Lab Report: Testing the Effect of R and C Variation on Firing Angle of SCR

1. Objective:

To study and test how variations in resistor (R) and capacitor (C) values in R and RC triggering circuits affect the firing angle (\u03B1) of an SCR.

2. Apparatus Required:

  • SCR (e.g., TYN612)
  • Resistors (1k\u03A9 to 100k\u03A9)
  • Capacitors (0.01\u03BCF to 1\u03BCF)
  • Function generator
  • AC Power Supply (0-230V)
  • Oscilloscope
  • Voltmeter
  • Load resistor
  • Breadboard and connecting wires

3. Theory:

R Triggering Circuit:

  • In an R triggering circuit, the firing angle is controlled by adjusting the resistance value.
  • A higher resistance delays the voltage rise at the gate, increasing the firing angle.

RC Triggering Circuit:

  • In an RC triggering circuit, the charging time of the capacitor affects the gate voltage rise.
  • The combination of resistor and capacitor values controls the firing angle by delaying the gate pulse.

Firing Angle (\u03B1):

The angle at which the SCR turns ON during the AC voltage cycle.

4. Circuit Diagram:

  • Provide circuit diagrams for both R and RC triggering configurations.

5. Procedure:

Step 1: Setup of R Triggering Circuit

  1. Connect the circuit with the SCR, resistor, and load as per the diagram.
  2. Connect the gate terminal to the function generator through a variable resistor.
  3. Connect the oscilloscope to measure the voltage waveform.
  4. Apply an AC voltage and gradually vary the resistance.
  5. Measure the firing angle (\u03B1) from the oscilloscope waveform.

Step 2: Setup of RC Triggering Circuit

  1. Connect the SCR with a resistor and capacitor in series across the gate and cathode terminals.
  2. Connect the circuit to the AC supply and the oscilloscope.
  3. Gradually vary the resistance and capacitance values.
  4. Measure the firing angle (\u03B1) for each combination of R and C using the oscilloscope.

6. Observations:

Circuit Type Resistance (R, k\u03A9) Capacitance (C, \u03BCF) Firing Angle (\u03B1, degrees)
R Triggering N/A
RC Triggering

7. Results:

  • R Triggering Circuit: Increasing resistance increases the firing angle.
  • RC Triggering Circuit: The firing angle is influenced by both R and C; higher RC time constants delay the gate pulse and increase the firing angle.

8. Conclusion:

  • The experiment successfully demonstrated that the firing angle of SCR can be controlled by varying R and C values in the respective triggering circuits.
  • RC circuits offer better control for smoother and more precise phase delay compared to R-only circuits.