Soldering Station January 2019

The most expensive part of a soldering station is a base which PWMs the sine signal from 12V or 24V transformer to regulate the temperature of the soldering tip. That got me thinking if I can make a much more affordable but a better designed base that would control quality and affordable Weller WMRP tips for micro soldering that integrate tip, thermocouple and heater in one piece and can be affordably bought for 25€.

The design would be consisting of the following parts:

220V to 12V transformer with fusing
+-8V, +- 5V linear and 3.3V switching regulator
Discrete solid-state relay
Zero-cross detection using optocouplers and comparator
Thermocouple filter and instrumentational amplifier for amplification
Physical potentiometer to set the temperature with zero latency
Temperature sensor for thermocouple connector temperature compensation
STM32F303 MCU with 12-bit ADC for thermocouple measurement
Nokia 5110 LCD display

PCB design

We had Altium Designer lessons at university which raised PCB design to another level, because Altium offers many functions and has extremely well thought out interface and has even compatibility for 3D mice such as 3Dconnexion's Space Mouse. There are some bugs that would need to be polished out but otherwise an excellent program. The schematic and final PCB can be seen on Figures 1-3.

Some care was put into proper routing of power and sensitive analog lines and isolation. Also digital/mixed-signal part was properly conditioned by placing a solid ground plane under the traces and breaking it only to connect top traces where absolutely necessary.

Analog part with instrumentational amplifier was routed with symmetry in mind to limit common mode noise, because the signals of thermocouple are low and in the range on millivolts. Analog ground was also excessively filtered which turned out to be a mistake and causing oscillations on all analog signal lines, therefore analog ground was in the end connected directly to a common point of digital ground.

Soldering Station -. Schematic.pdf

Figure 1: Schematic of Soldering Station

Figure 2: PCB of Soldering Station

Figure 3: Bottom side of PCB with continuous ground pour

MCU programming

To broaden my knowledge outside of pure Assembly programming I finally used a MCU with modern ARM architecture Cortex-M3. STM was chosen because of CubeMX code initialisation generator program and good documentation of MCU architecture and also high level HAL libraries. This has enabled me to quickly write desired functionality that is comprised of following parts:

Interrupt for handling zero crossing of 12V sine power line
Write a custom Nokia 5110 LCD driver library
Write a simple scheduling program with pooling cycling the following tasks:
- LCD print
- Calculate PID regulation
- ADC measure potentiometer
- ADC measure thermocouple
- Get keypress

The regulation was done using PID algorithm, where a Weller tip was characterised. The use of only proportional P part was decided upon since it delivered the fastest response and without oscillations.

The soldering iron was controlled using half-waves of power line sine where 25 sine waves represented 100% duty cycle and duty precision was 2%.

Temperature setting had to be responsive and intuitive therefore ADC for potentiometer was read very often and a cubic transfer function was applied to linear voltage levels of potentiometer. This means that the most often used temperatures around 325°C were slowly to change and extremes (100°C and 450°C) very quickly because they need less precision. The code of the main program can be found in the LINK.

Conclusion

This was my first more advanced project designed in Altium. It consisted of three important parts: power, digital and analog. It was also my first experience using a switching power supply and all the headaches such regulator causes due to switching, especially when using an extremely efficient one that changes frequency.

This caused that the circuit needed debugging since errors were made:

The isolated MOSFET relay driver did not operate without supply on the driver side therefore additional wounding was added to toroid transformer and rectified to get 10V isolated supply.
Ground filtering between digital and analog was a bad idea which caused oscillations and was therefore replaced with a solid connection
Because soldering iron was using a common ground for heater and thermocouple a huge temperature spike was measured when powering the heater. Unfortunately this was only mitigated by measuring when heater is not turned on.
Weller WMRP soldering tip was not designed to be run by efficient method of half-waves and solid state relay. This resulted in melting of the heater inside the tip after short-term use. See Figure 4 for disassembly of broken tip.

Unfortunately the project was not successful in the end and resulted in soldering tip breakage. But nevertheless a lot about analog and digital electronics was learned. An upgrade to simple relay switching would be a proper constant current switching regulator that would offer more precise control over tip heater power and proper current limit to protect the tip.

Figure 4: Disassembled Weller tip with broken heater

Figure 5: Collection of Weller soldering stations