Reed relays are deceptively simple devices in principle. They contain a reed switch, a coil for creating a magnetic field, an optional diode for handling back EMF from the coil, a package and a method of connecting to the reed switch and the coil to outside of the package. The reed switch is itself a simple device in principle and relatively low cost to manufacture thanks to modern manufacturing technology.
Kenvin Mallett, Pickering Electronics, Clacton UK
The reed switch has two shaped metal blades made of a ferromagnetic material (roughly 50:50 nickel iron) and glass envelope that serves to both hold the metal blades in place and to provide a hermetic seal that prevents any contaminants entering the critical contact areas inside the glass envelope. Most (but not all) reed switches have open contacts in their normal state.
If a magnetic field is applied along the axis of the reed blades the field is intensified in the reed blades because of their ferromagnetic nature, the open contacts of the reed blades are attracted to each other and the blades deflect to close the gap. With enough applied field the blades make contact and electrical contact is made. The only movable part in the reed switch is the deflection of the blades, there are no pivot points or materials trying to slide past each other. The reed switch is considered to have no moving parts, and that means there are no parts that mechanically wear. The contact area is enclosed in a hermetically sealed envelope with inert gasses, or in the case of high voltage switches a vacuum, so the switch area is sealed against external contamination. This gives the reed switch an exceptionally long mechanical life
Inevitably in practice the issues are a little more complicated. The ferromagnetic material is not a good conductor and in particular the material does not make a good switch contact. So the reed blades have to have a precious metal cover in the contact area, the precious metal may not stick to the blade material very well so an underlying metal barrier may be required to ensure good adherence. Some types of reed relay use mercury wetted contacts, consequently reed relays that use plated contacts are often referred to as “dry” reed relays. The metals can be added by selective plating or by sputtering processes. Where the reed blade passes through the glass envelope any plating (in many cases there may be none) requires controlling to avoid adversely affecting the glass to metal hermetic seal. Outside the glass seal the reed blades have to be suitably finished to allow them to be soldered or welded into the reed relay package, usually requiring a different plating finish to that used inside the glass envelope.
The materials used for the precious metal contact areas inside the glass envelope have a significant impact on the reed switch (and therefore the relay) characteristics. Some materials have excellent contact resistance stability; others resist the mechanical erosion that occurs during hot switch events. Commonly used materials are ruthenium, rhodium and iridium– all of which are in the relatively rare platinum precious metal group. Tungsten is often used for high power or high voltage reed switches due to its high melting point. The material for the contact is chosen to best suit the target performance – bearing in mind the material chosen can also have a significant impact on manufacturing cost.
Another design variable on the reed switch is its size. Longer switches do not have to deflect the blades as far (measured by angle of deflection) as short switches to close a given gap size between the blades. Short reeds are often made of thinner materials so they deflect more easily but this clearly has an impact on their rating and contact area. Smaller reed switches allow smaller relays to be constructed – an important consideration where space is critical. The larger switches may be more mechanically robust and have greater contact area, improving their signal carrying capability.
It is these compromises in reed switch design that gives rise to the sometimes bewildering range of reed relays that are available with both subtle and not so subtle differences in performance.
Generating the magnetic field
To create a relay a magnetic field needs to be created that is capable of closing the reed switch contacts. Reed switches can be used with permanent magnets (for example to detect doors closing) but for the reed relays described in this book the field is generated by a coil which can have a current passed through in response to a control signal. The coil surrounds the reed switch and generates the axial magnetic field needed to close the reed contacts.
Different reed switches require different levels of magnetic field to close the contact, and this is usually quoted in terms of the ampere turns (AT) – simply the product of the current flowing in the coil multiplied by the number of turns. Again this creates a great deal of variation in the reed relay characteristics. Stiffer reed switches for higher power levels or high voltage switches with larger contact gaps, usually require higher AT numbers to operate, so the coils require more power.
Use of different wire gauges for the coil and number of turns creates relays with different drive voltage requirements and different coil powers. The resistance of the wire coil controls the amount of steady state current flowing through the coil and therefore the power the coil consumes when the contacts are closed. Whenever fine wires are used in Pickering relays, the termination leads from the coils are skeined with several strands of wire twisted together to increase their physical strength.
Larger coils can be used to reduce power consumption, but that increases the size of the relay. A significant factor in some designs is the ability to drive reed relays with standard CMOS logic, requiring that the coil is operated from 5V or 3.3V and that the current (power) requirements in the coil are minimized.
Protection against magnetic fields
The fact that reed relays are magnetically operated causes a potential problem for users when they are assembled in dense patterns on PCB’s
The magnetic field required to close the reed blades flows through the nickel iron reed blades and returns by field lines which are outside the reed relay body. If several relays are placed close together the external field lines can be drawn by the neighbouring reed blades and either reinforce of partially cancel the field in the reed, changing the current needed to close or open the contact. This can in some circumstances cause enough effect that the relay may either fail to close or open depending on the magnetic polarity. Some manufacturers suggest arranging the relays in different polarity patterns to mitigate the worst effect of the interaction, but this can become a complex compromise in dense arrays of relays where there are many near neighbours.
A much more sensible approach is to include a magnetic shield in the reed relay package, an approach used by Pickering Electronics for many years. The user is then free to use a layout pattern that best suits the application. The approach has the added benefit of improving the coil efficiency since it concentrates the magnetic field lines closer to the reed switch body, shortening the magnetic field length outside the reed blades and creating a larger field for a given number of ampere turns in the coil. Lower coil operating currents make coil driving simpler and improves other parameters like thermoelectric emf generation.
Mercury reed relays
There is a class of reed relays that has been historically very popular where the reed contacts include mercury that provides the electrical contact between the blades. The mercury is provided by a small reservoir which blade actuation tends to pump up a grooved surface on the reed blade to the contact area using mercury’s high surface tension to retain the material.
Selective chrome plating is often used in the construction since mercury and chrome do not stick together and this is used to help control the mercury.
The glass envelope of mercury relays is also highly pressurised (typically 12 to 14 bar) which helps to manage the switch materials and operation and to improve electrical parameters.
These relays are strongly preferred in some industries because they have a long contact life and bounce free contact closure – a feature that is particularly helpful under hot switch conditions. Stability of low contact resistance during their operational life is considered to be better than that of dry reed relays.
Most types of mercury reed relays are position sensitive – they can only be used in a vertical orientation. Some non position sensitive versions are also available which can be used in any orientation. Mercury wetted relays however are not RoHS compliant and national regulations may limit their use to certain critical applications where exceptions on RoHS have been granted.
High voltage reed relays
High voltage reed relays in addition to having to ensure high clearance distance (including the distance between the contacts in the reed switch) have to have a carefully match operating environment and different contact materials to resist the contact erosion that occur when switching the signals. High voltage reed switches commonly use tungsten or rhodium contacts.
The glass envelope for high voltage reed switches is normally a very hard vacuum to maximise the voltage rating for a given blade separation and to manage arc duration as the contacts open or close. Any loss of seal will rapidly degrade the switch operation so reed switches have to be carefully managed as they are packaged into reed relays.
Normally closed reeds
Most of the information in this book relates to normally open reed relays – by far the most common configuration of reed relay. However, normally closed relays can also be supplied where the blade is biased so it is normally closed and the application of a magnetic field opens the relay contacts.
The contact bias is created by adding an internal permanent magnet to hold the reed switch in a normally closed state. When the relay coil is energised it cancels out the magnetic field bias and the contacts open. If the coil voltage is increased substantially beyond its nominal voltage (typically greater than 1.5 times nominal) there is a risk that the contact will reclose.
Not surprisingly normally closed relays are more difficult to manufacture and have higher magnetic interaction due to the bias magnet.
Reed relays can be supplied with changeover switches – the reed switch has a normally closed contact (when no magnetic field is applied) and a normally open contact (which closes when the field is applied). The reed switch closed contact uses the blade as a spring bias with a non ferrous spacer to avoid completing a magnetic circuit. The coil field moves the blade to the normally open contact blade which does not have this spacer. As the reed relay switch blades transition between the two states for a brief period neither contact is closed – and important consideration in some applications.
The normally closed position relies on contact pressure being created by the spring bias of the blade. 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.
Two pole relays
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 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 betweeen 50 – 250 microSeconds between them. Failure in one (say a contact weld) will not stop the other contact from moving.
Reed-Relais sind im Prinzip täuschend einfache Geräte. Siebestehen aus einer Magnetspule sowie einem sich in der Mitte befindlichen Reedschalters. Oft sind Reedrelais nach außen magentisch geschirmt, so dass sie dicht gepackt werden können, ohne sich gegenseitig zu beeinflussen. Dabei erweisen sie sich selbst als eine einfache Vorrichtung und sind mit relativ niedrigen Kosten dank moderner Fertigungstechnologien einfach herzustellen.
Les relais reed sont en principe des appareils étonnamment simples. Ils sont constitués d’une bobine magnétique ainsi que d’un commutateur reed situé au centre. Souvent les relais reed sont protégés vers l’extérieur du point de vue magnétique, de sorte qu’ils puissent être positionnés très près les uns des autres sans s’influencer mutuellement. Ils se révèlent ainsi par eux-mêmes comme des dispositifs simples et grâce à des technologies modernes de fabrication, ils peuvent être produits à des coûts relativement bas.