The Electrical Relay

Thus far we have seen a selection of Input devices that can be used to detect or “sense” a variety of physical variables and signals and are therefore called Sensors. But there are also a variety of electrical and electronic devices which are classed as Output devices used to control or operate some external physical process. These output devices are commonly called Actuators.

Actuators convert an electrical signal into a corresponding physical quantity such as movement, force, sound etc. An actuator is also a Transducer because it changes one type of physical quantity into another and is usually activated or operated by a low voltage command signal. Actuators can be classed as either binary or continuous devices based upon the number of stable states their output has.

For example, a relay is a binary actuator as it has two stable states, either energised and latched or de-energised and unlatched, while a motor is a continuous actuator because it can rotate through a full 360o motion. The most common types of actuators or output devices are Electrical Relays, Lights, Motors and Loudspeakers and in this tutorial we will look at electrical relays.

Electrical Relays can also be divided into mechanical action relays called “Electromechanical Relays” and those which use semiconductor transistors, thyristors, triacs, etc, as their switching device called “Solid State Relays” or SSR’s.

The Electromechanical Relay

The term Relay generally refers to a device that provides an electrical connection between two or more points in response to the application of a control signal. The most common and widely used type of electrical relay is the electromechanical relay or EMR.

electrical relay circuit

An Electrical Relay

The most fundamental control of any equipment is the ability to turn it “ON” and “OFF”. The easiest way to do this is using switches to interrupt the electrical supply. Although switches can be used to control something, they have their disadvantages. The biggest one is that they have to be manually (physically) turned “ON” or “OFF”. Also, they are relatively large, slow and only switch small electrical currents.

Electrical Relays however, are basically electrically operated switches that come in many shapes, sizes and power ratings suitable for all types of applications. Relays can also have single or multiple contacts within a single package with the larger power relays used for mains voltage or high current switching applications being called “Contactors”.

In this tutorial about electrical relays we are just concerned with the fundamental operating principles of “light duty” electromechanical relays we can use in motor control or robotic circuits. Such relays are used in general electrical and electronic control or switching circuits either mounted directly onto PCB boards or connected free standing and in which the load currents are normally fractions of an ampere up to 20+ amperes. The relay circuit are common in Electronics applications.

As their name implies, electromechanical relays are electro-magnetic devices that convert a magnetic flux generated by the application of a low voltage electrical control signal either AC or DC across the relay terminals, into a pulling mechanical force which operates the electrical contacts within the relay. The most common form of electromechanical relay consist of an energizing coil called the “primary circuit” wound around a permeable iron core.

This iron core has both a fixed portion called the yoke, and a moveable spring loaded part called the armature, that completes the magnetic field circuit by closing the air gap between the fixed electrical coil and the moveable armature. The armature is hinged or pivoted allowing it to freely move within the generated magnetic field closing the electrical contacts that are attached to it. Connected between the yoke and armature is normally a spring (or springs) for the return stroke to “reset” the contacts back to their initial rest position when the relay coil is in the “de-energized” condition, ie. turned “OFF”.

Electromechanical Relay Construction

Electromechanical Relay


In our simple relay above, we have two sets of electrically conductive contacts. Relays may be “Normally Open”, or “Normally Closed”. One pair of contacts are classed as Normally Open, (NO) or make contacts and another set which are classed as Normally Closed, (NC) or break contacts. In the normally open position, the contacts are closed only when the field current is “ON” and the switch contacts are pulled towards the inductive coil.

In the normally closed position, the contacts are permanently closed when the field current is “OFF” as the switch contacts return to their normal position. These terms Normally Open, Normally Closed or Make and Break Contacts refer to the state of the electrical contacts when the relay coil is “de-energized”, i.e, no supply voltage connected to the inductive coil. An example of this arrangement is given below.

electrical relay contact tips

The relays contacts are electrically conductive pieces of metal which touch together completing a circuit and allow the circuit current to flow, just like a switch. When the contacts are open the resistance between the contacts is very high in the Mega-Ohms, producing an open circuit condition and no circuit current flows.

When the contacts are closed the contact resistance should be zero, a short circuit, but this is not always the case. All relay contacts have a certain amount of “contact resistance” when they are closed and this is called the “On-Resistance”, similar to FET’s.

With a new relay and contacts this ON-resistance will be very small, generally less than 0.2Ω’s because the tips are new and clean, but over time the tip resistance will increase.

For example. If the contacts are passing a load current of say 10A, then the voltage drop across the contacts using Ohms Law is 0.2 x 10 = 2 volts, which if the supply voltage is say 12 volts then the load voltage will be only 10 volts (12 – 2). As the contact tips begin to wear, and if they are not properly protected from high inductive or capacitive loads, they will start to show signs of arcing damage as the circuit current still wants to flow as the contacts begin to open when the relay coil is de-energized.

This arcing or sparking across the contacts will cause the contact resistance of the tips to increase further as the contact tips become damaged. If allowed to continue the contact tips may become so burnt and damaged to the point were they are physically closed but do not pass any or very little current.

If this arcing damage becomes to severe the contacts will eventually “weld” together producing a short circuit condition and possible damage to the circuit they are controlling. If now the contact resistance has increased due to arcing to say 1Ω’s the volt drop across the contacts for the same load current increases to 1 x 10 = 10 volts dc. This high voltage drop across the contacts may be unacceptable for the load circuit especially if operating at 12 or even 24 volts, then the faulty relay will have to be replaced.

To reduce the effects of contact arcing and high “On-resistances”, modern contact tips are made of, or coated with, a variety of silver based alloys to extend their life span as given in the following table.

Electrical Relay Contact Tip Materials

  • Ag (fine silver)
    • 1. Electrical and thermal conductivity are the highest of all the metals.
    • 2. Exhibits low contact resistance, is inexpensive and widely used.
    • 3. Contacts tarnish easily through sulphurisation influence.
  • AgCu (silver copper)
    • 1. Known as “Hard silver” contacts.
    • Better wear resistance and less tendency to arc and weld, but slightly higher contact resistance.
  • AgCdO (silver cadmium oxide)
    • 1. Very little tendency to arc and weld, good wear resistance and arc extinguishing properties.
  • AgW (silver tungsten)
    • 1. Hardness and melting point are high, arc resistance is excellent.
    • 2. Not a precious metal.
    • 3. High contact pressure is required to reduce resistance.
    • 4. Contact resistance is relatively high, and resistance to corrosion is poor.
  • AgNi (silver nickel)
    • 1. Equals the electrical conductivity of silver, excellent arc resistance.
  • AgPd (silver palladium)
    • 1. Low contact wear, greater hardness.
    • 2. Expensive.
  • Platinum, Gold and Silver Alloys
    • 1. Excellent corrosion resistance, used mainly for low-current circuits.

Relay manufacturers data sheets give maximum contact ratings for resistive DC loads only and this rating is greatly reduced for either AC loads or highly inductive or capacitive loads. In order to achieve long life and high reliability when switching AC currents with inductive or capacitive loads some form of arc suppression or filtering is required across the relay contacts.

Extending the life of relay tips by reducing the amount of arcing generated as they open is achieved by connecting a Resistor-Capacitor network called an RC Snubber Network electrically in parallel with an electrical relay contact tips. The voltage peak, which occurs at the instant the contacts open, will be safely short circuited by the RC network, thus suppressing any arc generated at the contact tips. For example.

Electrical Relay Snubber Circuit

relay rc snubber circuit


Electrical Relay Contact Types.

As well as the standard descriptions of Normally Open, (NO) and Normally Closed, (NC) used to describe how the relays contacts are connected, relay contact arrangements can also be classed by their actions. Electrical relays can be made up of one or more individual switch contacts with each “contact” being referred to as a “pole”. Each one of these contacts or poles can be connected or “thrown” together by energizing the relays coil and this gives rise to the description of the contact types as being:

  • SPST – Single Pole Single Throw
  • SPDT – Single Pole Double Throw
  • DPST – Double Pole Single Throw
  • DPDT – Double Pole Double Throw

with the action of the contacts being described as “Make” (M) or “Break” (B). Then a simple relay with one set of contacts as shown above can have a contact description of:

"Single Pole Double Throw – (Break before Make)", or SPDT – (B-M)

Examples of just some of the more common diagrams used for electrical relay contact types to identify relays in circuit or schematic diagrams is given below but there are many more possible configurations.

Electrical Relay Contact Configurations

electrical relay contact configurations

  • Where:
  •     C is the Common terminal
  •     NO is the Normally Open contact
  •     NC is the Normally Closed contact

One final point to remember about using electrical relays. It is not advisable at all to connect relay contacts in parallel to handle higher load currents. For example, never attempt to supply a 10A load with two relays in parallel that have 5A contact ratings each, as the mechanically operated relay contacts never close or open at exactly the same instant of time. The result is that one relay contact will always be overloaded even for a brief instant in time resulting in premature failure of the relay over time.

Also, while electrical relays can be used to allow low power electronic or computer type circuits to switch relatively high currents or voltages both “ON” or “OFF”. Never mix different load voltages through adjacent contacts within the same relay such as for example, high voltage AC (240v) and low voltage DC (12v), always use separate relays for safety.

One of the more important parts of any electrical relay is its coil. This converts electrical current into an electromagnetic flux which is used to mechanically operate the relays contacts. The main problem with relay coils is that they are “highly inductive loads” as they are made from coils of wire. Any coil of wire has an impedance value made up of resistance ( R ) and inductance ( L ) in series (LR Series Circuit).

As the current flows through the coil a self induced magnetic field is generated around it. When the current in the coil is turned “OFF”, a large back emf (electromotive force) voltage is produced as the magnetic flux collapses within the coil (transformer theory). This induced reverse voltage value may be very high in comparison to the switching voltage, and may damage any semiconductor device such as a transistor, FET or micro-controller used to operate the relay coil.

flywheel diode across relay coil

One way of preventing damage to the transistor or any switching semiconductor device, is to connect a reverse biased diode across the relay coil.

When the current flowing through the coil is switched “OFF”, an induced back emf is generated as the magnetic flux collapses in the coil.

This reverse voltage forward biases the diode which conducts and dissipates the stored energy preventing any damage to the semiconductor transistor.

When used in this type of application the diode is generally known as a Flywheel Diode, Free-wheeling Diode and even Fly-back Diode, but they all mean the same thing. Other types of inductive loads which require a flywheel diode for protection are solenoids, motors and inductive coils.

As well as using flywheel Diodes for protection of semiconductor components, other devices used for protection include RC Snubber Networks, Metal Oxide Varistors or MOV and Zener Diodes.

The Solid State Relay.

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While the electromechanical relay (EMR) are inexpensive, easy to use and allow the switching of a load circuit controlled by a low power, electrically isolated input signal, one of the main disadvantages of an electromechanical relay is that it is a “mechanical device”, that is it has moving parts so their switching speed (response time) due to physically movement of the metal contacts using a magnetic field is slow.

Over a period of time these moving parts will wear out and fail, or that the contact resistance through the constant arcing and erosion may make the relay unusable and shortens its life. Also, they are electrically noisy with the contacts suffering from contact bounce which may affect any electronic circuits to which they are connected.

To overcome these disadvantages of the electrical relay, another type of relay called a Solid State Relay or (SSR) for short was developed which is a solid state contactless, pure electronic relay.

The solid state relay being a purely electronic device has no moving parts within its design as the mechanical contacts have beeen replaced by power transistors, thyristors or triac’s. The electrical separation between the input control signal and the output load voltage is accomplished with the aid of an opto-coupler type Light Sensor.

The Solid State Relay provides a high degree of reliability, long life and reduced electromagnetic interference (EMI), (no arcing contacts or magnetic fields), together with a much faster almost instant response time, as compared to the conventional electromechanical relay.

Also the input control power requirements of the solid state relay are generally low enough to make them compatible with most IC logic families without the need for additional buffers, drivers or amplifiers. However, being a semiconductor device they must be mounted onto suitable heatsinks to prevent the output switching semiconductor device from over heating.

Solid State Relay

solid state relay


The AC type Solid State Relay turns “ON” at the zero crossing point of the AC sinusoidal waveform, prevents high inrush currents when switching inductive or capacitive loads while the inherent turn “OFF” feature of Thyristors and Triacs provides an improvement over the arcing contacts of the electromechanical relays.

Like the electromechanical relays, a Resistor-Capacitor (RC) snubber network is generally required across the output terminals of the SSR to protect the semiconductor output switching device from noise and voltage transient spikes when used to switch highly inductive or capacitive loads. In most modern SSR’s this RC snubber network is built as standard into the relay itself reducing the need for additional external components.

Non-zero crossing detection switching (instant “ON”) type SSR’s are also available for phase controlled applications such as the dimming or fading of lights at concerts, shows, disco lighting etc, or for motor speed control type applications.

As the output switching device of a solid state relay is a semiconductor device (Transistor for DC switching applications, or a Triac/Thyristor combination for AC switching), the voltage drop across the output terminals of an SSR when “ON” is much higher than that of the electromechanical relay, typically 1.5 – 2.0 volts. If switching large currents for long periods of time an additional heat sink will be required.

Input/Output Interface Modules.

Input/Output Interface Modules, (I/O Modules) are another type of solid state relay designed specifically to interface computers, micro-controller or PIC’s to “real world” loads and switches. There are four basic types of I/O modules available, AC or DC Input voltage to TTL or CMOS logic level output, and TTL or CMOS logic input to an AC or DC Output voltage with each module containing all the necessary circuitry to provide a complete interface and isolation within one small device. They are available as individual solid state modules or integrated into 4, 8 or 16 channel devices.

Modular Input/Output Interface System.

solid state relay interface module


The main disadvantages of solid state relays (SSR’s) compared to that of an equivalent wattage electromechanical relay is their higher costs, the fact that only single pole single throw (SPST) types are available, “OFF”-state leakage currents flow through the switching device, and a high “ON”-state voltage drop and power dissipation resulting in additional heat sinking requirements. Also they can not switch very small load currents or high frequency signals such as audio or video signals although special Solid State Switches are available for this type of application.

In this tutorial about Electrical Relays, we have looked at both the electromechanical relay and the solid state relay which can be used as an output device (actuator) to control a physical process. In the next tutorial we will continue our look at output devices called Actuators and especially one that converts a small electrical signal into a corresponding physical movement using electromagnetism. The output device is called a Solenoid.


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29 Responses to “Electrical Relay”

  1. sai sneha rathnam

    Hello Wayne,article is very good.Currently am working on a project called POWERED WHEEL CHAIR,in that am using some basic components like 1K,10K resistors,IN 4007 diodes and KN2222A transistors for switching purpose.We also have relay in our project which is of 12v and that must drive a 12v dc motor.Iam unable trace out what is the problem,if the problem is with relay.Can you suggest which one to use?

    • Wayne Storr

      There are literally tens of thousands of different relays available in the marketplace. Which one is suitable for your application or circuit depends upon many things. Coil voltage – your case 12VDC, contact type – SPST, DPDT, etc, Contact rating – how many amps and volts you want to switch, Switching frequency – ON-OFF times per minute, hour etc, Physical dimensions and weight, availability – can you easily buy it, the list goes on. Then I can not suggest a relay to use.

  2. AWR

    Hello Wayne,

    Thanks for the unselfish sharing, it’s helpful. Really appreciated.
    Here I have another question would like to consult with you, though, it might seem stupid. I am curious to know when the EM relay is placed in a natural cooling environment, i.e., no water, no fan…etc. Is there any decent method to dissipate the heat while the EM relay is conducted at 15~20A, 208Vac condition. The ones I can think of are shown as below:
    1). Add thermal pad between the top of the EM relay and the top cover of the case, which is made of Aluminum. The purpose is to dissipate the heat to the case through the thermal pad.
    2). Put thermal pad between the bottom of PWB and the bottom of the case.
    Same idea as item 1, just change the location of the thermal pad.

    Please help to advice on this while you’re free.

    • Wayne Storr

      Again, only the coil (as an inductor) would generate a very, very small amount of heat even when the duty cycle is 100%. Thermal pads are not required for electromechanical relays in normal conditions. However, they are required for solid state relays SSR’s, when fixing to a heatsink as the switching device is a semiconductor.

  3. AWR

    Hello Wayne, thanks for the article. Really appreciated. Though it might seem stupid, I am curious to know how to dissipate the heat from EM relay when it is used to deliver 12 to 15 amps current in a natural cooling environment. I am thinking of utilizing thermal pad on the top of the relay or on the bottom of the PWB. Any advice?

    • Wayne Storr

      Hello AWR, the only heat generated by an electromechanical relay would be in the coil. The relay does not deliver 12 to 15 amps but only switches it through its contacts. The small I^2R loss across the contacts will depend upon their closed resistance when the relay coil is energised.

  4. Mark

    Mr. Storr,

    This article was helpful and I do appreciate your taking the time to write and respond to questions.

    All below is my current design, where I still have time to change the relay type if need.

    My question is in regards to using a relay in a single phase, L1 to L2 208 VAC circuit. I am bring 208VAC from our breaker through a 4 wire conductor cable. This is coming off of a center tapped transformer, where the L1- to L2 gives me my 208VAC. I am using a 5VDC coil on a SPST NO Relay Contacts. I am bring the L1 into my contact and the out to my device, with L2 going straight to the device.

    I cannot find any documentation on whether this is bad practice. My thought process: I get the control I need by being able to switch on the L1 line. However, L2 is constantly energized and I would still get 104 VAC between L2 and neutral. Would I need to change from SPST to DPST and bring both the L1 and L2 into contact side, or is my current design adequate?

    • Wayne Storr

      Hello Mark, If I understand correctly, you are switching the load across a center-tapped 208V transformer (104-0-104). While this will work with regards to switching the 208V load it is bad practice with regards to the fact that the load still has 104 volts across it to earth when the relay contacts are open. Someone may receive an electric shock thinking that the load is not energised when the relay is de-energised but the load always has 104V (L1 or L2) to earth regardless of the position of the relay contacts. Feeding the load through a DPST relay would be safer.

  5. Rico

    Hi Wayne nice article

    I am busy with a application where I am switching 220V AC equipment with a microcontroller and contact arcing and the noise induced into my circuit is huge thorn in my side. I have seen the IC’s beeing restarted and SPI coms being interrupted because of it. The RC snubber looks like the best possible solution to this problem, the only problem is the leakage current the snubber circuit allows through. Is there any other design considerations that needs to be taken into account to minimize the effect of contact arcing on my PCB?

      • Rico

        Initially it was cost and the list of disadvantages mentioned in your article but that’s the road we are going down now.

        • Wayne Storr

          Hello Rico, The only real disadvantages of an SSR are cost, single channel switching and no normally-closed option (well AC ones anyway). Cost can be reduced by making your own solid state switch using an Opto-isolator and Triac, and duplicating for extra phases or channels.

  6. Jacob Neumann

    This is really helpful! So I have a quick question. Relays contacts can handle a specified amount of current the contact can handle (contact ratings). So if I have a 3PST Relay with a contact rating of 10 amps, does that mean that I can run 10 amps through each contact, for a total of 30 amps being switched through the relay? Or am I limited to 3.33 amps per contact for a total of 10 amps through the relay? Thanks!

    • Wayne Storr

      A relay’s contacts are rated at a maximum switching current, then yes in theory each set of contacts should switch 10 amps on its own. However, do not try to switch 30 amps through the 3 sets of contacts thinking that they can each switch 10 amps as the current will not be evenly distributed and the relay will eventually fail.

  7. Shaun Irwin

    Hi Wayne, thanks for writing this article. Very informative indeed. Quick question. Would a 12v dc relay coil outlast a 240vac coil, or are the switching contacts likely to fail first? I am using a basic relay control circuit to remotely control a 3 phase roller door motor just to pull the contractors in and disengage the hold in contacts when operating remotely to allow remote stopping (up/stop/down control box existing). The PCB I’m going to use for the wireless Relays is 12 volt switching but contacts rated at 240vac so I have the option of 12 volt dc relay coils or 240 vac. Any advice would be much appreciated.



    • Wayne Storr

      Hello Shaun, There is no difference between the two coils with regards to their operational lifespan, its the pitting and burning of the contact points or their constant mechanical movement that generally ends a relays life. Whether a 12VDC or 230VAC coil depends on your control circuit wiring as I assume additional manually operated up/down pushbuttons and door safety devices will be used. If you decide to use 12VDC relays, a flywheel diode maybe required to eliminate the back emf of the relays coil when de-energising, also an RC snubber circuit or MOV across the contacts will help reduce arcing when energising and de-energising the two 3-phase motor contactors.

      • Shaun Irwin

        Thanks heaps for your help Wayne. You really know your stuff… Some of that went way over my head. Unfortunately I don’t get to work with much control circuit wiring and electronics in my place of employment, but it’s something I’m extremely interested in. The information on this website is amazing, I’m glad I found it. Thanks again for your help.

  8. John

    This is an excellent article, Mr. Storr. Big thanks!

    In addition to the possible safety consequences of switching significantly different voltages in one relay (or a manually operated switch), one needs to consider reliability and failure physics of switching poles in different load regimes. This used to be thought about a lot in military and aerospace work, but seems to less and less considered as time goes on.
    [post edited]

  9. Linda

    I am trying to use SSRs to measure connectivity of a cable, checking shorts. Each wire is connected to an SSR, the other output being one side of the DMM. Same with the other end of the cable. Continuity measurement works fine, but when I run a short test, I get 600 ohms for each measurement when I am expecting high impedence. Could it be that there is leakage? Or am I using the SSR in the wrong way, not having a power source?

    • Wayne Storr

      The output switching device is either back-to-back thyristors or a single triac for AC SSR’s depending upon current and voltage rating, or a darlington power transistor for a DC SSR. Either way, both devices need an external supply voltage greater than that given on the SSR’s label to activate the internal drive circuitry to turn the thing “ON”, otherwise it would just float about at any value.

  10. John B

    How can I set up SSR with parallel control? What I have at present is 3 x SSR’s operating some lamps, being time controlled switching on and off by PLC 24 volt DC and want to have temperature controller also controlling the SSR’s

  11. ghoneim

    hi Storr
    it is very good article ,
    i have question
    i make project in pic16f84a my input 24v and output 24 v
    you know pic specs 5 v 20 ma

    so that i need the suitable compent in input tp get voltage 5 v and 20 ma
    and the same for output

  12. kiran ali

    i want to ask what is the difference between static and steady electric field? are they both are same?

    • Wayne Storr

      Hello Kiran, thank you for your question. My understanding is that static electric fields arise from electric charges between the plates of capacitors, as in electrostatic field. Whereas direct currents (d.c.) applied to a coil or inductor for example, will produce a constant and steady electric field.


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