Many electronics experimenters and builders take the lowly 7805 regulator for granted, and we pop it in a circuit without a second thought and move along. But for beginners who are wondering how to power their Arduino or other 5 volt project (such as the QRP Labs U3S) from a 12v-13.8v power supply, the 7805 might not be obvious. Some might argue that the 7805 regulator is a poor choice for various reasons, but overall it is simple, reliable, and easy to work with. On top of that, it’s electronically quiet in that it doesn’t introduce more noise into a circuit. So, let’s take a look!
How does it work?
The 7805 is a linear regulator. It requires at least 7.5 volts to power it, and it puts out 5 volts. The current draw (amperage) is linear- if your project is consuming 200ma at 5 volts, then the regulator is drawing 200ma from its power supply, whatever its voltage. But, where does the extra energy go?
Heat! The higher the input voltage, the more wattage has to be dissipated, and the hotter it gets. I’ve actually burned finger on a 7805 being fed by a 13.8v bench power supply. The solution is very easy though. A small washer attached to the regulator with a small bolt is enough to aid heat dissipation. Another cheap trick is to use a tiny binder clip. It works great.
How to build a 7805 based 5v power supply
There are three pins on the chip. When viewed from the front of the chip where the writing is, the pins are IN, GND, OUT. The GND pin goes to the negative side of the input and the output. The IN pin goes to 7.5 to 14 volts, and OUT is your 5 volt source for your project. Two small capacitors are required, and can be salvaged. I wrote about salvaging parts and also some inexpensive, useful tools here. One capacitor goes across the IN and GND, the other OUT and GND. It’s very simple and just about any small capacitor will work fine. If you use electrolytic capacitors as I did, make sure you orient them correctly. The capacitors are there to dampen any self-oscillations that can happen. There’s Plenty More Reading Here if you want to learn about why that is.
Here is what the schematic looks like:
Here’s the same schematic with a real chip, just for clarity:
And here’s how I built this exact circuit on some PCB Prototyping board I bought on Amazon. It’s got connectors on the input and output so I can power it from several types of supplies, and it puts 5v into an Arduino. You may wish to build it straight into your project.
You can see that I cut down the board from its full size with a pair of cutters. I saved the rest of the board for other small projects. I highly recommend the board set: