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J & K type temperture sensor

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garg29

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op07 thermocouple

Hi friends,
i want to design temperture controller for J & K type temperature sensors. I want to do it using micrcontroller. I think i need to make some wheatstone bridge and then to op-amp and futher to adc. can anybody provide me some circuit or idea of this Wheatstone bridge and opamp stage.

Thanks
With best regards,
Amit
 

thermocouple type k lm35

THNAKS BUT DOES ANYBODY HAVE THE IDEA OF THE CIRCUIT WITH 89C51 ..I WILL BE OBLIGED
WITH BEST REGARDS
AMIT
 

thermocouple j microchip

Try MAX6675.
It is Cold-Junction-Compensated K-Thermocouple-to-Digital Converter (0°C to 1024°C):
**broken link removed**
It can be directly interfaced to any 8051-compatibile microcontroller ..
Regards,
IanP
 

using a temperature sensing diode as a cjc

thanks for replying
does this max6675 support j type thermocouple also.and also which type of thermocouple(sensor) should i use to measure temperature upto 1600 C

with best regards
amit
 

thermocouple j &k

another ICs which convert the Thermocouple signal to 10mV/0C are AD594/AD595
It is easy to use
 

max6675 outputs all 1s or 0s

Type K (Chromel / Alumel)
Type K is the 'general purpose' thermocouple. It is low cost and, owing to its popularity, it is available in a wide variety of probes. Thermocouples are available in the -200 °C to +1200 °C range. Sensitivity is approx 41 µV/°C. Use type K unless you have a good reason not to.

Type E (Chromel / Constantan)
Type E has a high output (68 µV/°C) which makes it well suited to low temperature (cryogenic) use. Another property is that it is non-magnetic.

Type J (Iron / Constantan)
Limited range (-40 to +750 °C) makes type J less popular than type K. The main application is with old equipment that can not accept 'modern' thermocouples. J types should not be used above 760 °C as an abrupt magnetic transformation will cause permanent decalibration.

Type N (Nicrosil / Nisil)
High stability and resistance to high temperature oxidation makes type N suitable for high temperature measurements without the cost of platinum (B,R,S) types. Designed to be an 'improved' type K, it is becoming more popular.

Thermocouple types B, R and S are all 'noble' metal thermocouples and exhibit similar characteristics. They are the most stable of all thermocouples, but due to their low sensitivity (approx 10 µV/°C) they are usually only used for high temperature measurement (>300 °C).

Type B (Platinum / Rhodium)
Suited for high temperature measurements up to 1800 °C. Unusually type B thermocouples (due to the shape of their temperature / voltage curve) give the same output at 0 °C and 42 °C. This makes them useless below 50 °C.

Type R (Platinum / Rhodium)
Suited for high temperature measurements up to 1600 °C. Low sensitivity (10 µV/°C) and high cost makes them unsuitable for general purpose use.

Type S (Platinum / Rhodium)
Suited for high temperature measurements up to 1600 °C. Low sensitivity (10 µV/°C) and high cost makes them unsuitable for general purpose use. Due to its high stability type S is used as the standard of calibration for the melting point of gold (1064.43°C).
Click to expand...
To cover 1600°C range the best choice seems to be Type N thermocouple ..
MAX6675 is not suitable for this purpose ..

What you may need is Linearized Input Module similar to: **broken link removed**
Regards,
IanP
 

max6675 j type

These chips are too expensive
 

max6675.lib

The cheapest way to go is to do all the linearization in software. The smartest design that I have seen used an OP07 op-amp based amp with a built-in CJC using a silicon diode, an LM311 vtof convertor and a cheap micro to do all the other stuff. No doubt all these can be done in $14.
 

max6675 typ j

Thanks a lot "techie" for replying. I think the way you're telling is good enough. can you please provide me the circuit. I need that OP07 stage circuit.

Thanks once again,

With best regards,
Amit
 

max6675 with microchip c

Why don't you just read some of the replies above more carefully, and start thinking for youself. EDA board isn't a design bureau you know.

Put some effort in it yourself, you may actually learn something that way.
 

thermocouple lm311 circuit

thanks Gorilla for replying. actually i've read all above carefully, as i've to design for 1200 celcius i cant use max6675, i'm trying to search for something simple and also which could provide me the full range. as "techie" said using OP07 the thing can be done i just asked for the OP07 stage circuit, and i think this wasn't too much. and also this is the only way i'm trying to learn myself. Still if you think the otherway i'm sorry.
 

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op07 thermocouple circuit

I dont have the circuit at this time because I only saw the end product's PCB. What I can tell you is that OP-amps are fairly simple of you study the basic theory. What the designer had done is to use the 2mv/degc temperature coefficient of the silicon diode to generate the CJC volatge and feed it (with appropriate attenuation to the (-) input of the Op-amp. The (+) input gets teh thermocouple voltage. carefully calcualated gains can give you a CJC compensated and amplified volatge that can be fed to the digitizer and linearized in software.
 

mcs51 temp type r

Garg29

I did a similar project for my instrumentation class.

I did everything using OPAMP and a LM35 to do the cold junction compensation.

The main problem in using a OPAMP with thermocouples is offset voltage. So you HAVE to choose a OPAMP that has low offset voltage. I chose the INA118 from TI. Its offset voltage is around 50uV. (you loose 1 degree since your TC, type K, output is 41uV°C)

The INA118 datasheet has a application note that shows you how to do that using a RTD as CJC, but with some effort you can adapt a LM35 (10mv/°C).

Another choice is to use CHOPPER compensated OPAMP (Analog devices has a few). This king of OPAMPS also has very low offset voltage, but are hard (almost impossible) to find in my country. Analog devices has a OPAMP Application book that has this application (they shows a circuit using type K thermocouple and LM35) you can download it here:

https://www.analog.com/library/analogDialogue/archives/39-05/op_amp_applications_handbook.html

So take a look at INA118 datasheet and on that book. There you will find some answers.

Hope i could help!

mcoster
 

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    garg29

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op07 cjc

I think the original constraint was the cost. OP07 is less that a dollar. The ones you suggested cost much more.

True that OP07 has larger offset. But commercail temperature controllers dont really require that much accuracy.
 

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    garg29

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