By connecting this simple circuit to a soldering iron with a built-in thermoelement, you can easily regulate its temperature. The opamp functions as a comparator and compares
the voltage value at the thermoelement with the voltage at the potentiometer P1. As long as the voltage at the minus input is lower than the voltage at the plus input, the IC is positive and
T1 conducts. The relay then closes and switches on the power of the soldering iron.
The reference voltage is varied through P1 and at the same time it varies the temperature of the soldering iron. Calibration: Common thermoelements produce an output of
5 mV per 100°C. Turn P1 to the right until the relay closes and the soldering iron begins to heat up. Using a voltmeter, monitor the voltage at the minus input of the opamp and turn
P2 until the relay opens when the measured voltage is 20 mV which is equivalent to 400°C.
Electronic Circuits volume 1.0 - Circuit Nr. 45
The complete data of the electronic circuit described above can be found in the following book and is available from Amazon.com.
Click on the image to view the book.