current In, flows. So collector current Ic, flows.
But the collector current Ic, of transistor T1, is the base current IB2, of transistor T2. So base current IB2 flows and as a result, collector current Ic2 flows.
Now this current Ic2, flows through the base the first transistor T1, so collector current Ic1 increases.
This is a cumulative process As a result large current is conducted by the device in very short time (almost Instantaneously), the resistance of the device decrease too much and almost all the voltage is available to the load.
The load current flows which is limited by the resistance of the load. If the gate pulse is removed then also the SCR continues to conduct. Thus even if the anode-cathode voltage is applied, SCR cannot conduct unless the sufficient gate current flows and once the SCR conducts the gate has no control over the load current i.e. the gate looses the control.
👉Characteristics of SCR:
Forward and reverse characteristics are shown in figure.
Let us first study the reverse characteristic.
When anode is given negative voltage and cathode is given the positive voltage, very small reverse current flows. It increases very slowly with the increase in the reverse voltage.
But when this reverse voltage exceeds the value of the reverse breakdown voltage, then the break down of the junction occurs due to the avalanche or zener breakdown and the reverse current increases rapidly. If this reverse current is not limited, SCR is damaged.
Reverse characteristic before the reverse breakdown is called the reverse blocking region.
Now let us try to understand, the forward characteristic.
When SCR is forward biased, i.e. anode is given positive voltage with respect to cathode and let gate current Ig be zero, If the forward voltage is less than certain value, SCR will not be in a position to fire.
But when Ig=0,and if the forward voltage equal to the break over voltage is given, SCR becomes on at point B as shown in fig.
In SCR, this break over voltage is much higher than the operating voltage. This means the SCR can not be made ON at the operating voltage when Ig=0.
When the value of the gate current is increased from zero to I. SCR becomes ON at less forward voltage (C). Similarly for gate current I SCR becomes ON at still
Now when gate current is increased to high value, at what minimum forward voltage the SCR can become ON ? For gate current Ig3. This is shown by voltage Vmin. This means that the gate current may be high, but the forward voltage should be minimum of Vmin (E). Thus SCR can not become ON when the value of the forward voltage is less than Vmin even if the gate current is high.
The forward characteristic before the SCR becomes ON is called the forward blocking region. Where-as the forward characteristic after the SCR becomes ON is called the forward conduction region.
In order to make the SCR OFF, its forward current should become less than the holding current. This holding current is specified for the gate open condition. It is shown by Io. But when certain gate current flows, the value of the holding current can be less i.e. IH1, IH2, IH3.
The forward voltage at which the forward current starts flowing suddenly at specific the current is called the forward break over voltage.
It is the reverse voltage at which the reverse current suddenly rises to high value.
It is the maximum reverse voltage which can be applied to SCR without making it to conduct. It is shown by VROM
It is the maximum value of the average current which can be passed through SCR without damaging it
It is the RMS value of the average current which can be passed through SCR without damaging it
It is the minimum value of the forward current SCR to keep it in ON state at zero gate current which should flow through the
It is the minimum value of the forward current necessary to bring the SCR from OFF state to the ON state.Its value is about three times the holding current.
It is the voltage drop across the anode and cathode when the SCR is conducting.
It is the gate voltage to make the SCR ON.
It is the minimum value of the gate current which should pass through the gate cathode circuit to make the SCR ON.
When anode is given negative voltage and cathode is given the positive voltage, very small reverse current flows. It increases very slowly with the increase in the reverse voltage.
But when this reverse voltage exceeds the value of the reverse breakdown voltage, then the break down of the junction occurs due to the avalanche or zener breakdown and the reverse current increases rapidly. If this reverse current is not limited, SCR is damaged.
Reverse characteristic before the reverse breakdown is called the reverse blocking region.
Now let us try to understand, the forward characteristic.
When SCR is forward biased, i.e. anode is given positive voltage with respect to cathode and let gate current Ig be zero, If the forward voltage is less than certain value, SCR will not be in a position to fire.
But when Ig=0,and if the forward voltage equal to the break over voltage is given, SCR becomes on at point B as shown in fig.
In SCR, this break over voltage is much higher than the operating voltage. This means the SCR can not be made ON at the operating voltage when Ig=0.
When the value of the gate current is increased from zero to I. SCR becomes ON at less forward voltage (C). Similarly for gate current I SCR becomes ON at still
Now when gate current is increased to high value, at what minimum forward voltage the SCR can become ON ? For gate current Ig3. This is shown by voltage Vmin. This means that the gate current may be high, but the forward voltage should be minimum of Vmin (E). Thus SCR can not become ON when the value of the forward voltage is less than Vmin even if the gate current is high.
The forward characteristic before the SCR becomes ON is called the forward blocking region. Where-as the forward characteristic after the SCR becomes ON is called the forward conduction region.
In order to make the SCR OFF, its forward current should become less than the holding current. This holding current is specified for the gate open condition. It is shown by Io. But when certain gate current flows, the value of the holding current can be less i.e. IH1, IH2, IH3.
👉Some terms regarding SCR:
(1) Forward break over voltage :
The forward voltage at which the forward current starts flowing suddenly at specific the current is called the forward break over voltage.
(2) Forward blocking voltage :
At zero gate current, the maximum forward voltage which can be applied to SCR without making the SCR ON is called the forward blocking voltage. It is shown by VDOM
(3) Reverse break down voltage :
It is the reverse voltage at which the reverse current suddenly rises to high value.
(4) Reverse blocking voltage:
It is the maximum reverse voltage which can be applied to SCR without making it to conduct. It is shown by VROM
(5) Average forward current (IFAV) :
It is the maximum value of the average current which can be passed through SCR without damaging it
(6) RMS forward current (If) :
It is the RMS value of the average current which can be passed through SCR without damaging it
(7) Holding current (I HO):
It is the minimum value of the forward current SCR to keep it in ON state at zero gate current which should flow through the
(8) Latching current:
It is the minimum value of the forward current necessary to bring the SCR from OFF state to the ON state.Its value is about three times the holding current.
(9) Forward voltage drop between anode and cathode (Vt) :
It is the voltage drop across the anode and cathode when the SCR is conducting.
(10) Gate trigger voltage (Vgt) :
It is the gate voltage to make the SCR ON.
(11) Gate trigger current (Igt) :
It is the minimum value of the gate current which should pass through the gate cathode circuit to make the SCR ON.
👉SCR ratings :
Some important ratings of the SCR are listed below.
These are normally specified for sinusoidal voltage at a frequency between 50 Hz and 400 Hz. In the bracket are indicated the normal values of a medium power SCR.
VDOM : Maximum forward blocking anode voltage with gate open (600 V)
VROM : Maximum reverse anode voltage (600 V)
It : Rated forward anode current (16 A)
Vt : Forward voltage drop across anode cathode in conduction (1 V)
Igt : Gate trigger voltage (3) Gate trigger current (20 mA)
👉SCR applications:
Some are listed below.
1.It is used for the speed control of d.c.motors.
2.It is used for the temperature control of furnace.
3.It is used in resistance welding as line contactor and in heat control.
4.It is used in electronic ignition of automobiles.
5.It is used in poly phase rectifiers and inverters.
6.It is used in chopper.
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This is my view on What is SCR ( Silicon Control Rectifier ) ? | Construction of SCR | Working of SCR | Characteristics of SCR | SCR applications, After referring the book Basic electronics by Mr. R P Ajwalia.
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