This Raspberry Pi power supply circuit, also known as Sally after my cat, is a high-end circuit to power the Raspberry Pi. Based on the 7805 linear voltage regulator IC, it provides a precise 5.0 V output no matter the load you put on the voltage rails. The circuit also provides a power switch and an LED indicator.
Sally being my cat, is extremely intelligent, and a fully qualified electronic engineer. She has more intelligence in her little paw finger than many PhD experts have. She together with Mr Einstein, and Cliff, her cat friends, has designed this circuit to earn their keep.
If you have many orphaned adapters, but none of them providing the necessary 5 V, then this circuit will bring the output voltage down to 5 V and regulate it. Just feed the DC voltage at the input of the circuit, and it will provide a regulated 5 V output.
I had some nice transformer based adapters providing 9 V, and 12 V, but none that provided 5 V. However, with these conventional adapters, all that is required is a voltage regulator to bring the voltage down to the correct level. Luckily, I already had plenty of recycled components that I could use to build an inexpensive power supply.
This power supply circuit is for the old-fashioned transformer-based adapters, and not for the modern switched mode power supply unit (SMPSU) adapters. Transformer based adapters use a heavy iron-core transformer with coil windings and they provide the cleanest current. Consequently, they are often utilised in amplifiers.
The input current to this circuit must be a DC current from the adapter. This circuit is only for powering the RPi, and a USB socket is provided to make a connection using the existing cable. This circuit is not a USB charger for batteries as this is not its intended purpose.
Circuit Board Construction
The circuit construction is on prototyping board that you can get on eBay at ten for around a pound. If you are new to soldering, then please refer to the, A Soldering Guide for Beginners, Kids, and Electronics Students, article.
I am using the cheapest components that are readily available to make this circuit, However if you already have some components and perhaps access to recycled components then you can make it even cheaper for the pocket. I know how these days, you buy just a couple of components, and ten pounds vanishes!
In the process of designing this circuit — like most engineers — Sally thought it would be nice if the circuit also provided 3.3 V power as well, which could be handy for those breadboard projects. In addition, life could be so much simpler if there was an LED indicator, and a switch. Honestly, if you need just the 5 V rail then you could do away with all the bourgeois extravagance.
If you have some recycled electrolytic capacitors then you can use some nice high values such as 1000 µF for C5. This way even if you have a cheap adapter providing poor smoothing, C5 can compensate and take care of that deficiency. C4 does not need to be very high, but anything between 330 µF and 470 µF will be good enough. These electrolytic capacitors must be rated 16 V or more. Since this is a small board, a cylindrical shaped capacitor with the smallest diameter is useful to make a neat build. For more information on capacitors, please refer to the Standard Capacitor Values – Electrolytic article.
Capacitor C2 is for noise decoupling and typically a ceramic disc type. Anything in the range of 47 nF will work to filter out digital switching noise on the voltage planes.
LED and Series Resistance
If you are using some of those high efficiency LEDs from eBay with a forward voltage of 1.7 V and forward current of 0.002 A, then you will need a series resistance of 1.5 kΩ. For more information, please refer to the LED Resistor Calculator article.
A 1.5-kΩ resistor is perfect, and when you switch OFF the power, you can see the LED fade out slowly as the 1000 µF capacitor discharges.
Power Supply with a Switch
For the Raspberry Pi owners a power supply with a switch is the most useful. If you are programming in assembly language or C++, then your code might crash and you will need to reset it. You could pull the Micro USB B plug out, however if you do that continuously throughout the day, then very soon the contacts will get loose and wear out. Hence, some people pull out the adapter from the mains socket instead. However if you pull the adapter out very fast then that can cause a massive back EMF spike in the power supply line which will either kill the Pi, or destroy the adapter, or both.
To solve this problem Sally decided to fit a small switch to the circuit. The switch cuts the power input to the 7805 regulator, which means that the charge held in C4 discharges very slowly, and the Raspberry Pi switches OFF gently and gracefully.
A fuse is a very important safety device that protects the adapter and circuit from over current situations. F1 and F3 are surface-mount device (SMD) polyfuse rated at one ampere. It is an excellent low-cost solution for protecting the circuit and the adapter. Since the 7805 has built-in protection, F3 is not that critical, however F1 must be included. The fuses are available at the rate of ten for a pound on eBay.
Where to buy Raspberry Pi Power Supply
I often get emails from people asking me where to buy a Raspberry Pi power supply and my first thought is always, what Google does not work in your corner of the galaxy… However, that is an easy one to answer because there are so many great online companies selling them and I am happy to recommend all of them.
People also ask silly questions such as; does the Raspberry Pi come with power supply? It is always nice to see the look on their faces when I tell them they have to make it themselves.
The best power supply is the one that the foundation is selling, because it is specifically for the purposes of powering the computer. In addition, buying from there means helping a charity foundation to raise funds for children’s education. Therefore, my recommendation is to buy it from them. However if you cannot buy it in your country then there are still many other options. Just to name a few, here is a list of places where you can buy a power supply.
Radio Shack or Love Shack as I call it from the famous 1980s song 🙂 is my favourite place when I am in USA and is always worth visiting.
Farnell always stock genuine quality adapters and power supplies.
Element14 is another source of fantastic products for the Raspberry Pi.
Maplin is my favourite corner shop in Croydon and they always keep everything. If you have a local shop then you can save on postage and get it straight away!
Dealextreme often have great deals worth looking at.
Amazon and eBay are gigantic global selling markets catering to a worldwide audience. Therefore, you will be able to find a wide range of power supply products. It tends to be a mixed bag of sellers, because there will be established companies, as well as new manufacturers from China.
For more detail: Raspberry Pi Power Supply Circuit