If a dc voltage generated by a bridge rectifier & dc capacitor is 250vdc & has to be loaded about 3A,what is the u farad of the dc capacitor needed?
How to calculate?
Thanks!!

 

What is the voltage ripple you can tolerate? You need to specify the peak output current also.

I usually use about 2000 uF per Amp as a intital rule of thumb. You are still going to need add'l smaller caps as filters.

 

That depends on how much ripple you find acceptable. You also need to define the base line AC frequency, though that is not very critical, since 50Hz and 60Hz are pretty close to each other. I would assume single phase as well. The last assumption I would have to make is a purely resistive load, motors are reactive, so would load it down differently.

At 250V and 3A, your load would be 83Ω resistive. If your acceptable ripple is .1V, then figure what cap charged to 250VDC would discharge .1V through 83Ω in 10ms (½ of a 50 Hz period due to a full wave bridge). Using a ripple of .1 is probably too good, as that cap will be huge, but you get the idea.

 

(½ of a 50 Hz period due to a full wave bridge)

shouldn't this be 2X for a full wave

You are correct, a .1V ripple is very low for a 240V system unless this is fed into a regulator. 10% is a more usual requirement.

 

Full wave would double the freq,, halve the period. Each peak of the sine wave is now a pulse, so the period is halfed.

http://www.allaboutcircuits.com/vol_3/chpt_3/4.html

 

@Bill
I was reading you earlier post at an hour where my math functions were in sleep mode.