Constructing your own Variable DC Power Supply

This project provides the schematic and the parts list needed to construct a simple DC Power Supply from an input power supply of 7-20 V AC or 7-30V DC. This project will come in handy especially if you use a lot of batteries for your basic electronics project.

Two DC voltage outputs are available; one is a fixed regulated 5V for TTL use. The other output is variable from 5V upwards. The maximum output voltage depends on the input voltage. The specified maximum input DC voltage to the regulator is 35V. The minimum input voltage must be 2 volts higher than the regulated output voltage.

CIRCUIT DESCRIPTION

The DC Power Supply circuit is based around the 7805 voltage regulator. It has only 3 connections (input, output and ground) and it provides a fixed output. The last two digits of the part number specify the output voltage, eg. 05, 06, 08, 10, 12, 15, 18, or 24. The 7800 series provides up to 1 amp load current and has on-chip circuitry to shut down the regulator if any attempt is made to operate it outside its safe operating area. (If this happens to you, let the chip cool down & attach the heatsink.) It can be seen that there are in fact two separate circuits in this power supply. One 7805 is directly connected as a fixed 5V regulator. The second 7805 has a resistor divider network on the output. A variable 500 ohm potentiometer is used to vary the output voltage from a minimum of 5V up to the maximum DC voltage depending on the input voltage. It will be about 2V below the input DC voltage.)

The capacitor across the output improves transient response. The large capacitor across the input is a filter capacitor to help smooth out ripple in the rectified AC voltage. The larger the filter capacitor the lower the ripple.

For small applications the heat sinks will not be needed. The tab on the regulator will dissipate 2W at 25 oC just in air. (This is equivalent, for example, to an input voltage of 9V, an output of 5V and drawing 500 mA.) However, as your projects get bigger they will draw more current from the power supply and the regulators will operate at a higher temperature and a heat sink will be needed. You can easily add voltage & current meters to it and put it into a suitable plastic case connected to a transformer.

DC Power Supply Parts List

Trouble Shooting Procedure

An LED has been put into the output of the fixed 5V regulator to indicate that the circuit is working. Poor soldering is the most likely reason that the circuit does not work. Check that all the soldering is done properly. Check that all components are in their correct position on the PCB. Other items to check are to ensure that the regulators, electrolytic capacitor and bridge rectifier are inserted in the correct orientation.

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