This project provides the schematic & 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 in case you use plenty
of batteries for your basic electronics project.
Two DC voltage outputs are available; 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 maximum input voltage must be two
volts higher than the regulated output voltage.
Variable DC Power Supply Circuit Diagram
The DC Power Supply circuit is based around the 7805 voltage regulator.
It's only three connections input, output & ground & it provides
a fixed output. The last digits of the part number specify the output
voltage, e g. 05, 06, 08, ten, 12,15, 18, or 24. The 7800 series
provides up to one amp load current & has on-chip circuitry to close
down the regulator if any attempt is made to operate it outside its
safe operating area.It can be seen that there's in fact separate
circuits in this power supply. 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 maximum 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 giant
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 tiny applications the heat sinks won't be needed. The tab on the
regulator will dissipate 2W at 25 o C in air. (This is equivalent, for
example, to an input voltage of 9V, an output of 5V & drawing 500 m
A.) 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 basically add
voltage & current meters to it and put it in to an appropriate
plastic case connected to a transformer.
Trouble Shooting Procedure
An LED has been put in to 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 parts are in their correct position on the PCB. Other items to check are to make sure that the regulators, electrolytic capacitor & bridge rectifier are inserted in the correct orientation.
An LED has been put in to 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 parts are in their correct position on the PCB. Other items to check are to make sure that the regulators, electrolytic capacitor & bridge rectifier are inserted in the correct orientation.
