When special properties of the electromechanical relay are not taken into account in circuit development and they are treated like semiconductor elements, this can lead to malfunctions. This article explains the differences between the characteristics of electromechanical relays and semiconductors.
In the 21st century, electromechanical relays are used in very large numbers in a variety of applications for switching currents. Several billion relays are produced and distributed globally every year. The main advantages over semiconductor switches are the electrical isolation of the control and load circuit as well as the lower terminal resistances of the load side. Additionally, the relays from leading manufacturers are very reliable and robust, so long as they are used in accordance with their specification.
Through the electromechanical structure, the switching time, that is to say the time from applying the control signal to change the switching state, is considerably longer than it is for semiconductors. In addition, there may be significant differences between the on and off times. The switch-on time (also called response time) is taken from the inductive build-up time of the current in the relay coil and the transfer time of the relay armature.
The switch-off time (also known as drop-out time) is taken from the time required for the degradation of the magnetic field and, in turn, the transfer time of the relay armature. The relay switching times are in the one- to two-digit millisecond range, depending on the relay shape and size.
The switching times indicated by manufacturers are mostly characteristic values at 23°C and rated voltage. The switching time is influenced by the coil voltage and the coil temperature (see Graphic 1). The switch-off time can be increased through external coil suppression circuits. These external influences have a considerably higher influence on switching times than the production-related variations.
If relays are used in control systems with time-critical processes, these effects have to be considered and, at best, checked by the relay manufacturer.
Unlike semiconductors, with electromechanical relays the response and release voltage (also called energizing voltage) are temperature-dependent. The fundamental operation of electromechanical relays is based on the production of a magnetic field which moves a mass spring system armature, contact, spring). The magnetic field is proportional to the current which flows through the relay coil. The relay coils are almost always made of copper wire, the resistance of which is temperature-dependent, and therefore also the voltage that is required to close and/or open the magnetic circuit of the relay. The temperature resistance of copper is about 0.4% / K.
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