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thermocouples giving erratic readings

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Hey All,

Got a problem that popped up yesterday that I can't seem to wrap my head around.
Maybe someone else has seen this before...

I have a freezer that is controlled with a V280, three Type T thermocouples into an IO-ATC8.
(module four in a bank of five)

At the points being monitored by the T/Cs I typically have readings between -200 & 40 deg.
Yesterday, all three suddenly went whack, reading a couple hundred degrees higher that actual temp.

This morning I tried;
swapping out the IO-ATC8 module (thinking it went goofy), no change
reset/init of the PLC (to clear out any whacky numbers), no change
reloading the program into the plc (in case the module config somehow changed), no change
wiggle-jiggled the expansion connectors between modules, no change

Next stop, swap out the PLC, but I can't do that today as the freezer is in use.

There was an event yesterday morning, where one of my techs said the panels 24V supply
got shorted (stray whiskers on a terminal block), but the PS came back after he cleared the fault.
And it is giving me 24volts....

Thoughts or suggestions anyone??




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In my limited experience I have had similar issues.  

things to check/try. 
Even though your power supply is 24 VDC it may not be large enough.  Try a higher watt supply and a quality one.  

Check how close your thermocouple wire is to motor leads.  

Grounds, grounds, grounds.  We earth ground every tank, isolated pipe, panels, etc.  if it’s metal... earth it.  

good luck 

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As this problem happens after PS shortcut, you have to check power supply.

1. Check PC for output voltage, voltage fluctuation and ripple.

2. Check PC for load. Recommended to keep PS loaded not more than 80% of maximum load.

*If maximum load is 2.0A, then use PC rated for 2.5A.

3. Replace PS for new/good working one.

Best regards,


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  • MVP 2023
2 hours ago, John_R said:

Two used modules in the trash

This, in my opinion, has some logical explanation.
There is something that the module does not like in your installation.
And this is likely to happen again.
Having faulty modules in hands - I would try to understand the reason analyzing the internal scheme.
Therefore, if such a module worked for a long time and broke - you also need to change the sensors that are connected to it.
It is not too expensive but it is usually a cause of breakage.

If the module is installed in a new installation and fails - usually an error in installation, connection, power supply and grounding.

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  • 1 month later...

From the IO ATC8 spec. "Thermocouple Inputs"
-->  Absolute maximum rating ±0.6VDC

It is very easy to get a greater than ±0.6VDC differential between the thermocouple, and the module. All it takes is a poor ground.

Company I work for has  > than  a hundred of V350-35-TA24 with a similar maximum limit on thermocouple inputs.
As this is on portable equipment, nearly all failures have been due to;  too high of a voltage getting to the thermocouple input.

Example Failures...
Powered IR sensor,  input shorted to 24 vdc power for sensor.

Ground fault in heat cartridge where the actual ground was not sufficient, and the resistance through the  J contact thermocouple was lower than the ground connection..

RF sparks from improperly grounded servo motors.   (The problem here is the servo control and "actual" ground may be a long way from the servo motor, which may have a closer low resistance ground through the thermocouple. The spark may initiate a high current flow from another source.)
Regular static electricity sparks. 

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On 10/2/2020 at 2:38 PM, John_R said:

swapping out the IO-ATC8 module (thinking it went goofy), no change

John, can you clarify why first swap of modules did not fixed problem?

On 10/6/2020 at 9:39 PM, John_R said:

Back to square one and replaced the ATC8 module again with another used spare, and I'm back in business.



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Well, the two "spare" modules I had were actually used modules pulled from two other machines that were scrapped last year, stashed away in a box of other misc used parts😡

One of them worked, but of course the first one I grabbed didn't, and that steered me off looking for a different problem, it wasn't until my troubleshooting brought me back full circle to the module, which I then replaced with the other "spare" that did work....

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