Posted on 5th July 2018 in the categories: Magnetics

What Is a Reed Switch and What Magnets Operate Them?


Reed switch

A reed switch is an electromagnetic switch used to control the flow of electricity in a circuit. They are made from two or more ferrous reeds encased within a small glass tube-like envelope, which become magnetised and move together or separate when a magnetic field is moved towards the switch. Effectively, when the two reeds are in contact, electricity can flow around the circuit operating a device. Reed switches are controlled completely by invisible magnetic fields.

Types of reed switch

There are two types of reed switch:

1. Normally open

These type of reed switches consist of two reeds, made from ferrous material such as a nickel-iron alloy. These reeds are positioned so that they are not touching. However, when a magnet is moved near the switch it pulls the reeds together, completing the circuit. Once the magnet is removed the reeds return to their original position, breaking the circuit.

2. Normally closed

For normally closed reed switches, when no magnetic field is present the reeds are in full contact. Therefore, the electric circuit is complete, and the device is ‘on’. However, when a magnet is moved close to the switch, the reeds repel, split apart and break the circuit.

What type of magnets can be used with reed switches?

Magnets are an essential component in ensuring reed switches work successfully. Both the size and type of magnet required, solely depends on the type and assembly of the reed switch. Reed switches can either be hidden or fixed within an assembly and still be functioned by a magnet. Therefore, the distance between the magnet and the switch is a critical element. The wider the distance, the stronger the magnet will need to be to interact with the switch.

Although any permanent magnet will work with a reed switch, it’s important to note two different aspects:

  • Different materials have different strengths
  • Different sized magnets produce different sized magnetic fields

While neodymium magnets are the strongest type of magnets commercially available, ferrite magnets are preferred for reed switches. Despite being much weaker, ferrite magnets are hugely popular due to the deep magnetic field they produce. Therefore, when selecting a magnet for a reed switch application there are a few main factors to consider:

  • Shape of magnet
  • Magnet strength
  • Switch sensitivity
  • Distance
  • Angle between magnet and switch

What applications are reed switches used in?

Reed switches are most commonly used for proximity and sensing applications, including:

  • Alarm systems – The reed switch is used for sensing whether doors and windows are open or closed. Additionally, they are used to ensure systems are tamper-proof by placing magnets in covers so once removed the switch is activated, and the alarm is triggered.
  • Laptops – Here the reed switch and magnet work together to switch the device on and off, along with adjusting the screen. Due to the contact of the switch and magnet, the device knows the proximity of the cover or screen and reacts accordingly.
  • Safety –Some devices, such as food processors, contain proximity sensing reed switches. These particular reed switches cause such devices not to function if a lid is not correctly in place or if another safety precaution has not been taken.
  • Automotive – Several safety and sensing applications in cars rely on reed switches. Just a few include impact, speed, braking, door positions, fluid and fuel levels.
  • Refrigerators – Most modern refrigerators use reed switches to detect when the door is open or closed. As the door is shut, a magnet in the frame of the door is moved close to a fixed reed switch and the magnet forces the reeds apart, turning the light off.

Goudsmit UK

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