What are the purpose of relay in the electrical system?

A relay is an electrically operated switch.

Many relays use an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state relays. Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal.

The first relays were used in long distance telegraph circuits as amplifiers: they repeated the signal coming in from one circuit and re-transmitted it on another circuit. Relays were used extensively in telephone exchanges and early computers to perform logical operations.

A type of relay that can handle the high power required to directly control an electric motor or other loads is called a contactor. Solid-state relays control power circuits with no moving parts, instead using a semiconductor device to perform switching.

Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called “protective relays”. ‎

Control element – the role of the relay

The relay is an automatic switching element with an isolation function. When the change of the excitation amount in the input circuit reaches a prescribed value, the automatic circuit control device capable of causing a predetermined step change in the controlled power in the output circuit. So what are the functions of the relay as a control element?
1, integrated signal
For example, when a plurality of control signals are input to a multi-winding relay in a prescribed form, the comparison is integrated to achieve a predetermined control effect.
2, automatic, remote control, monitoring
For example, a relay on an automated device, together with other appliances, can form a program control circuit for automated operation.
3. Expand the scope of control
For example, when the time relay Xiaobian knows that the multi-contact relay control signal reaches a certain value, it can switch, disconnect, and turn on multiple circuits at the same time according to different forms of the contact group.
4, zoom in
For example, sensitive relays, intermediate relays, etc., can control very high power circuits with a very small amount of control.
Relays are widely used in remote control, telemetry, communications, automatic control, mechatronics and aerospace technology to control, protect, regulate and transmit information.

Solid-state Relays

As versatile as electromechanical relays can be, they do suffer many limitations. They can be expensive to build, have a limited contact cycle life, take up a lot of room, and switch slowly, compared to modern semiconductor devices. These limitations are especially true for large power contactor relays. To address these limitations, many relay manufacturers offer “solid-state” relays, which use an SCR, TRIAC, or transistor output instead of mechanical contacts to switch the controlled power. The output device (SCR, TRIAC, or transistor) is optically-coupled to an LED light source inside the relay. The relay is turned on by energizing this LED, usually with low-voltage DC power. This optical isolation between input to output rivals the best that electromechanical relays can offer.

Being solid-state devices, there are no moving parts to wear out, and they are able to switch on and off much faster than any mechanical relay armature can move. There is no sparking between contacts, and no problems with contact corrosion. However, solid-state relays are still too expensive to build in very high current ratings, and so electromechanical contactors continue to dominate that application in industry today.

One significant advantage of a solid-state SCR or TRIAC relay over an electromechanical device is its natural tendency to open the AC circuit only at a point of zero load current. Because SCR’s and TRIAC’s are thyristors, their inherent hysteresis maintains circuit continuity after the LED is de-energized until the AC current falls below a threshold value (the holding current). In practical terms what this means is the circuit will never be interrupted in the middle of a sine wave peak. Such untimely interruptions in a circuit containing substantial inductance would normally produce large voltage spikes due to the sudden magnetic field collapse around the inductance. This will not happen in a circuit broken by an SCR or TRIAC. This feature is called zero-crossover switching.

One disadvantage of solid state relays is their tendency to fail “shorted” on their outputs, while electromechanical relay contacts tend to fail “open.” In either case, it is possible for a relay to fail in the other mode, but these are the most common failures. Because a “fail-open” state is generally considered safer than a “fail-closed” state, electromechanical relays are still favored over their solid-state counterparts in many applications.

Time-delay Relays

Time-delay Relays

Some relays are constructed with a kind of “shock absorber” mechanism attached to the armature which prevents immediate, full motion when the coil is either energized or de-energized. This addition gives the relay the property of time-delay actuation. Time-delay relays can be constructed to delay armature motion on coil energization, de-energization, or both.

Time-delay relay contacts must be specified not only as either normally-open or normally-closed, but whether the delay operates in the direction of closing or in the direction of opening. The following is a description of the four basic types of time-delay relay contacts.

First we have the normally-open, timed-closed (NOTC) contact. This type of contact is normally open when the coil is unpowered (de-energized). The contact is closed by the application of power to the relay coil, but only after the coil has been continuously powered for the specified amount of time. In other words, the direction of the contact’s motion (either to close or to open) is identical to a regular NO contact, but there is a delay in closing direction. Because the delay occurs in the direction of coil energization, this type of contact is alternatively known as a normally-open, on-delay:

The following is a timing diagram of this relay contact’s operation:


Next we have the normally-open, timed-open (NOTO) contact. Like the NOTC contact, this type of contact is normally open when the coil is unpowered (de-energized), and closed by the application of power to the relay coil. However, unlike the NOTC contact, the timing action occurs upon de-energization of the coil rather than upon energization. Because the delay occurs in the direction of coil de-energization, this type of contact is alternatively known as a normally-open, off-delay:

The following is a timing diagram of this relay contact’s operation:


Next we have the normally-closed, timed-open (NCTO) contact. This type of contact is normally closed when the coil is unpowered (de-energized). The contact is opened with the application of power to the relay coil, but only after the coil has been continuously powered for the specified amount of time. In other words, the direction of the contact’s motion (either to close or to open) is identical to a regular NC contact, but there is a delay in the opening direction. Because the delay occurs in the direction of coil energization, this type of contact is alternatively known as a normally-closed, on-delay:

The following is a timing diagram of this relay contact’s operation:


Finally we have the normally-closed, timed-closed (NCTC) contact. Like the NCTO contact, this type of contact is normally closed when the coil is unpowered (de-energized), and opened by the application of power to the relay coil. However, unlike the NCTO contact, the timing action occurs upon de-energization of the coil rather than upon energization. Because the delay occurs in the direction of coil de-energization, this type of contact is alternatively known as a normally-closed, off-delay:

The following is a timing diagram of this relay contact’s operation:

Time-delay relays are very important for use in industrial control logic circuits. Some examples of their use include:

  • Flashing light control (time on, time off): two time-delay relays are used in conjunction with one another to provide a constant-frequency on/off pulsing of contacts for sending intermittent power to a lamp.
  • Engine autostart control: Engines that are used to power emergency generators are often equipped with “autostart” controls that allow for automatic start-up if the main electric power fails. To properly start a large engine, certain auxiliary devices must be started first and allowed some brief time to stabilize (fuel pumps, pre-lubrication oil pumps) before the engine’s starter motor is energized. Time-delay relays help sequence these events for proper start-up of the engine.
  • Furnace safety purge control: Before a combustion-type furnace can be safely lit, the air fan must be run for a specified amount of time to “purge” the furnace chamber of any potentially flammable or explosive vapors. A time-delay relay provides the furnace control logic with this necessary time element.
  • Motor soft-start delay control: Instead of starting large electric motors by switching full power from a dead stop condition, reduced voltage can be switched for a “softer” start and less inrush current. After a prescribed time delay (provided by a time-delay relay), full power is applied.
  • Conveyor belt sequence delay: when multiple conveyor belts are arranged to transport material, the conveyor belts must be started in reverse sequence (the last one first and the first one last) so that material doesn’t get piled on to a stopped or slow-moving conveyor. In order to get large belts up to full speed, some time may be needed (especially if soft-start motor controls are used). For this reason, there is usually a time-delay circuit arranged on each conveyor to give it adequate time to attain full belt speed before the next conveyor belt feeding it is started.

The older, mechanical time-delay relays used pneumatic dashpots or fluid-filled piston/cylinder arrangements to provide the “shock absorbing” needed to delay the motion of the armature. Newer designs of time-delay relays use electronic circuits with resistor-capacitor (RC) networks to generate a time delay, then energize a normal (instantaneous) electromechanical relay coil with the electronic circuit’s output. The electronic-timer relays are more versatile than the older, mechanical models, and less prone to failure. Many models provide advanced timer features such as “one-shot” (one measured output pulse for every transition of the input from de-energized to energized), “recycle” (repeated on/off output cycles for as long as the input connection is energized) and “watchdog” (changes state if the input signal does not repeatedly cycle on and off).

The “watchdog” timer is especially useful for monitoring of computer systems. If a computer is being used to control a critical process, it is usually recommended to have an automatic alarm to detect computer “lockup” (an abnormal halting of program execution due to any number of causes). An easy way to set up such a monitoring system is to have the computer regularly energize and de-energize the coil of a watchdog timer relay (similar to the output of the “recycle” timer). If the computer execution halts for any reason, the signal it outputs to the watchdog relay coil will stop cycling and freeze in one or the other state. A short time thereafter, the watchdog relay will “time out” and signal a problem.

  • Time delay relays are built in these four basic modes of contact operation:
  • 1: Normally-open, timed-closed. Abbreviated “NOTC”, these relays open immediately upon coil de-energization and close only if the coil is continuously energized for the time duration period. Also called normally-open, on-delay relays.
  • 2: Normally-open, timed-open. Abbreviated “NOTO”, these relays close immediately upon coil energization and open after the coil has been de-energized for the time duration period. Also called normally-open, off delay relays.
  • 3: Normally-closed, timed-open. Abbreviated “NCTO”, these relays close immediately upon coil de-energization and open only if the coil is continuously energized for the time duration period. Also called normally-closed, on-delay relays.
  • 4: Normally-closed, timed-closed. Abbreviated “NCTC”, these relays open immediately upon coil energization and close after the coil has been de-energized for the time duration period. Also called normally-closed, off delay relays.
  • One-shot timers provide a single contact pulse of specified duration for each coil energization (transition from coil off to coil on).
  • Recycle timers provide a repeating sequence of on-off contact pulses as long as the coil is maintained in an energized state.
  • Watchdog timers actuate their contacts only if the coil fails to be continuously sequenced on and off (energized and de-energized) at a minimum frequency.

Tips for selecting relays

Relays from Magnecraft & Struthers-Dunn come in a variety of shapes, sizes, and ratings including socket or DIN-rail mountable versions.

Capacitive loads want to remain at constant voltage. When the contacts are closed, a current surge can weld the contacts together.

Inductive loads want to keep a constant current. When relay contacts open, the voltage surges to keep the current constant. This surge causes arcing across the contacts.

Programmable logic controllers (PLCs) and computer-based controllers are great for handling logic and data-collection tasks for industrial controls. But when it comes to driving motors, heaters, valves, lamps, or power supplies, PLCs don’t output enough current. A better idea is to let PLCs drive plug-in relays.

In addition to switching current, relays have other benefits. A relay does an effective job of electrically isolating a PLC from power mains, transients generated by the load, and effects of a malfunctioning device. A typical PLC output drives the coil in a relay and the relay contacts switch the power loads. There is no electrical connection between the coil and contacts of a relay. Extra relay contacts can be used to report the relay status to the PLC.

Control systems de-signed with plug-in relays are easy to repair when a system is damaged by an external malfunction such as a power surge, lightning strike, or a brown-out. What’s more, the skill level required to replace a plug-in relay is considerably less than the skills required to rewire a damaged PLC.

Some relays also have the advantage of smart operation. These units have status lamps showing power to the coils and mechanical flags to indicate closed contacts. Also included for diagnostic testing are mechanical buttons to operate the output of the relays to test the driven devices. And a lock-out actuator on the mechanical button can hold the relay in the operating position.

PLCs with relays can greatly simplify design, operation, and reliability of a control system when relays are chosen properly. Consider these factors when choosing a relay for use in industrial controls:

  • Voltages driving loads are the first concern. The voltage rating of a relay must be greater than or equal to the voltage driving the load. The frequency of the switched voltage is also critical. Because ac current fluctuates from positive to negative crossing through zero, the switched voltage will vary between the maximum voltage and zero. Dc voltage, on the other hand, is always at the maximum value, causing maximum wear on the contacts with every switch. Typically, a relay rated at 240 Vac will be rated for only 24 Vdc.
  • The current required depends on the type of load. Most loads don’t draw a constant current. In fact, the current demand of most loads varies somewhat predictably.

It is also important to avoid switching currents that are too small for the relay to operate reliably. Proper operation of a switch relies, to some extent, on the switching of some minimum current. This current is often referred to as a wiping current because it will burn off traces of contaminants that may build up on the relay contacts. The lower limit of current that can be reliably switched is a function of several factors such as contact material, contact geometry, and mechanical sliding of the contact surfaces.

Relays with gold-plated contacts can reliably switch currents as low as 10 mA. Relays with bifurcated (split) contacts also switch lower-level currents in the 10-mA range. Sealed relays, reed relays, and mercury-wetted reed relays are intended for low-level applications. The advantages of gold-plated contacts are that the relay can be used for mixed loads where one set of contacts can switch rated currents and another set of contacts can switch low-level currents. Gold plating protects the contacts while the relay is on the shelf and while low-level currents are switched. The gold disappears from the surface when the full-rated current is switched.

  • Resistive loads exhibit no surge at turn-on. Like ideal resistors, they have the same value of resistance all the time. The most common example of a resistive load is a simple heater. If it is specified at 10 A, it can be switched safely with a relay rated at 10 A. Unfortunately, there are very few purely resistive loads. Most are a combination of two or more types.
  • Lamp loads have high input surges. The filament of an ordinary incandescent bulb has a high temperature coefficient. When hot, the resistance of a lamp filament is often 20 times the resistance of a cold lamp. Perhaps the most severe load fluctuation is an ordinary incandescent lamp. A common 75-W light bulb draws 0.625 A during normal operation. But when the filament is cold and the lamp is first turned on, the inrush current hits 13 A. This surge only lasts about a tenth of a second but the current must be accounted for if the lamp is turned on and off many times. A relay rated at 10 A resistive can safely switch a lamp that draws 0.625 A when hot.
  • Motor loads also exhibit high input surges. An ordinary singlephase, 110-V, 1⁄3-hp synchronous motor normally draws 4.1 A. But at start up or locked rotor, the same motor can draw over 24 A. If the mechanical load is released from the motor and it runs unloaded, the motor can draw 6 A. A relay with a resistive rating of 10 A has been tested by UL and rated for 1⁄3 hp at 120 Vac.
  • Capacitive loads exhibit high-current surges at turn-on. A capacitor tries to maintain a constant voltage. Switching a voltage to a capacitor which is at zero volts, the capacitor tries to short out the voltage to maintain the initial zero volts. This high current at turnon can weld contacts shut. Typical capacitive loads include dc power supply outputs and other filtered power sources.
  • Inductive loads have a soft turnon, which means the current rises slowly on turn on, but a voltage surges across the contacts when the load is switched off. An inductor tries to maintain a constant current. But when the load is switched off, the inductor tries to maintain the current by increasing the voltage across the contacts. The voltage can increase across the contacts so much that it arcs. Arcing can melt the contacts and degrade them with every occurrence. Typical loads with high inductance include solenoids and electrically operated valves.

Of course, components can be derated. This improves the life of a system by choosing components with more strength than that theoretically required to do the job. The derating equation for relays is:

where Rd = derating factor, Io = actual current, and IR = rated current.

Therefore, a 12-A relay being used at 10 A has a failure rate of 100/144 or about 70% of that of a 10-A relay operating at 10 A.


Selection of the circuit breaker

Selection of the circuit breaker The selection of the air switch
Circuit breaker, full name automatic air circuit breaker, also known as air switch, is a commonly used low-voltage protection appliances, can achieve short circuit, overload and other functions.
The circuit breaker is used in the home power supply for the total power protection switch or branch line protection switch. When the residential circuit or household appliances short circuit or overload, it can automatically trip, cut off the power, so as to effectively protect the equipment from damage or prevent accidents.
Households generally use two-pole (ie 2P) circuit breakers for total power protection, with a single pole (1P) for branch protection.
If the choice of the circuit breaker is too small, the circuit breaker is prone to frequent tripping, causing unnecessary power failure, such as the choice is too large, then the desired protection effect, so the home improvement circuit breaker, the correct choice of rated capacity current size is very important.
10A, 25A, 32A, 50A, 63A, 80A, 100A, etc .; then the general family how to choose or check the total load current of the total value of it?
1, first calculate the value of each branch current
① pure resistive load, such as bulbs, electric heaters, etc. with the specified power directly divided by the voltage that is,
Formula I = Power / 220v;
Such as 20w bulb, branch current I = 20W / 220 = 0.09A
Electric water heaters, electric heaters, rice cookers, electric woks, vacuum cleaners, air conditioners, etc. are resistive loads. Electric water heaters, electric heaters, electric cookers, electric cookers, vacuum cleaners,
How to choose a circuit breaker This is a professional technical problem. Briefly said that you can choose from the following 6: 1, first according to the rated voltage selection, the rated voltage to be consistent. 2, the rated current of the circuit breaker is greater than or equal to the rated current of the circuit used. 3, the rated breaking current of the circuit breaker is greater than or equal to the short circuit current of the circuit used. 4, according to environmental conditions, such as altitude, temperature, humidity, choose to meet the requirements of the circuit breaker. 5, according to the quality of the brand selection, cost-effective circuit breakers. 6, the special break off the situation, to check the circuit breaker. However, different loads should use different types of circuit breakers.
The principle of air switch selection
1, according to the requirements of the line to protect the circuit breaker to determine the type and form of protection – to determine the use of frame type, device type or flow limit and so on.
2, the rated voltage of the circuit breaker UN should be equal to or greater than the rated voltage of the protected circuit.
3, circuit breaker undervoltage release rated voltage should be equal to the rated voltage of the protected circuit.
4, the rated current of the circuit breaker and the rated current of the overcurrent release shall be greater than or equal to the calculated current of the protected circuit.
5, the circuit breaker limit breaking capacity should be greater than the line of the maximum short-circuit current RMS.
6, the distribution line in the lower and lower circuit breaker protection characteristics should be coordinated with the protection of the lower level should be located in the lower level of protection features and do not intersect.
7, the circuit breaker long delay trip current should be less than the conductor to allow continuous current.

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What is an Automotive Relay?

Automotive Relays
What is an Automotive Relay?

Automotive relays of different sizes are found in every type of land or sea vehicle. They are often used to enable a low amperage circuit to switch a higher amperage circuit on or off. An example would be turning headlights on. Automotive relays also allow items to switch at the same time by using a single output, therefore allowing one to simultaneously open and/or close continuity on multiple items.
Types of Automotive Relays

Choose from more than 200 automotive relays available from Binder Electric, offering one of the most comprehensive portfolios of SPST and SPDT automotive relays in the industry. Our parametric filters help refine your search results by Contact Arrangement, Contact Current, Coil Voltage and Termination Style. We offer a huge selection of Latching and Non Latching automotive relays in SPST, SPDT and DPDT combinations, with Coil Voltages from 5V to 120V (12V relays and 24V DC relays being the most common). The most common sizes for maximum contact current are 20 A and 30 A. We also carry automotive relays with maximum contact current as high as 300 A.
Automotive Relays from Binder Electric.

Binder Electric has a full selection of automotive relays from several manufacturers when looking for a relay in SPDT, SPST or DPDT combinations, PCB relays, a non latching automotive relay, a quick connect auto Relay, or for any circuit that might require an automotive relay. Simply choose from the automotive relay technical attributes below and your search results will quickly be narrowed to match your specific automotive relay application needs.

Automotive relays are found in several automobile applications including:
Car antennas
Power windows
Power seats
Car stereos
Door locks
Power sunroofs
Interior lighting
Intermittent wipers
Automotive Relay termination style is usually plug-in or quick-connect, and Binder Electric has a full selection of both versions, as well a wide choice of PCB mount automotive relays. Simply choose from the automotive relay technical attributes below and your search results will quickly be narrowed to match your specific automotive relay application needs.

Interface series relay

BRG2RV Interface Series

Cross Reference

Binder 6.2mm interface relays provide a compact solution for general purpose relay requirements. The BRG2RV series Interface relays are ideal for PLC and electronic systems, industrial automation, panel builders, assembly machine applications and other applications that require a high switching capability in a compact space.

The BRG2RV relays can be used as a universal interfaces between the controller and the actuator to switch small and medium size loads. Installation time is greatly reduced with pre-assembled relays and sockets. Replacement relays and sockets are also available.

Optional accessories include jumpers, spacers and marking plates to facilitate wiring and quick identifications.

If you are looking for Binder BRG2rv Series General Purpose Relays, please call us on (86) 577-62710579 or email us at export@binder-electric.com we will do our best to help you find the relay that you are looking for at the most competitive prices possible. If you are searching forBinder relay technical information (data-sheets) please use the datasheets or product selection guide page links.


interface relay and socket






Key Features:
•Space-saving 6.2mm width
•Only 70mm in height from DIN rail
•Gold-plated contacts
•Pre-assembled relay and DIN mount socket
•Universal screw terminals
•Universal AC/DC socket with built-in surge suppression and green LED
•Lever for easy locking and removal of relay
•Wide input voltage range: 6 to 240V
•High dielectric strength and impulse withstand voltages
•Reverse Polarity protected
•400V AC maximum switching voltage
•Spring clamp accepts 20-14 AWG solid or stranded wires
•1500VA maximum switching power
•RoHS compliant

Jumper combs come with 20 points, if shorter lengths are needed simply cut off the excess points.

Note: When using a cut jumper, please use a spacer on the cut side.

Air switch principle describes the role of air switch which

Air switch principle describes the role of air switch which
Air switch principle
Tripping method has three kinds of thermal, electromagnetic and double tripping.
When the line occurs when the general overload, the overload current can not make the electromagnetic release device action, but can make the heat element to produce a certain amount of heat, so that bimetal heating up bending, pushing the lever to hook and lock off, the main touch Head off, cut off the power. When the line short circuit or serious overload current, the short circuit current exceeds the instantaneous tripping set current value, the electromagnetic release device to produce a large enough suction, the armature suction and impact the lever, so that the hook around the shaft seat up and lock off , The lock in the role of the reaction under the spring will be three main contact breaking, cut off the power.
The tripping mechanism of the switch is a set of connecting rods. When the main contact is closed by the operating mechanism, it is locked in the closed position. If a fault occurs in the circuit, the associated release will act to release the latch in the trip mechanism so that the main contact is quickly broken off by the release spring. In accordance with the different protection, the release can be divided into over-current release and voltage relief and other types.

Siemens breaker

Siemens breaker

The role of air switch
The air switch is a very important electrical appliance in the low-voltage distribution network and the electric drive system, which combines control and a variety of protection functions. In addition to complete contact and breaking circuit, but still on the circuit or electrical equipment, short circuit. Severe overload and undervoltage protection, but also can be used to start the motor less frequently.
Under normal circumstances, the overcurrent release of the armature is released; in the event of a serious overload or short circuit failure, the main circuit in series with the coil will have a strong electromagnetic attraction to attract the armature down and open the hook, Disconnect the main contact. Undervoltage release work on the contrary, in the normal voltage, the electromagnetic suction suction armature, the main contact was able to close. Once the voltage has dropped or is de-energized, the armature is released and the main contact is disconnected. When the power supply voltage back to normal, you must re-closing before work, to achieve a loss of pressure protection.
Because there are many ways of insulation, there are switches for oil switches, vacuum switches and other inert gases (sulfur hexafluoride gas). The air switch uses the air to extinguish the arc generated during the switch. So called air switch.

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