Engineering & Technology


LM7805 Voltage Regulator

Many electronic circuits need a specific and steady supply voltage that doesn't fluctuate. A circuit or integrated circuit (chip) that is able to achieve this is called a Voltage Regulator. When a circuit is powered from a battery, as the circuit draws more current the voltage can drop. A voltage regulator helps to avoid this.

Most voltage regulators have three leads:
Input voltage
Output voltage

Voltage regulators are also used in circuits where more than one supply voltage is required. For example where a microcontroller (eg PIC) needs a 5 V supply voltage but motors connected to it may require 12 V or more.

Some common types are:

Input voltage:  5 V to 35 V
Output voltage: 5 V
Max output current: 1 A

Input voltage:  5 V to 35 V
Output voltage: 12 V
Max output current: 1A

I'm sure you can also work out the codes for voltage regulators that are able to produce 5 V, 6 V, 8 V, 9 V, 10 V ,12 V, 15 V, 18 V, and 24 V.

The L, LM or other letters before the 7805 are codes that indicate the manufacturer of the chip. LM, for example, means it is made by a company called national Semiconductor.

7805 Pinout

The voltage regulator has a metal tab that allows it to be screwed to an aluminium heatsink. This draws the heat away from it and prevents it from overheating.

LM7805 5V Regulator Circuit Diagram

LM317 Voltage Regulator

While the 7805 is a fixed voltage regulator, the LM317 is adjustable. The circuit here shows how the LM317 can be wired up to act as a avariable voltage supply.

The circuit includes a potentiometer, which when rotated, causes the output voltage of the regulator to change.

Like the 7805, the LM317 regulates the output, so even with increases in current demand from the circuit it is powering, the output voltage remains constant.

Provided that the input voltage is equal or above 28 V, the LM317 is able to produce an adjustable output voltage of between 1.2 V and 25 V.

Whereas the LM7805 is able to supply a maximum current of 1 A, the LM317 is capable of supplying 1.5 A.

Take note of the pin layout of the chip as it is not set out as logically as on the 7805.

LM317 Pinout

Take note that the pin layout is slightly different to the 7805

LM317 Variable Voltage Regulator Circuit