hi my name is EJ dagel I'm the director of Robotics and Manufacturing here at Dunwoody College technology and today we're going to talk about relay logic circuits um some topic learning objectives that I'd like to cover today uh first one is upon the successful completion of this topic uh traine should be able to describe the physical construction and operation of a relay uh the trainee should be able to describe how to use relays to control electronic devices and the trainee should be able to wire a simple relay control circuit so the first thing I want to talk about is there are several different types of relays out there that you may see I've got a few samples of them here um some of them are much larger and and they are what we call contactors um they can be used to start motors they can be used to uh to control large current devices um couple examples of those right here and then there's a lot of relays that are more of an ice cube type relay like this relay here and these are generally used in more of a control logic circuit and then there's solid state relays and and transistor based relays and a lot of other relays too but I just want to show at least a couple of samples there um now if we go over here this is the physical construction of a relay to kind of give you an example here I'll try to grab a different marker here give you a different color there are standard there are a set of terminals just for the coil so you see a couple of screw terminals here that you'll be able to pull wires up to when we get up into the lab um if I pull wires to here I've got to have a return path on the other wire um in some cases this coil inside of here is a DC coil and it's important that I use direct current with these terminals in other cases this might be an AC coil in which case it would be fine for me to use alternating current um with this particular relay um let's say for practical purposes this is a DC relay I would wire from a DC Source something like this maybe it's 24 volts DC and I would wire perhaps through a switch and maybe over to this contact now that would Supply current um through the coil and give me a path through that coil um but then I'd also need a return path so from the other side I would run over to here and I'd have a complete path When I close this switch I would get current through the coil and back to the power source now all the coil does is the coil is a big electromagnet and its sole purpose in life is to move this Armature up here this is an Armature here right here the idea is that this would be a magnet here an electromagnet that would energize and it would pull this Armature down and whenever there wasn't current through this coil the Armature would spring back up because of the spring that we see over here so we can see that this spring is actually attached to the Armature itself and the Armature pivots on this Pivot Point right here and what happens is when I energize the coil the Armature is pulled down by the magnet and when I release power to the coil the spring is going to pull back down on the Armature and bring it back up so basically this Armature is going to bounce between two points and this would be one point here and this would be one point here these are what we call our normally open and normally closed contacts so as I look at this this drawing here what I can see is right now my coil is deenergized so the switch is off the coil is deenergized the spring is pulling on this there's no magnetism to pull on the Armature and and what happens is this Armature flies up to the normally closed contact and what normally closed means this NC stands for normally closed it means that it's normally closed without power on the coil so in this particular example the Armature is sitting on the normally closed contact UM as soon as I energize this if I was to close this switch now I have current flow so the switch is closed closed and I'm going to get current flow up and through my coil and back to the source as soon as there is current flow this becomes an electromagnet at that point the Armature is pulled down and now I'm going to go down to the normally open contact so we'll use blue to represent that but the Armature would change States and it would now be sitting down here and it would be pulling on the spring a little bit so the second that I released this energy or open this switch the magnet goes away and the spring pulls back on it and takes it back up so that's what normally open and normally closed contacts mean normally closed meaning closed when power is on and normally open meaning it's open when power's on there's also a common terminal up here the common terminal is a point where I can wire two to deliver power through a contact to a control device so this is the basic relay construction let's move over to a control circuit and look at how this works in real life a lot of times when we're working with control circuits we'll work with push buttons you'll actually see an example of a push button right here um this is a push button station there is a start push button on this station and there's a stop push button on this station and in our example I'll talk a little bit about start and stop and how they're going to be used in this control circuit so we'll go back up to the control circuit this is a schematic diagram we like to call this a ladder diagram because it's got a power rail a power Rail and then it's got rungs like a ladder would have and if I wanted to add another circuit I would just extend the lines and add another rung um in this case you'll see that the stop push button much like a normally closed contact it's shown in its closed State all stop push buttons are going to be normally closed meaning they can deliver power without someone pressing them all start push buttons are going to be normally open meaning they will only deliver power when they are pressed so we show that it's not making contact when I press it it's going to go down and make contact and then I have my coil here this is the schematic representation of a relay coil so I can see my coil here is wired up in series with the stop in series with the start and back to the power rail making a complete circuit um also though I have the normally open contacts I've got two K1 this is the reference designator of my a lot of times we'll use K to reference a relay K1 K2 K3 whatever it might be in this case the contacts still keep that reference designator K1 um but I'm going to put A- n o or A- n o or in some cases a-n C to represent that it's a normally open or a normally closed contact I could also see a normally closed contact if I was to see something like this in a drawing because it'd be shown drawn in its deenergized state and normally closed would appear to be closed okay now let's talk about the way this works right now the way this circuit's going to work is there is power or current being delivered right to this point because the stop push button is closed it's normally closed so there's power sitting on start push button waiting to energize something um in this case the second I hit the start push button I press it down it's going to make contact when it makes contact it's going to deliver power over to the coil the coil is going to consume that energy it's going to become magnetized again and all of its contacts are going to switch positions so the normally open contacts are going to close just like we saw in our diagram a minute ago and any normally closed contacts would open which means they are also delivering power now like so there and there and now I have a buzzer down here that's going to sound so that's going to work great now if I release the start push button if I release this button what's going to happen is this line is going to go away but the buzzer is going to stay on and the reason why the buzzer is going to stay on is because we have a holding contact so right here we have a holding circuit those contacts are not going to change State until they lose power well they have not lost power cuz no one hit the stop push button so they haven't lost power so they're holding this control relay on so both of the contacts are going to remain closed and now if I want to shut this circuit off the only way to shut this circuit off is to press the stop push button which is a normally closed push button it'll look like this now that's going to release release all of the current we're not going to have any current we're going to remove current from our circuit there's no way for electrons to flow through here so now the holding circuit's going to go away this circuit down here is going to go away and all of these normally open contacts are going to go back to their normally open positions just like so so hopefully that gives you a good idea now if I release that stop push button at this point it goes back to normally closed but I don't have any current pass and we're in our normally open deenergized state so what I'd like to do now is at this point we'll go up into the lab and we'll take a look at how we'd wire this circuit up and how we'd make this circuit work in real life