Electric soldering iron rapid heating automatic control circuit

In cold winter or when the mains voltage is low, plugging in the soldering iron often causes inconvenience due to the slow heating. In order to reduce the waiting time, the author installed a circuit device on the soldering iron that can quickly heat up after power is turned on. The circuit can quickly heat up without manual operation, and automatically switch to the normal heating state when the soldering iron reaches a certain temperature rise, which brings convenience to use.

The attached figure is the schematic diagram of the power-on automatic rapid temperature rise circuit. When using the electric soldering iron, plug its power plug into the AC 220V socket. The 220V power supply becomes a pulsating DC voltage of 0.9×220=198V after being rectified by the full-bridge VD. Since the two ends of the VD output are connected to a filter circuit composed of a thermistor PTC and an electrolytic capacitor C in series, at the beginning of C starting to charge and discharge through the PTC, since the internal resistance of the cold PTC element is very small (approximately direct), the filtering efficiency of the P1℃-C filter circuit will reach the highest at this time, and the voltage across the load will rise to the theoretical peak value of about 220 (2 root) = 314V. Under this power supply voltage, the soldering iron core is heated up quickly.

After a certain period of rapid heating, the PTC will slowly heat up due to the charge and discharge current of C. Its internal resistance will increase accordingly, which will increase the charge and discharge time constant, directly reducing the filtering efficiency. Then the voltage at both ends of the soldering iron core will slowly drop to a certain level and remain constant. In order to prevent the voltage at both ends of the load from dropping too low after the internal resistance of the PTC is too large, a compensation resistor R1 is added at both ends of the PTC, so that the constant supply voltage of the soldering iron core is stabilized at 220V to maintain the normal working state of the electric soldering iron.

The circuit is simple and can be placed in a small box after assembly, and then strung in the middle part of the soldering wire (the soldering wire is first cut from the middle, then inserted from both ends of the small box and connected to the circuit), and then it can be put into use.

When selecting components, the PTC thermistor can be directly replaced by a color TV degaussing resistor (model 18RM270, cold resistance 18Ω). The rectifier bridge can also be replaced by four 1N4007 diodes, but it is better to make the circuit device more compact after welding.