Practical - 13 :- Experiment to Indentify and Check VDR And LDR

 

Required Apparatus

• A digital multimeter with resistance measurement functionality (Ohmmeter).

• A variable DC power supply.

• A breadboard for creating a simple circuit.

• Connecting wires.

• A light source (like a flashlight or a lamp).

• The VDR and LDR to be tested.

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Experimental Procedure

This single method involves a series of steps to systematically identify and check both components.

Step 1: Visual Inspection and Initial Resistance Measurement

1. Visual Cue: Visually inspect the two components. LDRs typically have a serpentine pattern on their top surface, which is the light-sensitive area. VDRs are often disc-shaped, resembling a ceramic capacitor, and are commonly blue or brown.

2. Baseline Resistance:

   • Set the multimeter to the resistance measurement mode (Ω).

   • Connect the multimeter probes to the leads of the first component.

   • Record the resistance reading in normal ambient light.

   • Repeat this process for the second component.

Observation and Inference: A functional LDR will show a certain resistance value that is likely to be in the kilohm (kΩ) range in ambient light. A functional VDR, at the low voltage applied by a multimeter, will exhibit a very high resistance, often reading as "OL" (over-limit) or infinite on the multimeter, indicating an open circuit.

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Step 2: Identification and Testing of the LDR

1. Light Test:

   • Take the component that showed a finite resistance in the previous step (the suspected LDR).

   • Shine a bright light directly onto its top surface.

   • Observation: The resistance reading on the multimeter should drop significantly, potentially to a few hundred ohms.

2. Darkness Test:

   • Cover the top surface of the same component with your finger or an opaque object to block out all light.

   • Observation: The resistance reading on the multimeter should increase substantially, possibly into the megaohm (MΩ) range.

Conclusion for LDR: If the component's resistance varies significantly with changes in light intensity, it is identified as an LDR, and this test also confirms its functionality.

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Step 3: Identification and Testing of the VDR  

1. Initial Indication: The component that initially showed an extremely high (or infinite) resistance and did not respond to the light test is the suspected VDR.

2. Voltage Dependence Test (Functional Check):

   • Caution: This step involves a power supply and should be performed with care. Ensure the initial voltage is set to zero.

   • Construct a simple series circuit on the breadboard with the variable DC power supply, the suspected VDR, and the multimeter set to measure current (ammeter). Connect a second multimeter in parallel across the VDR to measure voltage.

   • Slowly begin to increase the voltage from the power supply.

   • Observation: Initially, you will observe a very small, almost negligible, current flowing through the VDR. As you continue to increase the voltage and approach the VDR's rated "clamping voltage" (this value is specific to the VDR model), you will observe a sudden and sharp increase in the current flowing through it. This indicates that the VDR's resistance has dropped dramatically.

Conclusion for VDR: If the component's resistance remains very high at low voltages and then suddenly decreases, allowing a significant current to flow when a specific threshold voltage is reached, it is identified as a VDR, and its basic functionality is confirmed.