Interference and Screening

Spark suppression is applied in order to reduce the burning and arcing of contacts, which produce additional wear of contacts and might be a source of interference.

Spark suppression usually consists of R and C across the contacts. Typical values: R = 50, C = 0.22 µF.

Typical network for spark suppression of a contact.

Such RC networks are available ready-made in one unit. Spark suppression may reduce HF interference from the contacts. Very often the components are fairly bulky and also show too much inductance, e.g. due to long leads, which increases the radiation. In such cases, only sparking is reduced. Additional components have to be installed to cut down HF interference.

HF interference is produced at the contact points. The opening or closing of electrical contacts produce a square wave signal, which contains harmonics with frequencies in the VHF and UHF bands. Click noises in receivers are the result. These noises can also occur in audio equipment, especially when the interference affects the input of high-gain amplifiers.

Typical switches or relays which may require careful interference suppression are:
power switches of amplifiers, tape recorders, room lights, clocks, cue lights, air conditioning, mains voltage regulators.
Also, brushes for commutators or slip rings of motors or generators, e.g. an emergency power plant.

For the efficient suppression of HF interference, it is most important to fit a suitable HF filter (low pass) very close to the actual source of interference, e.g. switch contacts. This will prevent HF from being radiated from connecting leads which act as "antennas" for the interference transmission. These filters usually consist of HF chokes (wide-band types on ferrite core) and low inductance capacitors, forming a low pass filter.

Often just two HF chokes are sufficient, but they must be situated very close to the contacts. Ferrite beads slipped over the leads close to the contact points are another way of providing inductance at HF. Note that all leads to all contacts must carry chokes, including contacts which are connected to 0V or ground.

Typical arrangements of interference filters for contacts.

Interference and Screening