Differences Between Electromechanical Relays and Semiconductors

Anbieter zum Thema

MEV Elektronik Service GmbH

SCHURTER AG

Apacer Technology B.V.

TE Connectivity

Temperature

The temperature-critical components of an electromechanical relay are the plastic parts (base, cover, coil bobbin, actuator, etc.) as well as the isolation lacquer of the coil wire. Through that, compared to semiconductors, for a relatively high proportion of precious and semi-precious metals (mainly silver and copper) in relay load circuits, the heating time during power load is much longer.

As a result, electromechanical relays can be clearly overloaded much more heavily than semi-conductors. It is often possible, on a short-term basis, to conduct ten times the rated current of the relay without a negative effect. Relays are immune to short-term voltage peaks. In addition, the application of twice the rated voltage on the relay coil is possible over a longer period of time.

The influences of temperatures on switching times, actuating voltages and electrical service life is explained in the respective section.

Voltage, current

One advantage of non-polarized relays over semiconductors is that the energizing and the load is usually arbitrary, i.e. the relay can be operated with both polarities.

The maximum possible load current and maximum coil voltage are defined by the thermal limits of the relay (see Temperature). The maximum possible load voltage is the voltage at which the switch-off arc still extinguishes. For AC applications, this is the voltage at which the arc does not reignite itself again automatically. For the time being, the zero crossing of the arc is extinguished.

For DC voltages, the maximum possible load supply depends on the contact gap as well as the other possible methods of arc suppression, for example, a protective gas or blowing magnet.

Service life

The electrical service life of electromechanical relay is essentially defined by the number of switching cycles. When opening relay contacts, an arc is formed (except with low signal levels). Contact material "burns" in the arc. The contact erosion finally leads to a failure of the relay. The failure mode is either no longer closing contacts or it is no longer possible to open contacts. The possible number of switching cycles depends very heavily on the load which is switched.

Therefore, reliable information about the life of the relay is only possible if the parameters of the switching application are known. Make current, break current, load voltage and load inductance have substantial influence here. Also, the switching frequency and ambient temperature can affect the durability.

The service life and/or reliability of semiconductors is often specified in FIT (Failure In Time). For electromechanical relays, this specification does not make sense because the service life is almost independent of the operating time, and depends rather on the switching frequency. The service life of relays is usefully indicated as a statistical value of the potential number of cycles with a specified load (e.g. B10 value of a Weibull analysis). If the switching frequency of a system is known, a FIT value can also be computed from statistical service life.

For many switching solutions, both semi-conductor and electromechanical relays can be used. The optimal solution can depend on many parameters. It is important that its special features are considered during the application of electromechanical relays.

Relay applications note

What should be considered in relay diagnosis

* Olaf Lorenz works as Application Engineer for TE Relay Products in Berlin.

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