How Does A Relay Work – SPDT DPDT SPST Automotive Relay

How Does A Relay Work – SPDT DPDT SPST Automotive Relay


hi this dr. class here and in this video I will talk about relays and if you’ve been following my channel I’ve done some videos on installing car alarms car stereos hid installs and in those videos I talked about using relays and for those of you who are not familiar with it this video will help explain how it really works how to choose the relay that you want to use for your application so in front of me these four little devices you see are all relays as you can see they come in different shape size color packaging relays are essentially switches that you can turn on and turn off by applying electricity to a coil inside the relay when you energize that coil you’ll create a magnetic field they’ll attract that metal contact to mechanically close that switch when you take away the power source to the coil the magnetic field is gone and then the switch will open up the game when you’re looking for a relay there are a couple of things you need to pay attention to one is the voltage just required to energize a coil so for example this little one right here if you look closely it tells you that this is a 5 volt relay and means that it requires 5 volt DC to energize a kuo inside to close and open the switch here this is actually an AC relay right here it lists 120 volt AC so you need to apply hundred 20 volt AC to the coil for the relay the energize these two on the right are commonly used in automotive installs these are 12-volt relays and the coil inside this relay requires 12 volt to energize and they’ll open and close a switch in fact you’re good at the top here it tells you right here this is a 12 Vesey really the next thing you want to pay attention to is the rating of the switch so inside the relay their contacts that open and close and it can handle only certain amount of current and voltage so for example if you look at this little one here the print is kind of small but it tells you the contacts inside is rated for 5 amp at 250 volt AC below that is 5 amp at 30 volt DC so those are the rating for the contacts inside the relay here this is the 120 volt AC relay and it’s printed at the bottom here tells you that I can handle 10 amp 240 volt AC and 28 volt DC these automotive ones are typically rated at 30 amp or 40 amp so here let’s talk about this automotive relay I have here this is what they call a single-pole double-throw relay let me go over the pin outs on this on the bottom side of this relay this contact here at the bottom and this contact here at the top are used to energize this relay so when you apply 12 volt to these two terminals the relay will switch ON or switch off horizontally these three pins right here these are the actual contacts for your switch and the way you read this is this one over here is your common the one in the middle is normally closed and the one over here is normally open so let me draw it out still it make more sense to you for this relay we have a coil inside the relay on a switch side this is what we have a contacts so this is your common this is your normally closed this is your normally open and the coil is pin 85 and pin 86 the common is pin 30 normally closes 87 a and normally open is 87 and if you look at the bottom here there are markings at the bottom where the pins are when you don’t apply any power to the coil and the relay is not energized this contact between 30 and 87 a is closed once you energize this coil the contact inside will switch to this contact so what they mean by normally open is that when you apply no power to the coil here this contact is open it’s not closed and same as here it is normally closed meaning that when there’s no power applied to it this switch over here is closed of course if you energize a coil this switches the normally open side will not be closed and the normally closed side will not be open this is also what they call a single-pole double-throw because there is one contact that moves between these two over here so single cold is one pole between these two contacts is double-throw because there are two points over here so single-pole double-throw here I have two other relays that are single-pole double-throw if you look at the top marking on these relays the one on the Left made by Bosch as a marking of 20/30 a or 30-amp the one on the right has a marking of 30/40 amp what these two number mean is that because this is a single-pole double-throw relay the side that is normally closed or the contacts as closed is rated for the 20 amp when the relay switches over to the normally open contact that contact is rated for 30 amp for the Bosch 1 and 40 amp for the one on the right that’s why you see two ratings so the maximum current rated for the normally open side compared to the normally closed side are different here is the 5 volt relay this one is a very simple relay and this is what they call a single pole single throw and the way it works is there’s a coil inside and inside there’s a switch and that’s all this relay is so what happened is that when you energize this coil the switch closes now there is no contact out of here there are only two contacts that’s provided on this relay that’s why you see four leads on here two of them are for the coil side and two of them are for the switch side so two of the contacts are for these two points right here and it is normally open so when you energize this coil the switch closes and there’s contact made between these two points now let’s talk about this 120 volt AC double pole double throw relay what’s inside is a coil just like you saw earlier in the other relays however inside this relay we have two sets of contacts so this is my common this side is my normally closed this size normally open this is normally closed which is normally open so with this how it works is that when you energize this with under 20 volt AC these two contacts will switch from one side to the other and they’ll switch at the same time so go from normally close to the normally open side and same for this side so that’s why they call this the double pole double throw because we have two poles here for the contacts and a double throw the double represents the normally closed and normally open of each side so these are your throws now keep in mind these two are separate switches they’re not connecting anyway the only thing that’s connected is the fact that when this coil energizes it will switch both of these poles at the same time but physically inside the relay these two sets of contacts are totally separate before ending the video I want to show you a demonstration of how the relay actually works now this is a perfect example because this really happened to have a transparent enclosure where as this one is enclosed and you can’t see what’s going on inside if I was to apply 12 volt to the cold side you hear a click and that’s it you wouldn’t know what’s happening inside so let me show you this one and I’m going to apply 120 volt AC to the cold side and then you can see the mechanical switch that gets switch inside as you can see this terminal here and the one below that I’ve soldered and heat shrink this is going to be the hundred 20 volt AC now energize kuo inside this area is the coil over here you see three contacts here these are the switch side of the relay so this one over here that pointing to this contact goes through this black wire connects up here to this metal conductor it comes down and I’ll switch between this conductor and the other one right here so these two pins inside is connected to these two pins on the far right so again this is going to be my common goes up on this wire come back down and I’ll switch to either this output or this output and what you want to watch when it energizes relay is right here inside here you’ll see this middle conductor will click and switch between these two so here I’m going to turn it on one two three I’m gonna turn it off I’ll do it again one two three turn it off if I flip this over you’ll see the contacts are totally separate between the left side and the right side this is called a double pole double throw let me energize this side so you see it also I hope you found this video helpful and if you’re looking for relay now if you read something that says you know spst the PDT d PST you know what it means now if you have any questions leave a comment below if you like video don’t forget to click on the like and subscribe to my channel thank you


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