What Is a Reed Switch?
A reed switch is a switch with two magnetizable reeds in a glass tube that turns electrical contacts on and off.
When open, a typical reed switch opens with a small gap between the reed contact edges; when an external magnetic field (e.g., a magnet) is applied to the two reed contact edges, the reed contact edges attract each other and close, closing the circuit.
Uses of Reed Switches
Reed switches are used in the following applications:
- Rotation detection of water meters, motorcycle, and bicycle speedometers, etc.
- Open/close detection of doors and lids of white goods
- Detection of liquid level in tanks
- Open/close detection of hydraulic and pneumatic cylinders
Reed switches are turned on and off by magnets, so they can be used for non-contact detection. Reed switches are mainly used for non-contact detection of equipment status.
Principle of Reed Switches
The components of a reed switch include a glass tube, a reed strip, a magnet, and a contact.
- Two magnetic reed strips, which are easily magnetized, are enclosed in a glass tube with a certain overlap and spacing.
- When a magnet or other object is brought close, the contact edges associated with the two lead strips are attracted to each other, closing the circuit.
- When the magnet is moved away, the spring force of the leads opens the contacts.
- The contacts are sealed in a glass tube with nitrogen to insulate them from the external environment.
Since the contacts are not contaminated, reed switches have very low contact resistance. The contacts are mainly made of metal and have a short operating distance, giving them an open/close life of up to several hundred million cycles.
Other Information About Reed Switches
1. Failure of Reed Switches
Reed switches are components with moving parts. Therefore, there is no possibility of failure. The main possible failures include contact failure, contact sticking, and unexpected malfunction.
Contact Failure
Factors that can cause contact failure include deterioration of the magnet over time and carbonization due to chemical changes on the contact surfaces. Since carbonization of the contact surface is unlikely to occur in a nitrogen atmosphere, the glass capillary may be cracked.
Contact Sticking
Contacts may stick due to deterioration of the spring or welding of the contacts due to overcurrent. Deterioration of the spring is mainly due to aging, while welding of the contacts may be caused by an abnormality on the electric circuit side, so caution is required.
Unexpected Malfunction
Unexpected malfunctions may be caused by a random combination of the above two causes. In this case, it is very difficult to identify the cause.
Reed switches are components that are relatively unlikely to fail. To deal with failures, it is essential to design circuits with redundancy and prevent malfunctions. In addition, since reed switches are inexpensive, they are sometimes designed from the beginning on the assumption that they will be replaced at regular intervals.
2. Points to Note About Reed Switches
Reed switches sense the proximity of a magnet. However, because the operating range over which the switch operates is not uniform, there is a risk of chattering. Equipment design must take chattering into consideration.
On the other hand, the distance between the contacts of reed switches is very small and the contact pressure is very slight. Because they are precision components, they must be used strictly within their rated current values. In addition, even when used within the rated current value, care should be taken not to apply strong shocks.
In addition, using reed switches under a strong magnetic field may cause malfunctions. In such an environment, a magnetic shield should be provided to block external magnetic forces.