Voltage regulation is a very simple but important topic. Voltage regulators can be obtained from among several styles and output capabilities. The 78XX series of regulators is popular. The 79XX series provide negative power supply voltages. They are easy to use with minimum external components required. The last number in the chip code tells the output voltage. The input voltage must be at least 3V greater than the regulator output or otherwise the regulator does not work well. The regulators come in different packages. The TO-220 style (no suffix or T in the part number) is limited to less than one amp. The TO-3 style (K suffix) offers up to several amps. The small TO-92 (L suffix) packages supply up to 500 mA.

A circuit diagram of a 5V supply is shown in Figure 1.

Figure 1.

Pinout of the 7805 regulator IC.

1. Unregulated voltage in

2. Ground

3. Regulated voltage out

It is not necessary to bypass the output, although it improves transient response. Input filtering and decoupling is needed only if the regulator is located more than six inches from the input filter capacitors of the power supply. The exact values of the capacitors aren’t real important but they must be rated to handle at least the supply voltage.

The LM317 adjustable voltage regulator is also useful. It requires only two external resistors to set the output voltage. Line and load regulation is better than standard fixed regulators explained previously. The adjustment terminal can also by bypassed to achieve very high ripple rejection ratios. A typical circuit is shown in Figure 2.

Figure 2.

 These regulators are great but they are a bit wasteful because you have to provide at least 3 volts greater than the desired output. For circuits that can stand some fluctuation zeners do a reasonable job.

Author: Phillip Jacobsen