# regarding capacitor value calculation in power supplies

#### yahooamit24

Joined Mar 12, 2020
8
hi , i am newly joined fresher. i have a question, how to calculate capacitor in power supply? for ex: ac to dc conversion .

#### ericgibbs

Joined Jan 29, 2010
9,768
hi 24,
Welcome to AAC.
In order to calculate the required capacitor values for a power supply, you need to specify the design current/voltage specification of the PSU also the permissible voltage ripple etc ...

Do you have a PSU design specification, post an example.

E

#### yahooamit24

Joined Mar 12, 2020
8
yeah psu requirement is 3.3v , 500 mA . I know the working voltage should be at least 1.3 times of requirement but don't know the capacitance .if you have any standard method for this pls let me know.

#### ericgibbs

Joined Jan 29, 2010
9,768
hi.
For a 3.3Vdc supply at 0.5A.
You could use a AMS1117 3.3V 800mA series type regulator.
Do you have a mains step down transformer that you want to use, say 5Vac output or so rated at 10VA.?

E

#### Hymie

Joined Mar 30, 2018
797
The discharge rate of a capacitor is given by V = ( I x t)/C
Where
V is the capacitor volt drop
I is the capacitor discharge current
t is the time
C is the capacitance value in Farads
As an example, a full wave rectified voltage (at 50Hz) from a 12V transformer secondary with a constant current discharge of 500mA and a reservoir capacitor of 4,700µF would have a peak voltage of 12 x 1.414 = 17V. Over 10ms the V drop = (0.5 x 0.01)/0.0047 = 1.06V.
Note the above calculations ignore any bridge rectifier diode voltage drop.