Lab Report: Operation of Class D, E, and F Turn-off Circuits for SCR

1. Objective:

To perform and analyze the operation of Class D, E, and F commutation circuits for SCR, and understand their practical applications and working principles.

2. Apparatus Required:

  • SCR (e.g., TYN612)
  • DC Power Supply (0–30V)
  • Resistors (100Ω to 10kΩ)
  • Capacitors (0.1µF to 100µF)
  • Inductors (10mH to 50mH)
  • Load resistor
  • Function generator
  • Oscilloscope
  • Breadboard and connecting wires

3. Theory:

Class D (Auxiliary Commutation):

  • In this circuit, an auxiliary SCR is used to apply a reverse voltage across the main SCR for turn-off.
  • The auxiliary SCR conducts briefly, reversing the main SCR's current to zero and forcing it to turn off.

Class E (External Pulse Commutation):

  • In this method, an external pulse generator delivers a sharp negative pulse across the SCR, causing it to turn off.
  • No auxiliary SCR or resonant components are required.

Class F (AC Line Commutation):

  • In this circuit, the AC line voltage itself provides the natural commutation for the SCR during the negative half-cycle.
  • Commonly used in AC power control applications.

4. Circuit Diagrams:

Include diagrams for each circuit showing component connections and SCR configurations.

5. Procedure:

Step 1: Class D Turn-off Circuit (Auxiliary Commutation)

  1. Connect the main SCR in series with the load and DC power supply.
  2. Connect an auxiliary SCR with a commutating capacitor across the main SCR.
  3. Apply the triggering pulse to the main SCR.
  4. After the main SCR starts conducting, trigger the auxiliary SCR.
  5. Observe the commutation process and verify the turn-off of the main SCR using the oscilloscope.

Step 2: Class E Turn-off Circuit (External Pulse Commutation)

  1. Connect the SCR in series with the load and DC power supply.
  2. Use an external pulse generator to deliver a sharp reverse pulse across the SCR.
  3. Apply a triggering pulse to the SCR and allow it to conduct.
  4. Apply the external reverse pulse and observe the SCR turning off.

Step 3: Class F Turn-off Circuit (AC Line Commutation)

  1. Connect the SCR with the load across an AC power supply.
  2. Trigger the SCR during the positive half-cycle of the AC waveform.
  3. Observe that the SCR naturally turns off during the negative half-cycle due to the reverse voltage from the AC supply.

6. Observations:

Circuit Type Supply Voltage (V) Load Current (A) Turn-off Observation
Class D
Class E
Class F

7. Results:

  • Class D: Successful turn-off observed using auxiliary SCR.
  • Class E: Proper commutation achieved using external pulse.
  • Class F: Natural commutation observed during the negative AC half-cycle.

8. Conclusion:

The experiment demonstrated the operation of Class D, E, and F commutation circuits. Each technique is suitable for different power electronics applications, including DC motor control and AC power regulation.