PLC And Thermocouple

husnain.mahmood

Member
H
Join Date
Feb 2007
Location
home
Posts
66
:confused:Dear guys,
I need to use a simple thermocouple with analog input card, which can accept 0~10v, 0~5v and 4~20ma. I dont have transmitter with me so how can i use it with simple themocouple. If anyone can help me in developing a circuit then please help me.
Regards,
Husnain
 
You will need to use a transmitter. A 0-10V or 0-5V input is not sensitive enough to accurately handle the mV produced by a thermocouple. Even if it were, a thermocouple is not a linear device, so you would have to produce a translation table or calculation to only approximate your temperatures.
 
Actually my budget dosent allow me to purchase new transmitter if you have any circuitry for it please let me know it?
I am doing this project at my home this is not commercial project at all.
 
All of the below was bought for less than $300? This will allow what you are wanting. You must get the temp range you need. This was bought for reading in outside temp. so I could control heat in my plant based on outside air temp. I am not so sure you can get what you want with out something like this.

TP-306A-CF-H1141-L1000 Temp Probe Pt100 4 wire 1m cable to M12 Eurofast
CF-M-6-N1/4-A4 Compression Fitting 1/4" NPT to suit 6mm dia probe
TT-100-LI6-H1140 Temp Transmitter for TP probe 4-20mA 8-35VDC M12 Euro
RKC 4.4T-4 M12 Eurofast Female to flying leads 4m
 
A typical PLC analog input is a 12-bit analog to digital conversion. That means the span of measured values (0 - 10 volts) is divided into 4096 parts. The value of 4096 is the number of different combinations of ones and zeros when you have twelve bits.

When you divide 10 volts by 4096, you find the minimum voltage value that your analog input module can detect, in this case, 0.0024 volts, and that is with a perfectly noise-free signal. Now with a type J thermocouple, 2.4 millivolts can represent a range of temperatures in excess of 80 degrees Fahrenheit. That's the smallest detectable temperature difference with the system you're proposing.
 
REHJ said:
All of the below was bought for less than $300? This will allow what you are wanting. You must get the temp range you need. This was bought for reading in outside temp. so I could control heat in my plant based on outside air temp. I am not so sure you can get what you want with out something like this.

TP-306A-CF-H1141-L1000 Temp Probe Pt100 4 wire 1m cable to M12 Eurofast
CF-M-6-N1/4-A4 Compression Fitting 1/4" NPT to suit 6mm dia probe
TT-100-LI6-H1140 Temp Transmitter for TP probe 4-20mA 8-35VDC M12 Euro
RKC 4.4T-4 M12 Eurofast Female to flying leads 4m
Click to expand...

husnain.mahmood -- if you choose to go with this, be aware that this list of parts is for an RTD, NOT for a thermocouple. And it still includes a transmitter...
 
He was asking about thermocouples - you will need to use a transmitter for your thermocouple. Thermocouples are non-linear and an algorithm is required to linearise the curve.

Also, thermocouples only produce millivolts - not suitable for a PLC input unless into an input card that is designed for thermocouple inputs.

Different types of thermocouples have different curves - J, K, T, R, S, N all have different curves and require different algorithms and, therefore, different transmitters - unless you buy a universal, programmable transmitter.
 
If its just for a home project, then why not forget using the "aquired" thermocouple and buy a cheap and chearful thermistor that can work in the range you require (the higher the resistance the better to limit heating through the current flow). Then build it into a potential divider circuit using 1 or 2 resitors. Use the output of this to the 0-10v input on your PLC or Logic Relay.
 
If you aren't too fussy about accuracy you could use a regular Op amp to amplify the signal then scale it in the PLC. The problem with that you wouldn't have cold junction compensation, but that's OK if your termination is at a constant temperature.
A better way to go is use one of the single chip thermocouple amplifiers. Here's one I have used Analog Devices AD595 puts out 10mV/deg C and has built in CJ compensation.
http://www.analog.com/en/temperature-sensing-and-thermal-management/digital-temperature sensors/ad595/products/product.html
Good Luck
Roy

0-500 deg C = 0-5V DC
 
Last edited:
Unless you are going to use the system for volume production it isn't worth the time and money to try to roll your own. If you are going to have several hundred identical systems you can amortize the engineering to design and test your own system. If you only have a few units the time involved to do your own design is going to cost a lot more than the one or two hundred dollars each for transmitters. The resulting accuracy and reiliability will undoubtedly be superior with a standard signal conditioner.
 
Last edited:

Similar Topics

Tweeker
Thermocouple not Reading in Siemens PLC
I am operating a Siemens IM151-8 F Processor with 4 6ES7-134-4NB01-0AB0 2 channel thermocouple cards. I am getting no reading from any of the...
2 3
Replies
31
Views
6,151
Tweeker
Tweeker
R
How to connect thermocouple to a PLC input module
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...
2 3
Replies
41
Views
28,265
GlenGineer
G
C
PWM Signal from IR Thermocouple to trigger PLC Input
Hey All, Looking to set up a no-contact thermocouple to a piece of testing equipment that can basically just switch a NC input that shuts off the...
Replies
2
Views
3,779
BobB
BobB
L
thermocouple analog input for plc
hello i am trying to setup a 4-20 analog input signal for a plc, i am using a jenco 77a type j thermocouple converter with a 4-20ma output...
Replies
4
Views
2,705
Ashwini Sahoo
A
B
Need help for plc and thermocouple
hello all.! im just new here.. im searching how to know if my plc has its input module for thermocouples till i reached this page.. my problem is...
Replies
10
Views
3,735
beng
B
Back
Top Bottom