FREE CIRCUIT DIAGRAMS 4U: Variable power supply using 7805

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Friday, September 18, 2009

Variable power supply using 7805

This circuit diagram shows you how to make a 5V to 12V variable DC power supply from a fixed 5V regulator IC 7805. This is attained by adding two resistors R1 and R2 as shown in figure. When the resistors R1 and R2 are added the equation for the output voltage of 7805 becomes Vout= Vfixed + { R2 [ (V fixed/R1) + Istandby] } ,where Vfixed=5V and Istandby=Vfixed/R1.By varying the POT R2 you can adjust the output voltage between 5V and 12V.

Notes. * The circuit can be assembled on a vero board. * T1 can be a 230V primary, 9V/5A secondary stepdown transformer. * 7805 must be fitted with a heat sink. * F1 can be a 1A fuse.