How Do Switchgear Work? – Automated protective switchgear consists of a relay and circuit breaker. The relay acts whenever there is a fault. The relay closes the faulty circuit and disconnects the disrupted line. This ensures that the appliances do not get damaged and there is an uninterrupted supply of power.
- 1 What does protective relay provide?
- 2 What is protection components?
- 3 Which relays are used for the protection of lines?
- 4 How are relays damaged?
- 5 What is basic protection system?
- 6 What is protection in a circuit?
- 7 What are the 4 parts of a relay?
- 8 What are the 2 main parts of a relay?
What are the components of protection relay?
What is a Protective Relay? – The protective relay was invented more than 160 years ago. During the last 60 years, it has undergone considerable change, the most obvious of which is its reduction in size. A protective relay is a switchgear device that detects the fault and initiates the operation of the circuit breaker to isolate the defective element from the rest of the system.
- They are compact and self-contained devices which can detect abnormal conditions.
- Protective relays detect the abnormal conditions in the electrical circuits by constantly measuring the electrical quantities which are different under normal and fault conditions.
- The electrical quantities which may change under fault conditions are voltage, current, frequency, and phase angle.
Through the changes in one or more of these quantities, the faults signal their presence, type, and location to the protective relays, Having detected the fault, the relay operates to close the trip circuit of the breaker. This results in the opening of the breaker and disconnection of the faulty circuit.
- The protective relaying is used in electrical substations to give an alarm or to cause prompt removal of any element of the power system from service when that element behaves abnormally.
- The abnormal behavior of an element might cause damage or interference within the effective operation of the rest of the system.
The protective relaying minimizes the damage to the equipment and interruptions to the service when an electrical failure occurs. Along with some other equipment the relays help to minimize damage and improve the service The protective relaying scheme includes protective current transformers, voltage transformers, protective relays, time delay relays, auxiliary relays, secondary circuits, trip circuits, etc.
What does protective relay provide?
The Institute of Electrical and Electronic Engineers (IEEE) supplies the following definition of a protective relay: A relay whose function is to detect defective lines or apparatus or other power system conditions of an abnormal or dangerous nature and to initiate appropriate control circuit action.
How does a relay sense the fault?
Working Principle of Relay – It works on the principle of an electromagnetic attraction. When the circuit of the relay senses the fault current, it energises the electromagnetic field which produces the temporary magnetic field. This magnetic field moves the relay armature for opening or closing the connections.
- The small power relay has only one contacts, and the high power relay has two contacts for opening the switch.
- The inner section of the relay is shown in the figure below.
- It has an iron core which is wound by a control coil.
- The power supply is given to the coil through the contacts of the load and the control switch.
The current flows through the coil produces the magnetic field around it. Due to this magnetic field, the upper arm of the magnet attracts the lower arm. Hence close the circuit, which makes the current flow through the load. If the contact is already closed, then it moves oppositely and hence open the contacts.
What is a relay in power system?
What is a Relay – If you are from the electronics field, this word must be prevalent and if you are not, let us tell you all about it! Relays are the switches that aim at closing and opening the circuits electronically as well as electromechanically.
It controls the opening and closing of the circuit contacts of an electronic circuit. When the relay contact is open (NO), the relay isn’t energized with the open contact. However, if it is closed (NC), the relay isn’t energized given the closed contact. However, when energy (electricity or charge) is supplied, the states are prone to change.
Relays are normally used in the control panels, manufacturing, and building automation to control the power along with switching the smaller current values in a control circuit. However, the supply of amplifying effect can help control the large amperes and voltages because if low voltage is applied to the relay coil, a large voltage can be switched by the contacts.
What is protection components?
Circuit Protection Components are devices used to protect the circuit from overcurrent and overvoltage (static electricity) in the secondary circuits of electronic equipment. Chip type protectors to meet the requirements for high density surface mounting technology, and more suitable for making compact equipment.
Which relays are used for the protection of lines?
Introduction – A distance relay is a type of protection relay most often used for transmission line protection. Distance relays measure the impedance from the installation side to the fault location and operates in response to changes in the ratio of measured current and voltage. One challenging situation for distance protection relays is when the power system is exposed to significant power swings. Power swings are oscillations in active and reactive power flows on a transmission line which can consequentially induce large disturbances, such as faults.
- The oscillation in the apparent power and bus voltages are observed by the relay as an impedance swing on the R-X plane.
- If the impedance trajectory enters the relay zone and stays there for a sufficient period of time, then the relay will issue a trip command.
- The higher the impedance of the source, the larger the circle in the R-X plane, allowing for a higher resistance tolerance.
Since the mere existence of resistance at the fault location is a problem in the operation of the distance relays, the detection of power swings is solved by applying mathematical morphology.
How can protective relays prevent equipment damage?
A protection relay is a smart device that receives inputs like current, voltage, resistance, temperature, or even light, compares them to set points, and provides outputs such as visual feedback in the form of indicator lights and/or an alphanumeric display, communications, control warnings, alarms, and turning the power off and on.
- They are used for motor protection and ground fault protection in industrial settings.
- A regulating relay is a special class of protection relay that activates when an operating parameter deviates from preset limits.
- This FAQ contrasts and compares traditional electrotechnical and solid-state protective relays, look at how layers of protective relays are used to protect zones in electricity transmission and distribution networks, then reviews the operation of ground fault and arc flash protective relays.
Protective relays can be based on electromechanical or solid-state relay technology. In lower-power applications, electromechanical designs have mostly been replaced by solid-state devices. But electromechanical relays continue to be used in high-power applications in electricity transmission and distribution systems.
- Microprocessor-based protective relays can detect small changes in parameters like voltage, current, resistance, or temperature that can be used to identify the early stages of a problem before it is severe enough to trip conventional protective devices like circuit breakers or fuses.
- A protection relay can detect the cause of a fault, such as overcurrent, overvoltage, or increases in temperature, that conventional protection devices cannot identify.
And a protection relay can provide a variety of outputs, including local or remote alarms and visual indicators, and even shut down the equipment if the situation is severe enough (Figure 1). Figure 1: Protective relays can be designed to accept a variety of inputs and produce outputs based on fault settings or thresholds. (Image: Littelfuse ) In electrical engineering, especially in electricity transmission and distribution networks, a protective relay is used to trip a circuit breaker when a fault is detected. Figure 2: Electromechanical protective relays (glass enclosures) are still found in many electricity distribution networks. (Image: Wikipedia )\ Electromechanical protective relays are different Conventional electromechanical relays have fixed and relatively wide operating thresholds and times.
Electromechanical protective relays operate by magnetic induction or attraction and have well-defined, adjustable, and selectable time and parametric operating entries. To achieve this improved performance, protection relays can employ arrays of induction disks, magnets, coils, phase-shifting networks, and other techniques.
Protective relays are available with a range of architectures. Some respond to a simple magnitude of a parameter like voltage or current. Induction relays use two field coils to produce the product of two parameters, for example, current and voltage, and respond if the power level is too high.
Multiple locks can be used to bias the relay using a separate circuit to control the sensitivity of its response. Some protective relays have a permanent magnet in the magnetic circuit enabling the relay to respond to current in one direction differently from another direction. These are called polarized relays and can be used on DC circuits to detect reverse current in a generator.
Polarized relays can be bistable and maintain a closed contact when no coil current is flowing and opening the contact when reverse current is present. The principle can be used with a polarizing winding connected to a reference voltage in AC circuits.
- Protection zones Protective relays in industrial and utility systems can be used to protect zones within the power system.
- These zones are usually based on functionality such as generators, motors, transformers, distribution feeders and buses, and transmission lines.
- When using protection relays, the overall system is divided into sections to provide “zones of protection” that define the system boundaries that the individual relays are required to protect.
For a higher level of protection, the zones can be defined to overlap, enabling multiple layers of protection to be provided to critical equipment. This architecture has a primary protective relay and one or more secondary protection devices. Overlapping and backup protection ensures that all devices remain protected in the event of multiple failures. Figure 3: Multiple protection zones can be used in industrial installations to provide a higher level of protection. (Image: Eaton ) Ground fault relays There are a variety of ground-fault protective devices, including residual-current devices (RCD), earth leakage circuit breakers (ELCB), ground-fault equipment protectors (GFEP), and ground-fault circuit interrupters (GFCI); their primary function is the protection of people and equipment against ground faults by interrupting/disconnecting the defective circuit.
In the U.S., GFCIs must comply with UL 943 and are designed to protect people. GFEPs (which include ground fault relays) must comply with UL 1053 and are designed for equipment protection. GFCIs have a sensitivity of 5 to 6 mA and can be subject to nuisance tripping if used to protect equipment like variable-speed motor drives.
GFEPs have a sensitivity of 30 mA or higher and are resistant to nuisance tripping. GFEPs and ground fault relays (or sensors) are designed to sense low-magnitude ground faults. When the current magnitude and time reach a set point, a control system opens the relay to disconnect the circuit.
Ground fault relays protect against low-level ground faults and must be used in coordination with fuses or circuit breakers that can protect against higher-magnitude ground faults. Low-level ground fault protection is important since those faults will not trip circuit breakers or blow fuses but can cause major factory and industrial operations disruptions.
A ground fault can damage equipment or cause a fire. Ground fault protection is used in water treatment plants, pumping systems, marine and mining environments, and in wash-down areas in industrial plants. Ground fault protection is a very specific activity it is not intended to:
Prevent ground faults Protect operators from electrical shocks Protect from 3-phase, phase-to-phase or phase-to-neutral faults
An example of ground fault detection is to pass all the cables connected to a motor through the window of a ground fault current sensor. If there is no ground fault, all the currents will be balanced, and no current will be induced in the sensor. Unbalanced currents indicate the existence of a ground fault.
The current differential will be measured by the current sensor and amplified for use in the control circuit. If the current exceeds the ground fault current set point, the ground fault relay will be turned on, disconnecting the motor. In some systems, the current sensor is directly connected to the ground fault relay.
Arc flash protection relays An arc flash is the light and heat produced from an electric short-circuit arc. An arc flash can cause substantial damage to equipment, start a fire, or harm nearby personnel. Electrical arcs have incremental negative resistance as the temperature rises.
As the arc gets hotter, its resistance drops, carrying more current. As the cycle continues, an arc flash can reach nearly 20,000 °C. Arc flash protection relays don’t prevent an arc flash but are used to reduce the energy in an arc flash rapidly. By reducing the severity of the flash, an arc flash relay can reduce the personnel protective equipment (PPE) rating of an installation from a very dangerous level 4, requiring rubberized clothing that does not conduct electricity and other precautions to a more easily managed PPE level 2.
Instead of trying to measure increases in current flow, arc flash relays use optical sensors to “see” the flash more quickly. Arc-flash relays are microprocessor-based devices that can detect the onset of a flash as quickly as 125 μs. Once a flash is detected, a fast-switching can IGBT trip a circuit breaker in a few milliseconds and stop the flash before it becomes too severe (Figure 4), Figure 4: An arc flash protection relay can respond in milliseconds to quench a building arc and protect equipment and personnel. (Image: Littelfuse ) Summary Protection relays are used to safeguard equipment and operators. They use parameters like current, voltage, resistance, temperature, or even light, to determine unsafe operating conditions and provide outputs like control warnings, alarms, and turning the power off.
What are 4 protective relays can be designed to respond to?
Free 10 Questions 30 Marks 10 Mins Protective Relays:
It is a sensing device. It senses the fault or given situation when the current attains a certain predetermined value. It knows its position and finally, it gives the tripping command to the circuit breaker. The protective relay gives the command to the circuit breaker to disconnect the faulted element. Relays can be designed to respond to changes in resistance, reactance, impedance, voltage, current, light intensity and temperature.
Additional Information Overload relay: An overload relay is an electrical device used to protect an electric motor from overheating. It is mainly three types. Thermal relay: Thermal overload relay is a protective device, and that is mainly designed to cut the power whenever the motor uses too much current for an extended time period.
- Once extreme current supplies throughout the motor circuit, then the relay gets open because of the improved temperature of the motor.
- Magnetic relay: Magnetic overload relay can be operated by detecting the magnetic field strength which is generated by the flow of current towards the motor.
- This relay can be built with a variable magnetic core within a coil that holds the motor current.
Solid-state relay: Solid-state relays are semiconductor equivalents of the electromechanical relay and can be used to control the electrical loads without the use of moving parts. Last updated on Apr 20, 2023 UPPSC AE Marksheet released. This is for the 2021 cycle.
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How do relays improve safety?
Defined safety functions – Safety relays can detect faults at the input contacts in the safety circuit in the event of an earth fault. The safe function is guaranteed by specific internal circuits and relay technology. Safety relays perform defined safety functions : for example, they ensure that a movement is stopped in a controlled and therefore safe manner, the position of moveable guards is monitored and a closing movement is interrupted when there is intervention.
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How are relays damaged?
Contact components – Contacts are electrical contact parts used by relays to switch loads. Some products’ contacts are press-fitted by riveting. The main drawbacks are loose contacts, cracks in the contacts, or excessive size and position deviation. This will affect the contact reliability of the relay.
- The faults of contact components generally include contact overheating, wear, and welding.
- The causes of contact overheating include insufficient capacity, insufficient contact pressure, surface oxidation or dirty, etc.; causes of wear include too small contact capacity, too high arc temperature to oxidize the contact metal, etc.; the causes of contact fusion welding are too high arc temperature, or serious jumping of contact.
Common contact failures: 1. Due to the mechanical engagement of the contacts (the needle-shaped protrusions and pits formed on the contacts bite each other), welding or cold welding, the phenomenon that they cannot be disconnected occurs.2. A phenomenon in which the circuit cannot be connected normally due to increased contact resistance and instability.3.
The contact cannot be open or close the circuit due to excessive load, small contact capacity, or change of load nature.4. Because the voltage is too high, or the contact distance becomes smaller, the contact gap re-breaks down.5. Because the frequency is too high, or the contact gap is too large, a fault that cannot accurately breaking the circuit occurs.6.
Due to various environmental conditions that do not meet the requirements, errors in contact work are caused.7. Because there is no arc extinguishing device or measure, or improper selection of parameters, the contacts are worn out or unnecessary interference is generated.
- How to deal with: Check the surface condition of the contact.
- If the contact surface is oxidized, the silver contact is not required to be repaired.
- The copper contact can be filed with a polished file or gently scraped off the surface oxide layer with an electrician’s knife.
- If the surface of the contact is not clean, it can be cleaned with gasoline or carbon tetrachloride; there are burn marks on the surface of the contact, which can be repaired with a polished file or a knife, but do not use emery cloth or sandpaper to polish it to avoid residual sand and cause poor contact.
If the contacts are welded, they should be replaced. If the contact capacity is too small, replace the relay with a larger capacity. If the contact pressure is not enough, the spring can be adjusted or replaced to increase the pressure. If the pressure is still insufficient, the contacts should be replaced.
What is fault protection?
Fault protection is defined as: Protection against electric shock under. single fault conditions. Fault protection provides protection. against persons or livestock coming.
What is the main function of a relay?
Machine tool relay – A machine tool relay is a type standardized for industrial control of machine tools, transfer machines, and other sequential control. They are characterized by a large number of contacts (sometimes extendable in the field) which are easily converted from normally open to normally closed status, easily replaceable coils, and a form factor that allows compactly installing many relays in a control panel.
- Although such relays once were the backbone of automation in such industries as automobile assembly, the programmable logic controller (PLC) mostly displaced the machine tool relay from sequential control applications.
- A relay allows circuits to be switched by electrical equipment: for example, a timer circuit with a relay could switch power at a preset time.
For many years relays were the standard method of controlling industrial electronic systems. A number of relays could be used together to carry out complex functions ( relay logic ). The principle of relay logic is based on relays which energize and de-energize associated contacts.
What are the two types of protection devices?
There are two general categories of circuit protection: 1) Fuses 2) Electro-mechanical circuit breakers.
What is basic protection system?
Components – Protection systems usually comprise five components
- Current and voltage transformers to step down the high voltages and currents of the electrical power system to convenient levels for the relays to deal with
- Protective relays to sense the fault and initiate a trip, or disconnection, order
- Circuit breakers or RCDs to open/close the system based on relay and autorecloser commands
- Batteries to provide power in case of power disconnection in the system
- Communication channels to allow analysis of current and voltage at remote terminals of a line and to allow remote tripping of equipment.
For parts of a distribution system, fuses are capable of both sensing and disconnecting faults, Failures may occur in each part, such as insulation failure, fallen or broken transmission lines, incorrect operation of circuit breakers, short circuits and open circuits.
Protection devices are installed with the aims of protection of assets and ensuring continued supply of energy. Switchgear is a combination of electrical disconnect switches, fuses or circuit breakers used to control, protect and isolate electrical equipment. Switches are safe to open under normal load current (some switches are not safe to operate under normal or abnormal conditions), while protective devices are safe to open under fault current.
Very important equipment may have completely redundant and independent protective systems, while a minor branch distribution line may have very simple low-cost protection. A digital (numeric) multifunction protective relay for distribution networks. A single such device can replace many single-function electromechanical relays, and provides self-testing and communication functions.
What is protection in a circuit?
What is circuit protection? – Circuit protection is essentially a failsafe that’s designed to protect people and properties if an electrical fault occurs. Every electrical circuit has a maximum voltage or amperage. If this is exceeded, the wire overheats, potentially causing the wire insulation to melt and fire to break out.
Similarly, if an earth fault, short circuit, or an incorrect installation occurs, or if a circuit is overloaded, the consequences are the same, and the risk of serious injury remains high. Circuit protection prevents this from happening by ‘tripping the circuit’ and automatically disconnecting the supply of electricity to that particular circuit.
When this happens, excessively high currents cannot flow through the wire and potentially cause overheating and fires.
What component is protecting this circuit?
Overcurrent Protection Components – Overcurrent protection devices include fuses (and their multiple subsets), PTC thermistors and NTC thermistors. Fuses, by nature, are overcurrent protection devices and are used for nothing else. They include a variety of configurations, but the core technology concept is that heat generated by an overcurrent event can change the molecular structure of a metal wire to separate the circuit from the system.
- PTC thermistors are used primarily for circuit protection now, an application which includes the ceramic and polymer versions of this technology.
- However, PTC thermistors are also used for additional applications, such as heater elements.
- NTC thermistors are used primarily as temperature sensors (an application where their use has grown substantially), but a minor portion of NTC thermistor demand is for inrush current limiting, primarily in motors and digital electronics.
Fuses – electronic circuit protection devices that protect against overcurrent by melting or “fusing” open its conductive element when an excessive current flows through it. In the field of electronic fuses, the configurations employed are typically miniature and micro fuses that employ cylindrical bodies with ferrule end caps, and which have a rating from several milliamperes to tens of amperes, with voltage ranges from a few volts to 600 volts.
- Larger industrial fuses are also available, designed to handle extremely high currents at high voltage.
- PTC Thermistors – these Positive Temperature Coefficient thermistors are used primarily for overcurrent protection in a variety of electronic equipment applications, but are also available in specialty switch applications for degaussing circuits in TV sets and PC monitors; for inrush current limiting in lighting ballasts; and in motor start circuits for compressor motors.
These devices undergo a large, abrupt increase in resistance when an overcurrent of high ambient temperature heats them above a specific, predetermined point. There are two basic technologies employed to produce PTC thermistors and switches. These include both ceramic and polymer technology platforms.
The ceramic platform is generally considered an offshoot of MLC capacitor manufacturing and is based upon doped barium titanate materials. Polymer thermistors are a radical alternative to ceramic PTC thermistors, and the technology is based upon polymer doped with carbon black. NTC Thermistors – these components demonstrate a decrease in resistance when subjected to an increase in body temperature – a negative temperature coefficient.
This makes them unique in their functionality compared to other electronic components. As an electronic protection component, NTC thermistors are typically viewed as thermal protection devices used for sensing and compensation. In many applications NTC thermistors compete against electromechanical thermostats; however, NTC thermistors are replacing the more complex thermostats in many applications, especially in automotive electronic subassemblies, because of their lower cost.
What are the 4 parts of a relay?
Step 1: Parts & Design of a Relay – IMAGE:
- Relay inside its Plastic Case.
- Relay separated from its case using a screwdriver.
- Parts of the Relay.
- Relay Leads which can be soldered to a PCB
- Parts of the Relay
Start by removing the Plastic or PVC case of the relay by using a screwdriver. You can see the design and various parts of the relay. The main parts of the relay are: Armature, Spring, Yoke, Contacts & Coil. A simple electromagnetic relay consists of a coil of wire wrapped around a soft iron core, an iron yoke which provides a low reluctance path for magnetic flux, a movable iron armature, and one or more sets of contacts (there are two in the relay pictured).
The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts. It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the relay pictured is closed, and the other set is open.
Other relays may have more or fewer sets of contacts depending on their function. The relay in the picture also has a wire connecting the armature to the yoke. This ensures continuity of the circuit between the moving contacts on the armature, and the circuit track on the printed circuit board (PCB) via the yoke, which is soldered to the PCB.
What are the 2 main parts of a relay?
How Relays Work Learn the basics of relays to understand the main parts, the different types as well as how they work. Scroll to the bottom to watch the YouTube tutorial. For all your relay needs, check out Tele controls, who have kindly sponsored this video.
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You can contact them via email at or through linkedin to receive your free relay configuration cheat sheet. For more information click A relay is an electrically operated switch. Relays, traditionally, use an electromagnet to mechanically operate the switch.
- However, newer versions will use electronics such as solid-state relays.
- Relay Relays are used where it is necessary to control a circuit using a low-power signal, or where several circuits must be controlled by one signal.
- Relays ensure complete electrical isolation between the controlling and controlled circuits.
Relays are often used in circuits to reduce the current that flows through the primary control switch. A relatively low amperage switch, timer, or sensor can be used to turn a much higher capacity load on and off. We’ll see examples of this a little later in the article.
- There are two main circuits in the relay.
- The primary side and the secondary side.
- Primary and Secondary The Primary Circuit provides the control signal to operate the relay.
- This could be controlled by a manual switch, a thermostat or some type of sensor.
- The primary circuit is generally connected to a low voltage DC supply.
The secondary circuit is the circuit that contains the load which needs to be switched and controlled. When we talk about a load, we mean any device that will consume electricity such as a fan, pump, compressor or even light bulb. Primary and Secondary explained On the primary side, we find an electromagnetic coil.
- This is a coil of wire which generates a magnetic field when current passes through it.
- When electricity passes through a wire it creates an electromagnetic field, we can see that by placing some compasses around the wire, when we pass a current through the wire the compasses change direction to align with the electromagnetic field.
When we wrap the wire into a coil, the magnetic field of each wire combines together to form a larger, stronger magnetic field. We can control this magnetic field by simply controlling the current. By the way, we have covered how solenoids work and even how to make your own solenoid in our previous article.
- Do check that out,
- At the end of the electromagnet we find the armature.
- This is a small component which is pivoted.
- When the electromagnet energises it attracts the armature.
- When the electromagnet is deenergised, the armature returns to its original position.
- Typically, a small spring is used to achieve this.
Connected to the armature is a moveable contactor. When the armature is attracted to the electromagnet, it closes and completes the circuit on the secondary side. Armature We have two types of basic relay, the normally open and normally closed type. There are other types of relays and we’re going to look at those later in the article.
With the normally open type, no electricity flows in the secondary circuit, the load is therefore off. However, when a current is passed through the Primary Circuit, a magnetic field is induced in the electromagnet. This magnetic field attracts the armature and pulls the moveable contactor until it touches the terminals of the secondary circuit.
This completes the circuit and provides electricity to the load. With the normally closed type. The secondary circuit is normally complete and so the load is on. When a current is passed through the primary circuit, the electromagnetic field causes the armature to push away which disconnects the contactor and breaks the circuit, this cuts the supply of electricity to the load.
The operation of Solid State Relays or SSR’s is similar in principal, but unlike electromechanical relays, it has no moving parts. The solid state relay uses the electrical and optical properties of solid-state semiconductors to perform its input and output isolation as well as switching functions. With this type of device, we find an LED light on the primary side, instead of an electromagnet.
The LED provides optical coupling by shining a beam of light across a gap and into the receiver of an adjacent photo sensitive transistor. We control the operation of this type by simply turning the LED on and off. SSR The phototransistor acts something like an insulator and doesn’t allow current to flow, unless it is exposed to light.
- Inside the phototransistor we have different layers of semiconductor materials.
- There are N-type and P-type, which are sandwiched together.
- The N-type and P-type are both made from silicon but they have each been mixed with other materials to change their electrical properties.
- The N-type has been mixed with a material that gives it lots of extra and un-needed electrons, which are free to move around to other atoms.
The P-type has been mixed with a material that has fewer electrons, so this side has lots of empty space which electrons can move to. When the materials are joined together, an electrical barrier develops and prevents the electrons from flowing. Phototransistor However, when the LED is turned on, it will emit another particle known as a photon.
- The photon hits the P-type material and knocks the electrons, pushing them across the barrier and into the N-type material.
- The electrons at the first barrier will now be able to also make the jump and so a current is developed.
- Once the LED is turned off, the photons stop knocking electrons across the barrier and so the current in the secondary side stops.
So, we can control the secondary circuit just by using a beam of light. There are many types of relays, we’re going to consider a few of the main ones as well as some simple examples of how they are used. As we have seen earlier in this article, we have the simple normally open relay.
- This means the load of the secondary side is off until the circuit is complete on the primary.
- We can use this for example to control a fan by using a bimetallic strip as a switch on the primary side.
- The bimetallic strip will bend as it increases in temperature, at a certain temperature it will complete the circuit and turn the fan on to provide some cooling.
Normally Open Relay We also have a normally closed relay. This means the load on the secondary side is normally on. We could for example control a simple pump system to maintain a certain water level in the storage tank. When the water level is low, the pump is on.
- But, once it reaches the limit we require, it completes the primary circuit and pulls the contactor away, which cuts the power to the pump.
- Normally Closed Simple Example In a standard, normally open, relay, once the primary circuit is de-energised, the electromagnetic field disappears and the spring pulls the contactor back to its original position.
Sometimes, we want the secondary circuit to remain live after the primary circuit is opened. For that we can use a latching relay. For example, when we press the call button on an elevator, we want the light on the button to remain on, so that the user knows the elevator is coming.
- So, we can use Latching Relays to do this.
- There are many different designs for this type of relay, but in this simplified example, we have 3 separated circuits and a piston which sits between them.
- The first circuit is the call button.
- The second is the lamp and the third is the reset circuit.
- Latching Relay When the call button is pressed, it completes the circuit and powers the electromagnet, this pulls the piston and completes the circuit to turn the lamp on.
A signal is also sent to the elevator controller to send the elevator down. The button is released, this cuts the power to the initial circuit, but as the piston isn’t spring loaded, it stays in position and the lamp remains on. Once the elevator car reaches the lower floor, it makes contact with the off switch.
- This powers the second electromagnet and pulls the piston away, cutting the power to the lamp.
- Latching relays therefore offer the benefit of having positional ‘memory’.
- Once activated, they will remain in their last position without the need for any further input or current.
- Relays can have single or double poles.
The term Pole refers to the number of contacts switched when the relay is energised. This allows more than one secondary circuit to be energised from a single primary circuit. We could for example, use a double pole relay to control a cooling fan and also a warning light.
- Both the fan and lamp are normally off, but when the bimetallic strip on the primary circuit gets too hot, it bends to complete the circuit.
- This creates the electromagnetic field and closes both contactors on the secondary side, this provides power to the cooling fan as well as the warning light.
- Double Pole When dealing with relays, you will often hear the term “throws”.
This refers to the number of contacts or connection points. A double throw relay combines a normally open and normally closed circuit. A double throw relay is also called a changeover relay, as it alternates, or changes, between two secondary circuits.
- In this example, when the primary circuit is open, the spring on the secondary side pulls the contactor to terminal B, powering the lamp.
- The fan remains off because the circuit is not complete.
- Double Throw When the primary side in energised, the electromagnet pulls the contactor to terminal A and diverts the electricity, this time powering the fan and turning the lamp off.
So, we can use this type of relay to control different circuits depending on an event. A double pole, double throw relay or DPDT is used to control 2 states on 2 separate circuits. Here we can see a DPDT relay. when the primary circuit is not complete, terminals T1 and T2 are connected to terminals B and D respectively.
- The red LED and the indicator light are energised.
- Double Pole Double Throw When the primary circuit is closed then T1 and T2 connect to terminals A and C, the fan turns on and the green LED is energised.
- Something we need to consider when working with electromagnets is the back EMF, or electro motive force.
When we power the coil, the electromagnetic field builds up to a maximum point, the magnetic field is storing energy. When we cut the power, the electromagnetic field collapses and releases this stored energy very quickly, this collapsing field continues to push the electrons, which is why we get the back EMF.
This isn’t a good thing because it can produce very large voltage spikes which damage our circuits. Suppressor Diode To overcome this, we can use something like a diode to supress this. The diode only allows current to flow in one direction, so in normal operation the current flows to the coil. But, when we cut the power, the back EMF is going to push the electrons and so the diode will now provide a path for the coil to dissipate its energy safely, so that it doesn’t damage our circuits.
: How Relays Work