What is a Relay? How they Work & Different Types of Relay

Relays are the switches which aim at closing and opening the circuits virtually as well as machine made. It controls the opening and closing of the circuit contacts of an electronic circuit. When the relay contact is open (NO), it does not energize with the open contact. Relays are the primary protection as well as switching devices in most of the control processes or equipment.

Types of Relay:

1.Electro magnetic Relays

These relays are constructed with electrical, mechanical, and magnetic components, and have operating coil and mechanical contacts. Therefore, when the coil gets activated by a supply system, these mechanical contacts get opened or closed. The type of supply can be AC or DC.

DC vs AC Relays

Both AC and DC relays work on the same principle as electro magnetic induction, but the construction is somewhat different and also depends on the application for which these relays are selected. DC relays are employed with a freewheeling diode to shut off the coil, and the AC relays use laminated cores to prevent eddy current losses.

The very interesting aspect of an AC is that for every half cycle, the direction of the current supply changes; therefore, for every cycle the coil loses its magnetism since the zero current in every half-cycle makes the relay to continuous make and break the circuit. So, to prevent this – additionally one shaded coil or another electronic circuit is placed in the AC relay to provide magnetism in the zero current position.

Induction Type Relays

These are used as protective relays in AC systems alone and are usable with DC systems. The actuating force for contacts movement is developed by a moving conductor that may be a disc or a cup, through the interaction of electro magnetic fluxes due to fault currents.

Magnetic Latching Relays

Magnetic latching type uses permanent magnet with a high remittance to remain armature at same point as coil is electrified when coil power source is taken away.

2. Solid State Relays

Solid State uses solid state components to perform the switching operation without moving any parts. Since the control energy required is much lower compared with the output power to be controlled by this relay that results the power gain higher when compared to the electro magnetic relays. These are of different types: reed relay coupled SSR, transformer coupled SSR, photo-coupled SSR, and so on.

3. Hybrid Relay

These relays are composed of electro magnetic relays and electronic components. Usually, the input part contains the electronic circuitry that performs correction and the other control functions, and the output part include electro magnetic relay.

4. Thermal Relay

These relays are based on the effects of heat, which means – the rise in the ambient temperature from the limit, directs the contacts to switch from one position to other. These are mainly used in motor protection and consist of bimetallic elements like temperature sensors as well as control elements.

Overview of Solid-state Relays

A solid-state relay (SSR) is an electronic switching device that switches on or off when a small external voltage is applied across its control terminals. It may be designed to switch either AC or DC to the load. It serves the same function as a machine made relay, but has no moving parts.

There are three basic categories of SSRs that electricians recognize: Reed Relay Coupled, Transformer Coupled, and Photo Coupled. Looking for SSR Relay? Buy various types of relays from Rajendra Traders in India.

The Reed Relay Coupled SSR has a control signal applied directly to the coil of a reed relay. When the reed switch closes it activates the circuitry which will trigger the thyristor switch.A Transformer coupled SSR is an ON/OFF control device that controls output current of circuit from 0 to max or vice-versa by connect.

Photo coupled is very similar to a normal relay,It can switch in a millisecond, lasts virtually endlessly, requires only 5 Milli Ampere of pilotage, has exceptional insulation and is very small. Also the spare PhotoMOS can be easily replaced on the base, even with different patterns and possibly even with the cheap PhotoTRIAC to use for application in AC.

The Basics of SSRs (Solid-State Relays): Relay Switching Device

The SSR output is activated immediately after applying control voltage. Consequently, this relay can turn on anywhere along the AC sinusoidal voltage curve. The SSR is particularly suitable in application where a fast response time is desired, such as solenoids or coils.

Because a “fail-open” state is generally considered safer than a “fail-closed” state, machine made relays are still favored over their solid-state counterparts in many applications.

Does SSR need heat sink?

Adequate heatsinking, including consideration of air temperature and flow, is essential to the proper operation of a solid state relay (SSR). It is necessary that the user provide an effective means of removing heat from the SSR package.

What is the difference between a contactor and a relay?

Both are electromagnetic switches and operate under similar principles. The difference comes if we see from the application perspective. Contactors are used for high voltage switching purposes whereas relays are used for low voltage switching and more commonly used in single phase applications whereas Contactors are typically built for and used in 3-phase applications. A contactor joins 2 poles together, without a common circuit between them, while a relay has a common contact that connects to a neutral position.

Contactor Overview - Contactors & Overloads

Thermal overload relays are economic machine made protection devices for the main circuit. They offer reliable protection for motors in the event of overload. Overloads are much less common on relays. Overload defined as it is an electrical device mainly designed for imitating the heating prototypes of the electric motor, as well as breakups the flow of current when the heat detecting device in the relay attains a fixed temperature.

The designing of an overload type can be done with a heater coupled with generally closed connections that unlock when once the heater acquires too hot. The connections of an overload relay can be connected in series as well as placed among the motor & contactor itself to avoid the motor from restarting when the overload trips.

The safety cover of the magnetic contactor and thermal overload relay is optional. One contactor may have several auxiliary contacts, either normally open or normally close if required.

How do the timers & counters function in PLC?

Timers are blocks that count the time as specified by the user and the executes the algorithm based on the time. Counters are blocks that count the pulses. PLC timers are instructions that provide the same functions as on-delay and off-delay mechanical and electronic timing relays. A PLC timer provides a preset delay to the control actions.

A PLC counter is a function block that counts up or down until it reaches a limit and When the limit is reached the output is set. The thing is that counting is in fact widely used in PLC programming.

What is the difference between MCB, MCCB, ELCB, and RCCB?

MCB, MCCB, RCCB, and ELCB are circuit breakers but all of them are designed for serving a specific purpose. MCB is a miniature circuit breaker which is used to break small currents. The ratings usually end up to 100 A. MCCB is a Module case circuit breaker which has a rugged construction as it can break larger currents usually from 100-1000 Ampere.

ELCB is working based on Earth leakage current while RCCB only has the line and neutral connections and not having sensing of Earth, because above all Phase current is equal to the neutral current in a single phase.

Types of Switches | Rotary & Toggle Switch

The toggle switch is a type of electrical switch that is identified by the presence of handle or lever that makes it possible to control the flow of electric current/signal from a power supply to a device or within a device. It is a switch that has just two positions and a hinged switch that can assume either of two positions that is ON or OFF.

A rotary switch is a switch operated by rotation. These are often chosen when more than 2 positions are needed, such as a three-speed fan or a CB radio with multiple frequencies of reception or "channels".

How to select the right motor starter?

Follow these steps in order to select the proper motor starter for your requirements:

1. Contactor Size & Line Power

2. Overload Relay Range

3. AC Coil Control Power

4. Control Power

5. Enclosure Cover Buttons

Electrical Measuring Instruments | ammeter and voltmeter

A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. An ammeter is a measuring device used to measure the electric current in a circuit.

AC or DC coil for a solenoid valve

The response time for AC solenoid valves is 8-5 μs compared to the typical 30-40 μs for DC solenoid operation. Generally, DC solenoids are preferred to AC because a DC operation is not subject to original peak currents, which may cause overheating and coil harm with frequent cycling or accidental spool seizure.

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