How a Reed Switch is Used with a Permanent Magnet
Using Reed Switches in a sensing environment, one generally uses a magnet for actuation. It is important to understand this interaction clearly for proper sensor functioning. Sensors may operate in a normally open mode, a normally closed mode or a latching mode. A permanent magnet is the most common source for operating the Reed Switch. The methods used depend on the actual application.
Before we investigate each of these approaches, it is important to understand the magnetic fields associated with the various Reed Switch vs. magnet positions and their on/off domain characteristics. The actual closure and opening points will vary considerably for different Reed Switches and different sizes and strengths of permanent magnets.
Normally Open (Form A)
In the normally open mode, when a magnet is brought toward the Reed Switch (or vice versa) the reed blades will close. When the magnet is withdrawn the reed blades will open.
Normally Closed (Form B)
With the normally closed sensor, bringing a magnet to the Reed Switch the reed blades will open, and withdrawing the magnet, the reed blades will re-close.
Changeover (Form C)
A SPDT (single pole double throw) where a normally closed contact opens before a normally open contact closes.
Latching (Form E)
In a latching mode the reed blades may be in either an open or closed state. When a magnet is brought close to the Reed Switch the contacts will change their state. If they were initially open, the contacts will close. Withdrawing the magnet the contacts will remain closed. When the magnet is again brought close to the Reed Switch, with a changed magnetic polarity, the contacts will now open. Withdrawing the magnet the contacts will remain open. Again, reversing the magnetic polarity, and bringing the magnet again close to the Reed Switch the contacts will again close and remain closed when the magnet is withdrawn. In this manner, one has a latching sensor or a bi-stable state sensor.
Magnets are available in multiple specifications on the market. Almost all dimensions and geometries can be realized. To activate the reed switch a magnet (magnet field) is needed. The different magnet materials have either more positive or negative specifications, depending on the dimension and geometries as well as on the environment. Most preferred and used forms are cylinders, rectangles, rings and discs. Depending on the different requirements, magnets can be magnetized in many different ways (See Illustration).
Furthermore each magnet material has a different magnet force as well as a different flux density. In addition to dimension and material, other factors exist that define the energy of a magnet. These are mounting position, environment and other magnetic field witch influence the interaction between reed sensor/switch and magnet. In applications were a magnet is used to activate a reed sensor/switch, the environmental temperature needs to be considered (in the application as well as in storage). High temperatures can cause irreversible damage and will have heavy impact on the magnetic force and the long term stability. AlNiCo magnets are best suitable for applications up to 450°C.