The diploma thesis presents development and manufacture of a soldering station. The purpose is to build a system which performs the function of a soldering station and has two special features: it is compatible with different types of soldering handles and controlled by the development board for learning programming MISKO V2. The latter enables the user to program the station according to their own wishes, thus offering the opportunity to learn programming of microcontrollers and regulation techniques.
The soldering station consists of a power supply, a soldering iron temperature sensor reader, a unit for measuring and controlling the heater current, and a unit for communication with user and for MISKO connection. The core of the soldering station which manages the entire system is Atmels processor on the development board. The power supply is divided into two parts. The first part lowers the mains voltage to 24 V, rectifies it and smoothens it. This voltage is used to power the heater and the second part of the power supply. The second part is a step down buck converter which lowers and stabilizes the unstable 24 V DC to stable 5 V DC to power other parts of the system. Most soldering handles use thermocouple for measuring temperature. They provide temperature information in the form of voltage. The dedicated integrated circuit with built-in cold junction compensation is used to read the temperature of the thermocouple.
The current control circuit topology is similar to a buck converter with continuous current, controlled by pulse width modulation. That way of control prevents current spikes and emissions generated by the circuit. The magnitude of the heater current is measured as the voltage drop across the shunt resistor through which the current flows. The low voltage across the shunt resistor is adjusted with a differential amplifier to the measurement level of analog- to- digital converter.
The user controls the soldering station with two potentiometers and an incremental encoder which are located on its own printed circuit board. The values of settings, temperature and current measurements are displayed on the LCD screen, which is already integrated into the MISKO V2 development board.
The temperature measurement with the used soldering handle is not accurate in the entire working range of the soldering station, presumably due to the manufacturer’s cheaper design of the thermocouple. Additional errors in temperature reading are caused by current flow through the heater. Therefore, a double software temperature correction is made enabling correct measurement over the entire working temperature range, regardless of the current magnitude of the heater. The temperature regulation is performed programmatically with a PID controller which determines the value of the duty cycle of the PWM signal.
With the help of the Matlab application, the identification of the soldering heater response is made along with its mathematical model, on the basis of which the parameters for the desired operation of the controller are set. In our case the response and the speed of regulation were tuned and verified on a 70 W soldering iron.
The finished system functions as a soldering station and a platform for learning programming. It features compatibility with a fairly wide range of soldering handles, has 80 W of power, and additionally replacement of the development board is fast and easy. Achieved temperature regulation is stable with small overshoots and satisfactory speed.