Using Capacitors and Inductors as Filters for Power Supplies

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Most power supplies found in industrial electronic circuits have capacitors and inductors used as filters. A filter on the power supply circuit will reduce the amount of ripple to a point where the output dc voltage is nearly a straight line, or pure dc. It's important in some circuits where the dc voltage is converted back to ac voltage that all traces of the original frequency of the input voltage is removed.

Fig. 1 shows a diagram of a typical capacitor and inductor in the power supply circuit. The capacitor is connected in parallel with the load, and the inductor is connected in series with the dc voltage terminals. Recall from basic electrical courses that the capacitor will charge when voltage is supplied to it. It will then discharge the stored voltage when the supply voltage is less than the stored charge. The effect of the capacitor charging and discharging is to smooth out the area between the peaks of the full-wave dc output voltage. The waveform in Fig. 1 shows the effect of the capacitor filter. (It should be noted at this time that it's customary to show only the voltage waveform since it can easily be seen from an oscilloscope. The current waveform exists but it's difficult to view it directly.) The inductor provides essentially the same function to the current waveform as it stores energy in its magnetic field and releases it back into the output circuit. The effect of the inductor storing and releasing energy into the output circuit's to provide a slight phase shift, which smooths the area between the current peaks. Together the capacitor and inductor filter the dc full-wave output voltage and current to a smoother, near pure supply of dc power. The capacitor can be increased in size or several capacitors can be used together in parallel to increase the filtering capability of the circuit.

Electronic circuit that shows a capacitor connected in parallel & an inductor connected in series with the dc output part of the rectifier circuit.
Above: Fig. 1 Electronic circuit that shows a capacitor connected in parallel and an inductor connected in series with the dc output part of the rectifier circuit.

The inductor that is used for filtering is generally called a choke and it looks very similar to a small transformer except it will have two wires instead of four. In the rectifier circuit for larger motor drives, the capacitors in the filter will have a pre-charge circuit that limits the rate that voltage is supplied to the capacitors when power is initially applied. The capacitors also have a discharge resistor to ensure that all of the stored potential is removed from the capacitor when power is turned off. It's important to remember that these filter capacitors store a large amount of energy and it will take several seconds for them to discharge after power is removed.

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