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Study and Plot V-I Characteristic of SCR

Aim: Study and plot V-I Characteristic of SCR


Equipments Needed:


1. NV6530 SCR Characteristic Trainer

2. 2mm Patch Cords

Circuit diagram:


Circuit used to plot different characteristics of SCR is shown in the figure.


Procedure:

1. Connect terminal1 to terminal 4, terminal 2 to terminal 8 and terminal 3 to
terminal 12 as shown in figure
             







2. Connect Voltmeter across terminal 7 and 8 and Ammeter across terminal 9 and
10 as shown in figure 12.

3. Make short terminals 5 and 6.

4. Rotate the knob P1 and P2 fully in counter-clockwise.

5. Switch “ON” the power supply.

6. Set the value of Anode Voltage at 35V by using the knob P1.

7. Now Increases gate current Ig gradually by varying knob P2 and observe it.

8. At a certain value of gate current, voltmeter reading falls down to almost zero.
This action indicates the firing of SCR.

9. Note the gate current value at this position (firing of SCR).

10. Keep the gate current constant by shorting terminal 9 with 10 and connect
Ammeter to the terminal 5 and 6



11. Rotate the potentiometer P1 fully in counter-clockwise.

12. Rotate knob P1 (from the initial position to its maximum limit) gradually and record
Anode current for the respective value of anode voltages.

13. Plot the graph between anode voltage Va and anode current Ia.
Observation: At constant Ig=2mA






Latching Current and Holding Current:

Latching Current:-

Once the SCR is conducting a forward current, reverse biased
junction J2 no longer exists. As such, no gate current is required for the device to
remain in on state. Therefore, if the gate current is removed, the conduction of current
from anode to cathode remains unaffected. However, if the gate current is reduced to zero
before the rising anode current attains a value, called the latching current, the thyristor
will turn off again.

The gate pulse width should, therefore, be judiciously chosen to
ensure that anode current rises above the latching current. “Thus latching current
may be defined as the minimum value of anode current which it must attain
during turn-on process to maintain conduction when the gate signal is removed”.


Holding Current:-


Once the thyristor is conducting, the gate loses control. The thyristor
can be turned off (or the thyristor can be returned to forward blocking state) only if
the forward current falls below a low – level current called the Holding current.

“Thus holding current may be defined as the minimum value of anode current
below which it must fall for turning -off the thyristor. The latching current is
higher than the holding current”.

Result:

 I-V characteristic of SCR is studied

Precautions:


  1. all the connections should be tight.
  2. the wire is not to be cut from anywhere.
  3.         proper maintenance of voltage and current to SCR.


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