A reed relay is a type of relay that utilizes an electromagnet to control one or more reed switches. Consisting of a reed switch, a coil, optical diode, and connection terminals, reed relays are devices with a low-resistance metallic switch path that provides an inherent isolation between the control voltage operating the coil and the signal being switched.
The reed switch is equipped with two metal blades made of a ferromagnetic material, usually nickel iron, within a glass envelope that holds the blades in place and provides a hermetic seal. This configuration prevents any contaminants from entering the critical contact area inside the envelope. Furthermore, a majority of reed switches have open contacts in their normal state. If a magnetic field is applied along the axis of the reed blades, their ferromagnetic nature intensifies the field. Consequently, the open contacts of the reed blades are drawn to each other, and the blades deflect to close the gap. With enough magnetic force generated, the blades touch, and electrical contact is achieved.
The only movable part in the reed switch consists of the deflecting blades. Sealed within the glass envelope with inert gasses, or a vacuum in the case of high-voltage switches, the reed relay is sealed away from external contamination, giving the reed switch an extended mechanical service life. Another design variable that one must consider is size. Longer switches are not required to deflect the blades as they can close the gap with ease. Short reeds are made of thinner materials that allow them to deflect more easily at the cost of their rating and contact area. Moreover, smaller reed switches enable smaller relays to be constructed where space is critical. Larger switches, on the other hand, are more robust in design and have a greater contact area, enhancing their signal-carrying capacity.
With regard to the switch contact area, various plating materials and methods are employed. The most common materials utilized are rhodium, iridium, or ruthenium, all of which are rare platinum group metals. These materials are hard, wear-resistant elements with increased resistance stability. For applications tasked with handling high voltages, tungsten offers high melting points and resistance to welding. Reed switch contacts can also be coated using one of two methods: electroplating or vacuum deposition (sputtering).
To operate a relay, the generated magnetic field must be capable of closing the reed switch contacts. Reed switches can be used with permanent magnets whereas reed relays generate a magnetic field with the use of a coil which can have a current travel through it in response to a control signal. The coil envelops the reed switch and produces an axial magnetic field that can close the reed contacts.
It is important to note that different reed switches necessitate varying levels of magnetic fields to close the contacts. In contrast, stiffer reed switches for higher power levels or high-voltage switches with larger contact gaps typically require greater amperes to operate, meaning the coils need more power. Meanwhile, changing the wire gauge for the coil and the amount of turns produces relays with different drive-voltage requirements and coil capabilities. Keep in mind that the resistance of the wire coil controls the amount of steady-state current traveling through the coil and the power the coil consumes when the contacts are closed.
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