Working principle and production circuit diagram of constant temperature electric soldering iron

As shown in the figure. The temperature control range of the electric soldering iron is 100℃～400℃, the temperature adjustment mark indicates low, medium and high positions, the temperature control accuracy is nominally ±5%, and a thermocouple sensor is used. The control circuit adopts the AC mains direct voltage reduction, filtering and voltage stabilization power supply scheme.

　　The AC220V mains is used as the power supply voltage and temperature setting voltage source of the comparison device IC after being stepped down by R1, half-wave rectified by D1, clipped and stabilized by D2, and filtered by C1. Pin 3 of IC-A is the input terminal of the thermocouple detection voltage (corresponding to the temperature value); pin 2 is the temperature setting voltage. After the voltages at pins 2 and 3 are compared, the voltage is output from pin 1. The function of R5 is to feed back a small part of the input to the in-phase input terminal pin 3 so that the output is locked and unchanged when the small signal fluctuates. When the thermocouple detects a low temperature, the level of pin 3 is lower than that of pin 2, making the output of pin 1 also low. Then, the pin 6 of the IC-B amplifier is lower than the fixed bias pin 5, making the output pin 7 high. Since the voltage of pin 5 of IC-B is obtained by dividing the voltage of AC220V by R6 and R7, the frequency and phase are exactly the same as AC220V. After comparing with the DC voltage of pin 6, the AC voltage is output at pin 7. The AC voltage triggers the bidirectional thyristor through C2, D4, D3 and D4 in reverse parallel (functioning like a bidirectional diode), so that the corresponding voltage is added to the soldering iron heating wire to achieve the purpose of constant temperature.