Overview
The reed switch is an electromechanical switch operated by an applied magnetic field. It was invented in 1922 by professor Valentin Kovalenkov at the Petrograd Electrotechnical University, and later evolved at Bell Telephone Laboratories in 1936 by Walter B. Ellwood into the reed relay. In its simplest and most common form, it consists of a pair of ferromagnetic flexible metal contacts in a hermetically sealed glass envelope. The contacts are usually normally open, closing when a magnetic field is present, or they may be normally closed and open when a magnetic field is applied. The switch may be actuated by an electromagnetic coil, making a reed relay, or by bringing a permanent magnet near it. When the magnetic field is removed, the contacts in the reed switch return to their original position.
Structure and Terminology
The "reed" is the metal part inside the reed switch envelope that is relatively thin and wide to make it flexible, resembling the reed of a musical instrument. The term "reed" may also include the external wire lead as well as the internal part. This design ensures reliable operation within the hermetically sealed glass envelope, protecting the ferromagnetic flexible metal contacts from environmental factors.
How does a reed switch work?
A reed switch functions as an electromechanical device controlled by an external magnetic field. Its core structure consists of two ferromagnetic, flexible metal contacts housed within a hermetically sealed glass envelope. These contacts, known as "reeds," are designed to be thin and wide, providing the flexibility necessary for movement, akin to the reeds of a musical instrument. The glass envelope serves a dual purpose: it protects the contacts from environmental contaminants and provides electrical insulation between the reeds and the external circuit.
Magnetic Actuation and Contact Closure
When a magnetic field is applied to the reed switch, the ferromagnetic properties of the metal contacts cause them to become magnetized. Depending on the polarity and orientation of the field, the tips of the reeds attract each other. This magnetic attraction overcomes the mechanical spring force of the reeds, causing them to bend inward and make physical contact. This closure completes the electrical circuit, allowing current to flow through the switch. The switch can be actuated by various means, including an electromagnetic coil (forming a reed relay) or by bringing a permanent magnet in close proximity to the glass envelope.
Return Mechanism and Configurations
Once the external magnetic field is removed or weakened below a certain threshold, the magnetic attraction between the reeds diminishes. The inherent spring force of the flexible metal reeds then pushes the contacts back to their original, unactuated position, breaking the circuit. Reed switches are typically configured in one of two primary states: normally open (NO) or normally closed (NC). In a normally open configuration, the contacts are separated when no magnetic field is present and close when the field is applied. Conversely, in a normally closed configuration, the contacts touch in the absence of a magnetic field and separate when the field is applied.
Non-Ferromagnetic Contact Configurations
While standard reed switches rely on ferromagnetic reeds that both conduct electricity and respond to the magnetic field, specialized configurations exist. In some designs, the contact surfaces themselves may be made of non-ferromagnetic materials or plated to reduce contact resistance and wear, while the underlying reed structure remains ferromagnetic to ensure reliable actuation. This allows for optimization of electrical performance, such as reduced bounce or higher current carrying capacity, without compromising the magnetic sensitivity of the switch. The term "reed" encompasses both the internal flexible metal part and, in some contexts, the external wire leads extending from the glass envelope.
Construction and materials
Reed switches are constructed with a pair of ferromagnetic flexible metal contacts sealed within a hermetically sealed glass envelope. The metal components, known as "reeds," are designed to be relatively thin and wide to ensure flexibility, a physical characteristic that resembles the reed of a musical instrument. The term "reed" can refer to the internal metal part or include the external wire leads as well. The hermetic seal is critical for protecting the contacts from atmospheric oxidation and contamination, ensuring long-term reliability in various environmental conditions.
Manufacturing and Glass Selection
The manufacturing process involves the precise insertion of the metal reeds into glass tubes. A critical aspect of this process is the matching of the thermal coefficient of expansion between the glass envelope and the metal leads. Proper matching ensures that the hermetic seal remains intact during thermal cycling, preventing gas leakage or mechanical stress fractures. The glass used is often green-colored to absorb infrared radiation, which helps in managing thermal loads and protecting the internal components from external light interference.
Contact Materials and Variations
The contacts within the reed switch are typically made from ferromagnetic metals to respond to the applied magnetic field. Common materials include rhodium, ruthenium, iridium, and tungsten, chosen for their conductivity, durability, and magnetic properties. In some applications, mercury-wetted contacts are used to reduce contact resistance and improve performance. These mercury-wetted reed switches have specific orientation requirements to ensure the mercury properly wets the contact surfaces, which can affect the switch's operational characteristics.
What are the main types of reed switches?
Reed switches are classified primarily by the internal atmosphere within their hermetically sealed glass envelopes, which significantly influences their electrical performance and longevity. The two main categories are nitrogen-filled and vacuum-sealed reed switches. Nitrogen-filled switches contain an inert gas at a specific pressure, which helps to quench arcing between the contacts during switching operations. This makes them suitable for applications involving moderate current levels where contact erosion is a concern. In contrast, vacuum-sealed reed switches have the air removed from the envelope, creating a near-perfect vacuum. This environment minimizes oxidation and arcing, allowing for higher breakdown voltages and longer life cycles, particularly in low-current, high-voltage applications.
Breakdown Voltage and Sensitivity
The choice between nitrogen and vacuum sealing directly affects the breakdown voltage of the switch. Vacuum-sealed reed switches generally exhibit higher breakdown voltages because the absence of gas molecules reduces the likelihood of dielectric breakdown across the contact gap. Nitrogen-filled switches, while having slightly lower breakdown voltages, offer better performance in terms of contact resistance stability over time due to the protective gas layer. Sensitivity in reed switches is measured in ampere-turns (AT), which quantifies the magnetic field strength required to actuate the contacts. Typical pull-in sensitivities range from 10 to 60 AT, depending on the specific design and material properties of the reeds. The formula for sensitivity can be expressed as S=N⋅I, where N is the number of turns in the coil and I is the current flowing through it.
Impact of Reed Size on Sensitivity
The physical dimensions of the reeds also play a crucial role in determining sensitivity. Smaller reeds, being thinner and more flexible, require less magnetic force to close the contacts, resulting in higher sensitivity. This means that for a given magnetic field, smaller reeds will actuate more readily than larger, stiffer reeds. However, smaller reeds may have lower current-carrying capacities and may be more susceptible to mechanical wear over time. Engineers must balance sensitivity with other factors such as current rating, voltage handling, and expected lifespan when selecting the appropriate reed switch for a specific application. The interplay between reed size, internal atmosphere, and magnetic field strength allows for a wide range of customization to meet diverse engineering needs.
Applications in power and sensing
Reed switches serve critical roles in power systems and sensing applications due to their hermetically sealed glass envelopes and ferromagnetic flexible contacts. These devices operate reliably in diverse environments, making them indispensable for precise magnetic field detection and switching tasks.
Security and Consumer Electronics
In security alarms, reed switches function as door and window sensors. A permanent magnet attached to the moving part of a door or window brings the magnetic field close to the switch, causing the normally open contacts to close. This simple mechanism triggers alarms when the magnet moves away, breaking the circuit. Similarly, laptops utilize reed switches for sleep mode activation. Placing a magnet near the switch detects the lid's position, signaling the system to enter or exit sleep mode efficiently.
Mechanical and Motor Sensing
Speed sensors on bicycle wheels employ reed switches to measure rotational velocity. A magnet attached to the wheel passes near the switch, generating a pulse each time the contacts close. This data helps cyclists monitor speed accurately. In brushless DC motors, reed switches detect rotor position sensing. The magnetic field from the rotor actuates the switches, providing feedback for precise motor control. This application leverages the switch's ability to respond quickly to magnetic fields, enhancing motor performance.
Medical and Musical Applications
Endoscopic capsules use reed switches for internal sensing in medical diagnostics. These tiny devices navigate through the digestive tract, with the reed switch detecting magnetic fields to determine position and orientation. In musical instruments, reed switches are integral to pipe organs and Hammond organ pedal keyboards. They detect key presses by responding to magnetic fields, translating mechanical actions into electrical signals for sound production.
Diving Equipment
Diving equipment also benefits from reed switches. Their hermetic sealing protects against water ingress, ensuring reliable operation underwater. Divers use these switches in various instruments, such as depth gauges and compasses, where magnetic field detection is crucial for accurate readings.
Reed switches' versatility across these applications underscores their importance in modern technology. Their ability to operate reliably under magnetic actuation makes them a preferred choice for engineers designing precise sensing and switching mechanisms.
Reed relays and telephone exchanges
A reed relay is an electromechanical device that integrates one or more reed switches within the magnetic field of an electromagnetic coil. In this configuration, the coil serves as the actuator, generating the magnetic flux required to close or open the hermetically sealed ferromagnetic contacts. This design eliminates the need for external permanent magnets, allowing for compact and efficient switching mechanisms. Reed relays are characterized by their high operating speed, which stems from the low mass of the reed elements compared to traditional armature-based relays. They also exhibit excellent performance when handling small currents, making them ideal for signal processing applications where contact resistance and capacitance are critical factors. The hermetic glass envelope protects the contacts from oxidation and contamination, contributing to high long-term reliability and a long operational life cycle.
Historical Applications in Telephony
Reed relays played a significant role in the evolution of telephone exchange technology, particularly during the late 20th century. In the United Kingdom, the British Telecom TXE family of telephone exchanges utilized reed relays extensively throughout the 1970s and 1980s. These systems leveraged the reliability and speed of reed technology to manage voice and data signals more efficiently than earlier electromechanical systems. The TXE exchanges represented a transitional phase between bulky step-by-step switches and fully electronic digital exchanges, with reed relays providing a robust solution for crosspoint switching matrices. The compact size of the reed relay allowed for higher density packing within the exchange cabinets, reducing the overall footprint of the switching infrastructure.
Mercury-Wetted Reed Relays
For applications requiring even higher performance, mercury-wetted reed relays were developed. These variants feature a small amount of liquid mercury within the glass envelope, which wets the ferromagnetic contacts. The mercury layer reduces contact resistance and eliminates the elastic hysteresis of the metal reeds, allowing for faster switching speeds and better performance in high-speed counting circuits. The liquid metal also helps to break the arc during switching, reducing wear on the contacts. This technology was particularly valuable in digital logic circuits and high-frequency signal paths where consistent electrical characteristics were essential. The use of mercury-wetted reeds extended the utility of the basic reed switch concept into more demanding electronic environments, bridging the gap between purely mechanical and solid-state switching technologies.
Performance characteristics and limitations
Reed switches exhibit distinct performance characteristics that differentiate them from solid-state alternatives such as Hall effect sensors. A primary advantage is their low contact resistance, which can be as low as 0.05 ohms, ensuring minimal voltage drop in low-current circuits. This contrasts with the higher on-resistance often found in semiconductor switches. Additionally, reed switches typically demonstrate very low leakage currents when in the open state, a feature critical for precision measurement and low-power applications. Unlike many solid-state devices that require a continuous power supply to maintain their state or sense a field, reed switches have a zero power requirement for operation. They are passive devices that rely solely on the applied magnetic field to actuate the contacts, returning to their original position when the field is removed.
Wiring and Power Efficiency
The simplicity of the reed switch extends to its wiring requirements. A standard reed switch typically requires only two wires to function, connecting the two internal contacts. In contrast, many solid-state sensors, such as Hall effect sensors, often require a three-wire configuration: power, ground, and signal output. This two-wire requirement simplifies installation and reduces material costs in extensive sensor networks. The zero power consumption during operation further enhances their suitability for battery-operated devices and remote sensing applications where power harvesting or long-term battery life is critical. The hermetic sealing of the glass envelope also contributes to the longevity and reliability of the switch, protecting the internal contacts from environmental contaminants.
Environmental Suitability and Mechanical Limitations
The hermetically sealed glass envelope makes reed switches particularly suitable for use in explosive atmospheres. The internal contacts are isolated from the external environment, minimizing the risk of spark ignition if the switch is used in environments with flammable gases or dust. This feature is valuable in industrial settings where electrical safety is paramount. However, the glass envelope also introduces mechanical stress risks. The thin glass can be susceptible to fracture under mechanical shock, vibration, or thermal cycling. Excessive mechanical stress can lead to the breakage of the envelope, potentially exposing the internal contacts to oxidation or physical damage, thereby compromising the switch's performance and lifespan. Engineers must consider these mechanical limitations when integrating reed switches into dynamic or harsh environments.
Worked examples
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