How Noise Filters Work

There are several types of noise filters available. The two most common types of filters are: Passive Filters, and Isolation Transformers.

Passive noise filters use a series of resistors, capacitors, and inductors to create a band of acceptable frequencies that they will allow to pass. A low pass filter lets low frequencies through and blocks high frequencies. High pass filters let high frequencies through and block low frequencies. Low-pass filters are most frequently used for power circuits, since they eliminate high frequencies which are the source of most electrical noise problems. These types of filters are effective on normal mode noise only. They will not eliminate common mode noise conditions.

Isolation transformers use two sets of inductors coupled to a common magnetic core. These inductors are separated with magnetic shielding. This shielding reduces or eliminates normal mode electrical noise by transferring it back to the ground system. The neutral and ground wires are bonded together on the output side (or secondary) of the transformer, making them the same potential and eliminating common mode noise.