Lab Report: Operation of Class A, B, C Turn-off Circuits for SCR

1. Objective:

To perform and analyze the operation of Class A, B, and C commutation circuits for SCR and understand their 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:

Commutation in SCRs:

Commutation refers to turning off the conducting SCR by forcing its current to zero.

Types of Turn-off Circuits:

  1. Class A (Load Commutation):

    • The SCR is turned off by the natural oscillation of an LC circuit connected in series with the load.
    • Suitable for high-frequency applications.

  2. Class B (Resonant Commutation):

    • The SCR is turned off by resonating the load circuit using an LC combination.
    • External capacitors help reverse the current and turn off the SCR.

  3. Class C (Complementary Commutation):

    • Uses a second SCR to assist in turning off the main SCR.
    • Common in inverter applications.

4. Circuit Diagrams:

Provide circuit diagrams for Class A, B, and C commutation circuits, clearly showing connections of components.

5. Procedure:

Step 1: Class A Turn-off Circuit (Load Commutation)

  1. Connect the SCR in series with an LC circuit and the load as per the circuit diagram.
  2. Apply a triggering pulse to the gate of the SCR.
  3. Observe the current waveform using an oscilloscope.
  4. Verify the oscillatory behavior of the circuit and the turn-off condition of the SCR.

Step 2: Class B Turn-off Circuit (Resonant Commutation)

  1. Connect the SCR with an external capacitor in parallel and an inductor in series with the load.
  2. Apply the triggering pulse to the gate of the SCR.
  3. Monitor the voltage and current waveforms across the SCR.
  4. Record the reverse current flow causing commutation.

Step 3: Class C Turn-off Circuit (Complementary Commutation)

  1. Connect two SCRs in a complementary configuration with load and commutation components.
  2. Trigger the first SCR to conduct.
  3. Apply a gate pulse to the second SCR to turn off the first SCR.
  4. Observe the current waveforms to verify proper commutation.

6. Observations:

Circuit Type Supply Voltage (V) Load Current (A) Turn-off Observation
Class A
Class B
Class C

7. Results:

  • Class A: Successful turn-off due to natural LC oscillations.
  • Class B: Proper commutation observed through reverse current flow.
  • Class C: Complementary SCR successfully turned off the main SCR.

8. Conclusion:

The experiment demonstrated the successful operation of Class A, B, and C commutation circuits. Each circuit has unique characteristics suitable for different power electronics applications, such as inverters and high-frequency switching systems.