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Shunt capacitor filter

2015-01-11 04:36  
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Shunt Capacitor Filter and types of it:

This is the most simple form of thefilter circuitand in this arrangement a high value capacitor C is placed directly across the output terminals, as shown in figure. During the conduction period it gets charged and stores up energy to it during non-conduction period. Through this process, the time duration during which Ft is to be noted here that the capacitor C gets charged to the peak because there is no resistance (except the negligible forward resistance of diode) in the charging path. But the discharging time is quite large (roughly 100 times more than the charging time depending upon the value of RL) because it discharges through load resistance RL.

The function of the capacitor filter may be viewed in terms of impedances. The large value capacitor C offers a low impedance shunt path to the ac components or ripples but offers high impedance to the dc component. Thus ripples get bypassed through capacitor C and only dc component flows through the load resistance RL

Capacitor filter is very popular because of its low cost, small size, light weight and good characteristics.

Shunt Capacitor Filter Circuit

1. Half-Wave Rectifier With Shunt Capacitor Filter.

The waveforms of ac input voltage, rectified and filtered output voltages and load current are shown in figure. During the positive half cycle of the ac input, the diode of the rectifier is forward biased and so it conducts. This quickly charges the capacitor C to peak value of the supply voltage VSmax because of almost zero charging time constant. This is shown by point b in figure. After being fully charged, the capacitor holds the charge till input ac supply to the rectifier goes negative. During the negative half cycle, the diode gets reverse biased and so stops conduction. So the capacitor C discharges through load resistance RL and loses charge. Voltage across RL (VL) or across C (vc), both being equal, decreases exponentially with time constant CRL along the curve be, as illustrated.

Because of the large discharge time constant CRL,  the capacitor does not have sufficient time to discharge appreciably. Due to this fact the capacitor maintains a sufficiently large voltage across RL, even during the negative half-cycle of the input supply. During rectified voltage exceeds the capacitor voltage vc represented by point C in fig. The capacitor again gets quickly charged to to Vg max (or VLmax) as represented by point d in the figure.This process of charging and discharging is repeated for each cycle of input supply voltage. seen, from the figure, that nearly constant dc voltage appears across load resistance RL at all times and also the dc component of output voltage is increased considerably.

The worthnoting points about shunt capacitor filter are:

1.For a fixed-value filter capacitance larger the load resistance RL larger will be the discharge time constant CRL and therefore, lower the ripples and more the output voltage. On the other hand lower the load resistance (or more the load current), lower will be the output voltage.

2.Similarly smaller the filter capacitor, the less charge it can hold and more it will discharge. Thus the peak-to-peak value of the ripple will increase, and the average dc level will decrease. Larger the filter capacitor, the more charge it can hold and the less it will discharge. Hence the peak-to-peak value of the ripple will be less, and the average dc level will increase. But, the maximum value of the capacitance that can be employed is limited by another factor. The larger the capacitance value, the greater is the current required to charge the capacitor to a given voltage. The maximum current that can be handled by a diode is limited by the figure quoted by the manufacturer. Thus the maximum value of the capacitance, that can be used in the shunt filter capacitor is limited.

Full-wave rectifier with shunt capacitor filter:

The filtering action of shunt capacitor filter on a full-wave rectifier is shown here. In this case capacitance C discharges twice during one cycle. Because both the diodes conduct, non-conducting period has reduced. The result is that ripple voltage Vr has been reduced to half and Vdc has been increased relative to half wave rectifier. Voltage regulation in this case is better than that in half-wave rectifier.

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