Filter Configuration

Mains filters for suppression are made up essentially from combinations of capacitors and inductors where the network is designed to give the required amount of suppression without interfering with the flow of supply current; the greater the suppression required, the more complex the filter network.

The simplest configuration is a single capacitor used in the decoupling manner as a shunt element as shown in the first diagram. The second diagram shows a practical version of this suppressor as a delta connection of capacitors, where the L to N capacitor is X rated and L to E and N to E are Y rated for the supply voltage and frequency in question in accordance with international safety requirements.

Capacitors used are of the self healing type and are either of conventional leaded style or where high performance is required in Tempest or GHz plus applications, axial feed through types are used.

Because of the limits placed on the size of the capacitors to earth by leakage current safety requirements, it is usually necessary to introduce inductive elements to achieve the required level of suppression.

There are 5 main styles of inductor which may be chosen from, for various modes and types of interference as follows:
· Multiple wound flux cancelling - asymmetric mode
· Single wound saturating - general thyristor suppression
· Single wound or multiple wound flux aiding - symmetric mode
· Low inductance aircore or open loop - general vhf suppression
· Earth line chokes - single wound - asymmetric mode

The SF range of filter configurations is built up by optimising combinations of various styles of inductor with the appropriate capacitors and other components to give a comprehensive selection of suppression levels and other features to cover the range of electronic equipment currently manufactured to the various specification requirements.