Reed switch: operating principle and device

Switching devices or contacts are widely used in various electrical or electronic devices. To improve the operational properties of the equipment, to extend the service life and improve the reliability of the connections use special sealed magnetically controlled contacts, which are called reed switches.

The devices consist of a core made of soft magnetic material, to which contacts are connected through insulating gaskets. When a current flows through the coil, a magnetic field appears in the core, which acts on the contacts and causes them to close. After the supply of electrical current through the coil is terminated, the contacts are opened.

Despite the use of various devices, today the reed relay in many cases remains out of the competition due to the ease of use, the properties of dry contact, galvanic isolation from batteries. It is still used in a variety of devices and circuits.

The reed switch is simply irreplaceable, if it is necessary to ensure the durability and high reliability of the switching elements. Reed switches are part of various sensors, position switches and electromagnetic relays.

According to the functional features, the reed switches are divided into switching, closing and opening. According to the design technological parameters of the device are divided into two categories: with mercury and dry contacts. The latter variety is practically no different from ordinary contacts. Mercury cells have a drop of mercury inside the glass hermetic housing. This is necessary for wetting contacts during operation, which allows to improve the quality of the connection by reducing the junction, and also helps to get rid of the rattling of the contacts.

The reed switch generates vibration or bounce when the contacts are closed or opened. This results in a single trip to multiple signal switching and a significant increase in the response time. When such devices operate in digital microcircuits, measures are taken to suppress side effects using triggers or RC-chains. In modern microcontroller boards, the bounce, which creates a reed switch and other contacts, is suppressed using software methods. However, this generally reduces the speed of the system.

Any reed switches in their design are presented in the form of a glass cylinder, in the middle of which there are contacts. These elements are made, like magnetic cores, which are welded along the ends in a balloon. The outer ends of the core are required to connect the device to the external electrical circuit.

The most common are reed sensors, the contact group of which operates on a short circuit. In such devices, each contact is made of an elastic ferromagnetic wire, flattened to a rectangular shape. To reduce the transition resistance and to improve corrosion resistance, the contact surfaces are covered with gold, rhodium, palladium or silver and their alloys.