Does the soldering iron tip have a thermocouple?
Temperature measurement and heating principle suitable for T12 soldering iron tip
You may wonder how the T12 soldering iron tip only has two wires to achieve heating and temperature measurement.
In fact, it is very simple, because there are two conductors on the soldering iron tip, which forms a thermocouple, and we know that thermocouples can measure temperature. Let’s look at the conditions for a thermocouple to form:
When there are two different conductors or semiconductors A and B forming a loop, and the two ends are connected to each other, as long as the temperatures at the two nodes are different, the temperature of one end is T, which is called the working end or hot end, and the temperature of the other end is T0 , called the free end (also called the reference end) or the cold end, an electromotive force will be generated in the loop, and the direction and size of the electromotive force are related to the material of the conductor and the temperature of the two contacts. This phenomenon is called the "thermoelectric effect", the loop composed of two conductors is called a "thermocouple", these two conductors are called "hot electrodes", and the electromotive force generated is called "thermoelectromotive force".
So at this moment, measure the temperature of the soldering iron tip and find that the temperature is low, then energize the soldering iron tip to heat it, turn off the heating for a while, then measure the temperature of the soldering iron tip, and then heat it again. Repeat the measurement and heating alternately.
You will know by looking at the control panel below. There is an op amp on the control that forms a comparator to control the MOS tube switch. When the voltage on the thermocouple is greater than the reference voltage, the MOS tube is turned off and heating stops. The following is a suitable T12 control board made by a forum expert for your reference.
Applicable to the temperature measurement and heating principle of JBC soldering iron tips.
The soldering station suitable for JBC also integrates the soldering iron head and the heating core. The difference is that the soldering iron tip suitable for JBC has 3 contacts. It separates the heating and temperature measurement, and can realize heating and temperature measurement at the same time. Therefore, the soldering iron The lower the head temperature will be detected faster, the temperature will return faster.
The three contacts of the soldering iron tip are connected to the red wire, the green wire, and the blue wire respectively. Then, the thermocouple between the red wire and the green wire is directly connected to the thermocouple temperature measurement chip, which outputs the temperature value in SPI mode. Between the blue wire and the green wire is the heating wire, which is connected to the thyristor circuit on the lower left side. It uses 15.5V AC (actually it should use 24V AC). The MCU controls the thyristor to open through an optocoupler. The upper left side is the zero-crossing detection circuit, which needs to give a conduction signal to the thyristor when it crosses zero to achieve precise control of the conduction angle.
Of course, there is another control method, which is to use a complete AC cycle and control the proportion of the number of AC cycle conductions to the total number to control the temperature.
The circuit is also very simple. It uses H-bridge control to detect the zero-crossing signal. It will continue to conduct before the next zero-crossing, and then it will be judged whether it should continue to conduct at the next zero-crossing.
The homemade circuits of the soldering station are very simple, the only difficult part is the PID temperature control in the program. Understand this reason, and you will know why DIY soldering stations sometimes tend to burn out when using soldering iron tips, but the original ones do not. In short, poor temperature control will lead to out-of-control temperature and burnout.
