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[Applause]
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hey there guys paul here from
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engineeringmindset.com in this video
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we're going to be looking at relays to
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understand the main parts
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the different types as well as how they
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work for all of your relay needs
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check out telecontrols who have kindly
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sponsored this video
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tele controls are one of the leading
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manufacturers in the automation industry
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since 1963. they offer some of the best
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or through linkedin to receive your free
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relay configuration cheat sheet
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for more information see the link in the
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video description down below
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a relay is an electrically operated
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switch
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traditionally relays use an
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electromagnet to mechanically operate
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the switch
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however newer versions will use
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electronics such as solid state relays
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relays are used where it is necessary to
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control a circuit
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using a low power signal or where
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several circuits must be controlled by
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one signal
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relays ensure complete electrical
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isolation between the controlling
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and the controlled circuits relays are
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often used in circuits
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to reduce the current that flows through
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the primary control switch
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a relatively low amperage switch timer
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or sensor
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can be used to turn a much higher
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capacity load on and off
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we'll see examples of this a little
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later in the video
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there are two main circuits in the relay
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the primary side
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and the secondary side the primary
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circuit provides the control signal
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to operate the relay this could be
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controlled by a manual switch
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a thermostat or some other type of
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sensor
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the primary circuit is generally
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connected to a low voltage
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dc supplier the secondary circuit
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is the circuit which contains the load
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which needs to be switched and
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controlled
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when we talk about a load we mean any
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device that will consume electricity
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such as a fan a pump a compressor or
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even a light bulb
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on the primary side we find an
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electromagnetic coil
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this is the coil of wire which generates
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a magnetic field
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when current passes through it when
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electricity passes through a wire
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it creates an electromagnetic field we
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can see that by placing compasses around
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the wire
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when we pass a current through the wire
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the compasses change direction
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to align with electromagnetic field
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when we wrap the wire into a coil the
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magnetic field of each wire combines
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together
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to form a larger stronger magnetic field
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we can control this magnetic field by
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simply controlling
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the current by the way we have covered
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how solenoid coils work
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and even how to make your own solenoid
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in our previous videos
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do check that out links can be found in
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the video description
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down below at the end of the
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electromagnet
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we find the armature this is a small
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component which is pivoted
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when the electromagnet energizes it
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attracts the armature
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when the electromagnet is de-energized
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the armature
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returns to its original position
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typically a small spring is used to
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achieve this
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connected to the armature is a movable
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contactor
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when the armature is attracted to the
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electromagnet it closes
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and completes the circuit on the
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secondary side
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we have two types of basic relay the
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normally open
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and the normally closed type there are
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other types of relays
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and we're going to look at these a
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little later in the video
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with the normally open type no
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electricity flows in the secondary
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circuit
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the load is therefore off however
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when a current is passed through the
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primary circuit a magnetic field is
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induced in the electromagnet
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this magnetic field attracts the
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armature and pulls the movable contactor
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until it touches the terminals of the
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secondary circuit
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this completes the circuit and provides
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electricity to the load
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with the normally closed type the
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secondary circuit is normally complete
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and so the load is on when the current
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is passed through the primary circuit
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the electromagnetic field causes the
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armature to push away
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which disconnects the contactor and
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breaks the circuit
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this cuts the supply of electricity to
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the load
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the operation of solid state relays or
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ssrs
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is similar in principle but unlike
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electromechanical relays
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it has no moving parts the solid state
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relay
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uses electrical and optical properties
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of solid-state semiconductors
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to perform its input and output
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isolation as well as switching functions
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with this type of device we find an led
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light on the primary side
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instead of an electromagnet the led
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provides optical coupling
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by shining a beam of light across a gap
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and into the receiver
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of an adjacent photosensitive transistor
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we control the operation of this type of
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relay
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by simply turning the led on and off
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the photo transistor acts something like
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an insulator
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and doesn't allow current to flow unless
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it's exposed to light
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inside the photo transistor we have
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different layers of semiconductor
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materials
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there are n-type and p-type which are
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sandwiched together
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the n-type and p-type are both made from
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silicon
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but they have each been mixed with other
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materials to change their electrical
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properties the n-type has been mixed
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with a material
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that gives it lots of extra as well as
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unneeded electrons
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these are free to move around to other
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atoms
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the p-type has been mixed with a
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different material that has fewer
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electrons
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so this side has lots of empty space
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where electrons can move too
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when the materials are joined together
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an electrical barrier develops
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and prevents electrons from flowing
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however
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when the led is turned on it will emit
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another particle
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known as a photon the photon hits the
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p-type material
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and knocks the electrons pushing them
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across the barrier
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and into the n-type material the
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electrons of the first barrier
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will now be able to also make the jump
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and so a current is developed
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once the led is turned off the photons
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stop knocking the electrons across the
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barrier
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and so the current in the secondary side
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stops
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so we can control the secondary circuit
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just by using
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a beam of light
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there are many types of relays and we're
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going to now consider
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a few of the main ones as well as some
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simple examples of how they are used
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let me know in the comments section how
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and where you've seen relays used
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or even better tell me what ideas you
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have for their application
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or any project you're working on where
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they could be applied
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as we have seen earlier in this video we
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have the simple
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normally open relay this means the load
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of the secondary side
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is off until the circuit is complete on
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the primary
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we could use this for example to control
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a fan
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by using a biometallic strip as a switch
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on the primary side
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the biometallic strip will bend as it
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increases in temperature
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at a certain temperature it will
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complete the circuit
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and turn the fan on to provide some
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cooling
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we also find normally closed relays this
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means the load on the secondary side
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is normally on we could for example
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control a simple pump system to maintain
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a certain water level
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in the storage tank when the water level
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is low
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the pump is on but once it reaches the
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limit we require
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it completes the primary circuit and
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pulls the contactor away
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which cuts the power to the pump
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in a standard normally open relay once
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the primary circuit is de-energized the
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electromagnetic field disappears
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and the spring pulls the contactor back
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to its original position
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but sometimes we want the secondary
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circuit to remain
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live after the primary circuit is
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reopened
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for that we can use a latching relay for
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example
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when we press the call button on an
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elevator we want the light on the button
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to remain
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on so that the user knows the elevator
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is coming
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so we can use latching relays to do this
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there are many different designs for
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this type of relay
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but in this very simplified example we
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had three
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separated circuits and a piston which
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sits between them
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the first circuit is the call button the
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second is the lamp
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and the third is the reset circuit when
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the call button is pressed
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it completes the circuit and powers the
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electromagnet
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this pulls the piston and completes the
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circuit to turn the lamp on
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a signal is also sent to the elevator
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controller
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to send the elevator down the button
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is released this cuts the power to the
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initial circuit
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but as the piston isn't spring-loaded it
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stays in position
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and the lamp remains on
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once the elevator car reaches the lower
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floor
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it makes contact with the off switch
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this powers the second electromagnet
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and pulls the piston away cutting the
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power to the lamp
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latching relays therefore have the
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benefit of having positional memory
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once activated they will remain in the
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last position
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without the need for any further input
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or current
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relays can have single or double poles
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the term pole refers to the number of
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contacts switched when the relay is
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energized
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this allows more than one secondary
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circuit to be energized
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from a primary circuit we could for
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example
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use a double pole relay to control a
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cooling fan
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as well as a warning light both the fan
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and the lamp are normally off
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but when the biometallic strip on the
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primary circuit gets too hot
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it bends to complete the circuit this
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creates the electromagnetic field
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and closes both contactors on the
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secondary side
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this provides power to the cooling fan
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as well as the warning light
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when dealing with relays you'll often
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hear the term
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throws this refers to the number of
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contacts or connection points
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a double throw relay combines a normally
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open and a normally closed circuit
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a double throw relay is also called a
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changeover relay
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as it alternates or changes between two
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secondary circuits
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in this example when the primary circuit
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is open
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the spring on the secondary side pulls
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the contactor to terminal b
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powering the lamp the fan remains off
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because the circuit is not complete
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when the primary side is energized the
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electromagnet pulls the contactor
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to terminal a and diverts electricity
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this time powering the fan and turning
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the lamp off
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so we can use this type of relay to
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control different circuits
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depending on an event
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a double pole double throw relay or dpdt
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is used to control two states on two
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separate circuits
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here we can see a dpdt relay
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when the primary circuit is not complete
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terminals t1
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and t2 are connected to terminals b and
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d
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respectively the red led and the
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indicator light
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are energized when the primary circuit
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is closed
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then t1 and t2 connect to terminals a
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and c the fan turns on and the green led
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is also energized
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something we need to consider when
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working with electromagnets
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is the back emf or electromotive force
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when we power the coil the
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electromagnetic field
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builds up to a maximum point the
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magnetic field
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is storing energy when we cut the power
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the electromagnetic field collapses and
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this releases the stored energy
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very quickly this collapsing field
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continues to push the electrons
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and this is why we get the back emf
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this is not a good thing because it can
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produce very large
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voltage spikes which damage our circuit
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to overcome this we can use something
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like a diode to suppress this
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the diode only allows current to flow in
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one direction
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so in normal operation the current flows
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to the coil
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but when we cut the power the back emf
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is going to push the electrons
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and so the diode will now provide a path
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for the coil to dissipate its energy
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safely so that it doesn't damage our
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circuit
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okay that's it for this video but to
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continue your learning on electrical
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engineering then check out one of the
00:13:46
videos on screen now
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and i'll catch you there for the next
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lesson don't forget to follow us on
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facebook
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the engineeringmindset.com
