All You Need to Know About a Relays

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Introduction: All You Need to Know About a Relays

About: I'm an Electronics Engineer who likes to make Doze Lamps, Lumen Powered Thingamajigs, Almighty Brainy Buttons, Tweeting Fart Detectors and share them on Instructables.

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

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

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

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

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

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.

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    57 Discussions

    hi

    I have a need for a 120v 30a RELAY, a SPDT, at least I believe that's what it should be:

    ...

    120v single wire power from TWO different possible sources(Inverter, Generator)

    at 30amp maximum pass thru

    ...

    I want the RELAY to 'rest' on the Inverter, position B(NC) - it will always be providing power

    ...

    but

    ...

    I want the RELAY to 'switch' to the Generator, position C(NO) when it STARTS providing 120v power

    ...

    ...

    is the SPDT Relay the correct device?

    ...

    thanks : )

    hi

    I have a need for a 120v 30a RELAY, a SPDT, at least I believe that's what it should be:

    120v single wire power from TWO different possible sources(Inverter, Generator)

    at 30amp maximum pass thru

    I want the RELAY to 'rest' on the Inverter, position B(NC) - it will always be providing power

    but

    I want the RELAY to 'switch' to the Generator, position C(NO) when it STARTS providing 120v power

    is the SPDT Relay the correct device?

    Are questions allowed here? If so, how long can the coil on a dpdt relay be energized? I am asking because I want to use 120vac to power the coil and upon power loss want the dpdt contact to close contacts to allow a 12 vdc system to kick in. I’ve been looking around. I know the design will work but not sure how long the coil will last, being energized 24/7. Thank you

    1 reply

    As long as you do not exceed the Maximum/Absolute Ratings of the relay, you should be fine.

    What are the connections to the relay?

    The relay coil may be burnt if its Absolute Ratings are exceeded

    Can someone please explain me
    why do we need full relay module with lots of other elements, because what i read makes me think a relay is complete in itself and has seperate connection for 5v DC and 220v AC.

    we use aurdino or rpi to supply it 5v power and to activate or deactivate it due to 5v current provided to them, then why do we have all other components on relay module

    1 reply

    Flyback Diode is possibly the only additional component you may need to drive the relay

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

    Again thank youLuis

    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.

    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?

    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.
    0
    None
    OachM

    1 year ago

    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 ?

    1 reply

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

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

    4 replies

    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.

    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.

    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