Silicon Controlled Rectifier - SCR

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The SCR stand for Silicon Control Rectifier, it is used in industries because it can handle high values of current and voltage.

SCR construction and Symbol

SCR Construction

The Silicon Control Rectifier SCR is consist on four layer of semiconductor which forms PNPN or NPNP structure, it has tree junction J1

J2 and J3 and also three terminals, the anode terminal of Silicon Control Rectifier SCR is connected to the P-Type of material of PNPN structure and the cathode terminal is connected to the N-Type layer of the Silicon Control Rectifier while the gate of the Silicon Control Rectifier SCR is connected to the P-Type of material near to the cathode, as shown in the given Figure.

SCR

SCR Operation / Working

The Silicon Control Rectifier SCR start conduction when it is forward biased. For this purpose the cathode is kept at negative and anode at positive. When positive clock pulse is applied at the gate the SCR turns ON.

When forward bias voltage is applied to the Silicon Control Rectifier SCR, the junction J1 and J3 become forward bias while the junction J2 become reverse bias. When we apply a clock pulse at the gate terminal, the junction J2 become forward bias and the Silicon Control Rectifier SCR start conduction.

The Silicon Control Rectifier SCR turn ON and OFF very quickly, At the OFF state the Silicon Control Rectifier SCR provide infinity resistance and in ON state, it offers very low resistance, which is in the range of 0.01O to 1O.

SCR Firing & Triggering

The Silicon Control Rectifier SCR is normally operated below the forward break over voltage (VBO). To turn ON the Silicon Control Rectifier SCR we apply clock pulse at the gate terminal which called triggering of Silicon Control Rectifier, but when the Silicon Control Rectifier SCR turned ON, now if we remove the triggering voltage, the Silicon Control Rectifier SCR will remain in ON state. This voltage is called Firing voltage.

Turning OFF methods of Silicon Control Rectifier - SCR

There are two methods through which Silicon Control Rectifier SCR can be turned OFF,

Anode current interruption method

SCR Anode Current interruption Method

In this method a parallel or a series switch is used to turn OFF the Silicon Control Rectifier (SCR electronics) by turning OFF the switch.

Forced Commutation method

SCR turn off by forced commutation method

In this method a reversed polarity battery is connected, so the current through the Silicon Control Rectifier SCR is reduced and it turn OFF.

Power Control of DC motor using SCR

In the Given ckt a Single phase controlled Rectifier is shown. which is used to control the DC motor speed. This ckt is consisting of four SCR and four Diodes. The DC voltage is supplied to the motor winding through four diodes while the DC voltage supplies to the armature through four Diodes.

In This ckt the motor is controlled through armature voltage, while the field voltage is at a constant value.

In the ckt the SCR 1 & SCR 4 are triggered at same time in first cycle, while in next half cycle SCR 2 & SCR 3 are conduct.

The motor armature receives a continuous or Discontinuous voltage depends on the firing angle of the SCR, Power factor.

In Discontinuous armature current , SCR 1 & SCR 4 are fire at angle a due to which the current flow path is form a to ß , as shown in the Fig:

The motor armature receives voltages form a to ß, while form ß to p + a the motor can't receive current.

Through this way the speed of a DC motor is controlled.

Single Phase Two Quadrant SCR driver

Power Control of AC motor using SCR

Speed control by Frequency Changing or Variable Frequency control method.

The frequency method is very important through which the speed of the induction motor is controlled.

Ns = 120f/P

Form the above formula, it is clear that the speed of the induction motor directional proportional to the frequency. It means that the frequency of the supply voltage changes , the speed of the indication motor also changes , increases the in put frequency of the supply increases the speed of the induction motor also increase and vice versa.

In the frequency controlled method, the three phase supply is controlled through a controlled rectifier. The voltage is applied on the forced Commutated Bridge rectifier, which produce a variable frequency for control of the induction motor.

Basic Controlled Rectifiers Circuits.

The Basic Controlled Rectifiers Circuits have main two types which are given below.

Speed Control Voltage and waveform

Half Wave Controlled Rectifiers Circuits with Resistive Load.

This type of rectifier CKT consist of a single SCR and a Load Resister as shown in above figure, The AC supply is feed from the secondary of the Transformer, The value of the supply is less then the forward biased voltage of the SCR. The given CKT Operation is given below.

Single Phase Half Wave Length Thyristor

  1. In given Figure (a), when the negative half cycle is applied, the anode terminal of the SCR is more negative w.r.t cathode, therefore there is no voltage flow through the Load Resistor (RL). During the negative half cycle if a plus is applied at the Gate terminal of the SCR can't conduct.
  2. When a positive half cycle is applied at the SCR input ,the anode of the SCR is more positive w.r.t Cathode ,This way the SCR forward biased but Stop the Load current IL until a plus is applied at the Gate terminal of the SCR, With this plus the SCR start conduction.
  3. At the Special angle a plus is applied at the Gate terminal, the SCR start conduction and Load Resistor (RL) receive the total voltage.(The SCR only drop one Volt) As shown in  the Figures b i and b ii.

At the delay angle the value of the out put voltage is at maximum level. In the case of the resistive load, the load current is in Phase to the Load Voltage. The time after which the SCR start conduction is called Delay angle. If the SCR is replaced with the Diode then the time at which the diode start conduction is called delay angle. The time at which the SCR start conduction like a diode is called delay angle or firing angle.

When SCR conduct at angle a, then voltage from 0° to a is zero volts. But from a to p total voltage is flow through the Load as shown at b (iii), the voltage change across the SCR is shown in b (v) diagram.

Application of SCR

There are the following applications of the SCR in Electronics.

  1. Phase control of the signals
  2. For Switching of Electronics devices.
  3. Triggering the IC's and different timing circuits.
  4. Used as controlled Rectifier.

SCR Video