Since relays are switches, the terminology applied to switches is also applied to relays; a relay switches one or more poles, each of whose contacts can be thrown by energizing the coil. 1) Normally-Open(NO): The circuit is disconnected i.e. Open when the relay is inactive.
Relay Working
In this article, the basics of a relay like energized relay and de-energized relay are explained in detail. Also, the design, construction, working, applications, and also relay selection is explained in detail.
What is a relay?
We know that most of the high end industrial application devices have relays for their effective working. Relays are simple switches which are operated both electrically and mechanically. Relays consist of an electromagnet and also a set of contacts. The switching mechanism is carried out with the help of the electromagnet. There are also other operating principles for its working. But they differ according to their applications. Most of the devices have the application of relays.
Why is a relay used?
The main operation of a relay comes in places where only a low-power signal can be used to control a circuit. It is also used in places where only one signal can be used to control a lot of circuits. The application of relays started during the invention of telephones. They played an important role in switching calls in telephone exchanges. They were also used in long distance telegraphy. They were used to switch the signal coming from one source to another destination. After the invention of computers they were also used to perform Boolean and other logical operations. The high end applications of relays require high power to be driven by electric motors and so on. Such relays are called contactors.
TAKE A LOOK : TYPES OF RELAYS
TAKE A LOOK : HOW TO TEST A RELAY
Relay Design
There are only four main parts in a relay. They are
- Electromagnet
- Movable Armature
- Switch point contacts
- Spring
The figures given below show the actual design of a simple relay.
It is an electro-magnetic relay with a wire coil, surrounded by an iron core. A path of very low reluctance for the magnetic flux is provided for the movable armature and also the switch point contacts. Â The movable armature is connected to the yoke which is mechanically connected to the switch point contacts. These parts are safely held with the help of a spring. The spring is used so as to produce an air gap in the circuit when the relay becomes de-energized.
How relay works?
The relay function can be better understood by explaining the following diagram given below.
Relay Design
The diagram shows an inner section diagram of a relay. An iron core is surrounded by a control coil. As shown, the power source is given to the electromagnet through a control switch and through contacts to the load. When current starts flowing through the control coil, the electromagnet starts energizing and thus intensifies the magnetic field. Thus the upper contact arm starts to be attracted to the lower fixed arm and thus closes the contacts causing a short circuit for the power to the load. On the other hand, if the relay was already de-energized when the contacts were closed, then the contact move oppositely and make an open circuit.
As soon as the coil current is off, the movable armature will be returned by a force back to its initial position. This force will be almost equal to half the strength of the magnetic force. This force is mainly provided by two factors. They are the spring and also gravity.
Relays are mainly made for two basic operations. One is low voltage application and the other is high voltage. For low voltage applications, more preference will be given to reduce the noise of the whole circuit. For high voltage applications, they are mainly designed to reduce a phenomenon called arcing.
Relay Basics
The basics for all the relays are the same. Take a look at a 4 – pin relay shown below. There are two colours shown. The green colour represents the control circuit and the red colour represents the load circuit. A small control coil is connected onto the control circuit. A switch is connected to the load. This switch is controlled by the coil in the control circuit. Now let us take the different steps that occour in a relay.
- Energized Relay (ON)
As shown in the circuit, the current flowing through the coils represented by pins 1 and 3 causes a magnetic field to be aroused. This magnetic field causes the closing of the pins 2 and 4. Thus the switch plays an important role in the relay working. As it is a part of the load circuit, it is used to control an electrical circuit that is connected to it. Thus, when the electrical relay in energized the current flow will be through the pins 2 and 4.
Energized Relay (ON)
- De – Energized Relay (OFF)
As soon as the current flow stops through pins 1 and 3, the relay switch opens and thus the open circuit prevents the current flow through pins 2 and 4. Thus the relay becomes de-energized and thus in off position.
In simple, when a voltage is applied to pin 1, the electromagnet activates, causing a magnetic field to be developed, which goes on to close the pins 2 and 4 causing a closed circuit. When there is no voltage on pin 1, there will be no electromagnetic force and thus no magnetic field. Thus the switches remain open.
Pole and Throw
Relays have the exact working of a switch. So, the same concept is also applied. A relay is said to switch one or more poles. Each pole has contacts that can be thrown in mainly three ways. They are
- Normally Open Contact (NO) – NO contact is also called a make contact. It closes the circuit when the relay is activated. It disconnects the circuit when the relay is inactive.
- Normally Closed Contact (NC) – NC contact is also known as break contact. This is opposite to the NO contact. When the relay is activated, the circuit disconnects. When the relay is deactivated, the circuit connects.
- Change-over (CO) / Double-throw (DT) Contacts – This type of contacts are used to control two types of circuits. They are used to control a NO contact and also a NC contact with a common terminal. According to their type they are called by the names break before make and make before break contacts.
Relays can be used to control several circuits by just one signal. A relay switches one or more poles, each of whose contacts can be thrown by energizing the coil.
Relays are also named with designations like
- Single Pole Single Throw (SPST) – The SPST relay has a total of four terminals. Out of these two terminals can be connected or disconnected. The other two terminals are needed for the coil to be connected.
- Single Pole Double Throw (SPDT) – The SPDT relay has a total of five terminals. Out of these two are the coil terminals. A common terminal is also included which connects to either of two others.
- Double Pole Single Throw (DPST) – The DPST relay has a total of six terminals. These terminals are further divided into two pairs. Thus they can act as two SPST’s which are actuated by a single coil. Out of the six terminals two of them are coil terminals.
- Double Pole Double Throw (DPDT) – The DPDT relay is the biggest of all. It has mainly eight relay terminals. Out of these two rows are designed to be change over terminals. They are designed to act as two SPDT relays which are actuated by a single coil.
Relay Applications
- A relay circuit is used to realize logic functions. They play a very important role in providing safety critical logic.
- Relays are used to provide time delay functions. They are used to time the delay open and delay close of contacts.
- Relays are used to control high voltage circuits with the help of low voltage signals. Similarly they are used to control high current circuits with the help of low current signals.
- They are also used as protective relays. By this function all the faults during transmission and reception can be detected and isolated.
Application of Overload Relay
Overload relay is an electro-mechanical device that is used to safeguard motors from overloads and power failures. Overload relays are installed in motors to safeguard against sudden current spikes that may damage the motor. An overload relay switch works in characteristics with current over time and is different from circuit breakers and fuses, where a sudden trip is made to turn off the motor.
The most widely used overload relay is the thermal overload relay where a bimetallic strip is used to turn off the motor. This strip is set to make contact with a contactor by bending itself with rising temperatures due to excess current flow. The contact between the strip and the contactor causes the contactor to de-energize and restricts the power to the motor, and thus turns it off.
The most widely used overload relay is the thermal overload relay where a bimetallic strip is used to turn off the motor. This strip is set to make contact with a contactor by bending itself with rising temperatures due to excess current flow. The contact between the strip and the contactor causes the contactor to de-energize and restricts the power to the motor, and thus turns it off.
Another type of overload motor is the electronic type which continuously watches the motor current, whereas the thermal overload relay shuts off the motor depending on the rise of temperature/heat of the strip.
All overload relays available to buy comes in different specifications, the most important of them being the current ranges and response time. Most of them are designed to automatically reset to work after the motor is turned back on.
Relay Selection
You must note some factors while selecting a particular relay. They are
- Protection – Different protections like contact protection and coil protection must be noted. Contact protection helps in reducing arcing in circuits using inductors.  Coil protection helps in reducing surge voltage produced during switching.
- Look for a standard relay with all regulatory approvals.
- Switching time – Ask for high speed switching relays if you want one.
- Ratings – There are current as well as voltage ratings. The current ratings vary from a few amperes to about 3000 amperes.  In case of voltage ratings, they vary from 300 Volt AC to 600 Volt AC. There are also high voltage relays of about 15,000 Volts.
- Type of contact used – Whether it is a NC or NO or closed contact.
- Select Make before Break or Break before Make contacts wisely.
- Isolation between coil circuit and contacts
Easy step by step procedure to test an automotive relay, this information pertains to all relay controlled circuits.
Difficulty Scale: 4 of 10
Step 1 - A relay is used to control (switch) a high amperage electrical circuit with a low amperage one, for example a radiator fan can pull up to 25 amps when in use, which would burn the computer circuit that controls it, a relay is used to bridge this circuit to prevent electrical damage.
Step 2 - Before beginning tests, use a test light and check all fuses and replace any that have failed. - Learn more
Step 3 - Many vehicles supply relay location and identification information on the lid of the PDC. If this information is not available, check the owners manual or Google Images
Step 4 - Once the relay has been identified, gently grasp or touch the relay in question, have a helper turn the ignition key to the on position, then crank the engine over, the relay should click in one of the ignition switch positions. If so the trigger circuit of the relay electrical system is working, if not continue to next step. ( Note: If the relay clicks and the circuit is still not working there is a good chance the contacts inside the relay have shorted.)
Step 5 - Next, remove the relay for inspection, grasp the relay and pull outward while slightly wiggling the relay housing, note the orientation of the relay, it must be installed the correct direction.
Step 6 - Once the relay has been removed, inspect the relay terminals for signs of extreme heat or corrosion.
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Step 7 - The relay is mounted in four electrical terminals housed in plastic and when overheated (due to overload or resistance caused by a poor connection) can distort and melt.
Step 8 - Use as small metal scribe or tool to scrape clean any corrosion to ensure a good connection once the new relay is installed.
Step 9 - Most relay's describe the internal circuit by an illustration on the side of the relay.
Relay Terminal Identification
- Terminals 86 and 85 are the primary side of the relay, which utilizes an electromagnet to close (connect) the secondary electrical circuit inside the relay. This electromagnet is activated by a simple power (+) and ground (-) much like a light bulb circuit.
- Terminals 87 and 30 are the secondary side of the relay which acts as the 'switch' that connects electrical current from one terminal to the other.
- Terminal 87a is not widely used and does not need to be connected for the relay to operate. 87a can be used for many different things such as relay activation monitoring or connecting a separate circuit that uses power when the relay is not in use.
Step 10 - Each terminal is identified at the relay base.
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Step 11 - To test the trigger or primary side of the relay set up a test light by connecting a scribe to a test light clamp, being illustrated in the picture below while testing the tail light socket.
Step 12 - Once the test light is set up connect each end to terminals 86 and 85. Now start the vehicle and operate the accessory switch, the test light should illuminate, if not the switch or circuit ground has shorted. ( Note: if the circuit is computer controlled a delay could be programmed into the operation of the accessory, additionally if a cooling fan is being tested the engine must reach operating temperature before the computer will trigger the circuit.) Use the test light grounded to check for power, and then switch the test light lead to the power side of the battery to check for circuit ground.
Step 13 - Next, use a piece of wire automotive wire (20 to 16 gauge) and strip both ends exposing the copper wire. Turn the ignition to the 'ON' position and jump terminals 87 and 30 the relay is now jumped and the accessory should activate, if so the relay has failed. If no power is observed at either 87 or 30, the fusible link or maxi fuse has shorted. Example: Testing the radiator cooling fan relay, the cooling fan should be operating.
Step 14 - When replacing a relay be sure to match up the terminal location from the old relay to to the new unit.
Step 15 - When installing the new relay be sure the orientation is correct or the relay will not work.
![Medications Medications](http://electricalacademia.com/wp-content/uploads/2017/08/11-pin.gif)
Step 16 - Once proper relay operation has resumed, reinstall the relay (PDC) cover.
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Additional Testing
When a particular two wire accessory is not operating, use a grounded test light to check for power at the wiring harness (either wire). If there is no power, the electrical system must be tested starting with the fuse then the relay. If power is present use the test light connected to battery power to check the ground circuit. if these tests check out, the accessory is bad and needs to be replaced.
Some relay's can differ from this configuration but follow the same principle. To confirm the wiring configuration consult a wiring diagram from Google Images or a service manual.
Intermitted Failure
Step 1 - To test for intermitted relay failures which are common, remove the relay in question, take a small wire strand, about two inches long and insert it into the relay connector terminal 87 or 30.
Step 2 - Next, reinstall the relay while keeping the wire strand inserted and clear of any other terminals or metal (ground).
Step 3 - With the wire strand secured in the relay terminal, attach the wire to a small automotive bulb and socket and ground. ( Note: A side marker bulb and socket work great because of its small size, make the wires long enough so the bulb can be seen while driving.)
Step 4 - Temporarily mount the small bulb in a visible area to be seen while driving, masking taped to the hood or dash works well.
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Step 5 - This test can be used to test all aspects of the wiring circuitry by moving the bulb ground to power. The bulb will now illuminate when the relay is use, and will go out to signal a failure.
Helpful Information
A relay is switch that utilizes an electrical trigger signal to activate. Once activated the relay connects an electrical supply to a particular accessory. These accessories can range from the main computer PCM (powertrian control module), radiator fan, fuel pump, door locks etc. There are two tests that should be considered when dealing with a relay problem, is the problem with the relay itself or is the problem a power or ground issue. A relay is prone to failure when used for a long periods of time (hot) or when the amperage of the accessory has increased beyond its designed use.
A rely should be considered as two separate halves, the primary side which utilizes an electromagnet to close the secondary electrical circuit. This electromagnet is activated by a simple power (+) and ground (-) much like a light bulb circuit. The second half of the relay is the 'switch' that controls power to a particular accessory like a fuel pump or ignition system.
In short, when the primary side of the relay (electromagnet) is activated, it closes the contacts (switch) to supply power to operate the accessory.
Tools and Supplies Needed
![Common Pin Relay Common Pin Relay](/uploads/1/2/5/8/125863591/780475061.png)
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- Test light
- Small piece of automotive wire
- Small automotive bulb and socket
Common Problems:
- When a relay warm up as in normal operation, the electrical contacts inside the relay can short circuit causing the electrical flow to stop, when the relay contacts cool it will resume the flow of electricity.
- When excessive amperage has been drawn through a relay circuit it can cause the relay contacts to 'stick' not allowing the power to be shut off to the accessory. Example: When an ABS system motor ages it will draw excessive amperage causing the control relay to 'stick'. This condition will run down the battery until corrected.
- Moister can get inside a relay hindering the relay operation.
- When testing relay circuits for power, ground is accidentally contacted causing the fuse to fail.