Analysis of Control Principle of Control Circuit of T12 White Photoelectric Soldering Iron
T12 white photoelectric soldering iron, the heating wire inside the soldering iron head is connected in series with the thermocouple, and the two pins are not only the input line of the power supply, but also the output line of the thermocouple signal. The circuit schematic diagram of the general control circuit is generally as follows:
The working process of the circuit is as follows:
①In the circuit, pin 2 of the operational amplifier LM358 presets the signal voltage value (mV level) of the thermocouple at the set temperature. When the circuit is powered on, the voltage of pin 3 of the LM358 is lower than that of pin 2, and the output of the operational amplifier Terminal 1 pin outputs low level, diode T4 clamps pin 6 of another operational amplifier to low level, and its output terminal 7 pin outputs high level. This high level is transmitted to the triode 2N5551 through the resistance of 103, the triode will be saturated, and its collector potential will decrease. The G pole of the two MOS transistors TPC8107 connected to the C pole of the triode is lowered to turn on the MOS tube, and the power supply starts to supply power to T12 through the D pole to the S pole.
② When the power supply is powered by T12, the power supply voltage charges the capacitor C103 through the resistance of 103, which can make the level of pin 3 of the op amp higher than that of pin 2 in a short time, and pin 1 of the op amp is flipped to a high level.
③T4 isolates the high level of pin 1, and pin 6 slowly charges the capacitor of 104 through the resistor of 205, and when the delay is delayed until the potential of pin 6 of the op amp is higher than that of pin 5, pin 7 of the op amp outputs low level, and the triode is cut off, TPC8107 Also cut off, stop supplying power to T12.
④Once the TPC8107 is turned off, stop supplying power to T12. Then the 3-pin voltage of LM358 will quickly become low level. But this low level is actually equal to the signal voltage of the thermocouple transmitted from the thermocouple inside T12 through the resistance of 103. If the signal voltage of the thermocouple is still lower than the preset op amp pin 2 level at this time, the circuit state will repeat the process of ①, and carry out the cycle of ①—②——③—④. If the signal voltage of the thermocouple is higher than the preset pin 2 level of the op amp, it means that the temperature of the soldering iron tip has reached the preset value, the LM358 will not flip, and the circuit will not supply power to T12. This power failure process has been waiting for the level of pin 3 of the op amp to drop below the preset value of pin 2 again, and then start to turn over to supply power to T12 again. Warm up the soldering iron tip. This action maintains the constant temperature of the T12 soldering iron tip.
All in all, the control circuit supplies power to T12 in a pulse state, and the pulse width depends on the speed at which the resistor 205 on pin 5 of the op amp charges the capacitor 104. The pulse interval is very small, depending on the speed at which the charge of the 103 capacitor on the 3 pin of the op amp is discharged through the resistance of 103 and then through the heating wire of T12. During the pulse interval, the circuit actually detects the signal of the thermocouple to determine whether the pulse power supply should continue, so as to realize the constant temperature of the soldering iron tip.
