Introduction: All You Need to Know About a Relays

Picture of All You Need to Know About a Relays

What is a Relay?

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.

Step 1: Parts & Design of a Relay

Picture of Parts & Design of a Relay

IMAGE:

  1. Relay inside its Plastic Case.
  2. Relay separated from its case using a screwdriver.
  3. Parts of the Relay.
  4. Relay Leads which can be soldered to a PCB
  5. 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.

Step 2: Working of a Relay

Picture of Working of a Relay

IMAGE:

  1. Armature & Insulated Coil of Relay.
  2. Relay without Insulated Coil.
  3. Contacts Of the Relay when No current is Applied across the terminals of the relay.
  4. Contacts Of the Relay when Current is Applied across the terminals of the relay.
  5. Spring of the Relay.

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.

When an electric current is passed through the coil it generates a magnetic field that activates the armature, and the consequent movement of the movable contact(s) either makes or breaks (depending upon construction) a connection with a fixed contact. If the set of contacts was closed when the relay was de-energized, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open. When the current to the coil is switched off, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position. Usually this force is provided by a spring, but gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low-voltage application this reduces noise; in a high voltage or current application it reduces arcing. When the coil is energized with direct current, a diode is often placed across the coil to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate a voltage spike dangerous to semiconductor circuit components. Some automotive relays include a diode inside the relay case. For instance when a relay switches in your car the voltage spike can cause interference on the radio, and if you have a faulty battery or are silly enough to disconnect it with the engine running it can damage the ECU etc.

Step 3: Pole & Throw of a Relay

Picture of Pole & Throw of a Relay

IMAGE: 1. Circuit symbols of relays. (C denotes the common terminal in SPDT and DPDT types.)

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 bethrown by energizing the coil in one of three ways:

Normally-open (NO) contacts connect the circuit when the relay is activated; the circuit is disconnected when the relay is inactive. It is also called Form A contact or "make" contact. NO contacts may also be distinguished as "early-make" or NOEM, which means that the contacts close before the button or switch is fully engaged.

Normally-closed (NC) contacts disconnect the circuit when the relay is activated; the circuit is connected when the relay is inactive. It is also called Form B contact or "break" contact. NC contacts may also be distinguished as "late-break" or NCLB, which means that the contacts stay closed until the button or switch is fully disengaged.

Change-over (CO), or double-throw (DT), contacts control two circuits: one normally-open contact and one normally-closed contact with a common terminal. It is also called a Form C contact or "transfer" contact ("break before make"). If this type of contact utilizes a "make before break" functionality, then it is called a Form D contact.

The following designations are commonly encountered:

SPST – Single Pole Single Throw. These have two terminals which can be connected or disconnected. Including two for the coil, such a relay has four terminals in total. It is ambiguous whether the pole is normally open or normally closed. The terminology "SPNO" and "SPNC" is sometimes used to resolve the ambiguity.

SPDT – Single Pole Double Throw. A common terminal connects to either of two others. Including two for the coil, such a relay has five terminals in total.

DPST – Double Pole Single Throw. These have two pairs of terminals. Equivalent to two SPST switches or relays actuated by a single coil. Including two for the coil, such a relay has six terminals in total. The poles may be Form A or Form B (or one of each).

DPDT – Double Pole Double Throw. These have two rows of change-over terminals. Equivalent to two SPDT switches or relays actuated by a single coil. Such a relay has eight terminals, including the coil.

Step 4: Change-over (CO) or Double-throw (DT) Relay

Picture of Change-over (CO) or Double-throw (DT) Relay

A Change Over type relay is much like a Single Pole Double Throw( SPDT) relay.

Inorder to explain the working of a Change Over Relay, I have compared it to a SPDT relay.

A SPDT relay configuration switches one common pole to two other poles, flipping between them. Consider a SPDT relay with a common pole 'C' and let the other two poles be 'A' and 'B' respectively. When the coil is not powered(inactive), the common pole' C' is connected to the pole 'A'(NC) and is in resting position. But when the relay is powered(active) the common pole 'C' is connected to pole 'B'(NO) and isn't in resting position. Hence only one position is resting position while the other position needs the coil to be powered.

Step 5: Voltage & Current Parameters of a Relay

Picture of Voltage & Current Parameters of a Relay

IMAGE: 1. Voltage & Current Parameters of the relay in-scripted on the Case of the relay.

2. Voltage & Current Parameters of the relay in-scripted on the Case of the relay.

Most relays are available in different operating voltages like 5V, 6V, 12V, 24V, etc. If the required operating voltage is supplied to the relay, the relay is activated. The operating voltage of a relay is generally in DC.Small signal relays and low voltage power relays are usually in DC, but mains control relays and contactors quite frequently have AC coils.The rest of the terminals of a relay are used to connect either a AC(generally 50/60Hz) or DC circuit. The switching and contact pins of the relay have their respective Maximum voltage and current ratings/Parameters. These Parameters are generally in-scripted on the plastic or PVC case of the relay.On the contact ratings, they will frequently have something like 5A@250VAC / 10A@12VDC. These are the figures you have to be within. Having said that you can run a higher current than stamped on it if your voltage is lower, they aren't directionally proportional though and the datasheet for the relay should be consulted. If a relay is overloaded, it can burn out and damage the circuit or appliances connected to it. Be sure to choose a relay that can handle your voltage and current requirements to ensure the relay coil doesn't burn out and your circuit doesn't get damaged.

Step 6: RECYCLE AND REUSE OLD RELAYS

  1. Relays Can be desoldered from any old or exisiting circuit and can be re-soldered/ Soldered back on any new circuit or project since relays do not get burnt by excessive soldering.

2. The Windings of the coil can be reused as Jumper wire in Various Circuits.

3. The contacts and Screws, Nuts, Bolts, Washers of the relay can also be reused.

If you like this instructable feel free to vote for it. Follow me on instructables so you can get updates to any of my other instructables. Post queries and questions in the comments section below and I will certainly answer all of them. Thanks for reading.

Comments

Lifeseeker (author)2017-10-31

Thank you for posting this instructible. It did clear a few doubts that I had.

Again thank youLuis

Andy handy (author)2017-08-13

hello you guys. Can you help? I am trying to upgrade my caravan amber side lights to work as daytime running lights and indicator lights all in one. I thought I could do it via a couple of relays. My idea was to have one relay working the right hand side of the van and one relay on the other side. But of course this doesn't work for multiple reasons. Would be grateful if anybody can suggest a way, and maybe provide a drawing. Thank you in anticipation.

QuantumW1 (author)2017-05-23

Hi, um, I have so much of these lying around in my house but I don't know what to do with them since I don't know electronics (even the basics). Any suggestions? Maybe I can make something out of them that is not related to circuits/electronics?

GautamR22 (author)2017-03-20
I am confused about relay. which relay to use. I have
inductive sensor as input with nominal voltage is 10 to 30
volt and nominal current around 250 mA and in output
circuit I have four electromagnet in parallel which require
24 volt and 0.5 A each so 2A in total. so, can you help me
with my problem.

Thank you in advance.
OachM (author)2016-11-25

why they made transparent case for relay? I knew there is not transparent one with same structure inside the case. Which one should I choose ?

techydiy (author)OachM2017-02-01

The transparent case is so that you can inspect the contacts for wear and perform maintenance before failure.

nayeem_ruet_eee (author)2015-11-25

How can I find out which relay is required for my dc motor?
Relay is used only for dc motor?
Thanks.

fred_ (author)nayeem_ruet_eee2016-02-07

One that is DC rated for the voltage you're going to power it with and locked rotor current rating the motor draws at that voltage. If you're going to energize the relay coil with an electronic doodad instead of a switch put a reversed bias diode across the coil so you don't fry your controller when the relay turns off. The other thing to look at is what the coil wants for voltage and current. Your doodad might need a little help with a transistor on its output to power the relay coil.

It kinda comes down to how big the motor is, what voltage it wants, and what voltage you have to power the relay coil.

If its a toy motor (mabuchi sp?) type at say 12 volts, then any 12 volt automotive parts place relay and pretty much any available switch in your parts bin should work well enough. If it's smaller than that say a 6 volt motor using flashlight batteries for power look at smaller relay that use 5 volts for coil power. Smaller than that say a cell phone vibrator motor you could use a reed relay in it's power line an just put a magnet near it to turn it on.

Jonathanrjpereira (author)fred_2016-02-07

Good Information. Thanks

fred_ (author)nayeem_ruet_eee2016-02-07

I'm new to instructables I didn't see a way to edit a comment any who...

Most relays you come across are going to expect DC to power the coil and not care which way you hook them up. (exception those with coil diode "snubbers" built it)

The contacts (the same for switch contacts) don't care AC or DC when they are closed or open. The issue is when they are opening or closing. DC relies on the opening gap to stop the flow. AC helps this along as it was going to hit zero as it oscillates anyway. IOW typically a switch or a relay is derated for DC as the arc lasts longer and tends to pit the contacts more. For fooling around with small motors and low voltage have fun and go play those details don't matter too much :)

The motors current consumption should be within the current parameters of the Relay. Otherwise the relay could get damaged.

ybaggi (author)2014-05-22

So in the "Change Over" type. Does either of the positions need the coil to be powered or does tthe coil simply need to be activated a shot period of time to switch over to the other contact?

In other words, are both positions resting positions that don't need power through the coil to stay?

thanks

Jonathanrjpereira (author)ybaggi2014-05-23

A Change Over type relay is much like a Single Pole Double Throw( SPDT) relay. A SPDT relay configuration switches one common pole to two other poles, flipping between them. Consider a SPDT relay with a common pole 'A' and let the other two poles be 'B' and 'C' respectively. When the coil is not powered(inactive), the common pole A is connected to the pole B(NC) and is in resting position. But when the relay is powered(active) the common pole A is connected to pole C(NO) and isn't in resting position. Hence only one position is resting position while the other position needs the coil to be powered. I will add this concept to this instructable after some time. I hope I have clarified your doubt. If you liked this instructable feel free to vote for it and don't forget to follow me so that you can get updates related to any of my instructables. Thanks!.

ybaggi (author)Jonathanrjpereira2014-05-23

Thanks you have clarifited it very well.

Now my question is: are there relays that act like seletors. Imagina stove knob wher zou turn between severa positions but each position does not need to have active power in the coil, just the change over does need an impulse.

Does that exist?

and then can you do an instructable or a 2.0 version of this one to cover solid state relays?

thanks a lot

fred_ (author)ybaggi2016-02-07

They do but there are more modern ways to do it in most cases.

Google sequence relay, stepper relay etc.Before Programmable Logic Controllers all manner of relays were used for programming. Google "relay logic"

or "ladder logic". You can wire a normal relay to be a latching relay but it looses its memory when the power is cycled. Other relays use a set and a reset coil and remember their state.

Jonathanrjpereira (author)ybaggi2014-05-23

I don't think such a relay exists. But you can try using the IC 4017 as a decade counter. I can make an instructable about this. Thanks for the idea. I'll send you the link to it when I do.

Jonathanrjpereira (author)ybaggi2014-05-23

I have added a step explaining the working of a Change Over Type Relay. Do check it out at: https://www.instructables.com/id/All-You-Need-to-Know-About-Relays/step4/Change-over-CO-or-double-throw-DT-Relay/

I hope this has answered your question. Thanks for Reading.

stephenfitton (author)ybaggi2014-05-22

if the changeover needs to permanent usually select a relay with an extra set of contacts and tie that back to the circuit that activated the relay,bearing in mind that that circuit has to be broken somewhere before activation or a locked circuit remains,locked.

As for solid state relays, I'm planning on doing another instructable which includes SSR's, Reed switches/ reed relays, mono stable and bistable relays too!. Thanks. I'll send you a link to it once completed.

Obaidhanan (author)2015-01-18

I have 12v battry and 500w invertor with charger, when the main power down I use them manually, now plz let me know how to install relay among them....

leanne_martinau (author)2014-06-08

So strange, now, to remember that the earliest computers of World War 2 were made of relays and thermionic valves - in fact the term 'bug' in relation to computers was when a moth got caught in a relay in one just after the war. Hot damn, youngens - showing my age . . .

Jonathanrjpereira (author)2014-06-03

If you liked this instructable, you can also check out this Electronic Sensor & Component Tester. https://www.instructables.com/id/Electronic-Sensor-Component-Tester/ .Feel free to vote/favorite for it too.

daniel.nicholls (author)2014-05-25

Good job, but the operating voltage of a relay isn't always in DC. Small signal relays and low voltage power relays are usually in DC but mains control relays and contactors quite frequently have AC coils.


On the contact ratings, they will frequently have something like 5A@250VAC / 10A@12VDC. These are the figures you have to be within... having said that you can run a higher current than stamped on it if your voltage is lower, they aren't directionally proportional though and the datasheet for the relay should be consulted.

On back EMF which you briefly touched upon, you should always use a diode even where the relay isn't driven by a semiconductor. For instance when a relay switches in your car the voltage spike can cause interference on the radio, and if you have a faulty battery or are silly enough to disconnect it with the engine running it can damage the ECU etc.

Lastly, when looking at how a relay works, there will typically be a throw current and hold current. So when the coil reaches it's throw current it will cause the relay to actuate, supplied voltage can then be reduced (reducing the coil current) and the relay will hold until the hold current is passed, it is at this point the relay will return to it's neutral position. This is interesting to note as it can mean that the power supply can be smaller and allow some drop in output voltage when supplying high transient loads before effects on the relay are seen. (for example when starting your car a significant voltage drop is often seen)

oh the pages that could be written on such a simple device...

You make a good point. I will make the necessary changes. Thanks for the constructive advice. If you liked this instructable do vote for it and follow me on instructables so that you can get instant updates to any of my future instructables. I will send you a link to the corrections once completed. Thanks for reading.

ii_awesum (author)2014-05-24

Relay magnet wire can be used to make invisible trip wires for various detection or anti-tampering schemes. You don't necessarily have to have a circuit (alarm) involved, just observation that the wire was broken is an aid to tell if someone was snooping where they shouldn't be. Worked for me!

Cool Idea. A few ways you can recycle relays and prevent e-waste. Thanks for readinhg

Justb3 (author)2014-05-24

Great guide!. Btw is there a relay that toggles state? I mean when it receives a pulse is open and when recives another pulse it changes to closed?.

Jonathanrjpereira (author)Justb32014-05-24

A latching relay can do the job. I hope this solves your question. If you like this instructable do vote for it. I will add a step explaining latching relays in some days. I will send you a link once completed.Follow me so that you can get updates to any of my instructables. Thanks for reading.

Justb3 (author)Jonathanrjpereira2014-05-24

Thanks a lot!

mculliso (author)2014-05-22

It would be nice to know about input and output voltages. These determine whether or not you can use the relay for a specific purpose. Also, the type of voltage is important, AC or DC. Be sure to select the appropriate relay for your circuit.

Hey, I just added a step explaining the voltage and current parameters of a relay. Here's a link to it: https://www.instructables.com/id/All-You-Need-to-Know-About-Relays/step5/Voltage-Current-Parameters-of-a-Relay/ I hope I have answered your question. Thanks for Reading!.

Thanks for the suggestion. I will add step about I/p, o/p, voltage, AC, DC, etc to this instructable in some days time. If you liked this instructable feel free to vote for it and follow me so that you can get updates to any of my instructables. Thanks.

Gene Bond (author)2014-05-22

Nice job! Some things we take for granted. It's nice to see someone break down the basics for those who are just learning a new area.

Just some clarifications:

I saw a spelling mistake under the 2nd group of pictures, Amature rather than Armature :)

I would label the N/O contacts in the last diagram as A rather than B, as they are in the form C beside them for clarity... Since Form A is N/O, form B N/C.

Thanks for pointing out those clarifications, I did correct the spelling. The only thing is I didn't really understand the question you have put forward. If you could rephrase it, it would be really great and I will certainly clarify any mistake I made. Thanks.

Let's see if I can post the picture I marked up. Again, great job!

Thanks for the reference picture. I found the correction to be right and so I have edited the picture accordingly. Thanks for the constructive advice. Check the edit out at: https://m.instructables.com/id/All-You-Need-to-Know-About-Relays/step3/Pole-Throw-of-a-Relay/

Kevanf1 (author)2014-05-22

I'm pretty well conversant with the way relays work as I am interested in electronics and also cars. However, I have to say that this is probably the best tutorial as to how different relays work that I have come across, well done :)

Are you considering doing a follow up explaining how electronic (solid state) relays work? I for one would love to see it if you do one.

Thank you for an excellent 'Instructable'.

Yes certainly, I would love to do a follow up explaining the working of Solid State Relays(SSR). I'll send you the link to it when I finally do it. You can also follow me on instructables so that you can get instant updates related to any of my future posts. Thanks.

Thank you :) I am very much looking forward to it and thank you once again :)

Take care.

Kevan

gbaker11 (author)2014-05-22

Pretty explanatory for the short forum you wrote it in. Is there a link inthese instructables to contact authors ? Would like to run some design options by you wlightning3@yahoo.com

If you want to contact me you can write on my instructables 'Orangeboard'. I will certainly try my best to help. You can also follow me on instructables so that you can get updates to any of my future posts. If you liked this instructable feel free to vote for it. Thanks.

Jan_Henrik (author)2014-05-12

Very usefull, thank you! :)

Thanks. Do vote for this instructable if you liked it.

marianok (author)2014-05-19

nice quick and simple introduction to relays !!! great ZOOM pictures !!!

Thanks !!!

Thanks. Do vote for this instructable if you liked it.

Tommy_tux (author)2014-05-22

You forgot, REED and SSR relais... ;-)

I will be posting an instructable about the working of monostable and bistable relays, reed switches/ reed relays and also SSR's. I'll send you the link to it once completed. You can also follow me on instructables so that you can get updates to any of my future posts. Thanks.

rick6213 (author)2014-05-22

Nice guide to relays? Would have loved to see also the working of other types of relays like bistable and monostable relays.

I will be posting an instructable about the working of monostable and bistable relays, reed switches/ reed relays and also SSR's. I'll send you the link to it once completed. You can also follow me on instructables so that you can get updates to any of my future posts. Thanks for the advice.

About This Instructable

123,059views

884favorites

License:

Bio: I'm Jonathan Pereira, a novice Electronics Engineer. I like to make Doze Lamps, Lumen Powered Thingamajigs, Almighty Brainy Buttons, Tweeting Fart Detectors and share ... More »
More by Jonathanrjpereira:LUMOS: Smart Lamp for Better SleepAdruino Serial PlotterTweeting Weather Station
Add instructable to: