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Topic: Reading a thermocouple ? (Read 619 times) previous topic - next topic

Gadget999

I want to read a thermocouple/pyrometer

i have one of these lying about so can use it

http://www.morshine.com.tw/upload_pdf_c/257.pdf

how do i wire it up ???

what else can i use to read thermocouples

Grumpy_Mike

Well first off that is not a thermocouple.
Second it is a very poor data sheet, in fact it is not a data sheet at all. There are 5 connections and it doesn't tell you what any of the connections are. So there is not enough information in the sheet to answer your question.

Try contacting them and asking for a real data sheet not a specifications sheet.

Gadget999

They were not 100% helpful

I managed to find this

http://www.s-products.com/site/specifications/RT82000.pdf

it has an output of 4-20 ma ?

am i right to assume it will work as a variable resistor ?

there are 5 pins

1 +12V
2 gnd

3 - tc
4 + tc
5 no markings


pwillard

4-20 mA is not a variable resistor... it is called "current loop" communication and it still remains popular for industrial sensor interfacing.

It's been discussed before... have a look here:  http://arduino.cc/forum/index.php/topic,19613.0.html

retrolefty


4-20 mA is not a variable resistor... it is called "current loop" communication and it still remains popular for industrial sensor interfacing.

It's been discussed before... have a look here:  http://arduino.cc/forum/index.php/topic,19613.0.html


Well in fact a 2 wire 4-20ma industrial 'transmitter' does indeed act just like a variable resistor. You power the 'current loop' with a typical +24vdc power supply and the 'transmitter' automatically adjusts it's output resistance to maintain the loop current at a specific value corresponding to it's measurement value within it's 4-20ma measurement output range. A typical way to measure the loop value is to wire a 250 ohm resistor at the 'receiving end' wired in series with the ground end of the loop power supply and it will drop a value of between 1-5vdc across the resistor suitable for measurement by a typical analog input pin.

Lefty

Gadget999

thanks guys for the info

i will power the device using 12v

so basically i can put a 220 ohm resistor between the plus and minus and then read into an arduino pin on the plus

will the 12v overload the arduino pin ?

Grumpy_Mike

Quote
will the 12v overload the arduino pin ?

Yes you can not let the input to the arduino exceed the supply voltage.

Gadget999

so it appears the arduino can not use this device because it is designed to operate on a minimum of 12v

i will look to see if i can find a thermocouple transducer that outputs 0-5 v


retrolefty

#8
Nov 09, 2012, 01:33 am Last Edit: Nov 09, 2012, 01:36 am by retrolefty Reason: 1

thanks guys for the info

i will power the device using 12v

so basically i can put a 220 ohm resistor between the plus and minus and then read into an arduino pin on the plus

will the 12v overload the arduino pin ?


No that doesn't sound correct. You wire from negative of 12 (or 24) volt PS to 250 ohm resistor, other end of resistor to negative of transmitter, then from + on transmitter to positive 12vdc on power supply. Then wire arduino ground to power supply ground and finally from the ungrounded end of 250 ohm resistor to an arduino analog input pin.

This creates a current loop for the transmitter to operate and the resistor senses the value of the current loop to measure as a 1-5vdc input voltage for the arduino.

Lefty

retrolefty


so it appears the arduino can not use this device because it is designed to operate on a minimum of 12v

i will look to see if i can find a thermocouple transducer that outputs 0-5 v




That device can be used with an arduino even when using a 12 or 24 volt 'loop supply'. What the arduino analog input 'sees' is only the voltage drop across the resistor and if sized at 250 ohm resistor will only see a maximum 5 vdc if the loop is at it's maximum measurement value of 20 ma. The negative of the loop supply must also wire to the arduino ground.

Lefty

Gadget999

Hi Lefty

thanks for putting me straight on that

i will give it a go with the resistor

ty

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