How to connect thermocouple to a PLC input module

[Rashwan_Mzhr]

I'm doing a small project to read the room's temp by using thermocouple type J. The PLC controller that I have is 1769-L36ERM controller, and the Analog input module is 1769-IF4XOF2. Anyone here can tell me how to wire up my sensor to the PLC module ? And if possible hints about ladder logic programming. Thanks in advance

 

[GlenGineer]

If nobody answers by Monday night I might be able to contribute. We have a couple of systems that are wired and programmed similarly. I didn't program it but I've had to modify them. With the thermocouple one side is always +....I believe it is the magnetic one. I always consult a chart. Once you determine which is positive and which is negative connect them to the appropriate ports on the analog card (I've also seen thermocouple cards used....my guess is that they are more accurate). I believe the logic that is then used is scaled with parameters. You have to know the multiplier. There are charts available but that will just get you close. Next you need to calibrate it with and ice batch and boiling water. You then adjust the multiplier based on these readings. However the most important thing is to have the thermocouple reading correctly in the normal operating temperature range. This can be done with an accurate instrument that is properly calibrated. There might be more to it that I am forgetting but I will refresh my memory on Monday.

Just one last note we (my company) have had odd side effects by putting grounded thermocouples on systems designed for ungrounded and visa versa.

Hope this helps

 

[Ken Roach]

You will not get usable results using a general-purpose analog input to attempt to read the microvolt signal from a thermocouple.

Thermocouple signal conditioners are a whole category of industrial sensor electronics. There's a cold-junction sensor, the thermocouple type (J,K,C, etc), and the linearization table all to consider.

Omega Engineering are the premium company for temperature measurement and signal conversion products, but there are many dozens of other vendors in the market.

 

[Rashwan_Mzhr]

Dear GlenGineer,
you mean, when I determine which side is negative and which side is positive, I will be able to connect it directly to the PLC module without any power supply?
And for the celebration stuff, after I will get the sensor wired up correctly to the module, How I can convert the 0 to 10 V that my analog input accepts as an inputsignal, to a temperature scales for example from -100 to +100. You said we need to use ice and boiling water and that make sense because the sensor nothing but differential resistance so it gives me different resistance value within certain temp. It looks little bit confusing for me but I will try my best. Thanks for your comment

 

[Mickey]

Rashwan Mzhr,
You need to do some reading on how thermocouples work. See link below.

Also from your post above resistive temperature sensors are not thermocouples they are
"RTD" ( resistive temperature detectors) or thermistors, also see link below for technical reference material about these devices.

http://www.omega.com/temperature/Z/zsection.asp

 

[Ken Roach]

When I arrived at my current company, I had minimal experience with thermocouples and we rely very heavily on them for careful temperature control of composite curing machinery.

My bookshelf reference is the slim but informative book Practical Thermocouple Thermometry, by T.W. Kerlin.

I mentioned Omega Engineering also because their product guides have a very simple, no-nonsense approach to explaining how their thermocouples work and how you can interface them with control systems.

 

[BobB]

I would not use a thermocouple for reading room temperature at all but a 100 ohm 3 wire RTD. I would also use a direct connect RTD input card. If you have to use a general analogue card and thermocouple you will need a transducer/transmitter to convert the signal to 4-20ma. I would NEVER use a type J thermocouple anyway - one on the wires is iron and a lousy but of moisture makes it rust away. Type K is much more robust. Also bear in mind your accuracy will generally not be as good with thermocouple at low temperatures as an RTD. Depends how accurate you require the reading to be.

 

[realolman]

I'm with Bob ... I think you would be better off to use something other than a thermocouple I have used these with computer inputs. I have never used them with a PLC but I bet you could.

LM34 Fahrenheit
LM 35 Celcius

http://www.ti.com/product/lm34

 

[NetNathan]

If you are not using a TC PLC module, I would highly recommend a signal converter that converts the TC temperature to 0-10V.

If you insist on doing it with a regular analog input module ...
You will need 2 inputs...1 for TC and 1 for "Cold Junction" measurement.
You will also need a good RTD to measure "Cold Junction" temperature accurately.
The Cold Junction (CJ) is where the TC connects to the PLC module (because you are going from TC material to copper at the PLC terminals). The millivolts generated at this "junction" need to be subtracted from the true TC signal.
A 0-10V range will be way too large. You will need a millivolt range.
Max output of Type J is only 70mV, and around 1.277mV at 77F, and the voltage goes negative below 32F.
Here are the TC millivolt tables in C:
http://www.ni.com/white-paper/4231/en/
Fast way to convert C to F (that you can do in your head) is: (C x 2) -10% +32=F.

TC voltage output to temperature is not very linear at all. You will need a large lookup table for accuracy....a lot of work in PLC code.


Good Luck

 

[Bit_Bucket_07]

Anyone here can tell me how to wire up my sensor to the PLC module ?

It's already been said in a roundabout way, but the correct answer to your question is: do not.

As to polarity, in the US all thermocouples have a red wire, and that red wire is always the negative wire. This color code is non-intuitive and many electricians who are familiar with red representing "positive" in a DC circuit therefore often terminate thermocouple wiring incorrectly.

As to power supplies, you do not need a power supply for a thermocouple, since it generates it's own voltage.

If you want to connect a thermocouple to a standard analog input module, then you need to purchase an instrument that will convert the non-linear voltage signal from the thermocouple to a linearized voltage or current signal for your analog input module. Otherwise, you should simply purchase a thermocouple input module for your PLC. It might be interesting to connect a thermocouple to a standard analog voltage input and play around with attempting to infer temperature from that signal, but it is not a practical nor cost effective endeavor.

 

[Bit_Bucket_07]

I'm doing a small project to read the room's temp by using thermocouple type J. The PLC controller that I have is 1769-L36ERM controller, and the Analog input module is 1769-IF4XOF2. Anyone here can tell me how to wire up my sensor to the PLC module ? And if possible hints about ladder logic programming. Thanks in advance

This is slightly off-topic, but I have occasionally encountered situations in which I wanted to temporarily monitor an electrical cabinet temperature and I have simply run thermocouple extension wire into the cabinet and twisted the ends of the wire together to form a makeshift thermocouple junction. It works pretty well for a temporary application.

 

[danw]

Go ahead, hook up your thermocouple.

But you won't get a room temperature reading, you'll get 0mV, which, if you convert the 0mV to a temperature according to the thermocouple tables, you'll get 0 Deg C, the freezing point of water. That's exactly correct and it is NOT a problem with the PLC, its analog input card or the thermocouple, it's just the wrong way to read a temperature.

A thermocouple measurement actually requires two temperature measurements
- the mV gradient across the thermocouple (hot end minus cold end) and
- the temperature of the connection terminals, also known as cold junction or ice point compensation.

At room temperature, both ends are at the same temperature so the temperature gradient across the thermocouple is non-existent. Hence, the EMF output is 0.0mV.

You can test this by putting a voltmeter across the wires of thermocouple sitting on your workbench or desk. The reading will be 0.0mV, because there is no temperature gradient; there's no difference in temperature to generate an EMF.

Your attempt to make a thermocouple-only measurement with a DC volts AI card (assuming you do the work to convert to temperature), will always have large negative offset error, of about 15-10 Deg C. It will always read low by the difference between terminal block temperature and the freezing point of water. At room temperature, it should and will read 0 Deg C.

It is the cold junction (CJ, or ice point compensation) circuit on a thermocouple card that measures the temperature of the terminals (usually close to room temperature) and that CJ signal gets added to the thermocouple signal for a conversion to temperature.

So a temperature reading by a thermocouple card is the cold junction temperature measurement PLUS the thermocouple reading.

At room temperature, when the thermocouple is 'isothermal' (no temperature difference), its output is 0mV (the freezing point of water on the thermocouple tables). 0mV = 0 Deg C.

You need either
- a thermocouple input card to use your J thermocouple as direct input, or
- a temperature transmitter for a J type thermocouple which provides a linear, high level (4-20mA/0-5V/1-5V) signal that is compatible with your AI card.

 

[Rashwan_Mzhr]

This is the temp sensor that I am using

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[Bit_Bucket_07]

This is the temp sensor that I am using

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None of the available models of this device employ a thermocouple.

 

[shooter]

what is in the sensorbox?
a thermocouple is only a comparing device between 2 connections.
the result is only some millivolts.
so your input card must be able to accept the low voltage signal and it must have a temperaturesensor.