The Magnetic Symphony: Applications and Types of Reed Switch Devices

 

 Reed Switch Devices

Reed switch is a simple electrical switch which is operated by magnetic field. It is commonly used as a proximity sensor or as a contactless switch. This electrical device works on the simple principle of magnetically operated contacts in a hermetically sealed glass envelope. Reed switches were one of the earliest technological application of electricity and magnetism and are still used in wide variety of devices and applications even today.

What is Reed Switch?

A Reed switch comprises of contacts in the form of thin, flexible ferromagnetic metal contacts enclosed in a hermetically sealed glass capsule. These metal contacts are separated by a small gap, usually less than 1 mm. When there is no magnetic field present, the contacts remain separated due to spring tension. However, when an appropriate magnetic field is introduced near the switch, the contacts attracted towards each other and close the switch. Once the magnetic field is removed, the spring tension reopens the contacts and breaks the circuit again. This basic working principle allows Reed switches to be used as simple on/off proximity sensors without the need for physical contact.

Construction and Operation

The essential components of a basic Reed switch includes a glass capsule, reed contacts and sealing agent. The glass casing hermetically seals the contacts under vacuum or inert gas and protects them from environment. The reed contacts are made of ferromagnetic metal alloy such as nickel-iron alloy which responds strongly to magnetic field. These contacts are cantilevered like springs and closely spaced inside the glass capsule. In absence of magnetic field, the contacts are kept apart by spring action. When brought near a magnet or electromagnet, the magnetic field exerts force on the contacts and draws them together closing the circuit.

Working Principle

The working principle of a reed switch is based on underlying scientific concepts of magnetism, electricity and material properties. Ferromagnetic metals like nickel-iron alloy experience strong magnetic force when brought near a magnet. This is due to atomic magnetic domains of the metal aligning themselves with the external magnetic field creating induced magnetism. The induced magnetism causes an attractive force between the magnetic field and the metal contacts of reed switch. When this force overcomes the spring tension holding contacts apart, they snap together electrically closing the circuit. Once the magnetic field is removed, the spring pulls the contacts back to open position breaking the circuit again. By sensing magnetically induced contact closure, reed switches act as magnetic field detectors and proximity sensors.

Applications of Reed Switch

Due to their simple and rugged design making them highly reliable, reed switches have found applications in wide range of sectors. Some common uses of reed switches include:

- Security and Access Control: Reed switches are used in door/window contacts, motion detectors, alarm systems to detect opening/closure or movement.

- Vehicle Security Systems: Reed switches are integrated in car alarms, anti-theft immobilizers to sense intrusion or movement.

- Appliance Controls: Reed switches enable magnetic activation of appliances like ovens, washing machines, ACs and enhance child safety.

- Medical Devices: Medical implants like pacemakers use reed switches in reed relay to trigger electrical pulses on detection of magnet.

- Test Equipment: Oscilloscopes, signal generators, electrical test meters use reed switches for triggering output waveforms.

- Fluid Level Sensors: Reed Switch Device detect fluid levels in tanks by integrating a floating magnet.

- Robotics: Proximity sensors using arrays of reed switches help robots sense objects and navigate environment contactlessly.

- Smoke/Fire Alarms: Presence of smoke or heat activates reed switch to trigger audible alarm.

- Musical Instruments: Electronic organs, electronic pianos use reed switches interfaced with keys and pedals.

Advantages of Reed Switches

The importance of reed switches stems from their distinct advantages over other types of switches:

- Contactless and Wearless Operation: Since contacts are never in physical contact, reed switches have extremely long operational life.

- Hermetic Seal: Glass casing provides reliable seal against environment ensuring longevity.

- Compact Design: Thin capsule design occupying minimal PCB real estate.

- Fast Switching Speeds: Reed switches can perform millions of open-close operations reliably.

- Low Power Switching: Reed switches require very little energy to change state.

- Durable: Withstanding shock, vibration and wide temperature variations ranging from -50°C to 150°C.

- Fail-safe Operation: Contacts spring back to open position if power or magnetic field is interrupted.

- Cost Effective: Inexpensive to manufacture despite rugged construction.

Areas of Improvement

While reed switches are very reliable, some limitations have opened up areas of further improvement:

- Contact Bounce: As contacts slam together rapidly on closure, transient contact bounce occurs briefly.

- Leakage Current: Very small current continues to flow even in open state across gap due to imperfect insulation.

- Magnetic Shielding: Strong external magnetic fields can accidentally trigger reed switch.

- Lifetime: After millions of operations, metal fatigue degrades switching performance over time.

- Sensitivity to Vibration: Excessive vibration or shock can cause false triggering or failures.

Research is being carried out on addressing these limitations through new reed switch designs, advanced materials and manufacturing techniques while retaining their inherent advantages.

Overall, simple yet rugged reed switch construction has ensured its relevance even in electronic era as a basic yet indispensable electrical component across various industries. Its important role as a contactless switch/sensor will continue in foreseeable future as long as electromagnetism remains fundamental to technology.

 

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