Filtering utilizes capacitors and inductors. Capacitors conduct AC but block DC, while inductors conduct DC but block AC. The ideal 12-volt source is pure DC. Noise, on t he other hand, is AC, either generated by the alternator or generator, or picked up by power leads acting like antennas.
a simplifed filter design
To reduce the amount of AC in a DC supply, we place capacitors across the + and – leads and inductors in series with the leads. Capacitors for 12-volt DC applications should be rated at 50 working volts (50 WVDC) minimum. Large capacitors are generally polarized electrolytics, so make sure you observe proper polarity. Inductors are rated by the maximum continuous DC current they can carry, as well as the value of inductance.
For electronic navigation equipment, a 5-amp rating is generally sufficient. For DC motors (pumps and autopilots), a 10-amp rating is common. Alternators and generators require ratings that match or exceed their peak output ratings. Inductors with 50-, 70-, and 100-amp ratings are common. In the case of noise on power leads, the unprotected length of lead serves as an antenna, picking up the noise. Thus, it is important to install the filter as closely as possible to the noise generator or the receiver. Sometimes a single capacitor or inductor will reduce the noise to an acceptable level.More powerful solutions are provided by a variety of special commercially available filters, combining capacitors and inductors, as shown in the figure given above.
