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
handle.
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.
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
