Filtering

This is why the 'smoothing' block is required. Its because the output come from bridge rectifier is not suitable for power supply. So to give the output more smooth we use filtering circuit by using only one capacitor as shown in figure 1 below.

The output waveform in figure 2 shows how smoothing works.  During the first half of the voltage peaks from the rectifier, when the voltage increases, the capacitor charges up.  Then, while the voltage decreases to zero in the second half of the peaks, the capacitor releases its stored energy to keep the output voltage as constant as possible.  Such a capacitor is called a 'smoothing' or 'reservoir' capacitor when it is used in this application.

If the voltage peaks from the rectifier were not continually charging up the capacitor, it would eventually discharge and the output voltage would decrease all the way down to 0V.  The discharging that does occur between peaks gives rise to a small 'ripple' voltage.  The amount of ripple is affected by a combination of three factors:

• The value of the capacitor.  The larger the capacitor value, the more charge it can store, and the slower it will discharge.  Therefore, smoothing capacitors are normally electrolytic capacitors with values over 470μF.
• The amount of current used by the circuit.  If the circuit connected to the power supply takes a lot of current, the capacitor will discharge more quickly and there will be a higher ripple voltage.
• The frequency of the peaks.  The more frequent the voltage peaks from the rectifier, the more often the capacitor will be charged, and the lower the ripple voltage will be.

To calculate the ripple voltage, it will use this formula below.

 V_r = the ripple voltage in Volts

 I = the current taken by the circuit in Amps

 C = the value of the smoothing capacitor in Farads

 F =the frequency of the peaks from the full-wave rectifier, in Hertz.

The ripple voltage should not be more than 10% of Vs - if it is, increase the value of the smoothing capacitor.

So in this project, the value capacitor for filter circuit is 470uF ( as picture below ).

capacitor 470uF