Скачать или смотреть EMI RF Filter 220V

The electrical power network carries both useful energy at fifty hertz and unwanted noise components. These noise components are usually divided into two categories: differential-mode interference and common-mode interference.

Differential-mode interference appears when noise voltage is induced between the phase conductor and the neutral conductor. This type of noise flows in the same path as the useful load current, but with higher frequency content. Sources include switching power supplies, motor drives, and electronic lighting systems.

Common-mode interference appears when both phase and neutral conductors are driven together with respect to the environment, the protective earth, or surrounding structures. In this case, the wires act as parallel antennas, radiating or receiving electromagnetic energy. Common-mode noise is especially problematic because it can spread over large areas through shared ground and can couple strongly into sensitive electronics.

The presented passive filter is designed to transform and absorb these disturbances. The principle is based on two safety capacitors, type X2, connected asymmetrically from phase and from neutral to a central node. Because the capacitors have different values, the symmetry between phase and neutral is broken. As a result, pure common-mode voltages are no longer balanced and are partly converted into differential voltages at the central node.

From this node, a flameproof resistor of fifty to one hundred ohms provides a low-impedance path for the high-frequency current. At mains frequency of fifty hertz, the impedance of the capacitors is very high, so the current into the resistor is negligible and no power is wasted. At high frequency, however, the impedance of the capacitors becomes small. The noise energy is guided through them and dissipated as heat in the resistor instead of propagating into the electrical network.

In effect, the circuit performs three simultaneous actions:

Bypass – it diverts high-frequency noise away from the load.

Conversion – it converts part of the common-mode energy into differential form, where it can be absorbed.

Dissipation – it transforms the noise energy into harmless heat in the resistor.

This simple arrangement reduces both types of interference: differential-mode noise is short-circuited through the capacitors, and common-mode noise loses its symmetry and is absorbed by the resistor. The result is a quieter power line, less electromagnetic radiation from the wiring, and more stable operation of sensitive devices such as computers and precision instruments.