The electric relay is one of the most frequently used devices in modern technological systems. It can be found in cars, washing machines, microwave ovens, medical equipment, aircraft, and ships. Practically no industry would function without relays. In some complex automatic control systems in the industry, the number of relays is estimated at hundreds and even thousands. Due to its wide usage, it is essential to have a clear understanding of how relays function. First, you have to know the construction of the relay.
Construction of a relay
An electromagnetic relay is the simplest, most ancient, and most widespread type of relay. What are its basic elements? As a rule, most people asked this question would probably name the following:
A winding, a magnetic core, an armature, a spring, and contacts.
This all is true of course, but if you begin to analyze how a relay works, it might occur to you that something is missing. What is the purpose of a magnetic system? Apparently, it is used to transform the input electric current into the mechanical power needed for contact closure. And what does a contact system do? It transforms the imparted mechanical power back into an electric signal.
Don’t you think that something is wrong here?
Everything will become more obvious if the list of basic components of a relay includes one more element, which is not so obvious from the point of view of the construction of a relay, for example, a coil or contacts. Very often, it is not just one element, but several small parts, that escape our attention. Such parts are often omitted on diagrams illustrating the principle of relay operation.
Construction of a simple electromagnetic relay: 1 — springs; 2 — contacts; 3 — armature; 4 — core; 5 — winding; 6 — magnetic core; 7 — insulator.
I am referring to an insulation system providing galvanic isolation of the input circuit (winding) from output one (contacts). If we take such an insulation system into account, it becomes clear that an input signal at the relay input and the output signal at the relay output are not the same. They are two different signals that are completely insulated from each other electrically. Note that if you use the above figure, which is often used to illustrate principles of relay operation, as the only guide while constructing a relay, the relay will not operate properly since its input circuit (the winding) is not electrically insulated from the output circuit (the contacts). In simple constructions used for work at low voltage, insulating bobbins with winding provide basic insulation (apart from an insulator). In a relay with a free bobbin coil, it is necessary to use a special insulating baffle pin between the armature and the contacts.
How does a relay work?
Relays transfer signals through mechanical action.
The working principle of a relay:
- An electrical signal is applied to the terminals of the relay.
- The current flows to the magnet (coil) to magnetize the core.
- Magnetism causes the armature to be attracted to the core.
- When the armature is attracted to the core, the moving contact touches the fixed contact and the current is transferred to the load that is connected to output contacts.
- After the current to the magnetic (coil) is cut off, the force of attraction is lost, and the force of the release spring returns the armature to its original position.
- When the armature returns to its original position, the contacts separate.
Other explanations about the operating principle of a relay:
Contact sets may be normally open or normally closed, and both types may be fitted on the same relay mechanism. The arrangement is determined by the operating sequence required: i.e., make, break, change-over, make-before-break, break-before-make. The contact size and material must be chosen in accordance with the rating and electrical characteristics of the circuits controlled.
Ideally, the contacts should operate cleanly and with no bounce. They should be of adequate size and of the most suitable material. In extremely low-voltage circuits contact resistance is usually an important consideration and special precautions may also have to be taken to ensure reliable operation under conditions of vibration or shock.
Similarly, in cases of high-current switching, it may be necessary to ensure a wide separation of the contacts or even to arrange for several gaps to operate in series. In some cases, it may be necessary to use arc-suppressing circuits.
The number and type of the contacts and springs determine the switching operation to be performed by the relay; this factor also determines the work to be done by the magnetic circuit. It follows, therefore, that the choice of a suitable coil and iron circuit design is determined by the contact arrangement of any particular relay. Various configurations of magnetic circuits and materials are used in the relays under review, depending upon their particular application. For example, in high-sensitivity relays, where the air gap has to be kept to a minimum, it is necessary to use materials having a very low residual magnetism and high permeability.
The power required to operate the relay is determined by the spring-set arrangement and the magnetic circuit. The method of construction is important since it largely determines the safe operating temperature of the winding and this, in turn, governs the coil power and the maximum pull available at the armature. By increasing the area of the flux path while maintaining the ampere-turns and coil power constant, the total air gap flux, and therefore the armature pull, can be increased and the increased coil area will permit cooler operation of the coil. This may, however, lead to a relay that is physically larger than can be tolerated. In practice, it is more reasonable to build a relay of a given size and to use other means to amplify the controlling power.
The continuous power input to a given relay coil is limited only by the maximum temperature that the coil insulation can withstand without breakdown. This temperature is governed by the environmental conditions as well as by the coil construction and the quality of the insulating material.