How to connect thermocouple to a PLC input module

[osmanmom]

below 3kohm,

1769sc-IR6I

 

[Lancie1]

The PLC range that I have 0 to 10 V. so, I guess the device that you suggest doesn't work. Thanks

Wait! 10 volts is well within the operating range of 5-to-30 volts DC for the LM34. It should run just great using your 10 volt DC power supply. Go buy a LM34 and stop messing around with this simple temperature reading.

 

[Rashwan_Mzhr]

Wait! 10 volts is well within the operating range or 5-to-30 for the LM34. It should run just great using your 10 volt DC power supply.

Looks reasonable however, the power supply that I have is 24 V. And what about the ladder logic programming ?

 

[Lancie1]

Looking at the spec sheet, the output of the LM34 is only 0 to 3 milivolts for 0 to +300 degrees F. It will not work very well with a 0-10 or 0-5 volt analog input module.

 

[realolman]

Im not home right now. I know i used these before with a labjack and i dont remember using any kind of amplifier I will look into it when i get home.
I am sorry if i misled you, but it looks right now as if i did

I know i used them ...but right now i have no explanation

 

[Lancie1]

There are several different models of that device. I saw a LM35 for use with AC voltage. There could be one that has a 0-10 DC voltage output.

 

[chelton]

Looking at the spec sheet, the output of the LM34 is only 0 to 3 milivolts for 0 to +300 degrees F. It will not work very well with a 0-10 or 0-5 volt analog input module.

Looking at the spec sheet the output is 10mV/degF. So output would be 0-3000mV for 0 to +300 deg. Still this would only be using 30% of the analogue input range for full scale of the temp sensor.

 

[TConnolly]

For economical room temperature ranges I would use the temperature IC. The LM34 has been mentioned, look also at the LM35. The chip typically costs around $1.00 US. Unlike a thermocouple, this sensor can be connected to the PLC analog input that you already have and it does not require a linearization polynomial like a thermocouple would if it was connected to a non thermocouple input. The LM35 will however require a 5V to 10V power supply.

 

[realolman]

I don't know what I was thinking... I know I used LM34 s with a labjack which I believe measures up to 10 vdc, but everything I can find indicates that it would NOT be suitable for a PLC input.

It seems that I am full of baloney. I apologize.

 

[GlenGineer]

Its sounds like you got a lot of responses. I just want to make a correction to my earlier post.
I said that we have an application the uses an analog module. I did some digging. As it turns out we have a universal analog input module. 1746sc-ni8u which can be set up to do voltage, amperage, rtd, or thermocouple input. The input into the program is a temperature. We then manipulate this number through some math functions and run it through a PID function. The resulting number is then outputted to an actuator.

The voltage generated by the thermocouple is really low. For a type j thermocouple 368 degrees only generates 10 mv. Each degree change in temperature results in like 0.025 (roughly ...its not linear) mv increase. So without any sort of signal conditioning it would be very difficult to do with a normal analog card.

 

[Rashwan_Mzhr]

Its sounds like you got a lot of responses. I just want to make a correction to my earlier post.
I said that we have an application the uses an analog module. I did some digging. As it turns out we have a universal analog input module. 1746sc-ni8u which can be set up to do voltage, amperage, rtd, or thermocouple input. The input into the program is a temperature. We then manipulate this number through some math functions and run it through a PID function. The resulting number is then outputted to an actuator.

The voltage generated by the thermocouple is really low. For a type j thermocouple 368 degrees only generates 10 mv. Each degree change in temperature results in like 0.025 (roughly ...its not linear) mv increase. So without any sort of signal conditioning it would be very difficult to do with a normal analog card.

Dear GlenGineer, I still working on my idea. right now somehow I did connect the thermocouple to the analog input card correctly. I'm able to see different decimal values based on the applied temp on the thermocouple. Next step in to build ladder logic program to normalize these valuse. For example, when I heat up the thermocouple the decimal value is 15000, and when I cool it up the decimal value is 1000. What I suppose to do is to convert these readings to Celes. Do you think there is a certain instructions to do that such as SCP in RSLogic 500? what I have is just RSLogic 5000.

 

[GlenGineer]

Well if I was programming this I would do a couple of things:

1) I would get the manual for the Input module. Find out how it calculates values. I think most calculate linearly but I don't know that for sure. You must compare the range to the range of the thermocouple. (There are tables available on line)
2) I would calculate a general formula for the input value or else I would find somebody that has the equation.
3) If I could not do 2 I would make a localized algerbreic equation based on charts for the range I want the most control in.

So I would multiply the values I get in by the equation(s) I mentioned above. Once again these values need to be calibrated (double checked by other devices or methods).

However concidering most analog voltage input modules are 0 - 10v I do not know if you will get accuracy out of it. It might be that it will be accurate within 10 degrees f or within 50.

Glen