For my next project, I’ve decided to build a reflow oven. It will be a very useful tool to have if I want to use components on my PCBs that have exposed pads or no leads, as well as make soldering much easier, faster and more consistent.
Toaster ovens are most often used for this project; they provide ample room and the heating elements provide the heat necessary for the solder to reflow. However, after looking around I decided to go with the Black + Decker TO2050S. It’s fairly cheap, has 4 heating elements as opposed to the usual 2, and has a fan built in that circulates the air, allowing for more even temperature across the PCB, which will prevent parts from getting too hot or cold. I bought my oven at Bed Bath & Beyond, but you can also find the unit on Amazon.
The project is relatively simple from an electronics point of view: you have an oven modified so that a microcontroller can turn the heating elements on or off, along with a thermocouple that the microcontroller continuously monitors. If the temperature in the oven gets too hot, turn the elements off; if too cold, turn it back on. The temperature that the microcontroller tries to set the oven varies over time and is set by the thermal profile you’re looking for, which will largely depend on the solder you’re using.
The microcontroller will control the heating elements using an electromechanical relay. A lot of projects I’ve looked at use a solid state relay, but I couldn’t find one that fit my vision for the project. My oven is a 1350 W unit at 120 V, which means it will draw 11.25 A. Most solid state relays I’ve found are rated for that much current, but they require heatsinks because the load causes the relay to get hotter over time. The heatsinks are substantially larger than the relay itself, and it adds to the cost of an already expensive relay. I’ve therefore decided to go with a traditional relay since it is cheaper and does not require a heatsink. One drawback that the electromechanical relay has is that it can only drive resistive loads; the fan in the oven is an inductive load, so I had to modify the wiring. The fan will be controlled by the timer dial of the oven, while the heating elements will be controlled by the timer and the relay in series. The timer, in addition to controlling the fan, will act as a fail-safe so that the oven cannot run longer than the set amount of time. This will protect the oven from running forever in case the microcontroller does something stupid.
I’ve decided to keep the microcontroller simple; I’ll be using an Arduino Pro Mini with 3.3 volt logic. The built-in bootloader, the simple nature of the algorithm and lack of need for specialized hardware make the Arduino fine for this application. The Arduino will be talking to a thermocouple (3.3V) module through SPI in order to read the temperature of the oven, and provide a user interface through an LCD (5V) and buttons.
Here’s a block diagram of the circuit:
Note that the relay is actually on the relay board, but the FBD does not reflect that for clarity.
I have to order most of the parts for this board (LDO, relay, thermocouple module, buttons), but I’ve started on the wiring:
You can see I removed 2 of the three dials, added a USB wall adaptor that’s been modified to output 5 volts over wires, and the two wires that are hanging near the bottom center of the oven connect to the relay. I’m thinking of putting the relay board above the USB adaptor, but I would have to see how big the board will be first. For the sake of safety, I’m trying to keep all the 120 V parts inside the oven. I’m planning on placing the User Interface Board where the two removed dials used to be. Also note that I’m using crimps and quick connect terminals instead of solder due to the high voltage and current in the system.
Once the parts come in, I can make the circuit and do a quick test to see how quickly the temperature can ramp up and what the maximum temperature the oven can get to is. There are a couple of ways to improve both these: adding insulation, adding more heating elements, or changing the wiring to draw more power. In addition, you can improve the solder quality by using a more complex control algorithm, where the top two and bottom two elements are controlled separately. I’ll have to see if these are necessary after achieving basic functionality.
EE Diary 