all right ladies and gentlemen we're going 
to talk about relays today, try to polish   up a couple things on the ever popular 90 340 
relay. That is 9-0-3-4-0, so 90-340 relay and   this is what we got right here in front of 
us. You can see here this is a White Rogers.   A white rogers relay, it says type 91 but 
right here it says 9340 and that's what   this particular relay is. It's a very 
common relay for things such as your   indoor fan motor and you can see here there 
there's some other pertinent details on the   data plate as well.

There's one on the front 
and also on the back. So just as a refresher,   what information do we need to know about 
a relay in order to replace one that you've   diagnosed is bad? Anytime you do a replacement 
there's things that you need to know about the   component in order to match it properly. The first 
thing you need you're going to need to know is   the coil voltage. We typically 
use 24 volt coils in our field   to control the switches and things that run 
our system so we need to know the coil voltage.   The second thing we need to know is the number 
and type of switches we're using.

Are we using one   normally open switch and one normally closed? Are 
we using two normally opens? Two normally closed?   There's a plethora of different options out there 
so you need to know what type of switches you need   and how many of that type. Okay, last but not 
least once you know the information about your   switches you need to know how many amps they can 

So switch and ampacity. How many amps   are going to be going through the normally open 
or normally closed switches that you do need?   Okay. So it's very much like a contactor. 
Simple information that you need to know,   coil voltage, number and type of switches, and 
then the ampacity of those switches. So I'm not   going to focus too much but I am going to show 
you the data plate that you've already taken a   look at it once.

This is our White Rogers brand, 
a 90-340 relay and you can see here on the front   it tells you that the coil is 24 volts. So this 
will work with most of our residential style   heating and air equipment. If 
I were to flip this unit over   you can see the back side this is going to 
give us our ampacity and voltage rating on   these switches. So you can see across the 
top here I've got 12 FLA, full load amps. 60 lock rotor amps (LRA) at 125 volts so if 
I run 125 volts through here I can handle in   the case of a fan motor. Fan motors are an 
induction load, they are not like electric   heat that is a resistive load.

pexels photo 5835359

The fan motors 
are going to pull a higher amperage on startup   and basically die down, or slow down, to a 
cruising speed you know their normal rotation.   So as we look here you can see that on the 
startup amps I can handle up to 60 on the   lock rotor (that initial inrush current high amp 
draw load) and then once the motor starts going,   then as the motor dies down to its running 
amperage, I can have no more than 12 full load   amps on this. Most of the time our blower is only 
going to run four or five amps or so (depending   on different factors for that particular motor) 
but this 12 full load amps can definitely handle   most of our indoor motor needs that we're 
going to run across.

So where full load amps,   your lock rotor amps, and then your voltage. 
You can see here if I ran up to a 277 volt   through this switch then I could have 
six full load amps and 35 lock rotor amps   okay so high in rush current the lock road ramps 
at 35 and then I can get the six full load amps   your your running amps basically we can go more 
into depth uh later on that if i was going to   run something a resistive load through here not 
necessarily an electric heater but something of   the sort maybe a small i don't know if this is a 
refrigerator maybe you have a little heater in the   door frame to keep any moisture build up around 
the seal or what this is a 15 amp resistive load   your resistive load is a steady load it 
doesn't have the inrush current and then   the the running amperage that you would normally 
have with an induction load so basic information   there not trying to rehash this i want to 
go over more detail about the switches so   you can see here and I'll try to get the light 
just right you can see here on the top that we've   got several switches going on I've got terminal 
one right here as indicated on the right there by   the little shadow underneath my little pointer 
terminal one is right here terminal 2 is here   on this middle terminal a little bit lower and 
then terminal 3 here this is one set of switches   now the way that they have laid out this little 
diagram on top of the relay you can see that these   switches are connected so I've got three terminals 
and two switches on or between those terminals   okay from one to three is a normally open and 
you can see right there is that normally open   indication that little schematic view and then 
from one to two is normally closed so one to   two is normally closed one to three is normally 
open and these are one set of switches they act they act when the 24 volt coil is energized 
you know of course from the thermostat so   in the case of an indoor fan relay when the 
thermostat sends 24 volts down the green wire   it's going to go ultimately here to this coil how 
does it do that well if you look right here this   is it's so so simple right these terminals right 
here on the end they are not labeled like your   switches are but down here at the bottom we've got 
a set of uh double stake homes right here and then   over here we've got another set this is going to 
be your g going in your 24 volts coming in on one   side doesn't have to be the right side about 
as I'm holding it here it could be on the left   side but whatever side the g comes in the opposite 
side is going to be where your common hooks up and   returns back to power to transformer okay the 
relay coil is easily identified by the copper   wire that is coming off the bottom of these male 
stake-ons and wraps around and goes to the bottom   of the coil so you can see right here there's a 
little copper wire that is soldered into this male   stake on connection here and the same thing over 
here try to get a little bit closer there we go   you can see that copper wire as it's uh in there 
on both sides so that's our coil on the top of   this 9340 relay they label the switches and I've 
already gone through terminals one two and three   but there is a identical kind of sister set of 
switches that's four five and six okay it is the   same layout as the the first set of switches we 
went over and it acts at the same time but it's   a completely separate circuit so i could run two 
different loads through here different voltages if   need be but they are completely separate I'll try 
to throw up a little picture a little schematic   but it's it's fairly easy to understand uh 
one two and three is one set of switches and   four five and six is another set of switches 
most people don't have a confusion with that   i know personally when i first started 
learning about relays years ago when I   got into the field i didn't have a eureka moment 
until I realized one big thing about this relay   most people uh and I'm gonna say here in the 
western world okay we read left to right and   for whatever reason we get in our mind that 
power has to flow through here from left to   right and i think once you know the three basic 
things that you need to replace this component uh   the coil voltage capacity of the switches and the 
number and type of switches that you have on that   relay the second thing you really have to get in 
your head is understanding you have to understand   circuits more so than the relay this relay is just 
a switch either normally open or normally close   of course when the coil gets voltage 24 volts it's 
going to make those switches do the opposite okay   you get all the bells the whistles the clicks the 
pops but a lot of people they get caught in their   head they get tunnel vision that power has to come 
in here on terminal one and out on number three   because that's the way you see it and that's the 
way you read you read left to right top to bottom   but that's not the case with relays once you 
get into the field and you really understand   you can see or you'll learn that not only can i 
come in and and flow through here bring power in   on terminal one and out on terminal 3 i can 
actually do it backwards i can do it in on   terminal 3 and out on terminal 1.

So that's a 
big deal I'm not trying to push that I'm just   trying to to give you something to think about 
with this but for me when i wrote down in my   little clipboard as a young heating and air tech 
the first thing that I had to write down and go   home and study and really commit to learning was 
this relay and I think a lot of people are thrown   off by it and it's just the way it is until you 
learn but you have to put in your time and effort   and and try to understand electricity and circuits 
and diagrams and and get into it okay so i hope   you get into it but just think about this not not 
only does power or can power flow in terminal 1   out terminal 3 the left to right as we're 
talking about i can actually bring it in   terminal 3 and out terminal 1.

Truth be told a 
lot of times when we're dealing with a heat strip   system that is interlocked with the blower circuit 
uh we're actually bringing power in terminal three   and out terminal one to go to a certain fan 
speed on a PSC motor so make sure that you   think about that it's not left to right it can 
be right to left but no matter what no matter   how this is working or if you're only using one 
set of switches instead of both set of switches   I've got two sets of switches they are 
identical in their makeup normally open   normally closed and they both react since 
I've got two sets of switches on this relay   both act at the same time instantaneous with that 
124 volts with that 124 volt signal that we have   so keep that in mind the last thing that 
I'm going to really go in and and show you   is voltage checks so I'm going to go out here 
on a practice board I've got a 9340 relay   wired up I'm going to do some voltage checks 
across this switch because not only in the   the schematic reading and learning switches 
and the whole left to right that we mentioned   checks across the switch throw a lot of 
technicians so I'm going to go wire up   a little practice board take you along and 
hopefully this will make some sense to you   all right here we go I've got a 
practice board we use here it's cool   and what I've done is I've wired up real simple 
24 volts to the fan relay coil and we're going   to use this 9340 as a fan relay but I've wired 
up to 24 volts coming from our thermostat and   to replicate a motor we've got a 120 volt light 
okay so real simple when i turn on the fan   from the auto to the on position on the thermostat 
the light comes on when i turn it back to auto   the light goes off so we're sending 24 volts 
to the coil when the coil gets power it's   going to close our connection our switch from 
terminal one to three and it's going to open   from terminals one to two if i were to add 
in another wire on the one to two terminal   then i could easily control a second light as you 
read this this set of switches from left to right   terminal 1 is bringing power in and terminal 3 
is power out terminal 2 would also be power out   because we're looking at it as we read 
from left to right so you can see here that   going through the normally closed from terminal 1 
out terminal 2 I've got a normally closed switch   and that brings on a light when i energize 
my 24 volt coil by calling for the fan   now you can see that the lights shift my normally 
closed from one to two is now open and shut the   light off and the switch from terminals one 
to three is closed so i turn on this second   light once i take power away from g everything 
goes back and forth so as we read the relay   from left to right i can take one power source in 
on terminal one and control two different loads   depending on whether or not I'm using the normally 
open or normally closed switch but this isn't   typical of what we do for our residential heating 
and air typically we use it backwards we have two   power sources coming in on terminal 3 and another 
on terminal 2 and it is going out on terminal 1   towards our fan motor to our psc motor for a 
high medium or low speed so I'm only going to   run through this one example here because 
i mainly want to talk about the voltage so   uh here we go i've got my volt meter set up 
right here i'm going to turn the light on   i think that'll come into play so right now i 
have got no power zero volts on my 24 volt coil   and because of that i am flowing electricity 
through the normally closed switch   so it's real simple if i were to check 
voltage across that switch right now   one to two i am getting better yet there we 
go from terminal one to two i get zero volts   and my light is on a lot of technicians i feel 
don't understand why it is because power coming in   on terminal 1 is coming from your l1 we'll say 
and it is immediately flowing out terminal 2.   so i've got l1 coming in and l1 coming out what 
is the difference between l1 and l1 it is zero   okay that switch is closed if i were to do 
the same check from one to three right now   i'm gonna come over here and do terminal one to 
terminal 3 you can see that i've got 122 volts   but that light is off it's 
because that switch is open   voltage checks across the switch if you know the 
full voltage of your power source in this case 120   volts if you check across a switch and it's closed 
you will get zero volts and your light will be on   that's because you're passing l1 power in to the 
switch and also out of the switch and l1 versus   l1 which is what our voltmeter does it doesn't 
tell us if the power's on or off it tells us   the potential difference between the two point 
the two places that we reference with our leads   so the difference in 120 volts and 120 volts 
is nothing it's a simple math problem okay so   voltage here zero volts on the switch light is 
on 122 volts across the normally open switch   which is terminal one to three is i've got 
and a half volts over here and my light is off   a technician in the field that doesn't understand 
switches is going to say i've got power on these   two terminals why isn't my light on and the 
answer simple it's because your switch is open   as soon as i apply 24 volts things change so 
down here at the bottom on my 24 volt coil   i've got 26.8 volts so my coil is now energized 
you can hear it click as these switches move   my voltage on terminals 1 and 3 which 
go to this light bulb here that's on   is going to be zero why because l1 power's coming 
in and l1 power's going out when i reference l1 to   l1 there's no difference between them it's 120 
volts going in and 120 volts going out so there   is no difference in that potential so light bulb 
on that definitely means that the switch is closed   when technicians think deeper than that and they 
wonder about the voltage across the switch that's   when we oftentimes get confused i don't know 
how many motors i've seen replaced because the   technician measured voltage across the switch 
and the motor wasn't on it's not the motor   it's the switch so i hope this helps a little 
bit uh i'm going to leave this lead in real   quick and run through a couple dry runs here 
so 26 volts this normally open switch is now   closed because the coil is energized and the 
light turns on as soon as i take that away i dropped down to just a couple volts on this coil 
so it is not energized and now the connection from   one to two is is back normally closed and 
i'm passing power to the other light bulb   so voltage across a switch if you get zero volts 
there's two possibilities one the power's not on   two the switch is closed if the switch is closed 
i will power that load when i apply power to   the coil all the switches move normally open 
becomes closed and normally closed becomes open   and i break this circuit in this case from the 
one to two terminal and i stop sending power   that voltage across that 
switch is going to be 120 volts   why is that it's because the switch is open now 
and because the switch is open the light bulb   is not shiny so i hope this makes a little bit 
of sense there is of course a lot more that we   could go in depth with but i'm trying to give 
you something to look at and also think about   as you try to learn how things in this field work 
so voltage gets a lot of people especially across   a switch so let me know how it goes and we'll 
uh we'll try to explain it more if we need to

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