RC Filter
[caption id="" align="aligncenter" width="490"]RC filter[/caption]
In the above figure two sections of RC filter are shown. These are connected between the input capacitor and the load resistor. The value of R should be at least 10 times greater than the capacitive reactance Xc. Therefore the ripple is dropped across the series resistors instead of across the load resistor. Each section reduces the ripple by a factor of 10. Therefore ac components are removed and at the output we get a steady dc voltage.
The main disadvantage of RC filter is the loss of dc voltage across each R. Therefore RC filter is suitable only for light loads. i.e. small load current.
LC Filters
[caption id="" align="aligncenter" width="392"]
LC filter[/caption]
In this type inductor L is in series and capacitor C is in shunt with load. The choke (L) allows the dc component to pass through easily because its dc resistance R is very small. The capacitive reactance Xc is very high for dc and it acts as open circuit. All dc current passes through across which dc output voltage is obtained.
The inductive reactance XL = 2pfL is high for ac components. Therefore the ripples are reduced. Even if any ac current passes through L, it flows through the capacitor because of its low capacitive reactance.
Advantages of LC Filter
- In choke input filter, current flows continuously. Therefore the transformer is used more efficiently.
- Ripple content at the output is low.
- It is less dependent on the load current.
- DC voltage drop across L is much smaller because its de resistance R is very small.
Disadvantages of LC Filter
- Large size and weight of inductors,
- More cost,
- External hold is produced by inductor.
For providing smoothest output voltage p type (capacitor input) filter can be used.
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