Filters Questions and Answers for Viva

Answer-1: A filter circuit is a device that converts pulsating output of a rectifier into a steady dc level. Hence, it becomes essential to reduce the ripples from the pulsating dc supply available from rectifier circuits to the minimum. This is achieved by using a filter or smoothing circuit which removes (or filters out) the ac components and allows only the dc component to reach the load. Obviously, a filter circuit should be placed between, the rectifier and the load.

Answer-2: A filter is generally a combination of inductors L and Capacitors C. The filtering action of L and C depends upon the facts that an inductor allows only dc and a capacitor allows ac only to pass. So a suitable L and C network can effectively filter out (or remove) the ac components from the rectified output.

Answer-3: The common types of filter circuits are Series Inductor filter, Shunt Capacitor Filter, Choke Input Filter and Capacitor input or pie filter.

Answer-4: In this arrangement a high value inductor or choke L is connected in series with the rectifier element and the load. The filtering action of an inductor filter depends upon its property of opposing any change in the current flowing through it. The function of the inductor filter may be viewed in terms of impedances. The choke offers high impedance to the ac components but offers almost zero resistance to the desired dc components. Thus ripples are removed to a large extent.

Answer-5: In this arrangement a high value Capacitor is connected in parallel with the rectifier element and the load. The function of the capacitor filter may be viewed in terms of impedances. The capacitor offers zero impedance to the ac components but offers high resistance to the desired dc components, so C bypasses the dc.Thus ripples are removed to a large extent.

Answer-6: A simple shunt capacitor filter reduces the ripple voltage but increases the diode current. The diode may get damaged due to large current and at the same time it causes greater heating of supply transformer resulting in reduced efficiency. In an inductor filter, ripple factor increases with the increase in load resistance RL while in a capacitor filter it varies inversely with load resistance RL.From economical point of view also, neither series inductor nor shunt capacitor type filters are suitable.

Answer-7: Practical filter-circuits are derived by combining the voltage stabilizing action of shunt capacitor with the current smoothing action of series choke coil. By using combination of inductor and capacitor ripple factor can be lowered, diode current can be restricted and simultaneously ripple factor can be made almost independent of load resistance (or load current). Two types of most commonly used combinations are choke-input or L-section filter-and capacitor-input or Pi-Filter.

Answer-8: Choke-input filter consists of a choke L connected in series with the rectifier and a capacitor C connected across the load. This is also sometimes called the L-section filter The choke L on the input side of the filter readily allows dc to pass but opposes the flow of ac components Any fluctuation that remains in the current even after passing through the choke are largely by-passed around the load by the shunt capacitor However, a small ripple still remains in the filtered output and this is considered negligible if it than l%.

Answer-9: Such a filter consists of a shunt capacitor C1 at the input followed by an L-section filter formed by series inductor L and shunt capacitor C2. This is also called the ? -filter the input capacitor C1 is selected to offer very low reactance to the ripple frequency. Hence major part of filtering is accomplished by the input capacitor C1. Most of the remaining ripple is removed by the L-section filter consisting of a choke L and capacitor C2.

Answer-10: In pi-filter the dc output voltage is much larger than that can be had from an L-section filter with the same input voltage. In pi-filter ripples are less in comparison to those in shunt capacitor or L-section filter. So smaller valued choke is required in a pi-filter in comparison to that required in L-section filter. In pi-filter, the capacitor is to be charged to the peak value hence the rms current in supply transformer is larger as compared in case of L-section filter. Voltage regulation in case of pi-filter is very poor, as already mentioned. So n-filters are suitable for fixed loads whereas L-section filters can work satisfactorily with varying loads provided a minimum current is maintained. In case of a pi-filter PIV is larger than that in case of an L-section filte

Answer-11: Circuit which allows a certain band of frequency is called a filter.

Answer-12: Circuit which allows high frequency above the cut off frequency.

Answer-13: Circuit which allows only low frequency below the cut off frequency.

Answer-14: It is a frequency of applied AC below or above which the circuit allow the frequency.

Answer-15: Cut off frequency of line pass or high pass filter f = 1 / 2πRC

Answer-16: It is a frequency at which stop band changes to pass band or vice versa
Or, It is a frequency at which the power level reduced to -3db
Or, It is the frequency at which the voltage gain reduced to 70.7% of max value.

Answer-17: It is the graph of voltage gain against frequency.

Answer-18: When X_c = R