Full transcript
0:04Hey there guys, Paul here from theengineeringmindset.com.
0:08In this video, we're going to be looking at relays to understand
0:12the main parts, the
0:13different types, as well as how they work.
0:16For all of your relay needs, check out Telecontrols, who have
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0:45through LinkedIn to receive
0:46your free relay configuration cheat sheet.
0:49For more information, see the link in the video description down below.
0:55A relay is an electrically operated switch.
0:59Traditionally, relays use an electromagnet to mechanically operate the switch.
1:04However, newer versions will use electronics, such as solid state relays.
1:10Relays are used where it is necessary to control
1:13a circuit using a low power signal, or where
1:16several circuits must be controlled by one signal.
1:20Relays ensure complete electrical isolation
1:23between the controlling and the controlled
1:25circuits.
1:27Relays are often used in circuits to reduce the current that
1:31flows through the primary
1:32control switch.
1:34A relatively low amperage switch, timer, or sensor
1:37can be used to turn a much higher capacity
1:40load on and off.
1:42We'll see examples of this a little later in the video.
1:48There are two main circuits in the relay, the primary side
1:52and the secondary side.
1:54The primary circuit provides the control signal to operate the relay.
1:59This could be controlled by a manual switch,
2:01a thermostat, or some other type of sensor.
2:05The primary circuit is generally connected to a low voltage DC supply.
2:10The secondary circuit is the circuit which contains the load which
2:14needs to be switched
2:15and controlled.
2:16When we talk about a load, we mean any device that will consume
2:20electricity, such as a fan,
2:22a pump, a compressor, or even a light bulb.
2:26On the primary side, we find an electromagnetic coil.
2:29This is a coil of wire which generates a magnetic field when
2:33current passes through it.
2:35When electricity passes through a wire, it creates an electromagnetic field.
2:40We can see that by placing compasses around the wire.
2:43When we pass a current through the wire, the compasses change direction
2:48to align with the
2:49electromagnetic field.
2:51When we wrap the wire into a coil, the magnetic field of each
2:55wire combines together to form
2:57a larger, stronger magnetic field.
3:00We can control this magnetic field by simply controlling the current.
3:05By the way, we have covered how solenoid coils
3:08work and even how to make your own solenoid
3:11in our previous videos.
3:12Do check that out.
3:13Links can be found in the video description down below.
3:17At the end of the electromagnet, we find the armature.
3:21This is a small component which is pivoted.
3:24When the electromagnet energises, it attracts the armature.
3:28When the electromagnet is de-energised, the
3:31armature returns to its original position.
3:35Typically a small spring is used to achieve this.
3:38Connected to the armature is a movable contactor.
3:42When the armature is attracted to the electromagnet,
3:45it closes and completes the circuit on the
3:47secondary side.
3:51We have two types of basic relay.
3:54The normally open and the normally closed type.
3:57There are other types of relays and we're going
3:59to look at these a little later in the
4:01video.
4:02With the normally open type, no electricity flows in
4:05the secondary circuit.
4:07The load is therefore off.
4:10However, when a current is passed through the primary circuit, a
4:14magnetic field is induced
4:15in the electromagnet.
4:17This magnetic field attracts the armature and
4:19pulls the movable contactor until it touches
4:22the terminals of the secondary circuit.
4:25This completes the circuit and provides electricity to the load.
4:30With the normally closed type, the secondary
4:32circuit is normally complete and so the load
4:35is on.
4:37When the current is passed through the primary circuit,
4:40the electromagnetic field causes
4:42the armature to push away, which disconnects the contactor
4:46and breaks the circuit.
4:47This cuts the supply of electricity to the load.
4:57The operation of solid state relays, or SSRs, is similar in principle.
5:02But unlike electromechanical relays, it has no moving parts.
5:06The solid state relay uses the electrical and optical properties
5:11of solid state semiconductors
5:13to perform its input and output isolation as well as switching functions.
5:18With this type of device, we find an LED light on the primary
5:22side, instead of an electromagnet.
5:25The LED provides optical coupling by shining a
5:29beam of light across a gap and into the
5:31receiver of an adjacent photosensitive transistor.
5:34We control the operation of this type of relay by simply turning
5:39the LED on and off.
5:41The photo transistor acts something like an
5:44insulator and doesn't allow current to flow
5:46unless it's exposed to light.
5:49Inside the photo transistor, we have different layers
5:52of semiconductor materials.
5:54There are n-type and p-type, which are sandwiched together.
5:58The n-type and p-type are both made from silicon,
6:02but they have each been mixed with other materials
6:04to change their electrical properties.
6:07The n-type has been mixed with a material that
6:10gives it lots of extra as well as unneeded electrons.
6:13These are free to move around to other atoms.
6:17The p-type has been mixed with a different material that has
6:20fewer electrons, so this
6:22side has lots of empty space where electrons can move too.
6:27When the materials are joined together, an electrical barrier
6:30develops and prevents electrons
6:32from flowing.
6:33However, when the LED is turned on, it will emit another particle
6:38known as a photon.
6:40The photon hits the p-type material and knocks the electrons, pushing
6:44them across the barrier
6:46and into the n-type material.
6:48The electrons of the first barrier will now be
6:51able to also make the jump and so a current
6:54is developed.
6:57Once the LED is turned off, the photons stop knocking the
7:00electrons across the barrier
7:02and so the current in the secondary side stops.
7:05So we can control the secondary circuit just by using a beam of light.
7:13There are many types of relays and we're going
7:15to now consider a few of the main ones
7:17as well as some simple examples of how they are used.
7:21Let me know in the comments section how and where you've seen relays used.
7:25Or even better, tell me what ideas you have for their application
7:29or any project you're
7:30working on where they could be applied.
7:33As we have seen earlier in this video, we have the
7:36simple normally open relay.
7:38This means the load of the secondary side is off until the
7:42circuit is complete on the
7:43primary.
7:45We could use this, for example, to control a
7:47fan by using a bimetallic strip as a switch
7:50on the primary side.
7:52The bimetallic strip will bend as it increases in temperature.
7:56At a certain temperature, it will complete the
7:58circuit and turn the fan on to provide
8:01some cooling.
8:05We also find normally closed relays.
8:08This means the load on the secondary side is normally on.
8:13We could, for example, control a simple pump
8:15system to maintain a certain water level in
8:18the storage tank.
8:20When the water level is low, the pump is on.
8:23But once it reaches the limit we require, it completes the
8:27primary circuit and pulls
8:29the contactor away, which cuts the power to the pump.
8:34In a standard normally open relay, once the primary circuit
8:38is de-energised, the electromagnetic
8:40field disappears and the spring pulls the contactor back
8:44to its original position.
8:47But sometimes we want the secondary circuit
8:49to remain live after the primary circuit is
8:52reopened.
8:53For that, we can use a latching relay.
8:56For example, when we press the call button on
8:58an elevator, we want the light on the button
9:01to remain on, so that the user knows the elevator is coming.
9:06So we can use latching relays to do this.
9:09There are many different designs for this type of relay, but in this very simplified
9:14example we had three separated circuits and a piston which sits between them.
9:20The first circuit is the call button, the second
9:23is the lamp, and the third is the reset
9:25circuit.
9:26When the call button is pressed, it completes the circuit
9:30and powers the electromagnet.
9:32This pulls the piston and completes the circuit to turn the lamp on.
9:37A signal is also sent to the elevator controller to
9:40send the elevator down.
9:42The button is released, this cuts the power
9:45to the initial circuit, but as the piston
9:48isn't spring loaded, it stays in position and the lamp remains on.
9:53Once the elevator car reaches the lower floor, it makes contact
9:58with the off switch.
9:59This powers the second electromagnet and pulls
10:02the piston away, cutting the power to the
10:05lamp.
10:06Latching relays therefore have the benefit of having positional memory.
10:11Once activated, they will remain in the last position without the
10:15need for any further
10:16input or current.
10:21Relays can have single or double poles.
10:23The term pole refers to the number of contacts switched when
10:27the relay is energised.
10:29This allows more than one secondary circuit
10:31to be energised from a primary circuit.
10:35We could, for example, use a double pole relay
10:38to control a cooling fan as well as a warning
10:40light.
10:41Both the fan and the lamp are normally off, but when the
10:45bimetallic strip on the primary
10:46circuit gets too hot, it bends to complete the circuit.
10:50This creates the electromagnetic field and closes both contactors
10:55on the secondary side.
10:56This provides power to the cooling fan as well as the warning light.
11:02When dealing with relays, you will often hear the term throws.
11:07This refers to the number of contacts or connection points.
11:11A double throw relay combines a normally open and
11:14a normally closed circuit.
11:15A double throw relay is also called a changeover relay, as it
11:20alternates or changes between
11:22two secondary circuits.
11:25In this example, when the primary circuit is open, the spring on the secondary side
11:30pulls the contactor to terminal B, powering the lamp.
11:34The fan remains off because the circuit is not complete.
11:39When the primary side is energised, the electromagnet
11:42pulls the contactor to terminal A and diverts
11:45electricity, this time powering the fan and turning the lamp off.
11:50So we can use this type of relay to control different circuits
11:54depending on an event.
11:58A double pole double throw relay, or DPDT, is used to control
12:03two states on two separate
12:05circuits.
12:07Here we can see a DPDT relay.
12:10When the primary circuit is not complete, terminals T1 and T2
12:15are connected to terminals
12:17B and D respectively.
12:20The red LED and the indicator light are energised.
12:23When the primary circuit is closed, then T1
12:27and T2 connect to terminals A and C.
12:31The fan turns on and the green LED is also energised.
12:38Something we need to consider when working with electromagnets is the
12:42back EMF, or electromotive
12:44force.
12:45When we power the coil, the electromagnetic
12:48field builds up to a maximum point.
12:51The magnetic field is storing energy.
12:54When we cut the power, the electromagnetic field collapses and
12:58this releases the stored
12:59energy very quickly.
13:02This collapsing field continues to push the electrons,
13:05and this is why we get the back
13:06EMF.
13:08This is not a good thing, because it can produce very large
13:12voltage spikes which damage our
13:14circuit.
13:15To overcome this, we can use something like a diode to suppress this.
13:20The diode only allows current to flow in one direction.
13:24So in normal operation, the current flows to the coil.
13:28But when we cut the power, the back EMF is going to push the
13:32electrons, and so the diode
13:33will now provide a path for the coil to dissipate its energy safely,
13:38so that it doesn't damage
13:39our circuit.
13:40OK, that's it for this video, but to continue your learning
13:44on electrical engineering, then
13:45check out one of the videos on screen now, and I'll catch you
13:48there for the next lesson.
13:50Don't forget to follow us on Facebook, LinkedIn, Instagram, Twitter,
13:54as well as TheEngineeringMindset.com