Hi friends, today we are going to learn some basic filter circuits like RC filter and LC filter.
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.
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 = 2πfL 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:
1) In choke input filter, current flows continuously. Therefore the transformer is used more efficiently.
This post provides an information about series inductor filter. Before going to study series inductor filter it is necessary to understand what is meant by filter circuit?
Filter Circuit – Most of the electronic circuits require a dc voltage that is constant, similar to the voltage from a battery but the rectifiers cannot provide ripple free dc voltage. They provide a pulsating dc. The circuit used for filtering or smoothing out the ac variations from the rectified voltage is called as `Filter circuit.
Series Inductor Filter:
The circuit diagram and waveforms of series inductor filter is shown below:
Series Inductor Filter – An inductor opposes any change in the current flowing through it. Whenever the current through an inductor tends to change, a back emf is induced in it. This prevents the change in current.
Inductive reactance XL = 2π* f*L. For dc, f = 0, therefore direct current easily passes through inductor to the load. Only opposition to dc is due to internal resistance of choke. The reactance increases with frequency. Therefore ac component is opposed. The output waveform shows a large dc component and a small ac component.
The operation of a series inductor filter depends upon the current through it. The higher the current flowing through it, the better is its filtering action. An increase in load current reduces the ripples.
It consists of a source of light (tungsten filament lamp), lens to make the light beam parallel, filter of wavelength selection, cuvette with sample holder for keeping the solution to be analyzed, mirror for incident the part of light beam onto reference photocell, two photocells (one as reference and other as measuring), potentiometers for zero and span adjustments and a recording device (galvanometer).
In double beam photometer, the light rays from the source are made parallel and passed through a filter. It is divided into two parts; one part passes through the sample solution cuvette and falls on the measuring photocell and the other part passes directly onto the reference photocell. The galvanometer receives opposing currents from the two photocells.
Steps in experiment:
1) With the lamp off, the galvanometer is adjusted to zero mechanically.
2) The potentiometer R1 is adjusted for T=1 or A=0.
3) Then with lamp on blank solution is placed in the light path of measuring photocell and potentiometer R2 is adjusted until the galvanometer reads zero.
4) The solution to be analyzed is then placed in the light path of measuring photocell and R1 is adjusted until the galvanometer reads zero, keeping R2 unchanged. The absorbance or transmittance can be read directly on the scale of potentiometer R1. Since the potentiometer R1 is calibrated in terms of transmittance and absorbance.
In double beam filter photometer errors due to fluctuations of the lamp intensity are minimized also the scale of potentiometer R1 can be made much larger in size than the scale of meter in single beam filter photometer.