Capacitors and chokes:

Another way to smooth out the direct current from the rectifier is to use an extremely large inductance in series with output. This is done in conjunction with the parallel capacitance. The inductance, known as filter choke, is on the order of several henrys. If coil should carry a lot of current, it will be bulky physically. At times the capacitor is placed ahead of the choke. This circuit is a capacitor input filter.  If the coil comes ahead of capacitor, circuit is a choke-input filter.

Figure--   Capacitor-input  (A) and choke-input (B) filtering.

Engineers may use capacitor input filtering when load is not expected to be very large. The output voltage is higher with the capacitor-input circuit than with the choke-input circuit. If the supply is required to deliver large or variable amounts of current, the choke input filter is a better choice, as the output voltage is much more stable.

If a supply should have a minimum of ripple, 2 or 3 capacitor/choke pairs might be cascaded, or connected one after the other. Each pair is called as section. Multisection filters can consist of capacitor-input or choke-input sections, but the 2 types are never mixed togather.

Figure--Two choke-input filter sections in cascade.