W. B. Ellwood invented the Reed Switch at Bell Telephone Laboratories in 1936. In the 1940's it began to find itself in demand as a sensor and Reed relay. Here it was used in a variety of stepping/switch applications, electronic and test equipment. Western Electric began using Reed relays in the late 40's in their central office telephone switching stations, where they are still used in some areas even today. The Reed Switch played quite an important role in the development of telecommunications technology. Over the years, several manufacturers have come and gone and in general, they produce good quality, reliable products that contribute to its significant growth today.
Today, Reed Switch technology is used in many markets such as in telecommunications, automotive, security appliances and test and measurement technology. An example of a reed switch's application is to detect the opening of a door, when used as a proximity switch for a burglar alarm. It has a very strong growth rate, in fact so much so that the world output cannot keep up with demand. The Reed Switch can be used in almost any environment due to the fact that it is hermetically sealed (completely sealed and airtight). Though it is structurally simple, it does touch upon many technologies in its manufacturing process.
What is critical to its quality and reliability is the glass to metal hermetic seal. The glass and metal used needs to have very exact linear coefficient of expansion (referring to the relative change in length of a material per degree temperature change) for it to avoid cracking which results in poor seals. Over the years, the Reed Switch has shrunk in size from approximately 50 mm (2 inches) to 6 mm (0.24 inches). These smaller sizes have opened up even more applications.
Reed Switches are low cost and very versatile. They can be great to use with PLC and other logic devices. Often having built in current limiting circuit, providing protection against current surges common to PLC's and long wire runs. Large surges of current common in inductive loads can severely limit switch life span.
A Reed Switch consists of two ferromagnetic blades which is usually made of iron and nickel. These blades are hermetically sealed in a glass capsule. The blades overlap inside the glass capsule, with a gap between them. These blades make contact with each other when there is a suitable magnetic field. Where the blades make contact with each other, this area is usually plated or sputtered ( a deposit, metal on a surface by using fast ions to eject particles of it from a target) with a hard metal like Rhodium or Ruthenium. The hardness of these metals contributes to the longevity of a Reed Switch if these contacts are not overloaded. Within the capsule there is a gas, which is usually Nitrogen or some equivalent inert gas. In the case of some Reed Switches, they have an internal vacuum to increase their ability to switch and standoff high voltages. The blades act as magnetic flux conductors when exposed to an external magnetic field from a permanent magnet or electromagnetic coil. Poles of opposite polarity are created and the contacts close when the magnetic force exceeds the spring force of the reed blades. As the external magnetic field is reduced so that the force between the reeds is less than the restoring force of the reed blades, the contacts open. The Reed Switch described above is a 1 Form A (normally open) or Single Pole Single Throw (SPST) Reed.
There's generally two ways that are used to close Reed Switch contacts. One is the use of a permanent magnet (as per diagram A below). The second is the use of a coil wound with copper (refer diagram B below).
A. The basic operation of a Reed Switch under the influence of the magnetic field of a permanent magnet. The polarisation of the reed blades occurs in such a manner to offer an attractive alternative force at the reed contacts.
There are a number of ways of extending the life of the reed switch. Reed switches are mechanical devices, normally open, but two ferromagnetic blades, sealed with an inert gas in a glass chamber, close when the magnet in the rotor passes and pass a current. Ideally, to maximise the reed switch life the customer should limit it to switch under 5V at 10 mA. Change of temperature will also have a role in reed switch life as fast changes in temperature will crack the glass body. To summarise what can be done to maximise the life of the reed switch:
For example, the FLOMEC® OM025 has the highest frequency of any of our meters at maximum flow rate of 67 Hz. This means if it is running close to the maximum flow rate specified continuously, the reed switch would have an expected life of 1 year for 2 billion pulses. The way to solve this is either through specifying a larger meter with lower frequency initially, or changing to the Hall effect sensor (for more information oh Hall Effect Sensor click here) which is not theoretically life limited by the frequency. Note that we do not recommend running at 100% of meter flow rate range as rotor bearing wear will be excessive, we would recommend the meter to run primarily below 80% of full range – a reed switch life of 18 months. So as you can see the reed switch life is within the bounds of expected use if they are running near the full flow rate range. Any higher switching voltage or current or rapid temperature change will also be contributing to early failure.
We hope that this techcorner article on reed switches has been informative. For assistance with anything discussed in this article or flow measurement in general, we have plenty of resources to assist you. Contact us for more information.
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