How do 12v relays work

Relays

Photo: A typical relay with its plastic outer case removed. You can see the two spring contacts on the left and the electromagnet coil (the red-brown copper-colored cylinder) on the right. In this relay, when a current flows through the coil, it turns it into an electromagnet. The magnet pushes a switch to the left, forcing the spring contacts together, and completing the circuit they’re attached to. This is a relay from an electronic, hot-water immersion heater programmer. The electronic circuit in the programmer switches the magnet on or off at preprogrammed times of day using a relatively small current. That allows a very much bigger current to flow through the spring contacts to power the element that heats the hot water.

Contents

What are relays?

A relay is an electromagnetic switch operated by a relatively small electric current that can turn on or off a much larger electric current.

The heart of a relay is an electromagnet (a coil of wire that becomes a temporary magnet when electricity flows through it). You can think of a relay as a kind of electric lever: switch it on with a tiny current and it switches on («leverages») another appliance using a much bigger current. Why is that useful? As the name suggests, many sensors are incredibly sensitive pieces of electronic equipment and produce only small electric currents. But often we need them to drive bigger pieces of apparatus that use bigger currents. Relays bridge the gap, making it possible for small currents to activate larger ones. That means relays can work either as switches (turning things on and off) or as amplifiers (converting small currents into larger ones).

Artwork: If relays were dogs: Suppose you had a huge, ferocious dog that slept so soundly it never woke up when it heard a noise. It wouldn’t be much use as a guard dog! But what if you bought a small, very alert dog as well? If the small dog heard a noise, it would start barking and wake the big dog, which could then attack an intruder. This is how relays work: they use a small electric current to trigger a much bigger one.

How relays work

Here are two simple animations illustrating how relays use one circuit to switch on a second circuit.

When power flows through the first circuit (1), it activates the electromagnet (brown), generating a magnetic field (blue) that attracts a contact (red) and activates the second circuit (2). When the power is switched off, a spring pulls the contact back up to its original position, switching the second circuit off again.

This is an example of a «normally open» (NO) relay: the contacts in the second circuit are not connected by default, and switch on only when a current flows through the magnet. Other relays are «normally closed» (NC; the contacts are connected so a current flows through them by default) and switch off only when the magnet is activated, pulling or pushing the contacts apart. Normally open relays are the most common.

Here’s another animation showing how a relay links two circuits together. It’s essentially the same thing drawn in a slightly different way. On the left side, there’s an input circuit powered by a switch or a sensor of some kind. When this circuit is activated, it feeds current to an electromagnet that pulls a metal switch closed and activates the second, output circuit (on the right side). The relatively small current in the input circuit thus activates the larger current in the output circuit:

Relays in practice

Photo: Another look at relays. Top: Looking straight down, you can see the spring contacts on the left, the switch mechanism in the middle, and the electromagnet coil on the right. Bottom: The same relay photographed from the front.

Suppose you want to build an electronically operated cooling system that switches a fan on or off as your room temperature changes. You could use some kind of electronic thermometer circuit to sense the temperature, but it would produce only small electric currents—far too tiny to power the electric motor in a great big fan. Instead, you could connect the thermometer circuit to the input circuit of a relay. When a small current flows in this circuit, the relay will activate its output circuit, allowing a much bigger current to flow and turning on the fan.

Relays don’t always turn things on; sometimes they very helpfully turn things off instead. In power plant equipment and electricity transmission lines, for example, you’ll find protective relays that trip when faults occur to prevent damage from things like current surges. Electromagnetic relays similar to the ones described above were once widely used for this purpose. These days, electronic relays based on integrated circuits do the same job instead; they measure the voltage or current in a circuit and take action automatically if it exceeds a preset limit.

Other types of relays

Photo: Four old-fashioned overcurrent protective relays pictured at an obsolete power substation in 1986, shortly before its demolition. Photo by courtesy of US Library of Congress.

What we’ve looked at so far are very general switching relays—but there are quite a few variations on that basic theme, including (and this is by no means an exhaustive list):

Who invented relays?

Photo: Professor Joseph Henry, photographed some time between 1860 and 1875. Photo courtesy of Brady-Handy photograph collection, US Library of Congress, Prints and Photographs Division.

Relays were invented in 1835 by American electromagnetism pioneer Joseph Henry; in a demonstration at the College of New Jersey, Henry used a small electromagnet to switch a larger one on and off, and speculated that relays could be used to control electrical machines over very long distances. Henry applied this idea to another invention he was working on at the time, the electric telegraph (the forerunner of the telephone), which was successfully developed by William Cooke and Charles Wheatstone in England and (much more famously) by Samuel F. B. Morse in the United States.

Relays were later used in telephone switching and early electronic computers and remained hugely popular until transistors came along in the late 1940s; according to Bancroft Gherardi, marking the 100th anniversary of Henry’s work on electromagnetism, there were an estimated 70 million relays in operation in the United States alone by that time.

Photo: Relays were widely used for switching and routing calls in telephone exchanges such as this one, pictured in 1952. Photo by courtesy of NASA Glenn Research Center (NASA-GRC).

Transistors are tiny electronic components that can do a similar job to relays, working as either amplifiers or switches. Although they switch faster, use far less electricity, take up a fraction of the space, and cost much less than relays, they generally work with only tiny currents so relays are still used in many applications. It was the development of transistors that spurred on the computer revolution from the mid-20th century onward. But without relays, there would have been no transistors, so relays—and pioneers like Joseph Henry—deserve some of the credit too!

How a Relay Works – How to Connect N/O, N/C Pins

Last Updated on June 5, 2022 by Swagatam 37 Comments

An electrical relay consists of a electromagnet and a spring loaded changeover contacts. When the electromagnet is switched ON/OFF with a DC supply, the spring loaded mechanism is corresponding pulled and released by this electromagnet, enabling a changeover across the end terminals of these contacts. An external electrical load connected across these contacts are subsequently switched ON/OFF in response to relay electromagnet switching.

In this post we learn comprehensively regarding how relay works in electronic circuits, how to identify its pinouts of any relay through a meter and connect in circuits.

Introduction

Whether it’s for flashing a lamp, for switching AC motor or for other similar operations, relays are for such applications. However young electronic enthusiasts often become confused while assessing the pin outs of the relay and configuring them with a drive circuit inside the intended electronic circuit.

In this article we’ll study the basic rules that will help us to identify relay pinouts and learn regarding how a relay works. Let’s begin the discussion.

How a Relay Works

The working of an electrical relay can be learned from the following points:

The following rough simulation shows how the relay pole moves in response to the electromagnet coil when switched ON and OFF with an input supply voltage. We can clearly see that initially the central pole is held connected with the N/C contact, and when the coil is energized, the pole is pulled downwards due to the electromagnetic action of the coil, forcing the central pole to connect with the N/O contact.

Video Explanation

Thus basically there are three contact pinouts for a relay, namely the central pole, the N/C and the N/O.

The two additional pinouts are terminated with the coil of the relay

This basic relay is also called a SPDT type of relay meaning single pole double throw, since here we have a single central pole but two alternate side contacts in the form of N/O, N/C, hence the term SPDT.

Therefore in all we have 5 pinouts in an SPDT relay: the central movable or switching terminal, a pair of N/C and the N/O terminals and finally the two coil terminals which all together constitute a relays pin outs.

How to identify Relay Pinouts and Connect a Relay

Normally and unfortunately many relays don’t have there pinout marked, which makes it difficult for the new electronic enthusiasts to identify them and make these work for the intended applications.

The pinouts that needs to be identified are (in the given order):

The identification of a typical relays pinouts may be done in the following manner:

1) Position the multimeter in the Ohms range, preferably in the 1K range.

2) Begin by connecting the meter prods to any of the two pins of the relay randomly, until you find the pins which indicate some kind of resistance on th meter display. Typically this may be anything between 100 ohm and 500 Ohm. These pins of the relay would signify the coil pinouts of the relay.

3) Next, follow the same procedure and proceed by connecting the meter meter prods randomly to the remaining three terminals.

4) Keep doing this until you find two pins of the relay indicating a continuity across them. These two pinouts will be obviously the N/C and the pole of the relay, because since the relay is not powered the pole will be attached with the N/C due to internal spring tension, indicating a continuity across each other.

5) Now you need to simply identify the other single terminal which may be oriented somewhere in across the above two terminals representing a triangular configuration.

6) In most cases the central pinout from this triangular configuration would be your relay pole, the N/C is already identified and therefore the last one would be your relay’s N/O contact or pinout.

The following simulation shows how a typical relay may be wired with a DC voltage source across its coils and a mains AC load across its N/O and N/C contacts

These three contacts may be further confirmed by powering the relay coil with the specified voltage and by checking the N/O side with the meter for a continuity..

The above simple procedure could be applied for identifying any relay pinout which may be unknown to you, or unlabelled.

Now since we have thoroughly studied how a relay works and how to identify the pinouts of a relay, it would be also interesting to know the details of the most popular type of relay which is mostly used in small electronic circuits, and how to connect it.

If you want to know how to design and configure a relay driver stage using a transistor, you can read it in the following post:

A Typical Chinese Make Relay PinOuts

How to Wire Relay Terminals

The following diagram shows how the above relay may be wired with a load, such that when the coil is energized, the load gets triggered or switched ON through its N/O contacts, and through the attached supply voltage.

This supply voltage in series with the load may be as per the load specifications. If the load is rated at DC potential then this supply voltage could be a DC, if the load is supposed to be an AC mains operated then this series supply could be a 220V or 120V AC as per the specifications.

You’ll also like:

About Swagatam

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials.
If you have any circuit related query, you may interact through comments, I’ll be most happy to help!

Reader Interactions

Comments

Have Questions? Please post your comments below for quick replies! Comments should be related to the above artcile Cancel reply

Last two days I’m watching videos on youtube trying to understand the relay. Luckily I found your valuable post on the internet and I’m really benefited from your post here. Thanks for this post from you.

Thank you, and Glad you found the post useful!

Dear Sir,
What is the transistor and diode values and I want to use this open sliding gate (Ac motor) with Fingerprint(DC 12v) device so please tell me how can do it
Thanks

Hi Chandana,
For a relay the transistor can be a 2N2222, and the flyback diode across the relay coil can be a 1N4007 diode.

Sure, you are welcome!

Good day to you
How do i wire a relay(N/C, N/O, COM) with positive and negative also on relay, to battery(+,-) and switch(COM, N/O) to make a buzzer work when switch is pressed.

Hi, The relay connections to the buzzer through N/O contacts will be like this:

Hows does this 12v relay works with transistor voltage drop?

I’m working my way through «Make: Electronics» by Charles Platt and came across the following schematic:

As someone who just recently got started with electronics, what I can’t wrap my head around is how R1 relay works after Q1’s voltage drop. I have built the same circuit only with different power source and relay, both 5V instead of 12V as suggested by the book and what seems to be happening is that the emitter voltage is bellow the relay trigger voltage (12V in the schematics and 5V in my case since I’m using 5V relay and source). From what I have been reading around the internet and communities like this, the emitter voltage in this configuration is supposed to be limited to the base voltage minus

0.4V or so. Due to the 10K resistor, once the transistor is triggered, the base voltage will be under 12V, thus the emitter voltage will also be under 12V and the relay will not close. That’s basically how my thoughts are at the moment and I need help spotting my mistakes.

Edit: I believe this question is being misunderstood. I’m not asking how a self-locking relay works. I’m asking how a 12V relay is being triggered when the voltage on its coil (from Q1 emitter) is bellow 12V (I’m checking it with a multimeter and theres a voltage drop across the transistor).

From a small traffic signal controller to a complex high voltage switchyard, relays can be found everywhere. To put it in general, relays are just like any other switch which can either make or break a connection, that is it can either connect two points or disconnect it, therefore relays are commonly used to turn on or off an electronic load. But, this is a very generalized statement, there are many types of relays and each relay behaves differently as required for its application, one of the most popularly used relays is the electromechanical relay and hence we will focus more on that for this article. Despite the differences in construction, the basic working principle of a relay is the same, so lets discuss more on basic relay operation and take a deeper look into its construction

What is Relay?

A Relay is an electromechanical device that can be used to make or break an electrical connection. It consists of a flexible moving mechanical part which can be controlled electronically through an electromagnet, basically, a relay is just like a mechanical switch but you can control it with an electronic signal instead of manually turning it on or off. Again this working principle of relay fits only for the electromechanical relay.

There are many types of relay and each relay has its own application, a standard, and generally used relay is made up of electromagnets which in general used as a switch. Dictionary says that relay means the act of passing something from one thing to another, the same meaning can be applied to this device because the signal received from one side of the device controls the switching operation on the other side. So relay is a switch which controls (open and close) circuits electromechanically. The main operation of this device is to make or break contact with the help of a signal without any human involvement in order to switch it ON or OFF. It is mainly used to control a high powered circuit using a low power signal. Generally, a DC signal is used to control the circuit which is driven by high voltage like controlling AC home appliances with DC signals from microcontrollers.

Construction of Relay and its operation:

The following figure shows how a Relay looks internally and how it can be constructed,

On a casing, a core with copper windings (forms a coil) winded on it is placed. A movable armature consists of a spring support or stand like structure connected to one end, and a metal contact connected to another side, all these arrangements are placed over the core such that, when the coil is energized, it attracts the armature. The movable armature is generally considered as a common terminal which is to be connected to the external circuitry. The relay also has two pins namely normally closed and normally opened (NC and NO), the normally closed pin is connected to the armature or the common terminal whereas the normally opened pin is left free (when the coil is not energized). When the coil is energized the armature moves and is get connected to the normally opened contact till there exists flow of current through the coil. When it is de-energized it goes to its initial position.

The general circuit representation of the relay is as shown in the figure below

An electromechanical relay is basically designed using few mechanical parts like Electromagnet, a movable armature, contacts, yoke, and a spring/frame/stand, these parts are showing in the internal pictures of Relay below. All these are arranged logically to form into a relay.

Here we have explained the internal mechanical parts of a Relay:

Electromagnet: An Electromagnet plays a major role in the working of a relay. It is a metal which doesn’t have magnetic property but it can be converted into a magnet with the help of an electrical signal. We know that when current passes through the conductor it acquires the properties of a magnet. So, when a metal winded with a copper wire and driven by the sufficient power supply, that metal can act as a magnet and can attract the metals within its range.

Movable Armature: A movable armature is a simple metal piece which is balanced on a pivot or a stand. It helps in making or breaking the connection with the contacts connected to it.

Contacts: These are the conductors that exist within the device and are connected to the terminals.

Yoke:It is a small metal piece fixed on a core in order to attract and hold the armature when the coil is energized.

Spring (optional):Few relays don’t need any spring but if it is used, it is connected to one end of the armature to ensure its easy and free movement. Instead of a spring, a metal stand like structure can be used.

Relay Working Principle

Now let’s understand how a relay works in a normally closed condition and normally open condition.

Relay in NORMALLY CLOSED condition:

When no voltage is applied to the core, it cannot generate any magnetic field and it doesn’t act as a magnet. Therefore, it cannot attract the movable armature. Thus, the initial position itself is the armature connected in normally closed position (NC).

Relay in NORMALLY OPENED condition:

When sufficient voltage is applied to the core it starts to create a magnetic field around it and acts as a magnet. Since the movable armature is placed within its range, it gets attracted to that magnetic field created by the core, thus the position of the armature is being altered. It is now connected to the normally opened pin of the relay and external circuit connected to it function in a different manner.

Note: The functionality of the external circuit depends upon the connection made to the relay pins.

So finally, we can say that when a coil is energized the armature is attracted and the switching action can be seen, if the coil is de-energized it loses its magnetic property and the armature goes back to its initial position.

You can check the live working of the Relay in below given animation:

Different Types of Relay:

Other than the Electromagnetic relay there are many other types of relays that work on different principles. Its classification is as follows

Types of Relay Based on the principle of operation

Electrothermal relay:

When two different materials are joined together it forms into a bimetallic strip. When this strip is energized it tends to bend, this property is used in such a way that the bending nature makes a connection with the contacts.

Electromechanical relay:

With the help of few mechanical parts and based on the property of an electromagnet a connection is made with the contacts.

Solid State relay:

Instead of using mechanical parts as in electrothermal and electromechanical relays, it uses semiconductor devices. So, the switching speed of the device can be made easier and faster. The main advantages of this relay are its more life span and faster switching operation compared to other relays.

Hybrid relay:

It is the combination of both electromechanical and solid-state relays.

Types of Relay Based on the polarity:

Polarized relay:

These are similar to the electromechanical relays but there exists both permanent magnet and electromagnet in it, the movement of the armature depends on the polarity of the input signal applied to the coil. Used in telegraphy applications.

Non-polarized relay:

The coil in these relays doesn’t have any polarities and its operation remains unchanged even if the polarity of the input signal is altered.

Pole and Throw combinations:

Switches can also be classified based on the number of pole and throw combinations. A pole can be considered as an input terminal and a movable part connected to it, whereas a throw can be considered as an output terminal. Its classification is as follows

Single pole, single-throw Relay (SPST):

It consists of only one pole and one throw. Generally, the path is either closed or opened (remains untouched to any terminal). A push button is the best example of this type. When we push the button, the contact is in the closed position and when released the contact is in the open position, which can be understood from the below image.

Single pole, double throw Relay (SPDT):

This type of switches consists of only one pole but has two throws. So, the contact is always made to either of the terminals. A slide switch can be considered as its example. The slider is always connected to either of the contacts i.e., a closed path always exists all the time if both the terminals are connected to a circuit.

Double pole, single throw Relay (DPST):

It has two poles and a throw. The contacts of it are either opened or closed which is done simultaneously. Toggle switch works on this property. When the switch is toggled from one position to another, both the contacts are moved simultaneously.

Double pole, double throw Relay (DPDT):

This type of switches has two poles but the individual pole has two throws. So, it is named as double throw and the switching action is done similarly and simultaneously for both the poles. A switch on a standard trimmer is of DPDT because while we are charging the trimmer and when the switch on the trimmer is in the ON state, it automatically stops charging means the switches are internally opened in the charging circuit.

Applications of Relay:

The applications of the relay are limitless, its main function is to control the high voltage circuit (230V circuit AC) with the low voltage power supply (a DC voltage).

Understanding Relays & Wiring Diagrams

What is a Relay and How Does it Work?

A relay is an electrically operated switch. They commonly use an electromagnet (coil) to operate their internal mechanical switching mechanism (contacts). When a relay contact is open, this will switch power ON for a circuit when the coil is activated.

The example relay diagrams below show how a relay works.

We stock a large range of relays & relay holders all available for purchase directly from our website.

Why use a Relay?

Why Use a Relay in a Car?

How to Understand the Electrical Specifications of a Relay

Why are Protection Devices used in a Relay?

Sample Wiring Diagrams for a 4 Pin Normally Open Relay

What’s the difference between 4 or 5 pin Relays?

The difference between a 4 and 5 pin relay is that a 4 pin relay is used to control a single circuit, whereas a 5 pin relay switches power between two circuits.

4 Pin Relay
4 pin relays use 2 pins (85 & 86) to control the coil and 2 pins (30 & 87) which switch power on a single circuit. There are 2 types of 4 pin relay available; normally open or normally closed. A normally open relay will switch power ON for a circuit when the coil is activated. A normally closed relay will switch power OFF for a circuit when the coil is activated.

5 Pin Relay
5 pin relays provide 2 pins (85 & 86) to control the coil and 3 pins (30, 87 & 87A) which switch power between two circuits. They have both normally open and normally closed connection pins. When the coil is activated, power will be switched from the normally closed pin to the normally open pin.