Want to convert Thermocouple voltage signal from millivolt to Volt

Thread Starter

arif.taibani

Joined Jan 16, 2020
4
Hi,

I have thermcouple for temperature measurement and it gives output in voltage ranging from 0 to 50 millivolt. I am using a DAQ system which can record the voltage signal in the range of +/-10 V. Hence, I am looking for pre-amplifier which can convert my signal from millivolt (0 to 50mV) into volt (+/-10V) and then transfer the voltage signal to DAQ system. Basically,I want to apply gain of 200 to my voltage signal. Please help me to build the circuit. I am mechanical engineer and don't have much idea about electronics circuits. Thanks
 

Thread Starter

arif.taibani

Joined Jan 16, 2020
4
What you need is an analog thermocouple amplifier. There are several made; however, the trend is definitely to digital. Here is an analog breakout board that uses the AD8495 chip: https://www.adafruit.com/product/1778

If you need a different voltage range, that can be easily changed, but it is probably easier to modify your DAQ system to deal with that range.
Thanks for the reply. I don't want to convert my voltage signal to temperature output because my DAQ system can take the signal in voltage only (can't use Temperature based DAQ due to sampling rate constraints). And my thermocouple is J-type. If you could suggest an amplifier which can apply gain and amplify my signal from mV to V.
 

pmd34

Joined Feb 22, 2014
507
I was about to say....
I have used an INA326 for this type of application and it works OK, but to be honest you are best using one of the thermocouple to serial data ICs as they have "cold junction compensation" as well as an internal lookup table so will give a more reliable accurate and consistent readout.

But actually @jpanhalt had THE answer, he has suggested an amplifier and it is built for thermocouples specifically.. see the data sheet used on the board he suggests:

https://www.analog.com/media/en/technical-documentation/data-sheets/ad8494_8495_8496_8497.pdf
 

MisterBill2

Joined Jan 23, 2018
6,725
Keep in mind that the thermocouple output is not that linear, and so to record temperature your DAQ system will need to do some math. If your application needs fast response then you will need an amplifier package made for the application. There are a lot of them available at different prices. It may also be that a fast thermocouple meter with a digital output could be tied to your DAQ system. AND many brands of DAQ hardware also sell thermocouple input modules. Check George Instrument and Dave Ray company websites to see what I am describing.
 

ebeowulf17

Joined Aug 12, 2014
3,274
Hi,

I have thermcouple for temperature measurement and it gives output in voltage ranging from 0 to 50 millivolt. I am using a DAQ system which can record the voltage signal in the range of +/-10 V. Hence, I am looking for pre-amplifier which can convert my signal from millivolt (0 to 50mV) into volt (+/-10V) and then transfer the voltage signal to DAQ system. Basically,I want to apply gain of 200 to my voltage signal. Please help me to build the circuit. I am mechanical engineer and don't have much idea about electronics circuits. Thanks
@pmd34 hit on the key point here, which is "cold junction compensation." If you simply amplify the thermocouple voltage, the voltage you get will represent the difference in temperature between one end of the thermocouple wire and the other. It won't tell you the actual temperature at the end of the thermocouple.

So, unless you're also measuring the actual temperature right where the thermocouple wire meets the amplifier you plan to build, then adding that temperature to the calculated temp from your thermocouple voltage, your numbers will be fairly meaningless.

Do you have a good plan for accomplishing this? If you don't already have cold junction compensation figured out, I strongly recommend you use one of the numerous existing thermocouple amplifier solutions that have built-in cold junction compensation.

I've personally used the exact product suggested by @jpanhalt in several of my own projects, and I highly recommend it, when it's suitable. It sounds like it may not work for you because you need J type thermocouple input, but there should be similar products for that type. I've personally also used the AD595 in a few projects with equal success, and it has a sibling chip specifically for type J, the AD594. Here's a table listing a variety of options from AD, although there are good options from several other manufacturers as well:
https://www.analog.com/en/parametricsearch/11146

Unless you really know what you're doing, I'd strongly advise against just building a simple amplifier yourself
 

ebeowulf17

Joined Aug 12, 2014
3,274
That series, AD8494..8497 is appropriate for both J and K TC's.
View attachment 196992
The main reason I mentioned the older product line, including the 594 specifically, is because it's available in a through-hole package as opposed to SMT. It's much easier for some people (including me) to work with for small, one-off projects... unless someone is making a breakout board for the other variations in the newer line.

So far I've only seen the type K version on a breakout board, so if type J and through hole are both requirements, the older line might be helpful.
 

Thread Starter

arif.taibani

Joined Jan 16, 2020
4
I was about to say....
I have used an INA326 for this type of application and it works OK, but to be honest you are best using one of the thermocouple to serial data ICs as they have "cold junction compensation" as well as an internal lookup table so will give a more reliable accurate and consistent readout.

But actually @jpanhalt had THE answer, he has suggested an amplifier and it is built for thermocouples specifically.. see the data sheet used on the board he suggests:

https://www.analog.com/media/en/technical-documentation/data-sheets/ad8494_8495_8496_8497.pdf
Thanks. This helps.
 

jpanhalt

Joined Jan 18, 2008
10,087
Thanks. I think I can use AD8494 for my system. With amplification, I would also like to use data conditioner or filter to reduce the signal noises. Could you please suggest the data filter circuit which can be used along with AD8494 amplifier? Thanks
I am doing something right now with TC's, calculating rates of change, and so forth. It's all done digitally, and my filter is simply an average. Perhaps a capacitor and resistor in a low-pass configuration (aka "integrator") would give you a sum over time and buffer out rapid changes in the TC voltage. Check out "integrator" here: https://www.animations.physics.unsw.edu.au/jw/RCfilters.html
 

Reloadron

Joined Jan 15, 2015
5,609
There are simple devices (turn key solutions) over the counter made to do what you are looking to do. I suggest you give this page a read to understand what CJC (Cold Junction Compensation) is about. The bottom of the page has a few examples of the Temperature Transmitters I mentioned. The idea being to take the signal from a thermocouple (milli-volts) and convert it to a signal you can use like a Voltage or Current proportional to the temperature being measured. Temperature Transmitters perform signal conditioning operations and their outputs can be scaled for a given temperature range from a given thermocouple type. I am not discouraging the use of the 849X family of Precision Thermocouple Amplifiers with Cold Junction Compensation from Analog Devices but merely pointing out a turn key solution.

Ron
 

Reloadron

Joined Jan 15, 2015
5,609
Thinking about this years ago I was part of a Type J to Type K conversion. The Type J (Iron Constantan) was not working well in a hot moist environment. Anyway during a refurbishment we removed a few dozen of the Temperature Transmitters I mentioned. These are early before the nice newer programmable units available now. These are setup for a Type J input and scaled for 35 F to 700 F or if you prefer C they are 1.7C to 371.1C. They output 4 to 20 mA for that range so placing a 500 Ohm resistor in the loop you get 2 to 10 volts scaled for a temperature thermocouple range of 35 to 700 F. When using with a DAQ system most data acquisition systems allow the user to set up a channel to be charted or recorded. These are Minco USA manufacture with an original part number of TT221J1HM below is a picture of one.

Temp Transmitter Type J.png

The LED illuminates for an Open Thermocouple indication.

Here is an example of plotting 3 temperatures which the voltage has been scaled for 0 to 600 F and some user annotations added.

TC Test 2.png

If you think you could use one of these just let me know and it's yours as long as you are in the US or Canada I'll send it USPS and it's on me. Fewer things for the wife and kids to worry about when I am gone. I have more of this stuff laying around than I will need or use in my lifetime so if it helps someone out or does someone any good more power to them. :)

The newer units are actually fully programmable as to input type, range and output.

Ron
 

MisterBill2

Joined Jan 23, 2018
6,725
Two things that were not asked but they do matter: How many of these do you need? AND is this for a hobby project, a non-profit public service project, or a "for profit company" product line, or maybe a one-off instrumentation setup in some research lab. The reason is that in some cases it will make far more economic sense to purchase a device.
 
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