Need capacitor for smoothing? hello friends if i have 6 volt peak full-wave rippled output wave form comes from full-wave rectifier. the what rating capacitor (in faraday) i connected with it to make it smooth dc wave. thanks a lot frequency is 50hz and 220volt input and out from rectifier and my load demands taking current about some mAmp
There always will be some ripple. You need to define how much ripple you can tolerate. The amount of ripple will also increase with the load current so you also need to define the maximum load current for the ripple specification.
As an example, with your supply and a 150 ohm load (40 mA), and a 1000 uF capacitor, the ripple would be about 0.8 volts p-p. You could use a larger capacitor, but the charging current for a bigger capacitor quickly gets very large. Even a 1000 uF capacitor needs about 500 mA charging pulses to charge it. It is usually better to use a voltage regulator, but you would need at least about 9 volts from the power supply to get 6 volts out and this would have to be filtered. However, the output of a voltage regulator can be almost perfect DC with only slight ripple and noise visible on an oscilloscope.
True, but even so you still need to be able to calculate how much ripple you'll have so that you don't go below the minimum voltage the regulator can regulate.