A reed relay is basically a combination of a reed switch and a coil for creating a magnetic field. Users often add a diode for handling any back EMF from the coil, but this is optional. The entire arrangement is very low cost and a simple device to be manufactured.
The most complex construction in the reed relay is the reed switch. As the name suggests, the switch has two reed-shaped metal blades made of a ferromagnetic material. A glass envelope encloses the two blades, holding them in place facing each other, and providing a hermetic seal preventing entry of contaminants. Typically, reed switches have open contacts in a normal state, meaning the two metal blades do not touch when not energized.
The presence of a magnetic field along the axis of the reed switch induces the reeds to magnetize, which attracts them to each other. The reeds, therefore, bend to close the gap. If the applied field is strong enough, the blades bend to touch each other, thereby forming an electrical contact.
The only movement within the reed switch is the bending of the blades. The reed switch has no part that slides past another or pivot points. Therefore, it is safe to say the reed switch has no moving parts that may wear out mechanically. Moreover, an inert gas surrounds the contact area within the hermetically sealed glass tube. For high-voltage switches, a vacuum replaces the inert gas. With the switch area being enclosed against external contaminants, the reed switch has an exceptionally long working life.
The size of a reed switch is a design variable. In longer switches, in comparison with shorter switches, the reeds do not need to deflect much to close a given gap between the blades. To make the reeds in more miniature switches bend more easily, they need to be made of thinner material, and this has an impact on the switch’s current rating. However, small switches allow for more miniature reed relays, which are useful in tighter spaces. On the other hand, larger switches are mechanically more robust, can carry higher currents, and have a greater contact area (lower contact resistance).
A magnetic field, of adequate strength, is necessary to operate a reed relay. It is possible to operate a reed relay by bringing a permanent magnet close to it. However, in the field, a coil surrounding the reed relay typically generates the magnetic field. A control signal forces a current through the coil, which creates the axial magnetic field necessary for closing the reed contacts.
Different models of reed switches need different levels of the magnetic field to make them operate and close the contacts. Manufacturers specify this in ampere-turns or AT, which is the product of current flow and the number of turns in the coil. Therefore, there is a huge variation in the characteristics of the reed relays available. A higher voltage or power level is necessary for stiffer reed relays and those with larger contact gaps. These require higher AT levels to operate, as the coils require more power.