How to connect a relay?
It is not necessary to be an expert installer to know how to connect a relay if we need to place one of these in our control panel or in our automation project.
However, to know how to connect a relay, we must first know some very basic concepts about the different types of relays.
It is not the same to connect an electromechanical relay as a solid state relay or a “reed” relay, for example.
What kind of relay will I install?
First of all, we have to look at the type of relay we have available to locate and differentiate in this the input or control terminals and the output or power terminals.
Basically an electromechanical relay always has two pins to which are welded the ends of the enameled copper wire with which its coil or solenoid is made that is wound around the iron core to form the electro magnet.
In the electromechanical relays, these two terminals constitute the input or control circuit, and when put in tension, we energize the coil and the electro magnet formed attracts the iron sheet to which the mobile contacts are integral.
A solid-state relay does not have a coil, so to know how to connect a solid-state relay, it must be noted that it has two pins, terminals or terminals to which the input of the electronics inside it is connected. It is isolated from the output electronics that opens or closes the power circuit.
What is the polarity and tension of the relay?
In these two cases, both for the electromechanical relay and for the solid state relay it is very important to take into account the polarity for the relays with direct current control circuit and also the nominal voltage or voltage range to which the Electromechanical relay coil or control electronics if it is a solid state relay.
If we do not check the working voltage we could “burn” and disable the control part of the relay, whether it is for direct current or for alternating current.
Sometimes some models of relays can have very wide operating ranges and admit control voltages for example from 90 to 250 volts in alternating current.
We will not always fix when connecting in the + and – signs that are usually marked next to the pins of the coil or the input circuit.
How are the relay terminals numbered?
The pins of the coil of an electromechanical relay are usually identified by the letters A1 + and A2- and they are usually drawn on the printed diagram on the side of the relay next to two segments that are joined to a square that carries inside another segment of one from its vertices to the opposite vertex.
The input terminals of the control electronics in a single-phase solid-state relay are usually named with the numbers 4 – and 3 + (also 1 ~ and 2 ~ if it is three-phase with control voltage in alternating current).
When the relay coil is designed to work in alternating current we must pay attention to the frequency of the network to which we are going to make the connection, since it can be 50 Hz (common in European countries) or 60 Hz (common in countries of America or other continents)
In a “reed” type relay, we will only find two wires or pins, but in this case they are always those of the output or power circuit, since there is no coil or electronics, so we will know how to connect a reed type relay as soon as it falls into our hands.
These relays are very simple and only carry sheets of normally open or normally closed contacts that change position to be near a magnetic field, which is usually produced by a permanent magnet of greater or lesser power, allowing or preventing the passage of current by the circuit to which they are connected.
What happens when we have a plug-in relay?
So far we have assumed that the cables of the input circuit are connected directly to the corresponding terminal of the relay by tightening the screw that catches the end of the conductor, but how to connect a plug-in relay that does not have terminals with screws.
We can say that this is usual in the case of solid state relays, but not for the vast majority of electromechanical relays.
Although in some very specific applications the ends of the cables are joined in the manner described above or are welded to the pins of the relay being permanently attached to them, it is normal that the relay is not connected in this way.
The vast majority of the industrial electromechanical relays that are commercialized are of the “plug-in” type, which means that they need a base, socket or socket to which they are plugged.
What is the base or socket for a plug-in relay?
In this way, the base has the corresponding terminals to which we can connect the cables of the input and output circuits by means of the captive screw when we manufacture the control cabinet.
Once we have made the connections of the conductors to the base, we only have to “plug” the relay on the housings available by matching each pin of the relay with its corresponding housing.
From each terminal, brass conductive sheets are placed inside the base, ending in the above-described housings and coming into contact with the relay pins.
Should we do a first activation test when connecting a relay?
We have answered the first part of the question, how to connect a relay ?, but we have not said anything about how to connect the contacts to the power circuit that we want to control.
In any case, once we have made the relay coil connections, we can make a test that the relay acts correctly before continuing with the rest of the installation.
For this, we will put voltage between the coil terminals and verify in the case of the electromechanical relay that the mobile contacts change position and go from being separated from the normally closed fixed contacts to join them.
In many industrial electromechanical relays that have a small LED indicator on its front face, we can also verify that this LED lights up when putting voltage between the coil terminals and helps us to know how to connect a relay without wiring errors.
When testing the solid state relays we can only verify that the front LED is illuminated, since, having no moving parts inside, we can not perceive any change of position or internal sound.
How to connect the contacts of the relay to the power circuit?
Now we will enter to answer our initial question how to connect a relay ?, but this time attending to the zone of exit or power.
We have already explained previously and therefore we must already know if the relay we have in our hands is an electromechanical relay, it is a solid state relay or it is a relay of another type.
In addition, we can also distinguish whether the relay has terminals to which the cables are connected directly, or needs a connection base to be coupled because it is of the “plug-in” type.
How many “group of contacts” does an electromechanical relay have?
The next thing we need to know about our relay is the number of contact groups available inside it if it is an electromechanical relay, or if it is for single-phase or three-phase use when it is a solid-state relay.
We can define in a simple way “group of contacts” in an electromechanical relay as each set of two or three metal sheets that end each of them in a small button of conductive material that are joined or separated to close or open the power circuit .
We have previously said “set of two or three metal sheets” since we can find different types of relays depending on whether the contact groups are two sheets or three sheets.
When the groups of contacts only have two sheets, one of them is fixed and immobile at all times (fixed contact) and the other sheet can move and change position when we put the relay to work (mobile contact).
If the relay is at rest, ie if there is no voltage between the coil terminals, it may happen that the fixed contact is next to the movable contact.
How to identify the normally open contact of a relay and the closed one?
In this case we say that the fixed contact is a normally closed contact (NC), since with the relay at rest the circuit is closed by passing the current through the moving plate that is connected by the contacts of its contacts. ends to the fixed.
In this type of relays, when we energize the coil, the mobile contact is separated from the fixed one and the circuit opens, remaining open while maintaining the voltage between the coil terminals.
If the relay is at rest, and the fixed contact is separated from the movable contact, we say in this case that the fixed contact is a normally open contact (NO), since the circuit is not open with the relay at rest. letting the current pass since the moving blade is now separated from the fixed one.
In this other type of relay, when energizing the coil, the movable contact is attached to the fixed one and the circuit closes, remaining closed while maintaining the voltage between the coil terminals.
How to know how to identify a relay with investor contacts?
When the groups of contacts have three plates, two of them are fixed and immobile at all times (fixed contacts), and the third plate (inverter mobile contact), can move and change position when we put the relay to work going from being together to one of the fixed contacts to separate from it and join the opposite fixed contact.
In this case, when the relay is at rest, we call the normally closed contact to which it is connected to the inverter contact and the normally open contact to which it is separated from the inverter contact.
This type of industrial electromechanical relays is the most common, since with the movement of the inverter contact we simultaneously handle the closing and opening.
It is very useful and versatile since we can use the same relay to open a circuit or to close a circuit when we excite its coil, depending on which fixed contact we have connected the cable, whether to the NO, or to the NC.
As we have already learned what a “group of contacts” in an electromechanical relay is and how it works, it is very easy to deduce how an electromechanical relay will work with two, three, or four groups of contacts.
It is important to say that all the mobile contacts of each of the groups of contacts available to the relay always move simultaneously.
That is, if for example a relay has four groups of contacts, when the coil is excited, the electromagnet will attract to its core the piece of iron to which the four moving plates are integral and this will move all at once, making the “Investment” in each of your contact groups.
The fact of having more than one group of contacts in a relay, is to take more advantage of the space and the excitation energy of the coil among other aspects.
In many industrial applications we need a circuit to open and another to close simultaneously (almost simultaneously) and this can easily be achieved by using a relay with two groups of contacts so that when the coil is energized one of the groups closes the first circuit and the other group opens the second circuit that was closed.
What is the identification of a relay contact according to his group?
The most usual numbering with which the contacts of the industrial electromechanical relays are indicated is with two digits forming a pair.
The first digit indicates the group and the second digit indicates the contact.
In this way, for a relay that has four groups of investor contacts, your contacts will be numbered in the way that we will explain below.
First group of contacts will be designated by the numbers 11, 12, 14 with 11 being the inverter contact, 12 the normally closed contact and 14 the normally open contact
Second group of contacts will be designated by the numbers 21, 22, 24, with 21 being the inverter contact, 22 the normally closed contact and 24 the normally open contact.
Third group of contacts will be designated by the numbers 31, 32, 34 with 31 being the inverter contact, 32 the normally closed contact and 34 the normally open contact
Fourth group of contacts will be designated by the numbers 41, 42, 44 with 41 being the inverter contact, 42 the normally closed contact and 44 the normally open contact.
We can see that it is quite simple to distinguish each one of the 12 contacts that we have to connect with this type of numbering.
We should note that investor contacts are always numbered by odd numbers, regardless of the group they are.
In turn, fixed contacts are always numbered by even numbers, regardless of the group they are, with the smallest of the two even numbers of each group being the normally closed and the largest being used to number the normally open contact.
What numbering has the base or base?
Logically, the same numbering recorded in the relay is also recorded in the connection base, so that it is very easy to reach the desired terminal with each of the conductors, avoiding errors that would prevent the correct operation of the installation.
It is not necessary to use all the groups of contacts, not even to use in the same group the three available contacts so that the relay works perfectly and fulfills its purpose.
In many industrial installations, the control cabinets are configured with enough space to be able to expand them in the future.
It is also usual to leave groups of contacts if used in some relays to be used later if the need arises to work with additional circuits that were initially foreseen.
How to connect a relay if it is a solid state relay?
Let’s see now how to connect a relay ?, when it is a solid state relay.
The vast majority of solid state relays available in the market can only close the power circuit when we apply voltage to the terminals of the control circuit.
That is, if voltage is not applied to the terminals of the control circuit and therefore the solid state relay is at rest, the power circuit is open, and no current passes between the terminals of the power circuit.
Therefore, the solid-state relay could be compared to a “two-blade” electromechanical relay for each group of contacts, in which the “fixed contact” is a normally open contact.
I have used the quotes because as we already know, in a solid state relay there are no metallic contacts like those of an electromechanical relay, but they use semiconductor materials that let pass or prevent the passage to the electric current according to the level of tension that we apply to the control circuit.
There are some models of solid state relays on the market that are pluggable to connection bases, although this is not usual.
In a conventional solid-state relay, terminals are usually available to connect directly to the relay the control circuit cables and the power circuit cables.
As in solid state relays, very important powers are handled, the naked eye immediately distinguishes which terminals are the power circuit or output circuit (very large terminals) and those of the control circuit (smaller terminals).
How to distinguish a relay of monofasic solid state from a three phase one?
It is also very easy to distinguish the single-phase solid-state relays of the three-phase solid-state relays, since in single-phase only four terminals will be found, two for the control circuit and two for the power circuit, while in the three-phase we will see eight terminals.
Two of the eight terminals that we see the three-phase solid-state relay are smaller and they are connected to the control circuit.
The other six much larger terminals are arranged in two rows of three terminals each to connect the inputs of the three-phase lines in one of the rows and the three conductors of the three-phase load, (motor, resistance, etc.) to the terminals of the other row.
What is the numbering of the solid state relay terminals?
As for the numbering of the terminals, there is not as much uniformity as in the case of electromechanical relays, although in most single-phase solid-state relays that can be found in the market, the terminals of the control circuit are numbered with 3 + and 4- if the control is in direct current, to take into account the polarity and with the numbers 3 ~ and 4 ~ if the control is in alternating current.
The power circuit terminals of the single-phase solid state relays are named with the numbers 1 ~ and 2 ~ when the relay is for AC loads, which are the most common or with the numbers 1+ and 2- when the relay is for direct current loads, to take into account the polarity of the output circuit.
For three-phase solid-state relays, the terminals of the control circuit can be named as explained for the single-phase and also some manufacturers use the letters A1 + and A2- for DC control and A1 ~ and A2 ~ for current control alternate
Regarding the six terminals of the output circuit, there are many options on the relays that are available in the market, but all of them use markings where it is very easy to recognize which are the input terminals and which ones the terminals to which we must connect load.
For example, the most used configuration is to number the inputs as L1, L2, L3 and the terminals of the load or output as T1, T2, T3 so that we immediately identify each “group of contacts”.
Other commonly used configurations are R, S, T for the inputs and U, V, W for the load, also A1, B1, C1 for the inputs and A2, B2, C2 for the load, and equally 1, 3, 5 for the inputs and 2, 4, 5 for the load.
How to connect a relay without forgetting some important things?
In the installation of solid-state relays, it is also very important not to forget the placement of fast fuses of nominal load somewhat lower than the maximum load that the relay admits in its output circuit.
This is because normally when a solid-state relay fails, its output contacts are short-circuited, even if we eliminate the voltage of the control circuit and therefore in this case we will always need a fuse to prevent the installation from being damaged.
On the other hand, we should never carry out the installation of a solid state relay without placing it on a sufficiently efficient heat sink to evacuate from the relay the heat power that will have to dissipate at full capacity in permanent connection.
In addition, the back plate of the solid state relay must be able to transmit heat perfectly to the heatsink, so between both surfaces we must place dissipating pads or thermal paste, for the conduction of heat is very good.
Another important recommendation, in this case aimed at electromechanical relays with DC coils, is to use diode modules connected on the base, which eliminate the reverse voltage peaks that occur when the relay is disconnected and could damage the relays. electronic devices that feed the relay, such as the output of an industrial programmer or PLC.
You have questions about How to connect a relay?
We hope to have contributed a little to the fact that the installation and connection of the relays in general, is somewhat clearer than at the beginning of the article for most of the users, and we will be happy to clarify any possible doubts that could arise in response to your messages in our email.