Hello. I just build the attached schematic. It works only with CA3130. I can't find on the local/internet shop the CA3130 (out of stock).




Then I decided to use 1/2 of LM358 as thermocouple amplifier. I know that it doesn't have the same performance as CA3130. I also added a resistor (1M) from +12V rail to the pin 5 of LM358 (non inverting input), for the protection of the soldering iron if the thermocouple doesn't work properly.





I have a few questions:
1. Is the schematic correct ?
2. Should I connect the 1M resistor from the +12V rail or from the K of the zener diode ?

I know that my shematic doesn't have cold junction compensation, but I used LM358 at a few thermocouple amplifiers for soldering stations and it seems to work, even if it is not perfect.

 

What voltage is coming from the op amp in the old one pin6, and the new one on pin 7. U3b?

 

The old one pin 6 = 11V
The new one pin 7 = 3V.

 

The old one pin 6 = 11V
The new one pin 7 = 3V.

You need something with rail-to-rail outputs.

CA3130: high input impedance, ground included in input voltage range, rail-to-rail outputs.

Unless the designer was ultra conservative, or clueless, LM358 is unlikely to ever be a reasonable substitute for CA3130.

 

I can't find CA3130 at the shop were I buy electronic parts. And I wanted to replace it with something which is cost effective.

 

Can you get a TL071?

 

Yes. I already tried with a TL071. It doesn't give the desired results, because the output stays always at about 11V.

 

Yes. I already tried with a TL071. It doesn't give the desired results, because the output stays always at about 11V.

TL071 doesn't have rail-to-rail outputs and the common mode input range won't include ground with the supplies the circuit uses.

 

One more question: I placed the 1M resistor (R22) from the +12V rail to the non inverting input of the thermocouple amplifier.
Will this resistor protect the heating element from the soldering iron if the thermocouple fails, by driving the LM393 pin 1 low to turn off the 2N2222 ?

 

Your life would be easier if you can make a negative power supply for the dual opamp.

Here is how I dealt with the problem in a similar amplifier:


A very good rail-to-rail opamp is the TLC27L2.