Temperature Controller - can I use thermocouple instead?

[gauravkothari23]

i have made a temperature controller using Seven segment display 89S52, ADC0804, and LM35. (image Attached)
i just want to ask and confirm that rather than using LM35 to measure the temperature, can i directly connect any K type thermocouple to ADC (pin 6[Vin+] and pin 7[vin-])to measure the temperature.

 

[KlausST]

Hi,

You want to replace the LM35 with a K type temperature sensor?

A thermocouple is working completely different to th LM35.
* it measures temperature difference only (hot_junction - cold_junction)
* it produces about 41uV per °C only, ( the LM35 about 250 times this value)

--> You may connect it, but you will not get meaningful results

Klaus

 

[Altaero]

I wouldn't recommend connecting thermocouple directly to the ADC. Thermocouples have relatively "low" output voltage compared to ADC resolution, plus ADC error. You need to amplify the signal. There are a lot of schemes with OpAmps connected to thermocouples, use them instead of direct connection.

 

[FvM]

There are ADC with internal PGA that can measure thermo couples. ADS1118 has e.g. about 0.2 K resolution and a few K absolute accuracy, also an internal temperature sensor suitable as cold junction reference. That's not mind blowing but suitable for many applications.