5 V PSU for USB Devices and Raspberry Pi

This is a power supply circuit, which provides a regulated 5 V output through a female USB socket. There are many USB stick type MP3 players, which connect to the USB socket for power, however, with the correct cable, it is also possible to connect a Raspberry Pi to draw power from it.

I create this to be a sort of “power hub” for devices to draw power from. It was at a time when such adapters were not available. Many Chinese firms saw this idea, and today adapters with a female USB socket are readily available.

Orphaned Adapters

I tested and found many adapters to have poor line regulation, and poor rectification. Some adapters that indicated 5 V DC output were in fact providing a far excessive voltage. Therefore, I figured that a circuit, which regulated the voltage to precisely 5 V and provided extra smoothing was the way forward.

The most common adapter voltages tend to be 5 V, 6 V, 9 V, and 12 V. As it turned out, I had a 9 V DC adapter providing a 1 ampere output which was ideal. It was a nice conventional type with a heavy transformer inside. The 7805 regulator brought the voltage down to 5.1 V, and it was operating in comfort powering the RPi.


The Circuit

I used a 1000 µF smoothing capacitor to provide a nice and clean steady voltage level for all the digital circuits connected to it. Digital devices usually create transient spikes on the voltage planes as gates switch ON and OFF, and therefore it is important to filter this noise if the device is to operate without crashing.

There is also an optional LED indicator with series resistance R to display the power level. Please use the LED Resistor Calculator article to learn how to use the LED. The diode D at the input is also optional as most voltage regulators have internal protection from reverse current. The coil is optional; however, it is easy to make one by turning a thick copper wire around a standard sized pencil.

Voltage Regulator

Voltage Regulator

The 5 V regulator is the most important component in this circuit, and there are many different packages available. A common one currently available from electronics stores is the TS7805, which is a three terminal device. Pin 1 is input, pin 2 is ground, and pin 3 is output. It can manage an input voltage in the range 5 V to 24 V, and maximum current of 1 A.

This regulator is a monolithic IC that operates without any additional components. It also has internal short-circuit current limiting protection.

USB socket Pinout

USB socket Pinout

This is a dual USB socket, as you would see it when plugging in devices. The Vcc and ground pins are on the outside. The two centre pins are for data, which are not used.

The idea for this project was to use 100 % recycled components, which came from old motherboards, and other circuits.