Reducing Interference from Electric Equipment

In general, an electric device causes noise first by direct radiation, and second by radiation over the supply wires. If the device is enclosed completely in a metal housing or cabinet, the direct radiation will be largely prevented. The radiation over the supply wires may be caused by the propagation of noise components between wires, or the propagation may be between wires and ground.

In Fig. 26 are shown inductors (choke coils) in series with the line wires, and capacitors between each line wire and ground. Also, it will be noted that these are within the metal shield, such as the metal housing or cabinet, in which the electric device is placed.

Figure 26. Method of shielding a noise-producing device so that it will produce the minimum of external radiation and resulting noise.

The values of the inductance and capacitance to be used and the current and voltage ratings will depend on circumstances. If audiofrequency interference is being created, then the inductance and capacitance should be greater than if the interference is of radio frequency. Also, it may not be necessary to use both inductors and capacitors, and, if one side of the supply circuit is grounded, as is often the case, then the arrangement may be simplified.