Filter is a circuit which converts pulsating dc output from a rectifier to a steady dc output. In otherwords, filters are used to reduce the amplitudes of the unwanted ac components in the rectifier.
Note: A capacitor passes ac signal readily but blocks dc.
Types of Filters
1. Capacitor Filter (C-Filter)
2. Inductor Filter
3. Choke Input Filter (LC-filter)
4. Capacitor Input Filter (Π-filter)
Capacitor Filter( C-filter)
• When the Input signal rises from o to a the diode is forward biased therefore it starts conducting since the capacitor acts as a short circuit for ac signal it gets charged up to the peak of the input signal and the dc component flows through the load RL.
• When the input signal fall from a to b the diode gets reverse biased . This is mainly because of the voltage across the capacitor obtained during the period o to a is more when comapared to Vi. Therefore there is no conduction of current through the diode.
• Now the charged capacitor acts as a battery and it starts discharging through the load RL. Mean while the input signal passes through b,c,d section. When the signal reaches the point d the diode is still reverse biased since the capacitor voltage is more than the input voltage.
• When the signal reaches point e, the input voltage can be expected to be more than the capacitor voltage. When the input signal moves from e to f the capacitor gets charged to its peak value again. The diode gets reverse biased and the capacitor starts discharging. The final output across RL is shown in Fig.
Advantages of C-Filter
• low cost, small size and good characteristics.
• It is preferred for small load currents ( upto 50 mA)
• It is commonly used in transistor radio, batteries eliminator etc.