907 soldering iron

907 Constant Temperature Electric Soldering Iron Circuit Diagram
Abstract:
         Electric soldering iron is one of the indispensable tools for us to repair and make circuit boards. It is a necessary tool for electronic geeks, electronic enthusiasts, circuit board repair practitioners and garbage men to solder electronic components when making and repairing. It is even one of the household tools in daily life. However, in the process of using it, it is often damaged due to long-term work, forgetting to turn off the power after the work is completed, or improper operation. Sometimes it is very lucky that only the heating core is damaged and it can be replaced with a heating core. However, when the circuit board is damaged, some novices (such as me) will not repair it. After changing N electric soldering irons, I decided to study the specific working principle. This project is only from the perspective of novices and whites. Don't spray it!
            The content of this article is only for learning and communication purposes, please do not use it for commercial purposes! !
        The content of this article is only for learning and communication purposes, please do not use it for commercial purposes! !
        The content of this article is only for learning and communication purposes, please do not use it for commercial purposes! !
Circuit introduction:
            After the 220AV voltage comes in, it is first reduced to power the LM358. LM358 collects the reference voltage provided by the temperature control circuit and cooperates with the thermocouple in the soldering iron core to control the bidirectional thyristor to energize the heating core. When the temperature collected by the LM358 through the thermocouple reaches the specified value of the temperature control circuit, the heating stops.
Circuit analysis:
          
Figure 1. This is drawn according to the original PCB board
 
 
Figure 2. This looks easy to understand.
             Step-down rectification AC220V voltage is connected to the L and N terminals, and after R11 step-down, D3 rectification, D1 (20V voltage regulator diode) voltage stabilization, C1 filtering, it waits for a DC voltage of about 20V.
             Power supply The rectified voltage is supplied to the 8th pin (LM358 V+) of LM358 (dual operational amplifier) ​​and the other way to the temperature control circuit.
             Temperature control circuit The 20v DC after step-down is divided by R8, R9, R10, R6 (R8, R9, R10 are potentiometers) to provide a reference voltage to LM358 IN1- (the negative input terminal of the first operational amplifier). R10 is the main temperature adjustment potentiometer, and R8 and R9 are the maximum and minimum values ​​for adjusting the temperature.
             Temperature measurement circuit LM358 IN1+ (the positive input of the first op amp) is connected to the thermocouple inside the soldering iron heating core through R4. The thermocouple is an active device. Its positive terminal voltage will output a positive voltage (millivolt level) as the temperature rises.
             Working principle When the LM358 IN1+ voltage is lower than IN1-, OUT1 (the output of the first op amp) outputs a low level, otherwise it outputs a high level. LM358 IN2+ (the positive input of the second op amp) is the AC power obtained by dividing the voltage by R3 and R2, and the IN2- voltage comes from OUT1. After power-on, since the IN1+ voltage is lower than IN1-, OUT1 outputs a low level. Because OUT1 is connected to IN2-, IN2- is a low level, and IN2- is lower than IN2+, so OUT2 outputs a high level. OUT2 is connected to the control pin of D2 bidirectional thyristor (BT136s) through C2 and LED1 (D4 plays a protective role). At this time, OUT2 is high level, causing D2 thyristor to conduct, and the heating core is powered and starts to heat. As the temperature increases, the thermocouple outputs a positive voltage. When the output voltage of the thermocouple reaches a certain value, IN1+ changes from a low voltage to a high voltage. At this time, the voltage of IN1+ is higher than IN1-, OUT1 outputs a high level, IN2- is connected to OUT1, IN2- becomes a high level, and IN2- is higher than IN2+, so OUT2 outputs a low level, the thyristor becomes cut-off, and the heating core is powered off and stops heating. The role of R5 is self-locking, so that the OUT1 voltage of LM358 does not change frequently, so that the switching between heating and stopping is not too frequent. When the temperature of the soldering iron is lower than the set temperature, repeat the above actions to keep the soldering iron at the set temperature.
 
 
C2 is 330uF.
I made a mistake before. With the attitude of not paying for something for free, I controlled the length of the original PCB above 100MM to 100MM and the width to only 13.6MM
. The potentiometer can use the one on the original circuit board. I don’t have any suitable
ones left to make something else. Don’t waste even a little bit of space.
I will introduce this circuit to novices like me from a personal perspective. If you have any good opinions or suggestions, please put them forward in the comment area
so that novices and I can learn more.
 
           The content of this article is only for learning and communication purposes, please do not use it for commercial purposes! !
        The content of this article is only for learning and communication purposes, please do not use it for commercial purposes! ! The content of this article is only for learning and
        communication purposes, please do not use it for commercial purposes! !