Lab Report: Observation of Output Waveform of Full-Wave Controlled Rectifier with R Load, RL Load, and Freewheeling Diode Using CRO

1. Objective:

  • To observe the output waveform of a full-wave controlled rectifier using a CRO for different load configurations:
    • Pure resistive load (R)
    • Inductive load (RL)
    • Freewheeling diode configuration
  • To determine the average load voltage for each case.

2. Apparatus Required:

  • SCRs (2 or 4 depending on bridge configuration)
  • Resistors (100Ω to 1kΩ)
  • Inductors (10mH to 50mH)
  • Freewheeling diode (e.g., 1N5408)
  • AC Power Supply (0–230V, 50Hz)
  • CRO (Cathode Ray Oscilloscope)
  • Function generator for SCR gate pulses
  • Breadboard and connecting wires

3. Theory:

Full-Wave Controlled Rectifier:

A full-wave controlled rectifier allows current to flow in both positive and negative half cycles of the AC supply. By controlling the firing angle (α\alpha) of the SCRs, the output waveform can be modified.

Key Configurations:
  1. R Load: Pure resistive load results in sharp current waveforms.
  2. RL Load: Due to energy storage in the inductor, current flow continues even after the supply voltage becomes zero.
  3. Freewheeling Diode: Helps maintain continuous current flow in RL loads by providing a path during the off-period of the supply.
Load Voltage Calculation:
Vavg=2Vm(1+cos⁡(α))Ï€V_{avg} = \frac{2V_m(1 + \cos(\alpha))}{\pi}

Where:

  • VmV_m = Peak AC voltage
  • α\alpha = Firing angle

4. Circuit Diagrams:

  • Bridge configuration using two or four SCRs for full-wave rectification
  • Show R, RL loads, and the freewheeling diode connected across the load.

5. Procedure:

Step 1: R Load Observation

  1. Connect the full-wave controlled rectifier circuit with a pure resistive load.
  2. Apply gate pulses to the SCRs using the function generator.
  3. Connect the CRO probes across the load resistor.
  4. Observe and record the output waveform for different firing angles (α\alpha).
  5. Measure the peak voltage and firing angle to calculate VavgV_{avg}.

Step 2: RL Load Observation

  1. Replace the resistor with a series combination of resistor and inductor.
  2. Observe and record the waveform across the RL load on the CRO.
  3. Note the continuation of current flow beyond zero voltage due to the inductor.

Step 3: Freewheeling Diode Observation

  1. Connect the freewheeling diode across the RL load in parallel.
  2. Observe the waveform on the CRO.
  3. Note the smoother waveform due to the diode conducting during the off-period.

6. Observations:

Configuration Firing Angle (α\alpha) Peak Voltage (VmV_m) Waveform Observation Calculated Load Voltage (VavgV_{avg})
R Load
RL Load
RL with Diode

7. Results:

  • For R load, the output waveform followed the input with controlled conduction periods.
  • For RL load, the current waveform continued beyond the zero crossing due to inductive energy storage.
  • With the freewheeling diode, the waveform was smoother, showing reduced ripple and continuous current flow.

8. Conclusion:

The experiment successfully demonstrated the operation of a full-wave controlled rectifier with different load configurations. The use of a freewheeling diode significantly improved waveform continuity and reduced ripple for RL loads.