Soldering station-Type K thermocouple amplifier

Thread Starter

mike_the_begginer

Joined Dec 7, 2019
112
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).
original_schematic.jpg



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.
ss_lm358_v2.jpg




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.
 
Last edited:

dl324

Joined Mar 30, 2015
14,475
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.
 

Thread Starter

mike_the_begginer

Joined Dec 7, 2019
112
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 ?
 
Last edited:

DickCappels

Joined Aug 21, 2008
8,710
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:
1651223195166.png

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