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Old September 11th, 2019, 06:03 AM   #1
Rson
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Testing thermocouple inputs

So I have a project with a couple of thermocouple cards on an Allen Bradley L33ER rack. The instruments aren't due in for a little while, and I want to test out the signal high/low. I have an analog signal generator for mA and 0-10, but thermocouples send in a mV signal, correct?

Any good way to bench test these inputs? Is it as simple as making a voltage divider with a potentiometer reducing the voltage to the mV range?
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Old September 11th, 2019, 07:04 AM   #2
gtsuport
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I have used this device with pretty good success.


https://www.amazon.com/Resistance-Cu...s%2C139&sr=8-3


Does quite a few different signal types.



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Old September 11th, 2019, 07:56 AM   #3
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You can buy a cheap thermocouple for less than $10.00.


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Old September 11th, 2019, 08:07 AM   #4
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This.

Do a 3 point check with a short piece of TC wire. 0C/room temp/100C.
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Old September 11th, 2019, 08:40 AM   #5
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Better to use a specific thermocouple simulator since the signal at the analog input is the difference between the mV generated in the hot junction minus the mV of the cold junction.
It could also be done with a simple mV generator if you apply the difference of mV explained above.
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Old September 11th, 2019, 10:26 AM   #6
NetNathan
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If you use the correct TC extension wire, and you have a millivolt output meter.

Run copper wire from meter and twist together with the TC extension wire. Make sure you remember that Type K wire colors are Red=Neg and Black = positive.
Now get a glass of ice water (lots of ice) and insert the junctions you twisted together in the glass of ice. Now you have created a "cold Junction" at 32 degrees. The water will stay at very near to 32 deg. till the ice melts.
You can use the millivolt reference table from here to test the PLC module. And now you do not have to correct for the different temperature errors at the copper to Type K wire junction.
https://www.thermocoupleinfo.com/the...nce-tables.htm
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Old September 11th, 2019, 10:49 AM   #7
Mickey
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Quote:
Originally Posted by NetNathan View Post
Make sure you remember that Type K wire colors are Red=Neg and Black = positive.
[/url]
FYI. In the US Type "K" is yellow(+) and red(-)

They vary though, see link

https://www.google.com/search?q=ther...hrome&ie=UTF-8
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Last edited by Mickey; September 11th, 2019 at 10:58 AM.
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Old September 11th, 2019, 12:58 PM   #8
NetNathan
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Quote:
Originally Posted by Mickey View Post
FYI. In the US Type "K" is yellow(+) and red(-)

They vary though, see link

https://www.google.com/search?q=ther...hrome&ie=UTF-8
My mistake....thinking about Type S which I use a lot of...
Yes..
Type K is Yellow =Pos and Red = Neg

The OP is from US, so i did not see a reason for confusing things by giving him any of the overseas colors..
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Old September 11th, 2019, 02:00 PM   #9
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If they are 4 channel thermocouple cards, not 8, you can:
Make a copy of the project.
Change the IO Config to 4 channel input analog input cards
Test the program using mA or voltage signals.
Any changes needed make to the original also, ,or change the IO back to thermocouple.

If 8 channel inputs and you have extra slots on the rack you could put 2 analog inputs for a thermocouple input & change some addresses.
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Old September 11th, 2019, 08:31 PM   #10
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You don't need to create a 0C cold junction or use TC extension cable.
Measure the ambient temperature near the module, then look in the TC table the mV that correspond to the ambient temperature, then look the mV for the temperature you want to simulate and apply the difference between the two voltages in the module terminals.
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