Gerkon: what is it and how does it work?

In a wide variety of electrical and electronic circuits, a radio component with the beautiful name "reed switch" is used. What is it and how does it work?

The name and its meaning

The name really sounds poetic, it is worthy of a beautiful flower. But the origin of the word is very prosaic, it stands for "hermetic contact". It is the lack of air or the replacement of its inert gas due to the advantages of the device in comparison with the usual contact groups. The principle of its operation is extremely simple, and is explained briefly by another part name: "magnetically controlled electrical connection". Inside the glass cone of small dimensions, two elastic metal plates are fixed, one of which is provided with a ferromagnetic patch. Sealing is achieved by tight fitting of the amorphous material of the body at the time of manufacture, in other words, the leads are simply fused from two sides.

Device construction

So, in the glass tube inserted a mechanical system, consisting of two spring plates, magnetic material and welded or soldered on them contact groups. In the normal state, the right and left components can be in galvanic contact, allowing the passage of an electric current (such reed switches are called normally closed), or, on the contrary, they can be open (reed switch). Then a vacuum is created inside the tube or an inert (chemically passive) gas is pumped into it. This is done to increase the life of the part. When the current passes, the contacts are heated and the process of oxidation, that is, the connection with oxygen, accelerates. If the metal is surrounded by inert medium, then such a reaction will not occur. Now the tube can be sealed and the device is ready.

Operation of the device, its advantages and disadvantages

To change the state of the contacts (opening or closing), apply the reed switch. What it is, what exactly the effect is expressed in, is clear from the second name of the device and from its device. To the cone, you need to bring a magnet, and one of the plates will move, bending or bending away from the second. As a result, the required direction will be switched. Simplicity and reliability of the details, cheapness (there is no need to apply silver or gold for contact groups) - these are its main advantages. But there are also disadvantages that the reed switch has. What is it? Such a glorious invention suffers from so-called "bounce" (due to the elastic properties of the metal), susceptibility to parasitic magnetic fields, which in any production is lacking, mechanical inertia and excessive fragility.

Application

And yet, despite the constructive fundamental defects, which it is almost impossible to completely exclude, the characteristics of the reed switches allow them to be used in many areas of human activity, in which flaws are not so important, and dignities prevail. For example, in a conventional computer keyboard in which a so-called "chatter" can be dealt with by including damping filters in the circuit, and then do not worry about the cleanliness of the contacts. These devices are also irreplaceable in alarm systems. There is nothing easier to install the sensor, which is based on the reed switch. The doors are closed - the contact is closed, and when they open the magnet attached to the doorpost is retracted, the strength of the magnetic field decreases, the circuit is opened, which serves as a signal for the operation of the electronic warning circuit. To determine the position of the elevator car is also often used reed switches. The lighting equipment of divers is also easy to control with the help of magnets, without fear that salty sea water will flow into the electric lights through the holes in the switching devices. In the circuits of electric meters, both single-phase and three-phase, reed switches are also present.

Hercotrons

In the study of high-voltage circuits, students and specialists sometimes face the term "hercotron", while from the context it is clear that in principle it is the same reed switch. What is it and what is the difference? In the characteristics, namely the voltage (up to 100 kV) and the current that can go on the contacts. The ability of insulation to resist the possibility of breakdown and the cross-section of the conductor, as well as the contact area - this is what distinguishes the hercotron from the reed switch. In everything else, and most importantly, in a principled arrangement, these devices are identical.