Building a Regulated 5V DC power supply
A regulated 5 volt DC supply is essential for powering micro-controller and TTL based circuits. The output of most wall-warts and adapters is to rippled and impure for use in digital circuits. Lets build an inexpensive power supply using some discrete components and a fixed voltage regulator IC.
You will need:
- a step down transformer [12V]
- four silicon diodes [1N4007]
- a resistor [47Ω]
- capacitors [2 x 220µF, 10µF and 100µF]
- three terminal voltage regulator IC [LM7805]
- a small general propose PCB, some wires, and a suitable output port (I use a
- audio connector).
The circuit consists of three main blocks, the rectifier, filter and regulator. The rectifier is used to transform the mains AC voltage to a suitable DC voltage. The output of the rectifier is however an impure DC signal so we use a filter to clean the signal and finally a regulator to deliver precisely 5 volts, irrespective of the load connected to the output.
It consists of a transformer and a diode bridge. The diodes are standard silicon 1N4007 diodes. We have chosen a 12V transformer because the regulator IC needs at-least 7.5V of input voltage to function properly.
We use a pi-filter here. The two 220µF capacitors and resistor form the filter. Pi-filters are great for light load applications like digital circuits. Be sure to check the polarity of the capacitors before connecting them. The resistor shown above is one rated to dissipate up-to 5W of power across it. You may use smaller 3W resistors, but availability may be an issue. Don't use the tiny 0.25 W or 0.5W ones though.
The voltage is regulated by three terminal voltage regulator IC – LM7805. This regulator provides stable 5V DC output against large fluctuations in input voltage and load. It also has internal protection circuits which 'brownout' the device when overloaded. To decide the pin-outs, hold the regulator with its face towards you and legs pointing upward, the pin to the right is the input, middle pin is ground and left most pin is output.
- The transformer can be made to snugly fit onto the PCB by bending the screw mounts under and around the board as shown in the picture below.
- A 6.3mm or 3.5mm female mono TRS connector (tip, ring, sleeve), also known as an 'audio jack' serves as a great output port. The power can then be delivered via standard audio cables.
- Since the power resistors is quite large, solder it vertically to save space on the board.
- Mount the entire board on thermocol when finished to prevent accidental short circuits.
The finished circuit fits neatly into a 2” by 1.5” PCB, excluding the output jack and plug. You can also include a small switch at the out-put for easy operation. The IC can be heatsink-ed on multiple layers of aluminum foil to dissipate the heat, or if you are enclosing this in a metal box, pin the IC to the sides of the box. This is not entirely necessary though.
Questions and suggestions are welcome, please use the comment form below.
Update: You can grab the Eagle CAD project files, including PCB layout here: http://www.mediafire.com/?mqjiwcneyy4
Note: If you have any question, please feel free to contact me.