Capacitor sizing?

[Ken9876]

How would you correctly size a capacitor for a DC power supply? I was building a 12vdc power supply at work the other day and wanted to know how to get the right size and not just throw caps in the circuit till it worked right.

 

[LarryFine]

A generality is to use 1,000 to 2,000 uf per amp.

 

[gar]

100831-2045 EST

Larry:

Why?

What is the % ripple, output voltage, frequency, and configuration of the rectifier for your 1000 to 2000 mfd?

If there is a fixed output filter capacitance and the load resistance is constant, then the % ripple remains constant independent of the output voltage. The output current is proportional to the voltage.

If output current is held constant while output voltage is varied and C is constant, then % ripple voltage changes. Constant current with a change of voltage means the load resistance is changed.

% ripple voltage is determined by frequency, rectifier configuration, and the RC time constant.

Thus, output voltage, allowed % ripple voltage, current, frequency, and rectifier configuration must be defined to determine the value of capacitance.

X microfarads per amp is not sufficient.

.

 

[dbuckley]

The size of the capacitor can be determined by a bit of maths, but the first important questions for a standard mains powered power supply followed by a full wave bridge is what is the current that will be drawn from the 12V supply, and then what ripple you are prepared to accept.

It may be that a simple capacitor filter isn't adequate for the ripple you will accept..