Theory - 11 :- VDR and LDR

Voltage Dependent Resistor (VDR)

1. Definition and Basic Function

A Voltage Dependent Resistor (VDR), also known as a Varistor, is a variable resistor whose resistance changes in relation to the applied voltage. Its defining characteristic is a sharp decrease in resistance when the voltage across it surpasses a specific threshold. This property makes VDRs ideal for overvoltage and surge protection in electronic circuits.

2. Working Principle

• Under normal operating voltages, a VDR exhibits very high resistance and allows only a small leakage current to pass.

• When the applied voltage exceeds the clamping voltage (threshold), the VDR's resistance drops dramatically.

• This creates a low-resistance path, causing a large avalanche current to flow through the varistor, thereby diverting the surge away from the protected circuit components.

3. Voltage-Current (V-I) Characteristics

The relationship between current and voltage in a VDR is non-linear and similar to that of a diode.

• Below the clamping voltage, the current is very small (standby/leakage current).

• Above the clamping voltage, the current increases sharply and non-linearly with a small change in voltage.

4. Types of VDR

• Metal Oxide Varistor (MOV): This is the most common type. It consists of a sintered matrix of zinc oxide (ZnO) grains, which collectively act like a network of series and parallel connected semiconductor junctions.

• Silicon Carbide Varistors: Made from sintered silicon carbide, these are suited for high-power and high-voltage applications. Their main drawback is a high standby current, which leads to significant power dissipation.

5. Key Performance Indicators

• Clamping Voltage: The voltage at which the varistor begins to conduct significantly.

• Maximum Rated AC/DC Voltage: The maximum continuous voltage the VDR can handle without damage.

• Maximum Pulse Energy: The maximum energy (in joules) the VDR can absorb from a single surge.

• Peak Current: The maximum surge current the VDR can withstand.

• Standby Current: The small leakage current that flows through the VDR under normal operating voltage.

6. Important Application Considerations

• Degradation: Repeated exposure to voltage surges can lower the clamping voltage of a varistor over time.

• Safety: This degradation can eventually lead to a short circuit, creating a fire hazard. To mitigate this risk, it is crucial to:

  1. Use a varistor with a sufficiently high clamping voltage for the application.

  2. Connect a thermal fuse in series with the VDR.

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Light Dependent Resistor (LDR)

1. Definition and Basic Function

A Light Dependent Resistor (LDR), also called a Photoresistor, is a resistor whose resistance varies with the intensity of incident light. Specifically, its resistance decreases as light intensity increases, and vice versa. This property allows LDRs to be used in light-sensing circuits to distinguish between light and dark conditions.

2. Working Principle (Photoconductivity)

• LDRs are made from high-resistance semiconductor materials (e.g., Cadmium Sulfide).

• In the dark, the material has very few free charge carriers, resulting in extremely high resistance (up to 1 MΩ).

• When light (photons) strikes the semiconductor, its energy excites bound electrons, allowing them to jump into the conduction band and become free charge carriers.

• This increase in free carriers dramatically lowers the material's resistance (down to a few hundred ohms in bright light).

3. Types of LDR

• Intrinsic Photoresistors: Made from pure, un-doped semiconductors like Silicon or Germanium. They generally have lower sensitivity.

• Extrinsic Photoresistors: Constructed from semiconductors doped with impurities. Doping enhances the photoconductive effect, resulting in higher sensitivity to light.

4. Key Performance Indicators

• Sensitivity: Refers to how the LDR's resistance changes relative to light intensity. This relationship is non-linear. The sensitivity can also be affected by temperature, so the Maximum Power Dissipation is an important parameter.

  • Dark Resistance: The resistance in complete darkness.

  • Typical Resistance: Resistance at a specified light level.

• Wavelength Dependence: LDRs exhibit varying sensitivity to different wavelengths (colors) of light, described by a spectral response curve. Most are designed to be sensitive to the visible light spectrum.

  • Peak Wavelength: The wavelength at which the LDR is most sensitive.

• Latency (Response Time): The change in resistance is not instantaneous.

  • Rise Time: It can take up to a second for the resistance to rise to its maximum value when light is removed (dark situation).

  • Fall Time: The resistance drops much faster, typically around 10 ms, upon illumination.

5. LDR vs. Photodiodes and Phototransistors

• Due to their high latency and susceptibility to thermal effects, LDRs are not ideal for applications requiring high precision or speed.

• Photodiodes and phototransistors are active components with true PN junctions. They offer much sharper sensitivity and significantly lower latency (faster response times).

• LDRs remain useful in simple, non-critical circuits where the goal is only to detect a general light vs. dark state. A unique application is in audio compressors, where their slow latency is beneficial for smoothing audio signals.