capacitor value after rectifier

[innovation1]

capacitor

hi.......... can u tell me how to decide capacitor val that is to be connected after rectifier stage.........

 

[Audioguru]

Re: capacitor

The load current causes ripple. A higher value capacitor smooths the ripple which reduces its p-p level.
I use a graph like this one:

 

[innovation1]

Re: capacitor

thanx for reply...... but i connected a 1000uF capacitor across a rectifier (with 18V 50 Hz i/p) it gave o/p (across capacitor) of 22-20V. is it bcoz of charging of the capacitor........???????

 

[Audioguru]

The AC voltage is 18V RMS which has a peak voltage of 25.5V. Then the rectifier reduces it to 23.5V. The ripple from the capacitor with a small size reduces the voltage more.

 

[innovation1]

thanx for reply .. bt the graph u hav posted is for fr=120hz.....
my i/p is 50hz i.e. fr=100hz????
so i just want to know the generalised relation to find value of C

 

[Audioguru]

The formula is on my graph but it is wrong.
Since 50Hz is 0.833 times 60Hz then the amount of ripple is 1.2 times more than my graph.