CHAPTER 19 (3): VOLTAGE REGULATOR

Overview

  • This chapter focuses on voltage regulators, critical components in electronic systems to maintain consistent output voltage despite varying input or load conditions.

Objectives

  • Understand the block diagram of a basic power supply.

  • Determine line regulation and load regulation.

  • Differentiate between positive and negative voltage regulators.

Basic DC Power Supply

  • Definition: A power supply converts standard AC voltage (230V, 50Hz) into a steady DC voltage for electronic devices.

  • Applications: Powers various devices including:

    • Television receivers

    • Stereo systems

    • VCRs

    • CD players

    • Laboratory equipment

Components of a Basic DC Power Supply

  • Transformer: Steps down the AC voltage.

  • Rectifier: Converts AC to DC voltage.

  • Filter: Smoothens the rectified signal to minimize fluctuations.

  • Voltage Regulator: Maintains a consistent DC voltage despite changes in the input or load.

  • Load: The device that consumes power from the power supply.

Block Diagram of a DC Power Supply

  • Components Involved:

    • AC input (230V, 50Hz)

    • Step-down Transformer

    • Rectifier

    • Filter

    • Voltage Regulator

    • Load

Voltage Regulation

  • Function: Regulators keep output voltage stable under varying conditions:

    • Line Regulation: Keeps output voltage stable when input voltage fluctuates.

    • Load Regulation: Keeps output voltage stable when the load changes.

Line Regulation

  • Definition: Percentage change in output voltage corresponding to a change in input voltage.

  • Voltage regulators must compensate for changes in AC input to maintain constant output voltage.

Load Regulation

  • Definition: Percentage change in output voltage corresponding to a change in load current.

  • The regulator adjusts the output voltage to stay constant as the load current increases or decreases.

Example of Load Regulation

  • Case Study: A voltage regulator has:

    • No-load output voltage (VNL): 12 V

    • Full-load output voltage (VFL): 11.9 V

    • Load Regulation Calculation:

      • Load Regulation: (VNL - VFL) / VFL x 100%

      • Results in a load regulation of approximately 0.840% from no-load to full-load conditions.

      • Expressed per mA change in load current as 0.084%/mA.

Linear Voltage Regulators

  • Basic Types: Three-terminal devices that provide fixed or variable positive and negative voltages.

  • Positive Linear Regulators: Commonly represented by the 7800 series, providing a fixed positive output voltage.

    • Terminals: Input, output, ground.

    • Capacitors: Output capacitor for transient response; input capacitor for stability.

  • Negative Linear Regulators: Represented by the 7900 series, functioning similarly to the 7800 but providing negative output voltage.

Dual-Polarity Power Supply

  • Components: Schematic includes transformers, diodes, and voltage regulators (7812 for +12V, 7912 for -12V).

Summary

  • Voltage regulators are crucial for maintaining constant DC output voltage.

  • Availability of three-terminal linear IC regulators enables the provision of either fixed or adjustable output voltages.

  • The 7800 and 7900 series are widely used for fixed positive and negative output voltages respectively.