SCR Characteristics Experiment

In this post I will tell you, how to perform the SCR Characteristics experiment.

Aim :

To study the IV characteristics of SCR.

Apparatus Used :

A SCR, 2 multimeters, a breadboard, resistors, variable DC power supply and some connecting wires, etc.

SCR Characteristics experiment Theory :

About SCR :

SCR or thyristor is a four layer, 3 junction, pnpn semiconductor switching device. It has 3 terminals – anode, cathode and gate. SCR Consists of 4 layer of alternate p-type and n-type Silicon semiconductor forming 3 junctions J1, J2, and J3.

The terminal connected to outer p region is called Anode (A), the terminal connected to outer n region is called cathode (K) and that connected to inner p region is called the gate (G). 

Now SCRs of voltage rating 10 kV and rms current rating of 3000 A with power handling capacity of 30 MW are available. Such a high power SCR can be switched on by a low Voltage Supply of having current of 1 A and 10 W. For such power handling, we need to mount heat sink on SCR.

SCR Characteristics
Figure:
Why we called it SCR ?

An SCR is so called because silicon is used for its construction and its operation as a rectifier can be controlled.

SCR characteristics Experiment Circuit Diagram
SCR Characteristics Circuit Diagram
I-V characteristics of SCR :

SCR is an unidirectional device that blocks the current flow from Cathode to Anode. Unlike the diode, a SCR also blocks the current flow from anode to cathode until it is triggered into conduction by a proper gate signal.

The circuit diagram for obtaining Typical SCR Characteristics has 3 basic modes of operation : forward blocking (off state) mode, forward conduction mode (on state) and Reverse Blocking Mode.

  • Forward Blocking Mode : In this mode, anode is positive with respect to cathode, with gate circuit open. The junction J1 and J3 are forward bias but junction J2 is reverse biased. In this mode a small current called forward leakage current flows, due to high impedance. Therefore, SCR works as open switch.

     

  • Forward Conduction Mode : When anode to cathode forward voltage increased with gate circuit open, reverse junction J2 will have on avalanche break down at a voltage called forward break over voltage VBO. After this breakdown SCR gets turned on with anode current (IA) sharply increased and voltage (VAK) sharply decreased. We can decrease forward break over Voltage by increases gate pulse.

  • Reverse Blocking Mode : In this mode cathode is made positive with respect to anode with gate open. Junction J1 and J3 are reverse biased whereas junction J2 is forward biased. The device behaves as if two diodes are in series with reverse biasing. A small leakage current of order of few micro ampere is flows. This mode is called off state. If we increase reverse bias voltage , then at a certain level reverse breakdown voltage occur due to avalanche breakdown at J1 and J3 and current increases rapidly. This may lead to damage of the SCR due to over heating. 

Procedure for SCR Characteristics Experiment:

Forward Bias :
  1.  Make the circuit of SCR Characteristics Experiment as given in figure.
  2. Set the gate current at 0 A.
  3. Now increase the anode to cathode voltage V(AK) in step of 3 V and note the corresponding anode current (IA).
  4. After reaching at 20 V, then gradually increase the voltage and at a certain level you will observe that voltage decreases and current changes from micro ampere to mA.
  5. Note the voltage carefully at which the current sharply increases.
  6. Now increase and set gate current Ig to certain level and follow the step from 2 to step No. 5.
Reverse Bias:
  1. Make the circuit of SCR Characteristics Experiment as given in figure.
  2. Now increase the reverse bias voltage or voltage across SCR and note the reading on table.

Finally Plot the graph of forward bias and reverse bias to obtain the scr characteristics. 

Observation :

SCR Characteristics Forward Bias Table:
At Gate Current Ig = 0 uAAt Gate Current Ig = ___ uA
S.No.Anode to Cathode Voltage V(AK)
(V)
Anode Current IA
(mA)
Anode to Cathode Voltage V(AK)
(V)
Anode Current IA
(mA)
1
2
3
SCR Characteristics Reverse Bias table
At Ig = 0 A
S. No. Anode to Cathode Voltage V(AK)
(V)
Anode Current IA
(mA)
1
2
3

Result :

  • The characteristics of SCR is plotted and studied successfully.
  • The forward break over voltage found to be___V at gate current __uA.
  • The holding current is __A.

Precaution And Source Of Error :

  • Connection should be as per the diagram of SCR characteristics experiment.
  • Verify the terminal of SCR before use.
  • While reaching around forward break over voltage, note the reading very carefully.
  • Plot the graph of SCR characteristics experiment carefully.

SCR Characteristics Experiment Video :

Watch SCR Characteristics experiment video:

3 thoughts on “SCR Characteristics Experiment”

  1. Pingback: Half wave Rectifier using Diode - electronicsGyan.online

  2. Pingback: Zener Diode- Working, Doping and Characteristics - electronicsGyan.online

  3. Pingback: Zener diode as a voltage regulator

Leave a Comment

Your email address will not be published. Required fields are marked *