LINEAR VOLTAGE REGULATORS 78XX

Here we would like to show you how to work with 78XX linear voltage regulators. We will explain how to connect them to a power circuit and what are the limitations of using voltage regulators.

Here we can see regulators for: 5V, 6V, 9V, 12V, 18V, 24V. To complete all the exercises you will need components listed below:

• LM7805, LM7812
• Li-Ion 7.4 V Battery Pack
• Li-Po 14.8 V Battery
• 01. and 0.33 uF electrolytic or ceramic capacitors
• Breadboard, Jumper Wires
• Arduino Uno

Pinout for LM78XX is the same for each of them. As you can see from the image above, the leftmost pin is input, middle pin and the big terminal on the top of the regulator are ground, and the rightmost terminal is output (regulated voltage).

• IN Here we connect the red wire (plus terminal) from the battery
• GND Here we connect the black wire (common ground) from the battery
• OUT Here we connect the power distribution circuit input (any device that we are charging), for LM7805 this pin will output 5V.

The circuit we are about to build is the same for all LM78XX voltage regulators. This circuit is for fixed output. We only need a regulator and two capacitors 0.1 uF and 0.33 uF to make it. Here is how the circuit looks like on a breadboard:

The wiring steps are as follows:

• Connect the LM78XX to the breadboard.
• Connect the 0.1 uF capacitor with the IN pin. If you are using electrolytic capacitors be sure to connect the - to the GND.
• Connect the 0.33 uF capacitor with the OUT pin.
• Connect the IN with the plus terminal of the power source
• Connect the GND with the minus terminal of the power source
• Connect the OUT pin with the plus terminal of the device you would like to charge.

The circuit for LM7805 will give as an output steady 5V current. Important thing here to consider is how big should the input be? The necessary voltage drop for the regulator to work properly is 2V that means that the minimum voltage should be 7V. Keep in mind that as the batteries deplete the voltage inside them drops. To learn more about batteries please refer to that section.

Here we are going to use 2x 3.7 Li-Ion Batteries in series. That will provide us with a mean value of 7.4 V. Which is perfect for our case, we will have voltage drop of 2.4 V. All the voltage dropped is turned into heat. So you want to keep the drop to the minimum.

Another perfect battery for this case would be 2S Li-Po battery, the issue here would be the connectors that usually come with these batteries. Please refer to the Battery or connector section to learn more.

As a last note: the most convenient battery to use would be 9 V Alkaline battery, just keep in mind that you are dropping 4 V from the battery if you use it. It’s the most convenient because it’s easily found in local stores.

The output current is used to charge Arduino Uno through a 5V I/O pin. The ground is connected with the common ground of the battery and the regulator. You can choose to power up as many 5V devices as you can find in this way.

The circuit for LM7812 differs from LM7805 circuit only in the input and output voltage. We still have a 2V drop, meaning that we need at least 14V. Perfect for this situation is the 4S Li-Po Battery which has voltage of 14.8 V.

Now we have a 12V power source, but for what can we use it? There aren’t many controllers such as Arduino that run on 12 V, or modules such as PS2 Joystick. They are all 5V or even 3.3V. Most obvious things that we power up with 12V are the motors. Let’s talk about that in the next section.

LM78XX regulators are great if we need to power up devices requiring low currents. Such as controllers, drivers, modules, sensors etc. We can also use them to power up weak motors such as servo motors SG90, mini-gearmotors. But if we need to power up typical motors used to move robots or racing cars we would need to have larger currents.

We almost never have only one motor on our robots, we tend to have about 4 motors, and they usually total a minimum of 3.5 A in steady current demand.

LM78XX voltage regulators have 1-1.5 A steady current rating, depending on the manufacturer. Just to be safe let’s say we have 1 A steady current limit. Peak current for these regulators would be 2.2 A, just to put it in contrast 4 gearmotors would have peak current of about 9.6 A.

As you can see we can’t really use these regulators for such practices. Keep in mind that we can’t put multiple regulators together to have higher current ratings.

We would like to summarize what we have shown here.

• LM78XX are used to create fixed voltage output
• All LM78XX have the same circuit
• We need to have 2V more on the input than what we expect to have on output
• Steady Current rating is 1 A or 1.5 A depending on the manufacturer

If you would like to know how to power up devices requiring more current, please refer to our section on DC-DC Converters.

You can download the models we have used in this tutorial from our GrabCAD account:

GrabCAD Robottronic Models

You can see our other tutorials on Instructables:

Instructables Robottronic

You can also check Youtube channel that is still in the process of kicking off:

Youtube Robottronic