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:
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.
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.
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.
Machine Tool Relay: It is standardized for machine tools, transfer machines, and sequential control.
Mercury Relay: This relay uses mercury as the switching element.
Mercury-wetted Relay: It is a form of the reed relay that is used to employ the mercury switch.
Overload-protection Relays: The electric motors require overprotection in order to prevent the damage from the overloading motor against the short circuits.
Reed Relay: It is a reed switch that is found enclosed in the solenoid.
Conclusion
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 on Relay Electronics
1. What is an electromagnetic relay and what is its main purpose in electronics?
An electromagnetic relay is an electrically operated switch. Its primary purpose is to use a small amount of electrical power in a control circuit to switch a much larger amount of power in another, completely separate circuit. This provides what is known as galvanic isolation, which enhances safety and protects sensitive control components from high voltages or currents.
2. How does a simple electromagnetic relay work?
The working principle of a relay is based on electromagnetism. When an electric current flows through the coil wrapped around a soft iron core, it generates a magnetic field. This magnetic field attracts a movable lever called an armature. As the armature moves, it mechanically pushes or pulls a set of electrical contacts, either closing an open circuit or opening a closed one. When the current to the coil is turned off, the magnetic field disappears, and a spring returns the armature to its original position.
3. What are the essential components of a standard electromagnetic relay?
A standard electromagnetic relay consists of several key parts working together:
- Electromagnet: A coil of wire wrapped around a soft iron core that creates a magnetic field when energised.
- Armature: A movable iron part that is attracted by the electromagnet and operates the contacts.
- Spring: A component that returns the armature to its default position when the coil is de-energised.
- Contacts: The set of electrical terminals that open or close to switch the secondary circuit. These are typically labelled as Common (COM), Normally Open (NO), and Normally Closed (NC).
4. What are some common types of relays and their specific uses?
There are many types of relays designed for specific applications, including:
- Latching Relay: Maintains its contact position even after control power is removed, requiring another pulse to reset. Used in applications where power conservation is important.
- Reed Relay: Features contacts enclosed in a sealed glass tube, making it fast and reliable for switching low-power signals.
- Overload Protection Relay: Specifically designed to protect electric motors from damage by sensing overcurrent conditions.
- Solid-State Relay (SSR): Uses semiconductor devices like thyristors to switch circuits with no moving parts, offering longer life and faster switching speeds.
5. Why is a relay often preferred over a simple transistor for switching high-power AC circuits?
A relay is often preferred over a transistor for switching high-power AC circuits primarily due to its robustness and complete electrical isolation. While transistors are faster, relays can handle higher voltage and current surges. Most importantly, the control circuit in a relay is physically and electrically separate from the switched circuit, preventing high-power faults from damaging the low-power control electronics, a critical safety feature not inherently present in a simple transistor switch.
6. What is the functional difference between a latching relay and a standard non-latching relay?
The key difference lies in how they maintain their state. A standard (non-latching) relay requires continuous power to its coil to keep its contacts in the switched position. As soon as the power is removed, it reverts to its default state. In contrast, a latching relay only needs a brief pulse of power to switch states. It then mechanically 'latches' into that position and stays there without consuming any more power, until a second pulse (often to a separate coil or with reversed polarity) unlatches it.
7. Why is a freewheeling diode or a snubber circuit necessary when using a relay?
When the current to the relay's coil is suddenly cut off, the collapsing magnetic field induces a large, potentially damaging, reverse voltage spike (known as 'flyback voltage'). A freewheeling diode placed across the coil gives this induced current a safe path to circulate and dissipate, protecting the control circuit's semiconductor components. Similarly, a snubber circuit (a resistor and capacitor) is often placed across the output contacts to suppress electrical arcs that can form when switching inductive loads, thus extending the life of the relay's contacts.
8. Can a relay designed for DC voltage be used with AC voltage? Why or why not?
Generally, a relay designed for DC cannot be used with AC. A DC relay coil fed with AC would cause the armature to 'chatter' or vibrate rapidly and likely overheat, as the magnetic pull would disappear 100 or 120 times per second (for 50/60Hz AC). Relays designed specifically for AC operation include a shading ring on the electromagnet's core. This ring creates a delayed, out-of-phase magnetic field that maintains a continuous pull on the armature, even when the main AC current passes through zero, preventing chatter.
