However the speed with which the reed switch contact approach one another does mean that contact-points collide with considerable energy so that they rebound, collide again etc for a while causing a phenomenon called contact bounce. The way in which the contacts bounce depends largely upon the size of the reed switch, the weight of the contact-elements, their elasticity, etc.
Obviously the bounce period significantly increases the wear of contact-elements. The contact bounce can give rise to arcing if current is being carried, especially if there is a capacitive or inductive element to the circuit being switched. Even when switching small currents for items like CMOS or other circuits, the capacitive element introduced by decoupling capacitors on the circuit can cause very high transient currents to occur which can significantly reduce the contact life.
When the external magnetic field is removed, the magnetisation of the nickel iron will also be removed. This will result in the magnetic attraction between the two contacts disappearing, and the spring in the contacts will force the contact apart. Rather than using a bar magnet as shown for the purposes of the explanation, a coil is normally used - this assembly then becomes the whole reed relay, i. The whole assembly is the reed relay.
As current is passed through the coil, so this creates a magnetic field and the contacts become magnetised, and are attracted to each other. As the field is increased, a point is reached where the contacts close. Removing the current removes the field and the contacts then spring apart.
Using a coil has the advantage that this can be driven by electronic circuitry to enable the reed relay contacts to be controlled or switched by an external electronic stimulus. In this way a small current can control a much larger current passing in the reed contacts. One of the issues that can occur with reed relays is that there is magnetic coupling from the coil.
Each reed relay assembly will have an associated magnetic field that extends beyond the mechanical confines of the relay itself. If the magnetic field is not contained, then the field from an adjacent relay or relays can oppose the field within the relay in question. The arrows on the field that pass through the centre of the coil are in the different direction to the external ones. As the internal field from the coil will be affected by the external field from an adjacent relay, these can be understood to oppose one another.
The field cancellation effect reduces the sensitivity, requiring a higher voltage to ensure reliable switching.
Poor screening can also give rise to excessive pickup in other circuits and this may impact the EMC performance. To overcome this issue, reed relays normally have a ferrous metal screen placed around them. The ideal properties for the shield are for it to have a high permeability and very low magnetic remnance.
The screen concentrates the magnetic field and this improves the efficiency of the relay and allows the devices to be closely stacked. Reed relays offer many advantages and can be used to good effect in a number of situations.
Like many technologies there is a balance to be made between the advantages and disadvantages to determine the applicability for any given situation. Reed relays are a very reliable form of relay that can be used in a variety of areas.
Often they are used in switching matrices were complete isolation and low contact resistance are required. Their comparative small size and the fact that these relays are often contained within small packages, some the size of a IC dual in line packages or even smaller, means that they are easy to use, convenient, and small enough to be used in virtually any electronic circuit. Development of the reed switch and reed relay The concept for reed switches was first proposed in by a professor at the Leningrad Electrotechnical University called V.
As well as being much harder to manufacture than normally open reed relays the two contacts, normally closed and normally open, can have quite different characteristics and stability. Experience is generally that they have a slightly less stable contact resistance than their simpler normally open counterparts.
Even so, they perform a useful function for many applications because unlike the use of two normally open reed relays used to create a changeover function they only need one coil drive and it is mechanically not possible to have both contacts closed at the same time. If you are interested in reed relays with changeover reeds please check out our Series range. There are a couple options available including 1 Form C and 2 Form C, both 3 watts at v. Reed relays can also be supplied as 2 pole relays where two reed switches are contained in the same package and operated by a common coil drive.
It is important to remember that these relays do not have an interlock mechanism between the two, it is unsafe to assume that the two poles operate at exactly the same time and the two reed switches are essentially independent.
There could be an operate time difference of between microSeconds between them. Normally open reed switch. When a magnet is moved close to a switch, or the switch close to a magnet, the reeds repel one another and split apart, breaking the circuit. There is a third configuration that has three contact points, rather than two. In this configuration, the current flows along a common lead which can be toggled between two contacts. The common lead will be in contact with one contact in its normal position until a magnetic field is introduced moving the common lead to be in contact with the other contact.
When the magnetic field is removed, the common lead reverts back to its original position. Without a magnet , a reed switch is redundant but introduce a magnetic field to the reed switch and the switch will spring into action. The size and type of magnet required depends entirely on the type of reed switch and how the reed switch is built into an assembly. Because a reed switch can be hidden or embedded within an assembly and still operated by a magnet, the distance between the magnet and the switch is all important.
The wider the distance between switch and magnet, the stronger the magnet will need to be to interact with the switch. Any permanent magnet will work with a reed switch but it is important to remember that different materials have different strengths and different sized magnets produce different sized magnetic fields.
Neodymium magnets are the strongest type of magnets commercially available, and therefore even tiny magnets can be effective. However, ferrite magnets , although much weaker, are popular because of the deep magnetic field that they produce. When selecting a magnet for a reed switch application there are several main factors to consider; shape of magnet, magnet strength, switch sensitivity, distance and angle between magnet and switch.
Bar magnet. The sensitivity of a reed switch is rated based on the magnetomotive force measured in the ampere-turns AT required to pull in or release the contact points. When a reed switch is manufactured, it is placed within a test coil with a specific number of turns of wire. As an electric current is passed through the coiled wire, it creates a magnetic field.
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