Week 16: Applications and implications

In the week14 , when we had the mechanical design week, I described about different parts of the final project. In this week we got to know several nice examples that had been done in the former classes of fab academy. Our task this week was to plan and document our final project. There were several good questions in Neil's keynotes for week 16 that could help us find the direction of the project.

For my final project I would like to make a soldering Iron with a temperature controlled soldering station. I am planing to make a stand for the soldering iron too with a place for sponge. The idea of making a soldering iron was based on Neil's suggestion in one of the sessions. I decided to make a temperature controlled soldering station and a stand too, because all of these could cover several skills in the lab. One of the other motivations was to work with the termal systems and learn how to control them with a PID controller. I am going to answer all of the questions here one by one.

What will the final project do?

The soldering Iron with a temperature controlled soldering station can be used to solder the components on the electronics boards. The temperature controlled soldering station helps the iron to stay in a fixed temperature. Also the temperature can be set to different values based on the needs in the defined range.

What are the former tasks done in this regard?

Soldering iron has been on the market for many years. So it is not something new. In this project I am planning to make a DIY (Do it yourself) soldering Iron. There has been some efforts done before for making the DIY versions. They are usually hacked versions although there are some DIY soldering stations version available. give the background here

What materials and components are required?

Along with answering this question I will explain how the system works. Based on that information it is easy decide about the materials and components. I can divide this project to three smaller parts and describe them one by one

• Soldering Iron

The main part in the soldering iron is the heating element. The heating elements are available on the market for a cheap price. I am planning to buy one. The heating element has one resistive element that takes care of the heating. It has also a thermocouple that senses the temperature. The body of the heating element is usually made of ceramics. Is it possible to build one?

The other part is the handle. I am planning to use molding and casting for the handle part. What material is the best to do that with

• Soldering Station

For the soldering station the main part is the controlling circuit. In the input part there is the thermocouple that senses the the temperature. There are also two buttons for decreasing or increasing the temperature. The data which is read from the thermocouple is an analoge value. Maxim has designed some ICs to convert this value from Analog to digital. Max6675 for the k-thermocouples and max31885 for K-, J-, N-, T-, S-, R-, or E type can be used to provide the digital value for temperature to the microcontroller. Both of these ICs connect through SPI.

For the output, I will be using an LCD to show the amount of temperature. Also the soldering Iron comes in the output which its temperature should be controlled using a PID controller. The PID controller can be implemented using an IC like or and other solutions. There is also the possibility to do a software based PID controller. For the AVRs the specification can be seen here. There is also a well explained article about PID controllers that Dave from Amsterdam sent to me. I will put it here too.

The PID controller is reading the temperature from the input which is the thermocouple. It is considered as the feedback as well. Based on this it will set a the duty cycle of a PWM signal. This PWM signal goes to a Mosfet to provide enough current for the soldering Iron. The PWM signal turns the Mosfer on and off. When it is high the Mosfet is on and it can conduct the current to the soldering Iron. When it is off there is no current.

The soldering Iron which is a resistive element is connected to the source voltage (for example 12 volts) from one side and from the other side it is connected to the Mosfet. When the PWM duty cycle increases, the Mosfet is on for more amount of time. That means current can go through the Iron more in average and more power will be delivered to it. On the other hand with lower duty cycle less current in average will be delivered to the Iron. That is how the temperature is controlled in the Iron.

Note1: the power of the iron: The power of the iron can be determined easily by the formula p=(v^2)/r, where v is the voltage on two side of the iron, p is the power of the iron and r is the resistance of the heating element. Therefore targeting a power, for example 20 watts, after measuring the resistance of the heating element we can define the amount of voltage.

Note2: the mosfet: It is very important to select a mosfet that can handle the current which is being delivered to the Iron. It is easy to measure the current from v=ir, where v is the voltage across the iron, r is the heating element resistance and i is the current going through the iron. It is obvious that having a higher voltage, less current can be used. Therefore a Power Mosfet can be chosen that can handle less current which is generally cheaper.

Note3: power supply: for selecting the power supply it is important to select the one that can deliver the amount of power which is needed. For example if we are willing to use a 12 volt power supply with a 20 watt iron, then we should be make sure that our power supply can at least source i= (20/12).

Note4: when the soldering iron is attached to the solder it will lose its heat soon, by transferring that to the solder. That is why it is better ot adjust the PWM signal to 50% when the iron reaches the desired temperature. Because with this setup the duty cycle can go higher when the iron is transferring heat to the solder to compensate.

Note5: the number of microcontroller input and output pins: 2 pins for 2 switches that increase and decrease the temperature. 3 pins of SPI for connecting the MAX6675 to the micro, 6 pins of micro for connecting the LCD, one PWM signal which goes to the power mosfet.

• Soldering Stand

The stand can be done from anything. Only the part which is holding the soldering iron should resist against being burned.