MEASURING THERMOCOUPLE WITH PIC16F628

[tonybg]

pic16f628 thermocouple

I'm tring to measure thermocouple without any external active elements like op amps.Tring to use one of the internal comparators as amplifier.is it possible?

 

[kender]

ppic16f628 thermocouple

Oh No. You need signal conditioning hardware between the thermocouple and PIC.

First of all, output of thermocouple is about 50uV/degC (note the micro-volts). For example, type-T thermocouple is 37uV/degC. This is too low to measure or compare without amplification.

Second, you need cold junction compensation.

 

[tonybg]

measuring thermocouples

What about this

 

[kender]

measure thermocouple

No, I don't think so. Let's do the math.

500 [mV] / 37 [uV/degC] = 13500 degC

The thermocouple will melt around 2000 degC
Your schematic still doesn't take care about the cold junstion compensation

Although, delta-sigma ADC with a PIC is a neat idea. Could you post a link to a page (or a book), where your diagram comes from?

 

[januar1983]

pic16f628 delta sigma

you need cold junction compensated to acomplished this. maxim has the ic max6675, cold junction is to compensated the temperature between thermocouple and the junction

 

[tonybg]

max6675 + microchip

Here is a link to the article delta sigma convertion.
http://ww1.microchip.com/downloads/...icle regarding cold-junction compensation. http://www.national.com/an/AN/AN-225.pdf#page=1

 

[un-2000]

max6675,microchip

is very easy if you used dedicated chips for this
search the maxim dallas MAX6675 IS A WONDERFULL CHIP
0 TO 1024 oC.
the samples is free
is for K thermocuple
for read used serial interface is easy
cold joint compensation is't necesary

 

[kender]

cold joint compensation is't necesary

Cold junction compensation is always necessary, if you want to acheive accuracy better than, say, 20 degC. However, MAX6675 has cold junction compensation on the chip.

 

[melc]

I'm tring to measure thermocouple without any external active elements like op amps.Tring to use one of the internal comparators as amplifier.is it possible?

FORGET IT.

 

[racno]

Hello Friends!!!

I am a project for PIC16F84A and I want to make a messure of temperature with max6675, but this pic don't have some things that other pic, but I know that it's posible to creat a Software in asemmbler for this pic can understand the serial protocol of max6675 and show the temperature in the display but i have a problem i don't know how to create the code, colud you please help me to make this code???

 

[doraemon]

Hello!

Just for your info, this thread is about 3 years old.
Now, what you need is a SPI port. You can emulate the SPI
port with any I/O. This is called bit banging.

To do this, you take the MAX6675 documentation. There is a timing
explaining what exactly should happen on the I/O. Just write it
exactly like explained on the timing, it will work.

Dora.