# Bench Test Help



## HeretoHear (Jul 10, 2015)

Hi there,
I have this control module. Its a 120v ac input that puts out 24v ac. Its from a big stand alone commercial freezer. Not to sure how to test the contacts (NO and NC) from the relay. I tried all sorts of combinations with my meter but could not get voltage during any combination. Any suggestions before I bench test again and find out if this thing actually works or not? Thanks.


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## The_Modifier (Oct 24, 2009)

:jester:After the drop test and the hammer test, I haven't got much to offer.


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## telsa (May 22, 2015)

HeretoHear said:


> Hi there,
> I have this control module. Its a 120v ac input that puts out 24v ac. Its from a big stand alone commercial freezer. Not to sure how to test the contacts (NO and NC) from the relay. I tried all sorts of combinations with my meter but could not get voltage during any combination. Any suggestions before I bench test again and find out if this thing actually works or not? Thanks.


Pull manufacturer's numbers, Google same.

Obtain wiring diagram.

Done.


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## splatz (May 23, 2015)

HeretoHear said:


> Hi there,
> I have this control module. Its a 120v ac input that puts out 24v ac. Its from a big stand alone commercial freezer. Not to sure how to test the contacts (NO and NC) from the relay. I tried all sorts of combinations with my meter but could not get voltage during any combination. Any suggestions before I bench test again and find out if this thing actually works or not? Thanks.


I am assuming the writing on your picture is according to the wiring diagram for the device. This is what I would expect to see - just an educated guess: 

Short the thermostat wires, apply 120VAC to the coil wires, you should have 24VAC across the NO terminals, and 0VAC across the NC terminals. 

Separate the thermostat wires, apply 120VAC to the coil wires, you should have 24VAC across the NC terminals, and 0VAC across the NO terminals.


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## Dark Knight (Jan 6, 2016)

splatz said:


> I am assuming the writing on your picture is according to the wiring diagram for the device. This is what I would expect to see - just an educated guess:
> 
> Short the thermostat wires, apply 120VAC to the coil wires, you should have 24VAC across the NO terminals, and 0VAC across the NC terminals.
> 
> Separate the thermostat wires, apply 120VAC to the coil wires, you should have 24VAC across the NC terminals, and 0VAC across the NO terminals.


Wouldn't those just be dry contacts and therefore no voltage should be present? Or is this something different? 

If that is the case I would just ping out the contacts and apply voltage with the thermostat wires twisted together to see if the contacts change.


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## splatz (May 23, 2015)

Dark Knight said:


> Wouldn't those just be dry contacts and therefore no voltage should be present? Or is this something different?
> 
> If that is the case I would just ping out the contacts and apply voltage with the thermostat wires twisted together to see if the contacts change.


Could be, I was reading that "24vac out" as this device supplies 24VAC. 

More normal would be wire 120VAC on leads; with thermostat wires open, see 120VAC across NC terminals and 0V across the NO terminals; apply 24VAC across the thermostat leads, and see 120VAC across the NO terminals, 0V across the NC terminals.


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## Dark Knight (Jan 6, 2016)

splatz said:


> Could be, I was reading that "24vac out" as this device supplies 24VAC.
> 
> More normal would be wire 120VAC on leads; with thermostat wires open, see 120VAC across NC terminals and 0V across the NO terminals; apply 24VAC across the thermostat leads, and see 120VAC across the NO terminals, 0V across the NC terminals.


Aah, yes, you're probably right. That would make sense. 

Best to google the part number and go from there. Or like you said, apply voltage and see what it does. If it works.


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## telsa (May 22, 2015)

Google's cheating database is so vast that it's the go-to for these queries.

I'm constantly amazed that fellas don't query Google -- straight off.

BTW, I wouldn't be surprised if the unit was some solid state puppy and that the control voltage is 24V and that the dependant voltage is 120VAC.

In sum, the OP has got everything backwards. He'll not get ANY results until he feeds it 3-24V... probably 3-24VDC.

THe unit looks like it's totally encapsulated in epoxy -- a dead give away that it's solid state and is triggered off of VDC. It can switch 120VAC, of course.

But, that's just a blind guess, as I can't really see much from the photo.

The idea that a low voltage would control line voltage -- isn't that how it's supposed to work ?

Such switches work with *3-24VDC* and are as cheap as all get out.

This is most unlikely to be a 120V to 24V power supply.

The switching logic will be all on and all off... and that it controls some blower... probably for the heat cycle -- after the fuel has warmed up the heat exchanger.


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## telsa (May 22, 2015)

Additionally, NC// NO are legends used by SWITCHES not by power supplies.

It would not surprise me if the unit needs to be fed a constant 24VDC at one pole -- and that the switching logic is driven by a thermostat// thermistor that is linked to the unit by the wire leads.

The other pole needs to be kept at 0VDC. 

It has this lay-out because it suits mass production... and field repair.


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## telsa (May 22, 2015)

I suspect that this puppy is interlinked to some sort of defrost cycle -- or it cycles on cold -- instead of hot, now that I've re-read your OP.

Google that dang part number.


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## gnuuser (Jan 13, 2013)

if this is a relay which i suspect you are all correct.
the only time i have seen 120 volt to control 24 volt was using the output of reject solenoids to trigger an ice cube relay. but there are other situations where you would see it, such as an emergency lighting system and power monitoring system.
switching a 24 volt signal for the networked production counters is common in many facilities to integrate older analog technology with newer digital.

those who work primarily in resi may seldom see a situation such as the above. but its just another facet of the gem we call electrician.


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## JRaef (Mar 23, 2009)

Could also be that those wires don't go to a thermoSTAT but rather a thermoCOUPLE. If so, closing them would do nothing, a thermoCOUPLE is sensing a milivolt level of feedback from a probe.


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## telsa (May 22, 2015)

JRaef said:


> Could also be that those wires don't go to a thermoSTAT but rather a thermoCOUPLE. If so, closing them would do nothing, a thermoCOUPLE is sensing a milivolt level of feedback from a probe.


I'd assume thermistor because they are dirt cheap and work wonderfully at moderate temperatures. ( Bleed off some VDC juice and run it through a 5-20ma analogue circuit and roll from there. )

But, it's easy to imagine either thermistors or thermocouples or even a thermostat being used as the master controlling signal. 

The gadget has been manufactured for the OEM marketplace -- so I assume that the design is flexible enough to adapt to all of the popular feedback schemes.


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## scotch (Oct 17, 2013)

what does it control....seems the 24vac would be going to the Tstat....and the switching may be solid state triac...l so don't look for open or closed contacts...you need to put a load thru' them ; a light bulb will do it .


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