Relay - a device for turning on and off electrical circuits, one of the "long-livers" among radio elements. Despite the relatively simple design, it has high efficiency and reliability. Even now, in some devices, there is no alternative. Despite the existence of power semiconductors, relay contacts are still the easiest way to switch heavy loads in low current circuits.
Destination
An elementary electrical circuit consists of a power supply, a switch and a load. Ideally, all three elements should match each other in voltage, and most importantly, in current. This is a prerequisite for the normal operation of the circuit. If the allowable current through the switch is greater than the load consumed, nothing bad will happen. Moreover, such a breaker will last much longer. When the current flowing through the switch exceeds the maximum allowable, problems begin.
They are expressed in the sparking of contacts, which ultimately affects their service life. It would seem that it is enough to install a switch corresponding to the load andeverything will be OK. This is true, but not always possible. The fact is that the higher the permissible current, the larger the dimensions of the circuit breaker. In this case, the load can be quite large, but it has to be controlled, for example, from a remote control on which there is no place for a large switch.
In this case, the relay is installed. It requires a relatively small current to turn it on. The load power can be very significant, while the relay can be taken out of the same control panel and installed in a place where the dimensions are not of fundamental importance.
Relay device
It should be noted right away that there is a wide variety of voltage control devices. The article will consider the most common electromagnetic relay. It consists of the following parts:
- electromagnetic core coil;
- anchor;
- switched relay contacts;
- return spring.
The relay is made in a closed, sometimes sealed case. This protects its mechanism from dust and moisture. To connect the device outside the case, there are terminals for contacts and coil windings.
Working principle
The key element of the relay is an electromagnetic coil, in this case it is called a winding. In the design, it performs the function of a solenoid. When current flows through the coil, a magnetic field arises, due to which an armature is attracted to the core, rigidly connected to the movable contact of the relay. He is movingcloses the electrical circuit. After the voltage is removed from the winding, the armature returns to its original position under the action of the spring, opening the relay contacts.
The resistance of the coil, and hence the number of turns, depends mainly on the power of the connected load. In accordance with this, the dimensions of the winding and the relay also grow. However, in any case, the current consumed by the coil is tens or even hundreds of times less than that switched by the contacts. This property allows you to use the relay as an intermediate one. First, the relay itself is powered by a low-current switch, and then it supplies voltage to the consumer with its contacts. This use of the device has become the main and most widespread. In this case, experts say that the load is connected through the contacts of the intermediate relay. Thus, the dependence of the switch on the power of the powered device is excluded.
What are contacts
With regard to the relay, this is not an idle question, as it might seem. The fact is that in this case, we mean not only mechanical contacts that switch inside the device. When they talk about a relay, they mean all the conclusions located on its body. They can be divided into two types:
- Winding contacts. Sometimes there can be more than two on the relay.
- Switched.
To avoid confusion, these pins are often referred to as relay connection pins. Sometimes their number can reach 10. At the same time, due to the lack of standardization, it is not always clear where to connect which circuit. Figure outthe pinout of the relay contacts, which is almost always applied to its body, will help. If not, you will have to look for a description. The winding contacts are connected directly to its terminals. A voltage is applied to them, from which the relay is activated. There can be several windings and each will have its own pair of contacts. Sometimes the coils can be interconnected with conductors if it is necessary to provide a certain algorithm for their operation.
Material of switching contacts
The service life of some relays is tens of years. At the same time, all its parts are under heavy loads, especially contacts. Firstly, they experience mechanical effects associated with the movement of the anchor. Secondly, they are negatively affected by high load currents. Therefore, the relay contacts must meet the following requirements:
- High electrical conductivity. Provides low voltage drop.
- Good anti-corrosion properties.
- High melting point.
- Small erosion. Contacts must be resistant to metal transfer, which is inevitable with constant closing and opening.
All listed qualities directly depend on the material used. Consider the base metals used to make the relay:
- Copper fully meets the requirements, except for corrosion resistance. Therefore, it is often used in sealed case relay contacts. In addition, copper has another advantage - a relatively low cost compared to other metals. the onlyits disadvantage is the tendency to oxidize during prolonged operation. Therefore, it is used where a short-term operation is provided, for example, in the contacts of the turn relay.
- Silver has excellent conductivity and wear resistance. Does not cause sparking when switching inductive loads. At the same time, silver contacts do not have sufficient arc resistance, therefore they cannot be used to control loads of significant power. In addition, they have a rather high cost. Therefore, the contacts have a combined design - copper with silver sputtering.
- Tungsten has great wear resistance and high temperature resistance. Contacts made from it are capable of switching very high currents (tens of amperes).
In addition to the material, the relay contacts differ in the way they are switched.
Normally open
These are the contacts that have been considered so far. In the neutral position, i.e. when the relay winding is not energized, they are open. After the voltage is turned on, the armature is attracted to the core and the contacts are closed. Normally open contacts are most often used in various electrical circuits, mainly as intermediate contacts.
Normally Closed
The algorithm of their work is just the opposite. The contacts are closed when the relay is de-energized, and turn off when voltage appears on the winding. This is used in the implementation of various interlocks and in signaling circuits. A typical example of the use of normally closedcontacts is a mechanical relay regulator. Let's briefly talk about his work below.
Through normally closed contacts voltage is applied to the excitation winding. Accordingly, when the armature is released, the generator generates an electric current. The battery is being recharged. As soon as the voltage in the on-board network exceeds the set value, the armature is attracted, the contacts of the relay-regulator are released, the excitation winding is de-energized. As a result, the voltage at the generator output decreases.
By the way, despite the fact that electronic relay-regulators have appeared for a long time, owners of old cars are in no hurry to put them instead of mechanical ones. This is due to the failure-free operation of the latter for many years. This is about reliability.
Switching
In this case, the relay has both normally closed and open contacts. And there are not four of them, as it might seem, but three. The fact is that one of them is common. In total, there are 5 contacts on the relay case (two winding outputs and three switched ones). Due to their versatility, radio elements of this type are widely used. Therefore, most modern relays have changeover contacts, sometimes even several groups.
Marking
All data on the technical characteristics of the relay, as a rule, are printed on its body. This is not at all superfluous information, because sometimes devices that look the same have different purposes and capabilities. Moreover, somedomestic relays are also called the same, differing only in the so-called passport. In this case, you have to refer to the description.
As for the imported relays, which are now a large number, the markings on their case, although it differs depending on the manufacturer, are intuitive. As a rule, there is information on the operating voltage of the winding and the maximum current passing through the switched contacts. In addition, the designation of the relay contacts is mandatory on the relay case.
