Lab Report: Determining Latching Current and Holding Current of SCR

1. Objective:

To determine the latching current and holding current of an SCR using its V-I characteristics.

2. Apparatus Required:

  • SCR (e.g., TYN612)
  • DC power supply (0–30V)
  • Resistors (100Ω to 1kΩ)
  • Load resistor
  • Ammeter and voltmeter
  • Function generator
  • Breadboard and connecting wires

3. Theory:

Silicon-Controlled Rectifier (SCR)

  • A four-layer, three-junction semiconductor device used for switching and power control.
  • It conducts when a positive gate pulse is applied, provided the anode-cathode voltage exceeds a certain threshold.

Important Parameters:

  • Latching Current (IL_L): The minimum anode current required to maintain conduction immediately after the SCR is triggered.
  • Holding Current (IH_H): The minimum anode current required to maintain conduction after the gate signal is removed.

4. Circuit Diagram:

Provide a simple circuit diagram with:

  • SCR in series with a DC supply and load resistor
  • Gate terminal connected to the function generator through a gate resistor
  • Measuring instruments (ammeter and voltmeter) connected appropriately.

5. Procedure:

Step 1: V-I Characteristics Measurement

  1. Connect the SCR in the circuit as per the diagram.
  2. Apply a small gate pulse using the function generator.
  3. Gradually increase the anode voltage from the DC power supply.
  4. Record the voltage and current values for various points on the V-I curve.

Step 2: Determining Latching Current (IL_L)

  1. After triggering the SCR, reduce the anode current by slowly decreasing the voltage.
  2. Observe the point where the SCR just starts conducting without turning off when the gate pulse is removed.
  3. Record this current as the latching current (IL_L).

Step 3: Determining Holding Current (IH_H)

  1. Once the SCR is conducting, continue reducing the anode current.
  2. Identify the point where the SCR turns off.
  3. Record this current as the holding current (IH_H).

6. Observations:

Voltage (V) Current (mA) Observation
Trigger point
Latching point
Holding point

7. Results:

  • Latching Current (IL_L) = [Insert Value] mA
  • Holding Current (IH_H) = [Insert Value] mA

8. Conclusion:

  • The latching current and holding current values were successfully determined for the given SCR.
  • These parameters are critical for designing reliable SCR-based power control circuits.