Reed Switch: Working Principle, Advantages and Applications

Reed switches are fascinating electromechanical devices that play a crucial role in various industries and applications. Reed switches are included in reed sensors or reed relays. The ferromagnetic circuit tongues move in relation to each other in the presence of an external magnetic field. This technology makes it possible to produce reliable, hermetically sealed switching elements with a small size for fast switching processes compared to conventional relays and contacts.

The main components of a reed contact are the switching tabs (paddles) made of a nickel-iron alloy (Ni approx. 48%) with the outer solder surface (approx. 2–6 μm tin or gold) and inner contact surfaces made of precious metal. A glass tube fixes and protects them and contains the inert gas filling (nitrogen/hydrogen or argon) or a vacuum in the case of switches for higher voltages. The reed switch originated in the USA and was developed there by Bell Labs at the end of 1930.

Understanding Reed Switches

A reed switch consists of two ferromagnetic switching tongues (usually nickel/iron alloy) that are hermetically sealed and melted into a glass tube. In the case of change-overs or openers, the end of one of the switching tabs is non-magnetic. The derailleur tongues overlap and have a small distance of a few micrometers to about 1 mm from each other. If an axial magnetic field acts on the switch, the two paddles move towards each other – the switch closes. The contact area of the two circuit tongues is coated with a very hard metal, usually rhodium or ruthenium, but also tungsten and iridium. These are applied either galvanically or by sputtering. The contact surfaces are important for the very long service life and good contact of a reed switch. Before melting, the existing air is replaced by nitrogen or an inert gas mixture with a high nitrogen content. For increased switching voltages (kV range), reed contacts are evacuated.

If the magnetic field generated by permanent magnets or coils is stronger than the spring effect of the paddles, the two contacts close. The field to be undercut to open is much smaller.

Due to the materials used and hermetically sealed design, reed switches can be used in almost all environmental conditions. Nevertheless, there are some points to consider that affect reliability. The glass bushing of the connecting wires, which is responsible for the tightness, is sensitive to breakage under bending loads and, due to the different expansion coefficients, is sensitive to thermal shock when soldered near the glass. The application of the contact material (rhodium or ruthenium) is carried out by sputtering or electroplating and requires high purity. Foreign particles, even in the smallest concentration, are the source of unreliability. Such precious metal contacts are not suitable for high switching capacities.

Over time, the length was reduced from the original 50 mm to 5 mm. In addition to miniaturization, this has opened up new applications, especially in high-frequency technology and through higher switching speeds.

Construction: Reed switches consist of two ferromagnetic contacts (reeds) encased in a hermetically sealed glass tube filled with inert gas, typically nitrogen. The contacts are coated with a thin layer of precious metals like rhodium or ruthenium to prevent oxidation and ensure reliable electrical conductivity.

Working Principle: Reed switches operate based on the phenomenon of magnetism. When a magnetic field is applied near the switch, the reeds are magnetized and attract each other, closing the circuit. Removal of the magnetic field causes the reeds to return to their original position, opening the circuit.

Types of Reed Switches: Reed switches come in various configurations, including normally open (NO), normally closed (NC), and changeover (SPDT). NO switches close the circuit in the presence of a magnetic field, while NC switches open the circuit. Changeover switches alternate between NO and NC states.

Applications of Reed Switches

If reed switches are used as position sensors (door contact, fill level, limit switches), permanent magnets are used for actuation. To ensure precise switching, the field must be axial, i.e. in the direction of the switching tongues.

If the field is exactly perpendicular to the shift tongues, the contact opens. This is exploited, for example, to maintain exact switching positions during reference runs of positioning drives: the contact initially closes when approaching, but opens when the tongues and magnet are in a T-shape to each other.

Reed contacts can also be manufactured with a bistable function (“latching”). With these, it is possible to change the switching state with a magnet or a coil. The sensor remains in the previous position until the reverse polarity of the external magnetic field. This is achieved by pre-magnetization, which is just enough to hold the contacts, but not to tighten them. If the external field cancels the pre-magnetization, the contact falls off. If both fields add up, it attracts.

A common and highly visible application in everyday life is as a speedometer sensor on the front fork of a bicycle, triggered by a spoke magnet running just past it. If you let the raised front wheel turn in a quiet environment, you can hear the click with your close ear, which occurs when the contacts close. E-bikes often have a reed sensor on the chainring, the crank with several magnets and on the rear wheel.

Reed switches, reed sensors and reed relays are produced for many different industries, such as mechanical engineering, automation technology, safety technology, the automotive industry, aviation, agriculture, test and measurement technology, medicine, telecommunications, household appliances and marine.

Reed switches, combined with a permanent magnet on a float, are used as a level sensor (float switch). Proximity switches are used to monitor doors, flaps and locks as well as to determine their position. Motion and acceleration sensors are other possible applications of the combination with permanent magnets.

Security Systems: Reed switches are commonly used in security systems, such as alarm systems and door/window sensors, to detect unauthorized entry. When a door or window is opened, the magnetic field is disrupted, triggering an alarm.

Proximity Sensors: Reed switches serve as proximity sensors in industrial and automotive applications, detecting the presence or absence of metallic objects. They are used in machinery, conveyor belts, and automotive systems for position sensing and object detection.

Medical Devices: In medical devices like infusion pumps and blood analyzers, reed switches provide reliable switching for controlling fluid flow, detecting cartridge insertion/removal, and other critical functions.

Automotive Electronics: Reed switches find applications in automotive electronics for controlling lighting systems, detecting gear shifts, and monitoring fluid levels. They offer durability, compactness, and resistance to harsh environments.

Consumer Electronics: Reed switches are utilized in consumer electronics products like reed relays, keyboards, and proximity switches. They provide a compact and reliable solution for user interface and control.

Advantages of Reed Switches

Reliability: Reed switches offer high reliability and long operational life due to their simple construction and absence of mechanical contacts, which are prone to wear and tear.

Low Power Consumption: Reed switches require minimal power to operate, making them suitable for battery-powered devices and applications where energy efficiency is essential.

Compact Size: Reed switches are compact and lightweight, allowing for easy integration into space-constrained electronic and mechanical systems.

Wide Temperature Range: Reed switches can operate over a wide temperature range, from sub-zero temperatures to elevated temperatures, without significant degradation in performance.

Insensitive to Contaminants: The hermetically sealed construction of reed switches makes them immune to dust, dirt, moisture, and other contaminants, ensuring reliable operation in harsh environments.