By the end of this article you will know what linear voltage regulators are, how they work, some very hot limitations, and why you should keep them around. 

Whenever we are building an electronic circuit there is one very important thing that we need to make sure is there. Of course I am talking about a power source! Since if we didn’t have any power to begin with our circuit would, well, do nothing at all. But this also brings up an issue that we may face, how to make sure our circuit has a clean and reliable power source. Another issue is that our circuit will be calculated to run at a certain voltage, say 5 volts for example, but what if the power brick we plug into the wall provides 12 volts?

We need some way to take a higher voltage and lower it for use in our circuit. Thankfully since this is a common thing to do there is an electrical component called a linear voltage regulator that does just that! If we take a look at one we will see that there are three pins and all we need to do is hook up the input to our voltage source, the common pin to ground, and the output pin to our circuit.

But wait a second, if we did this we might be welcomed with a very unpleasant surprise in the form of our voltage regulator burning up. You may be asking yourself why would someone make an electrical component that can burn up when doing the very thing it was designed to do. 

This has to do with conservation of energy. We can calculate the amount of energy that our circuit is using by multiplying the current and the voltage together. For instance if we had a 5v circuit drawing 2 amps of current we would be using 10 watts.

But now say we wanted to make ourselves a phone charger from the 12 volt supply from our car. We know that the current going through the circuit will not change, and on one side of the regulator we have 12 volts and 2 amps and the other side still has 5 volts and 2 amps. If we calculate the power of each side of the regulator we can see we are feeding 24 watts into the linear regulator and are only getting 10 out the other end. And this is because the linear voltage regulator is taking that extra energy and is converting it into heat. This means that the regulator will be responsible for dissipating the power difference which is 14 watts! See this page here to find a helpful calculator.

This brings us to the first main limitation of the linear regulator, which is that it is not very energy efficient. Because of this you will only really reach for this type of regulator if you are dealing with low power embedded circuits. But still these components are used in many electronic devices, although you will see that they generally are accompanied by a large heatsink or the PCB has been designed to dissipate the heat. If you were to look at the schematic of the Arduino development board you would see that it uses linear voltage regulators for its power needs!

Now let's look at how we can use a linear voltage regulator in our circuit. First we need to know what our desired output voltage and current are. Let's stick with 5 volts and 2 amps for this example. Next, we need to know what our input voltage is. We also need to make sure that input voltage is higher than the output voltage or else the linear voltage regulator won't work. If you need to increase the voltage you will need to use a boost converter.

There are two types of linear voltage regulators: fixed voltage and variable voltage. With a fixed voltage linear regulator it comes pre-set with a certain voltage for example some common part numbers are: L7805 for a 5 volt regulator and an L7809 for a 9 volt regulator. While these are great if we have a standard voltage there will be times when you need an odd voltage, or to change the voltage dynamically.

For these applications we can use a variable linear voltage regulator which uses resistors to give feedback to the regulator to set the desired output voltage. The datasheet will provide the various calculations depending on your circuit setup. 

As with most integrated circuits that we use in our own circuit designs we need to keep in mind that most of them recommend or require certain capacitors to be used to help keep the power supply reliable. These values can also be found in the datasheet.

I always keep a kit of assorted linear voltage regulators on my electronics bench since I always find myself reaching for them.