LC Filters

LC filters are one of the most commonly used filters. This type of filter is used primarily in radio receivers, small audio amplifier power supplies, and in any type of power supply where the output current is low and the load current is relatively constant.

The illustration below shows an LC capacitor-input filter and its associated waveforms. Both half-wave and full-wave rectifier circuits are used to provide the inputs.

LC capacitor-input filter and waveforms.

The waveforms shown in the figure represent the unfiltered output from a typical rectifier circuit. Note again, that the average value of output voltage (Eavg) for the half-wave rectifier is less than half the amplitude of the voltage peaks. This is indicated by the dashed line. The average value of output voltage (Eavg) for the full-wave rectifier is greater than half, but is still much less than the peak amplitude of the rectifier-output waveform. With no filter circuit connected across the output of the rectifier circuit (unfiltered), the waveform has a large value of pulsating component (ripple) as compared to the average (or dc) component.

A common type of LC filter is also shown in the illustration. C1 performs the same functions as discussed earlier by reducing the ripple to a relatively low level. L1 and C2 form the LC filter, which reduces the ripple even further (view C).

L1 is a large value iron-core inductor (choke.) It has a high value of inductance and, therefore, a high value of XL, which offers a high reactance to the ripple frequency. At the same time, C2 offers a very low reactance to the ac ripple. L1 and C2 form an ac voltage divider and, because the reactance of LI is much higher than that of C2, most of the ripple voltage is dropped across L1. Only a slight trace of the ripple appears across C2 and the load.

While the L1-C2 network greatly reduces the ac ripple, it has little effect on the dc. You should recall that an inductor offers no reactance to dc. The only opposition to current flow is the resistance of the wire in the choke. Generally, this resistance is very low and the dc voltage drop across the coil is minimal. Thus, the LC filter overcomes the disadvantages of the RC filter.

Aside from the voltage divider effect, the inductor improves filtering in another way. You should recall that an inductor resists changes in the magnitude of the current flowing through it. Consequently, when the inductor is placed in series with the load, the inductor tends to hold the current steady. This, in turn, helps to hold the voltage across the load constant.

The LC filter provides good filtering action over a wide range of currents. The capacitor filters best when the load is drawing little current. Thus, the capacitor discharges very slowly and the output voltage remains almost constant. On the other hand, the inductor filters best when the current is highest. The complementary nature of these components ensures good filtering over a wide range of current when size of components is a factor.

The LC filter has two disadvantages. The first is cost. The LC filter is more expensive than the RC filter because its iron-core choke costs more than the resistor of the RC filter. The second disadvantage is size, since the iron-core choke is bulky and heavy. Thus, the LC filter may be unsuitable for some applications but is still one of the most widely used.

Several problems may cause the LC capacitor filter to fail. Shunt capacitors are subject to open circuits, short circuits, and excessive leakage; series inductors are subject to open windings and occasionally shorted turns or a short circuit to the core.

The input capacitor (C1) has the greatest pulsating voltage applied to it, is the most susceptible to voltage surges, and has a generally higher average voltage applied. As a result, the input capacitor is frequently subject to voltage breakdown and shorting. The output capacitor (C2) is not as susceptible to voltage surges because of the series protection offered by the series inductor (L1), but the capacitor can become open, leaky, or shorted.

A shorted capacitor, an open filter choke, or a choke winding that is shorted to the core, results in a no-output indication. A shorted capacitor, depending on the magnitude of the short, may cause a shorted rectifier, transformer, or filter choke and result in a blown fuse in the primary of the transformer. An open filter choke results in an abnormally high dc voltage at the input to the filter and no voltage at the output of the filter. A leaky or open capacitor in the filter circuit results in a low dc output voltage. This condition is generally accompanied by an excessive ripple amplitude. Shorted turns in the winding of a filter choke reduce the effective inductance of the choke and decrease its filtering efficiency. As a result, the ripple amplitude increases.

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