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Reed Switch

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1. Reed Switch Construction

• A Reed Switch consists of two or three ferromagnetic blades (or reeds) hermetically sealed inside a glass envelope filled with inert gas. The construction ensures protection from the external environment.
• Three types are available: Form A (Normally Open), Form B (Normally closed) and Form C (Changeover). The operation is activated by either a permanent magnet or a current-carrying coil.
• Sensitivity of a reed switch is measured in ampere turns (A.T.) and it should be noted that lower switch (A.T) ratings are more sensitive as they require less magnetic field strength to operate them.
• Various voltage and current switching levels are available and contact plating materials can be varied to accommodate specific types of load.

 

2. Operating Principle

Application of magnetic field, generated by a permanent magnet or a coil, to the reed switch causes both reeds to be magnetized. This produces an N-pole at the contact area of one reed, and an S-pole at that of the other reed, in a manner shown on the drawing. If the magnetic attracting force overcomes by the resistive force caused by elasticity of the reed, the reeds come in contact (Pull-In) i.e, the circuit is closed. Once the magnetic field is removed, the reeds are separated again by the effect of elasticity of the reed (Drop-Out) i.e., the circuit is opened.

 

3. Application - Leads Cutting & Bending

Bending and/or cutting of switch leads may be necessary during its application. Incorrect handling could result in glass crack and over stress on the glass seal point. This could cause switch’s performance degradation or even damage. Therefore, proper support and clamping of the leads is necessary when bending and/or cutting. The following are the examples for proper & improper support and clamping.

It is recommended that the leads bending should be no less than 1.0mm from the glass seal and lead cutting no less than 3.0mm

As a result of leads cutting & bending, Pull-In and Drop-Out AT will increase. Please take this into your design.

 

4. Reed Switch Protection

 

• Inductive Loads

When switching inductive loads such as relays, solenoids and transformers, reed switch contacts require protection in order to insure long, dependable life. When current is interrupted, the inductance or electrical inertia of the load generates a large high frequency voltage, which appears across the switch contacts. If the voltage is large enough, it can break down the medium in the gap between them, making a conductive path. This phenomenon called “arcing” is the spark you see. Arcing can cause the contacts to burn, weld together or stick; thus, giving unreliable performance. The purpose of protection circuits is to prevent arcing, by shorting this voltage through an alternate parth.

        

 

• Conductive Loads

When switching conductive loads, such as capacitive, lamp loads or long cables, the switch contacts are subjected to high inrush currents which can lead to faulty operation and even contact welding. Therefore, protective circuits such as surge suppressors or current limiting resistors are recommended.

       

 

5. Mounting (Soldering & Welding)

• Excessive high temperature and exposure time during soldering may cause damage of the glass-to-metal seal (crack or leakage etc.). So quick & reliable soldering techniques or processes should be adopted. Recommended soldering conditions are 280~300°C within 3s for hand soldering and 250~300°C within 5s for wave soldering.
• When mounting a reed switch on PCB, care should be taken into consideration that the deformation or thermal expansion of PCB may damage the switch glass. It is recommended to provide adequate space bt. PCB and reed switch or the drop reed switch into an opening in PCB. (See Fig.)

 

6. Shock & Vibration

When a reed switch is dropped onto a floor (or a hard surface) from more than 30cm height, the switch's characteristics may be changed. Therefore, make sure its characteristics are still within the acceptable limits before use.

 

7. External Magnetic Interference

When reed switches and its actuating magnet or coil are located near strong magnetic interference sources such as steel plates, transformers etc, the reed switches’ characteristics may be changed or fault operation may occur.

When reed switches are located in close proximity one another, the mutual magnetic interference may cause switches’ characteristics change or fault operation. If such interference is observed, the reed switches should be spaced more than 15mm one another.