thermocouple temperature

[shaz]

thermocouple circuit diagram

dear frnz

i want to build a temperature controller project using thermocouple (it can be J,K,R, pt100(RTD) ) and display the output temperature using microcontroller and lcd.

i am totally new to this thermocouple thing. i've heard lot about non-linearity problems of thermocouples. please can anybody help me out on this topic .how to proceed wht should be the circuit diagram for this thermocouple based temperature controller.

i will be eagerly waiting for your replies.

thnx

 

[FoxyRick]

thermocouple project

Hi Shaz,

The first thing to remember is that a thermocouple has *two* junctions - the voltage from it is a *relative* measure of the temperature difference of these two. Hence the "cold-junction compensation" that is needed. Be aware also that solder joints and other connections can act as *another* junction.

The easiest way is probably to use an off-the-shelf ic, like the Maxim MAX6674 or 6675:

**broken link removed**
**broken link removed**

These do it all for you, including an MCU interface.

Or, search the web for full schematics of thermocouple amplifiers and theory, like these:

http://www.nhsouth.com/crafts/workbench/thermocpl.htm
http://www.efunda.com/designstandards/sensors/thermocouples/thmcple_theory.cfm?Orderby=Material DESC

I have some more info somewhere - I'll try to dig it out for you.

Cheers,
FoxyRick.

 

[shaz]

ina118 pt100

thanks for replying. i have gone through those maxim ics detail but if you see it's only either for j or k and what i need is a common system which would support j,k,r pt100(RTD). can you provide me that.

 

[msadek_eng]

lm35 cold junction

yes try www.cypress.com in the application note for the Psoc devices

 

[vicky]

linearisation of thermocouple

HI DEAR

NOW A DAYS I AM ALSO DOING WORK ON THIS PROJECT PLEASE TELL ME WHIC TYPE OF HELP YOU NEED. I AM DESIGNING A MICROCONTROLLER CONTROLLED OVEN WITH USING K TYPE FILTER RANGING FROM 0 TO 350C

 

[shaz]

6675 thermocouple pic

Vicky, actually i need the temperature to voltage conversion circuit part, the opamp stage, the linearization section. in some theory document i read linearization can be done either using extra circuit or using some polynomials in programming, but i can't get a clear picture of what i have to do.
as i have mentioned above i need to design temperature measurent system using thermocouple (it can be J,K,R, pt100(RTD) selectable ) and display the output temperature using microcontroller and lcd. which method of linearization should i adopt and how to proceed for that.
thanks a lot

 

[shollis]

thermocouple compensation lm35

Hi,

for some basic stuff see :
https://www.omega.com/thermocouples.html

I hope this helps, you will find volt charts and everything.

best regards
Steve

 

[pakitos]

thermocouple temperature display 12 volt

Hi are you obliged to use thermocouple? there are other temp sensors which can solve the linearity problem.

 

[BabyTiger]

thermocouple schematics

You can compensate cold junction of TC by a thermister.

 

[shollis]

thermocouple pic project

Hi Shaz,

for further info on CJC etc. see the following section from the Omega website:

**broken link removed**

We have a large website so it can be hard to find what you need at first, but it's worth having a look around.

Over the small temp range you need the linearity may not be a huge issue, have a look at the voltage tables to see if the actual non linearity over this range will cause any problem. You could split the curve into a section or two via a look up table rather than using the complex math for each value. It really depends on how accurate your readings need to be.

As another user suggested you could always use an RTD as thse are linear so the conversion from a voltage to temperature is simple.

I hope this helps.

Steve

 

[mcoster]

thermocouple type k pt100 howto

Hi Shaz,

I did a project that looked lot alike yours. I am telling you it is very hard because you have to keep the noise on the circuit very low since you are going to amplify DC signals, from the TC, in the order of µV.

In my project i basically used a instrumentation OPAMP called INA 118 from TI. Its main feature is that it has very low offset voltage. A lot people say it is expensive but it will save you some hair and sleep hours

You can find a lot of information inside INA118 data sheet itself. It even has a circuit for what you want. Check TI website for its documentation.

Analog Devices has a book on OPAMP´s that has a section about signal conditioning for TC's. Try it out. It has a great deal of theory to help you start. You can find this book online at: https://www.analog.com/library/analogDialogue/archives/39-05/op_amp_applications_handbook.html
It is free

For cold junction compensation i recommend using a LM35 IC. It will save a lot o time since it doens't need any calibation or linearization. It is just perfect !

Hope i could help !
mcoster

 

[yazou35]

thermocouple maxim

Hello,
you can simplify the problem by making a linearization of your high order polynomials by using lower order polynomials on smaller intervalls.

I send here a matlab program linc5 that make linearization of sensor data into polynomials. You can input the polynoimial order and the max accepted approximation error. There are two examples file, tc_b.dat for a thermocouple type B and pt46.dat for a platinum 46 ohm sensor.

I hope this will help you.

 

[RKNAIR]

thermocouple temperature display projects

Hi,
You have to be specific about the temperature range, the type of sensor and the accuracy level required. If it is just 0-150 degrees, then an LM35 or a similar IC would suffice. For higher temperatures one has to use a thermocouple or a RTD. The thermocouple voltage output is not linear in the whole range, but you can linearise it part by part. Cold junction compensation can be done using a dedicated ic or LM35 would do. RTDs are quite useful for measuring temperatures. Hope this will be of some help

 

[shaz]

omega thermocouple voltage charts

mr nair thanks but i think you did not read the question properly. i have asked for j k t r s e types temp sensor. and u r answering for lm35 . and rtds are quite useful i know that is why i want to make a temperature controller using that. please if you have some answer realting to thermocouples then only reply.

 

[zero2010]

lm35+cold+junction+compensation

Dear vicky,

can you please get me the ckt diagram for the project (thermocouple temperature) using any type of PIC e.g.:16F_series.

thank you