Change the power supply of the iron core transformer in the constant temperature soldering iron to a switching power supply

Figure 1 is the original circuit. The AC 220V power supply is stepped down by the power transformer T1. The heating power of the soldering iron core (HR1) is controlled by the bidirectional thyristor TR1. R15 is a potentiometer for setting the temperature. HE1 in the figure is a sensor (thermocouple) for detecting the temperature.

Figure 2 is the modified circuit. Optocoupler U10 replaces the original U1, and power field effect tube Q1 in Figure 2 replaces TR1 in Figure 1. R20 and R21 are added components. Because the power supply of the soldering iron core has been changed from AC to DC, the original thyristor cannot be used. Now the field effect tube is used to replace it. When the optocoupler U10 is turned on, it provides bias to Q1, making Q1 turn on and connecting the power supply of the soldering iron tip (HR1, HE1). LED1 is used to indicate the heating status, and when it is on, it means heating. The conduction voltage drop of the field effect tube is very small. The power consumption is also very small, so there is no need to install a heat sink.

Because the constant temperature soldering iron uses an iron core transformer, which is too heavy and inconvenient to carry, the method introduced in this article is to replace the original iron core transformer with a switching power supply, which can reduce the weight by three quarters.

Find a laptop power adapter (output voltage 17V~20v), take out the circuit board, replace the voltage divider resistor at the first foot of TL431, and change it to 24V output. Then install this board in the original control panel. Select a field effect tube with a current greater than 5A and a withstand voltage higher than 100V. It can be removed from an old PC motherboard, an old TV board, or an old switching power supply containing UC3842 (UC3842 drives field effect tubes). Another source is to cut off the power supply part of the old TV circuit board. A considerable number of these boards have a set of +24V~+27V power supplies, and this solution uses this set of voltages.