Relay Electronics

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Introduction to Relay

A Scientist called  Samuel Thomas von Sommerring, as a part of his electrochemical telegraph designed a component known as a relay in the year 1809. But later on, an American scientist named Joseph Henry claimed that he invented the relay in the year 1835 and the aim is to improve the version of the telegraph that was earlier developed in the year 1831. In the original 1840 telegraph patent, a simple device which is now called the relay was included. The mechanism of this device is described as a digital amplifier where the telegraph signal was repeating and thus the signals are propagated that is desired. It is used in electromagnetic operations since the year 1860. Let us learn more about the relay, how it is constructed, its working, applications, etc.

Electrical Relay Definition

Electrical relay definition is as follows, it is an electrically operated switch is known as a relay. It consists of a set of single or multiple control signals that are used as input terminals and a set of operating signals. These can have any number of contacts in any of the contact forms such as, make contact, break contact, or the combination of both. In the situations where the low-power one signal has to be controlled or in the cases where the several circuits are to be controlled by a signal, in such cases, relays are used. As signal repeaters in the long-distance telegraphs, the relays were found using for the first time. It transmits the signals that are coming from a circuit to another by refreshing it. 

The electromagnet is used in the traditional form of the relay in order to open and close the contacts. In the case of solid-state relays, semiconductor properties are used for the control without any moving parts. To protect the electrical circuits from overloads or faults a relay with the calibrated characteristics or multiple operating coils is used. In modern electrical power systems, all the above-mentioned functions are performed by the digital instruments that are known as protective relays. In the case of latching relays, in order to operate the switch persistently, the power is to be controlled in order to control the power a single pulse is required. Where another pulse is applied to the set of control terminals or to the terminals that have opposite terminals. 

About Relay Basic Design

A simple electromagnetic relay is made up of a soft iron core that is simply called a solenoid to which the coil of wire is wrapped. Along with this, it consists of an iron yoke that provides the path to the magnetic flux, one or more contacts, and a movable iron armature. The armature is connected to one or more moving contacts and it is attached to the yoke with the help of a hinge. This armature is held in a place with the help of spring such that whenever the relay is in the de-energized condition the air gap is present in the magnetic circuit. In these conditions of the sets of a relay are made closed and the other one kept open. 

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Electronic Relay Working

When the electric current is passed through the solenoid it activates the armature by producing the magnetic field and the movable contacts that are moving consequently either makes or breaks the contact depending on the construction. In the case when the relay is de-energized then the contact is opened by the movement and the connection breaks and a vice versa situation occurs if the contacts were open. The armature returns by a force when the current is switched off, this force is approximately half as that of the magnetic field in the relaxed position. Most of the relays are designed to operate quickly. 

In the cases when the coil s energized the resistor or a diode is placed across to it in order to dissipate the energy from the magnetic field that is collapsing at the deactivation else, it would be dangerous to the semiconductor circuit. It was not used before the application of transistors are relay drivers but when the germanium transistors were getting destroyed due to this reason the diodes were used to dissipate the energy. Whereas the resistors are more durable than compared to that of the diodes and are also less efficient in order to eliminate the voltage spikes that are generated by the relays. When the relay is driving the large reactive load then a similar problem of the surge currents occurs around the output contacts of the relay. In this case, in order to absorb the surge, a snubber circuit is used, which is a combination of resistor and capacitor that are placed in series with the contacts. 

If the coil is designed in order to energize with the alternating current, then a method is used to split the flux into the two out-of-phase components that are added together. This in turn increases the minimum pull during the armature in the AC cycle. This is done with the help of a small copper shading ring that is crimped around the portion of the core that creates the delay in the out-of-phase component. 

The contact materials that are used for the relays vary depending on the application. Material that has a low contact resistance may be oxidized in the air or it may stick instead of cleanly parting while opening. Contact material may be optimized in order to withstand the repeated operations or the high capacity to the heat of an arc. Silver or silver-plated contacts are used for signal switching. 

Types of Relay:

  1. Coaxial Relay: When radio receivers or transmitters share one antenna, a coaxial relay is used to transmit a relay that helps to switch the antenna from the receiver to the antenna. 

  2. Force-guided Contact Relay: These have relay contacts that are mechanically linked together. These are also known as positive-guided contacts, captive contacts, safety relays, locked contacts, or mechanically linked contacts. 

  3. Latching Relay: It is also called stay relay, simply latches or impulse, the main advantage is that one coil consumes power only for an instant while the relay is kept switched.

  4. Machine Tool Relay: It is standardized for machine tools, transfer machines, and sequential control.

  5. Mercury Relay: This relay uses mercury as the switching element. 

  6. Mercury-wetted Relay: It is a form of the reed relay that is used to employ the mercury switch.

  7. Overload-protection Relays: The electric motors require overprotection in order to prevent the damage from the overloading motor against the short circuits. 

  8. Reed Relay: It is a reed switch that is found enclosed in the solenoid.


From the above discussion, we can conclude that the relay is designed to prevent or protect the circuits from faults. Along with this, it has man applications, these relays are used in the control of the high power or the high voltage circuits when the galvanic isolations are required. It was first used in the long-line telegraphs. It is also used in Electromechanical switching systems such as Strowger and Crossbar telephone exchanges, in the logical control of the complex switching systems, electro-mechanical computers, etc. Since the relays are more resistant than that of the semiconductors it is used in the safety control logics such as control panels of the waste handling machinery. 

FAQs (Frequently Asked Questions)

1. Why Relay is Used?

Ans: The relay electronics are used as a switch in the electrical circuits. It is used to control the high power voltage circuits, to protect the circuits from electrical breakage, etc. 

2. What is a Relay?

Ans: Electrical relay definition is as follows, it is an electrically operating switch.

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