Article : Andy Collinson
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Description
You can buy ready made power supplies for the Raspberry Pi. These are good but like all switch mode power supplies, create havoc if you are a short wave listener. Here are two alternate projects to power your Raspberry Pi. The first one involves salvaging an old computer and the second one is a do-it-yourself power supply using a ready made step down converter available on Ebay.

 

PSU from Old PC

Before throwing out your old computer, consider its usefulness. The case is sheilded and makes an ideal housing for other projects, like a Raspberry Pi media centre. The ATX power supply can also be put to use powering the Raspberry Pi. Old ATX power supplies made before 1990 may require a load to keep the output voltage within 5% of rated voltage. Power supplies made after 1990, like the one in this picture maintain load voltage with no or light loads.

Connector

The ATX connector shown right (click to zoom) will have 20 or 24 pins depending on its age. To start PSU without a motherboard being connected, the green and black wires need to be shorted. The green wire is a sense wire and black wire is earth. You can connect these permanently with a short piece of wire, or wire them to a switch.
In the centre diagram I have marked the green and black wires with a black marker pin. With the ATX PSU plugged into an electric socket and green and black shorted, the power supply will start. Later models may have a cooling fan, but make no noise otherwise. If you have the 4 way molex connectors and a voltmeter you should measure 12Vdc between yellow and black wires. and 5Vdc between red and black wires.

Cheap Voltage Monitor

If you want to add a cheap voltage monitor, these are available from many places including Ebay and will show you the output voltage. They are not that accuate (about 5%) but a useful addition to the project.

 

I used an old ATX case to house my Raspberry Pi. Sorry about the photo quality in this image but the display reads 4.8V. I included a serial fuse between the 5V line and the raspberry Pi. Everything was fine for two years, then one day after a power cut, the ATX failed to start. There was no damage to the Raspberry Pi, so I made a new power supply with a transformer, details in next project, DIY power supply below.

DIY Power Supply

If you don't have an old computer, then you can build a more traditional power supply. You first need to consider load requirements. The early model A and B Raspberry Pi's used about 500mA. The later model 2 and model 3 Pi's can power up to 4 devices on the USB ports and recommended power supply is now rated up to 2.5A. See link below:
Raspberry Pi Power Requirements

The transformer primary should match your local line voltage, 230V for the UK and the secondary is rated at 15V 4amp. This allows for a 3 amp 5 Volt supply and a 12 Volt 1 amp secondary supply. Some peripherals e.g. external hard drives also require a 12V supply.

LM2596 Step down regulator.

The LM2596 or LM2596S is a DC to DC step down buck regulator. It has a maximum current output of 3A and can regulate from 37V down to 1.3V. Note that this regulator only steps down, and the input voltage must be a couple of volts higher than the output regulated voltage. This is why the transformer secondary is rated at 15V. These are widely available on Ebay and it is impossible to build a cheaper regulator than the assembled product. A multi turn adjuster is used to set the output voltage.

 

Connecting to your Raspberry Pi

All versions have the Raspberry Pi use a Micro USB connector. A close up of a micro USB plug is shown left. You can buy ready made USB to Micro USB cables from Poundworld as shown on the right (packaging may change).

To connect to the Raspberry Pi you can either use a USB socket or cut the cable in half and connect the wires to the Raspberry Pi.

There is a "spacer" in the plug of the USB2 plug. This ensures it only fits into the USB2 socket one way round. The outer two wires supply power to/from USB devices while the inner two wires carry the data signal. When you cut a USB charger cable in half, you may find that the wires colours are similar, if in doubt check for 5V on the red and black wires with a voltmeter.

Links and References
ATX Power Supply
ATX PSU Design
Ebay LED DMM
Ebay Molex
Poundland
More USB Cable Pinout