A reed relay is an electromechanical switch using a reed switch - two ferromagnetic reeds sealed in a glass tube filled with inert gas or vacuum. When a coil around the tube generates a magnetic field, the reeds magnetize, close, and complete the circuit. Removing the field opens the circuit. Reed relays are reliable, with lifespans up to 1 billion operations, due to sealed contacts coated with precious metals like ruthenium or tungsten. They’re used in proximity sensors, telecommunications, and low-level signal switching. Magnetic shielding prevents interference in dense setups, ensuring consistent performance in safety-critical applications.
Reed relays offer compact designs, fast switching speeds, and low power consumption, making them ideal for space-constrained applications like medical devices and telecommunications.
SPST relays: Most reliable, up to 1 billion operations.
SPDT relays: Shorter lifespan due to manufacturing complexities.
Contact coatings: Ruthenium, rhodium, iridium, or tungsten for high-power applications.
What Are Reed Relays? Reliable Electromechanical Switches
A reed relay is an electromechanical switching device that uses a reed switch as its core component to control electrical circuits.
How Reed Relays Work: Principle and Operation
The principle of reed relays is quite simple. The core component of a reed relay is a reed switch, which consists of two ferromagnetic reeds sealed within a glass tube. Typically, the reeds are composed of a nickel-iron alloy with a 50:50 ratio. The sealed glass tube is usually filled with an inert gas, which isolates the contacts from external pollutants and oxygen, effectively extending the lifespan of the contacts. Generally, reed switches are in a normally open state, though normally closed versions also exist.
As shown in the diagram, when a magnetic field is applied along the axis of the reeds, and the magnetic field reaches a certain strength, the reeds become magnetized due to the ferromagnetic effect. This causes the two contacts to attract each other, closing the circuit and allowing conduction between the two ends.
Magnetic Field Generation in Reed Relays
To close a reed switch, an external magnetic field is required. Some reed switches are used in combination with permanent magnets, functioning as proximity switches or for detecting the position of doors and windows. However, in reed relays, the magnetic field required to close the contacts is generated by a coil. This enables the control of the contact’s closing or opening by regulating the current through the coil. As shown in the diagram, the coil is wound around the outside of the reed switch’s glass tube, providing an axial magnetic field.
Different reed switches require different magnetic field strengths, typically measured in “ampere-turns” (AT), which is the product of the current flowing through the coil and the number of coil turns. This leads to a key characteristic of reed relays: relays with higher power or voltage ratings typically have stiffer reeds with larger gaps, requiring a stronger magnetic field (higher ampere-turns). Consequently, the power consumption of the relay coil increases.
Magnetic Field Shielding in Reed Relays
Since reed relays are driven by magnetic fields, this introduces potential issues. It’s important to understand that the magnetic field generated by a coil forms a closed loop, passing through the reeds of the reed switch and returning through the external environment. This means magnetic fields exist outside the coil and can overlap with others. When reed relays are densely arranged on a PCB, the overlapping magnetic fields can either reinforce or weaken each other in different areas. This may cause some relays, which were not intended to close, to close unexpectedly due to enhanced fields, while others, intended to close, fail to do so due to weakened fields. This interference can lead to serious malfunctions, rendering the equipment inoperable.
Some relay manufacturers suggest arranging relays with alternating polarities to minimize this issue. However, this requires users to account for magnetic field interference, which complicates mechanical design, especially in scenarios involving large-scale, dense relay arrangements. Another solution is to add magnetic shielding to the relay housing, which prevents magnetic field leakage. This allows users to arrange relays freely without worrying about interference between them.
Materials and Mechanical Lifespan of Reed Relay
The most reliable structure for reed relays is the single-pole single-throw (SPST) switch, also known as Form A. Single-pole double-throw (SPDT) switches (Form C) typically have a shorter lifespan due to manufacturing complexities and tolerances. For longer-lasting SPDT relays, combining two SPST relays to function as an SPDT relay can be considered. Reed relays generally have a very long lifespan. For example, relays manufactured by Greegoo for low to medium current applications can achieve up to 10^9 operations. The contacts of reed relays are sealed in a glass tube filled with inert gas or a vacuum, protecting them from external contaminants. The contacts are coated with precious metals, and the only moving part is the deflection of the reeds, with no rotating shafts or sliding components. This results in a long lifespan and eliminates issues like electrical arcing, making reed relays suitable for high-safety applications.
The precious metal coating on the contacts is critical to the relay’s performance. High-quality coatings can reduce erosion during hot-switching. The most commonly used contact materials are ruthenium, rhodium, and iridium—rare platinum-group metals. Tungsten is often used for high-power or high-voltage reed relay contacts due to its extremely high melting point. Different applications require different contact coatings, which significantly impact the relay’s performance and manufacturing cost. Reed relays are categorized into different grades based on cost and performance. Low-quality commercial relays typically use rhodium contacts, which can develop a polymer film on the contact surface, making them less suitable for switching low-level signals. Sputtered ruthenium contacts are better suited for low-level signal switching, as they avoid the polymer film issue seen with rhodium. For applications requiring high-quality low-level signal switching, instrument-grade ruthenium contact reed relays are the superior choice.
Reed relays are exceptionally durable, with lifespans reaching up to 1 billion operations (10^9) for low to medium current applications, as seen in Greegoo’s relays. Their longevity is due to a reed switch design: two ferromagnetic reeds sealed in a glass tube with inert gas or a vacuum, protecting contacts from contaminants and oxidation. Precious metal coatings like ruthenium or rhodium minimize wear during switching, and the absence of rotating or sliding parts eliminates mechanical degradation. Lifespan varies based on load, switching frequency, and contact quality, but SPST relays typically outlast SPDT due to simpler construction, making them ideal for high-reliability applications like telecommunications.
SPST (Single-Pole Single-Throw, Form A) reed relays feature one contact set, either normally open or closed, controlling a single circuit. They’re highly reliable, offering up to 1 billion operations due to simple design and minimal moving parts, ideal for low-current switching in sensors or telecommunications. Conversely, SPDT (Single-Pole Double-Throw, Form C) relays have one common contact switching between two circuits (one normally open, one normally closed), enabling multi-circuit control. However, SPDT relays have shorter lifespans due to manufacturing complexities and tighter tolerances. Combining two SPST relays can mimic SPDT functionality with greater durability, suitable for applications like signal path selection.
Ruthenium is a top choice for reed relay contacts, especially in low-level signal switching, due to its reliability. Unlike rhodium, which forms a polymer film that hinders low-level signal performance, sputtered ruthenium ensures clean, stable contacts, maintaining signal integrity. This platinum-group metal resists erosion during hot-switching, enhancing durability for applications like telecommunications or medical devices. Ruthenium’s balance of performance and cost makes it ideal for instrument-grade relays, outperforming alternatives like iridium in low-level scenarios. For high-power or high-voltage needs, tungsten may be used, but ruthenium excels in precision, ensuring long-lasting, high-quality switching without signal degradation.
Reed relays and high-voltage relays differ in design and application. Reed relays use a reed switch - ferromagnetic reeds sealed in a glass tube with inert gas to switch low to medium currents, offering up to 1 billion operations. They’re compact, reliable, and suited for low-level signal switching in telecommunications or sensors. High-voltage relays, designed for high-voltage circuits (e.g., kV range), use robust insulation and larger contact gaps to handle high power, often with ceramic or vacuum enclosures. They prioritize arc suppression over lifespan, which is typically shorter. Reed relays may use tungsten for high-voltage applications, but dedicated high-voltage relays are optimized for extreme voltages, not precision switching.
