c)Use of diodes

Use of diodes as surge suppressors

Contactors are generally designed in such a way that a slight contact welding during closing is forced open again by the back pressure springs of the contacts during de-energization. Diodes can reduce the drop-out speed of a magnet system to such an extend, that the abovementioned slight contact welding cannot not be opened anymore







In the extreme case the low drop-out speed caused by the diode can lead to a so-called two-step drop-out.

This means that following the first contact opening, another short, weak closing of the main contacts may happen during the breaking process. This second closing (during the arcing time) can lead to welded contacts.

Since two-step drop-out cannot happen with size S00 3RT1 contactors, diodes as surge suppressors are offered for this size only. From size S0 upwards diode assemblies (series connection of diode and zener diode), RC-elements or varistor have to be used as surge suppressors. Some companies on the market offer diodes for Siemens contactors. Their use on Siemens contactors leads to the above problems!