Power relay
Current relays are electrical switching devices that switch at a specified current flow. They can be operated electromagnetically , thermally or electronically.
Electromagnetic current relays
Electromagnetic current relays are often constructed like normal relays , but the coil consists of fewer, but larger cross-sectional turns of wire than normal relays, depending on the current. They are built for a specific response current strength. With designs similar to a clapper armature relay or a reed relay, the response current is significantly higher than the waste current. Because of this high switching hysteresis and the inaccuracy, these designs are only used for tasks in which, for example, the current consumption is monitored. Examples are the failure control of warning lamps or the brake light control in motor vehicles.
For more precise applications, rotary armature relays are built that have hardly any hysteresis. They can also be designed for alternating currents and are often operated in secondary circuits by current transformers .
Thermal current relays
Thermal current relays have bimetal switching elements (see also bimetal relays ), which are heated by resistance windings or themselves form the resistance in the current path. The current to be monitored flows directly through the resistor or current transformers are connected in between. If the limit current is exceeded, the switching process is initiated by heating and bending the bimetal. Thermal current relays have reaction times of seconds to many minutes, depending on the relative level of the overcurrent, so that they are particularly suitable as overload protection for motors and transformers. The response current is closely tolerable, since the current heat increases with the square of the effective current. The response current is adjustable for motor protection switches , among other things .
Electronic current relays
Electronic current relays contain a low-resistance resistor through which the current to be monitored flows. The resulting voltage drop is amplified by the electronics, which trigger a relay when the limit current is reached. The switching threshold of electronic current relays can be adjusted over a wide range and they work much more precisely than the other versions. They also have a smaller switching hysteresis, which is also adjustable in some relays. Electronic current relays are used for measurement and control tasks because of their accuracy. Disadvantages are the high price and the separate supply voltage.
Specialist literature
- Werner M. Köhler: Relays / Basics, Structures and Circuit Technology. 2nd edition, Franzis-Verlag , Munich, 1978, ISBN 3-7723-1602-6 .
- Réne Flosdorff, Günther Hilgarth: Electrical energy distribution. 4th edition, B. G. Teubner Verlag , 1982, ISBN 3-519-36411-5 .