Introduction

A voltage regulator is designed to take a variable input voltage and provide a fixed output voltage.

One very common family of voltage regulators is the 78XX series where the XX is replaced with the output voltage. For example a 7805 is a 5V regulator whereas a 7812 is a 12V regulator etc.

Overview

The 78XX series of voltage regulator are intended to provide a fixed voltage for use with a variety of different circuits. They are available in a range of different voltages and, although only the positive variety are considered here, there is a complimentary range of negative regulators that are essentially identical. The voltage regulators are capable of providing currents of up to 1.5A with adequate heat-sinking. Internal protection circuitry makes them almost indestructible. In other configurations and with extra components, these regulators can be employed as variable voltage sources or constant current sources

Specifications

The most common version of the voltage regulator is supplied in a three pin plastic package with a metal tab for connection to a heat sink. This is the standard TO220 package style. Note that the tab is connected to the common terminal and may therefore need to be electrically isolated from the heat sink

• High Output Current - Typically 1A
• Short circuit protection. Short circuit current limited to ≈200mA
• Thermal protection
• Low Quiescent current (I_Q) ≈5mA
• Maximum input voltage = 35V
• Minimum input voltage = Output voltage + 2V

Operation: The voltage regulator maintains a fixed voltage between the output terminal and the common terminal. For this to happen the input voltage should be approximately 2V higher than the output voltage.

Basic Regulator Circuit

The basic regulator circuit requires only two non-electrolytic capacitors to complete the circuit. The 'Common' is connected to the 0V rail and the input voltage must be at least 2V greater than the expected output voltage.

Increasing the Output Voltage

The regulator maintains a constant voltage between the Common and the Output which, in this case, is the voltage across R1. The current through R1 and the Quiescent current both flow through R2 and the resulting potential difference across R2 is added to the output voltage.

If the regulator's output voltage is V_XX and the Quiescent current is I_Q then the output voltage is given by:

For example, using a 7805 regulator with a Quiescent current of 6mA and R1 = 100Ω with R2 = 470Ω gives V_out = 31 volts. Obviously, by choosing R1 and R2 carefully, any regulated voltage above 5V can be achieved.

Constant Current Source

The regulator maintains a constant voltage across resistor (R) and therefore a constant current flows through R. This current, along with the Quiescent current, is the output current which is fixed for any load attached to the supply.

The output current depends on the voltage of the regulator, V_XX, and the value of the fixed resistor.

The output current is given by:

For example, a 7805 regulator with a Quiescent current of 6mA used with a 220Ω resistor fixes the output current at ≈30mA.

Heatsink Considerations

If the regulator has current flowing through it and a voltage drop between the input and output then the regulator will dissipate energy i.e. get warm. If more than a few tens of mW of power are being dissipated then a heatsink might be necessary. The equation for the power dissipation is:

Consider a 7805 being used to provide a 5V USB compatable power supply from a 12V battery. The maximum current required is 500mA. P_out = (12 − 5) × 0.5 = 3.5W ... a heatsink will definitely be required.