I Introduction

The basis of any reed relay is the reed switch itself - this is the core element of the reed relay. The reed switch consists of two reed contacts which are made from nickel-iron and then plated with materials to ensure the maximum working life span. The reed switch contacts overlap so when they close they make contact with each other. Commonly, the spacing in the open state is between 0.05 and 1 mm. The greater the spacing is, the greater the voltage resistance gets. The small gaps between the contacts enable fast switching speeds which depend primarily on the actual size of the contacts. Normally, the speed range is from around half a millisecond to a few milliseconds. Often the nickel-iron content of the reed contacts is around 52%. The materials used include ruthenium, rhodium, sometimes iridium, or tungsten, or molybdenum in the situation of high voltage. Rhodium is usually electroplated onto the reed element, while ruthenium is generally sputtered. There were also very few reed switches that used gold, often for audio.

The reed relay is surrounded by a glass envelope to make sure a hermetic seal so as to prevent the ingress of moisture and other contaminants. Most reed relays and reed switches are sealed in a glass tube which is cut from a long tube. The end of the tube containing each reed switch is melted to provide a hermetic seal. Commonly, the glass envelope is filled with an environment to prevent wear, oxidation and to better extinguish any sparks. It is usually filled with nitrogen and may have traces of helium. High voltage reeds may use a vacuum. 

Figure 1: Internal assembly of a reed switch

II Working principle

The operation of the reed relay is quite straightforward. It works by placing a magnetic field close to the reed switch contact, which makes each reed become magnetically orientated so as to make the ends of the reed attract each other and move together to close the contact. Without a magnetic field, the two contacts will not be magnetically orientated and the spring loading in the contacts will keep them apart. 

As the magnet close to the reed contacts which are made of magnetic material, typically nickel-iron, this starts to make the two contacts magnetically orientated. A north pole will appear in one contact and south in the other. 

Figure 2: Basic operation of a reed switch

As the magnetic field close to the contacts increases, the strength of its magnetization increases and it will reach a point ultimately to overcome the spring in the contacts. As the contacts move nearer, the strength of the attraction gradually increases, and ultimately a firm contact is made.

However, the speed at which the reed switch contacts approach each other does mean that the contacts collide with enormous energy, so they rebound and then collide after a while, which causing a phenomenon called contact bounce.

The way of contact bounce largely depends on the size of the reed switch, the weight of the contact elements, and their elasticity, etc. Obviously, the bounce time can significantly aggravate the wear of contact elements. The contact bounce can give rise to arcing if the current is being carried, especially when there is a capacitive or inductive element to the circuit is switched. Even when devices using CMOS or other circuits switch small currents, the capacitive element introduced by decoupling capacitors on the circuit can cause very high transient currents to occur, which can largely reduce the working life span of the contact.

Figure 3: Contact made between the reeds

When the external magnetic field is removed, the magnetization of the nickel-iron will also be removed. This will result in the disappearance of the magnetic attraction between the two contacts, and the spring in the contacts will be forced apart. When current passes through the coil, this will create a magnetic field. In this situation, the contacts become magnetized and are attracted to each other. As the field increases, a point where the contacts close is reached. Removing the current will remove the field and then the contacts apart.

Figure 4: Constitution of a reed relay operated by a coil

Using a coil has the advantage that this can be driven by an electronic circuit 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 the reed contacts.

Their comparatively small size and the fact that these relays are often contained within small packages. The small size means that they are easy to use, convenient, and small enough to be used in virtually any electronic circuit.

III Advantages and disadvantages of reed relays

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 relay advantages

1.Small size some are fitted into SIL, DIL packages, etc. or even smaller

2. Fast switching speeds 

3. Provide complete isolation between the switching current and the switched circuit 

Reed relay disadvantages

1.Generally a low current capability, i.e. not suitable for very high currents 

2.Not as fast as some solid-state switches, although many solid-state switches use optoisolator which is slow 

3.As electromechanical devices, they wear with use, especially the contacts 

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 in which complete isolation and low contact resistance are required. 

IV Development of the reed switch and reed relay

The concept for reed switches was first proposed in 1922 by a professor at the Leningrad Electrotechnical University called V. Kovalenko. He proposed the idea of what was termed a magnetically controlled contact which switched under the influence of a magnetic field.

The next developments occurred around 1936 when, in the USA, the Bell Telephone Laboratories launched research into reed switches expecting to use them in telephone exchanges, etc.

By 1938, an experimental switch was used to switch the center conductor in a coaxial cable. Two years later, the first production devices come into being.

Further use of these devices started to manifest in the 1950s when reed relays (reed switches with their associated electromagnetic coils) started to be used for automatically operated exchanges. These reed relays provided higher levels of reliability than their electromechanical relay counterparts. Moreover, they were also faster, smaller, and required less current.

In 1963, the Bell Telephone company introduced its first reed relay based on exchange. In the following years, the application of this technology for telephone exchanges increased.

As the availability of reed switches and reed relays increases, they start to be used in a variety of other applications. The switches could be used along with small magnets for various position sensing applications, as well as for use in test and measurement matrix switches. In addition, reed switches and relays are used in many other applications.

V  Precautions for the use of reed relays

From the aspects of handling, cleaning, PCB board mounting relay on a PCB board, connection protection, electromagnetic interference, capacitive interference, and drive relays through a semiconductor device, the precautions for the use of reed relays are explained.

1. Handling 

Impact force (for example, a drop height of 30cm or more) or high pulling or twisting force of the external terminal of the application may cause permanent damage to the reed relay.

1.Mounting the reed relay on the printed circuit board

Curved terminal

If it is a necessary relay terminal, the bending angle (between the PCB and the terminal) of the printed circuit board that is bent after insertion should not be greater than 45 degrees. An angle greater than 45 degrees may generate a vertical force that can cause damage to the relay mounted on the PCB. 

2.Position of the reed relay

Commonly, the reed relay is the noise resource in the circuit, so certain prevention measures should be taken:

Enlarge spacing as much as possible between the reed relay and semiconductor component.

Place the relay coil of the coil suppression circuit as close as possible.

Avoid approaching the noise resource of the coil circuit in the reed relay.

3.Soldering

Rosin-based flux is recommended for reed relays.

If soldering by hand, the soldering time must be kept to a minimum. Excessive temperature and/or brazing time may cause damage to the reed switch. The recommended procedure is at 280~300°C for 3 seconds, and the recommended procedure for maximum automatic soldering is at 250~260°C for 5 seconds.

4.Cleaning

Ultrasonic cleaning may cause permanent damage to the contacts of the relay.

Freon, alpha, ethanol, and iso-propyl alcohol are recommended. chlorosen and chlorosolder are acceptable or acceptable conditionally.