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Author Topic: thermocouple conditioning  (Read 3878 times)
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tonybg
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« on: December 19, 2007, 06:54:20 18:54 »

Hi,I'm having trouble conditioning k thermocouple to PIC16F88's ADC.I want to build cold junktion reference with ordinary diode in the thermo amplifier.Any suggestions?
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Gruber
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« Reply #1 on: December 19, 2007, 07:34:03 19:34 »

Hi Tonybg

Find a MAX6675 Cold-Junction-Compensated K-Thermocouple-to-Digital Converter (0°C to +1024°C).

Link: http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3149

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Gruber
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tonybg
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« Reply #2 on: December 20, 2007, 09:27:04 09:27 »

Sorry, no such animal in my country(bulgaria).No AD595,MAX6675.That's why I'm forced to do it the old fashion way.I'm using LM358 to aplify the signal to 4,8 mV per 1C.This is ADC resolution at 5V band.I know that ordinari diode has temp dependency - 2mV per 1C, but reversed.At 0C =0.600V,at 10C=0.580V,at 20C=0.540V.They use this tehnique in multimeters!But the schematics are very complicated because of the switches.Any help Undecided
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EDSONCT
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« Reply #3 on: December 20, 2007, 10:02:59 10:02 »


In http://www.eserviceinfo.com/  you will find schematics for multimeter, look for Protek 506.
It has a circuit for K thermocouple
Microchip has an application for it too.

Edson
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tonybg
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« Reply #4 on: December 20, 2007, 02:56:20 14:56 »

I'm newby.Not allowed to view the attachments Cry
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is_razi
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« Reply #5 on: December 20, 2007, 04:00:08 16:00 »

i propose you to use a lm35 thermometer ic for cold junction compensation, because it's more precise than a diode(in fact it is modeled as a zener diode,but there are compensation circuits in it for precision),so you can use an instrumentation amplifier ,such as an ad620, for amplifying the thermocouple signal and after that subtract the lm35 output from the ia's output. of course you must consider a potentiometer for dividing the lm35 output for calibration in 0 deg.
 Cool Cool
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tonybg
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« Reply #6 on: December 20, 2007, 05:21:53 17:21 »

I thought to use seperate input on the PIC for LM35 and then add the two in software, but was short in pins.That's why I'm trying to compensate in hardware.I Have one op amp in LM358 free.I can't figure a way to use it with the  diode?
« Last Edit: December 20, 2007, 05:51:15 17:51 by tonybg » Logged
BLK
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« Reply #7 on: January 02, 2008, 05:08:41 17:08 »

use op07
i try lm358 is not stabile at temperature variation, and have big offset
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EDSONCT
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« Reply #8 on: January 02, 2008, 05:24:48 17:24 »

lm308 is stabile in temperature
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« Reply #9 on: January 04, 2008, 10:08:58 10:08 »

I use this circuit and work OK, use god resistors
http://www.delabs-circuits.com/png_circuits/arc20050109.html
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DTiziano
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« Reply #10 on: January 31, 2008, 09:45:02 21:45 »

LM92 is like the LM35, but is digital, two PIC pins is enough to read it, no trimming or drift etc. very simple to use.
Put it close to the thermocopule clamp (must sense the connector temperature) and then add the read temperature to the one coming from the thermocouple.
A short circuit on thermocouple input give you the room themperature.
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fagari
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« Reply #11 on: February 02, 2008, 12:50:11 12:50 »

Hi Tonybg

You must use external voltage referance for VREF + and VREF - at PIC.

Example if your amp 2mV per 1C 0C = 3.600V,at 10C=3.580V,at 20C=3.540V..... 1024C = 1.552V

Connect 3.600 V for VREF+ and 1.552 V for VREF-.

Now you read ADC 0C = 1024 and 1024C = 0

For Thru result invert ADC result.
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