The common malfunction of a constant temperature electric soldering iron is temperature runaway, which results in the temperature of the soldering iron being too high. On the one hand, it leads to high-temperature oxidation of the soldering iron head (soldering tin is also oxidized at the same time); On the other hand, welding at high temperatures can also easily burn out electronic components. When an electric soldering iron works at high temperatures for a long time, it is also easy to cause damage to its internal circuit, causing permanent loss of control and even inability to use. During fault inspection, it will be found that the sliding contact of the temperature regulating resistor R2 is oxidized and causes poor contact, which is equivalent to the temperature being adjusted to the maximum upper limit, resulting in a high temperature of the soldering iron. There are two fundamental reasons for this: firstly, when the electric soldering iron is working, it transfers a portion of heat to the handle of the soldering iron (inside the circuit), causing the temperature of the working environment of the circuit to increase. After a period of time, it is easy to cause oxidation of R2's moving contacts; The second is that the current limiting resistor R1 in the rectification and filtering circuit dissipates heat, causing an increase in the working environment temperature of the circuit and easily causing oxidation of the R2 moving contact.

(1) Replace adjustable resistor R2 with a fixed resistor: First, adjust R2 to reach the optimal temperature point for normal use of the soldering iron, then measure the value of R2 and replace it with a fixed resistor.

(2) Transforming the rectifier circuit: The circuit structure is shown in the following figure.