WARNING!! This project involves hazardous voltages, high temperatures, sharp edges & risk of injury or death and is inherently unsafe. There are plenty of tutorials out there for anyone looking to build their own reflow oven, this post isn’t meant to be a tutorial.
With that said… here is my take on the reflow toaster oven.
I found myself in need of a small reflow oven that I could do low volume production and prototype quantities in. The small commercial units were priced in the thousands, and didn’t offer any features I couldn’t implement myself. So I browsed the various homebrew reflow ovens on the web and set off to make my own. I grabbed this little Black & Decker from Amazon and got to work.
It’s fairly small with a wire rack that measures about 11.5″ wide X 9″ deep. This allows me to run several small boards at once or, one or two medium sized PCB’s at a go. The small footprint fits nicely on the workbench too!
First order of business was to fire it up and see what it would do. Temperature control is via a simple bimetallic thermostat that regulates based on the air temperature behind the control panel, which is walled off from the oven chamber with a single piece of sheet metal. This struck me as an odd arrangement, but I’m replacing all of that anyway so it doesn’t really matter.
I grabbed my trusty Fluke meter and threw a thermocouple inside to see what the internal temps maxed out at. The bimetal thermostat opens at about 530 F, at which point the temp would drop a couple dozen degrees before closing again. Temperature regulation that is probably fine for reheating pizza, but it just won’t do for PCB assembly.
During initial testing I also noted the apparent lack of insulation and the single wall construction on the rear and bottom of the unit which made for an external temperature in the melt your skin off range. This issue will be addressed shortly.
A little probing around with the multimeter & tracing wires and I had a schematic to start with.
The three circles on the left represent the three knobs on the front panel. The four heating elements are shown at the top center of the page. The bimetallic thermostat switches the AC hot leg, and the timer switches the neutral. The function switch selects between various modes of operation. Note the diode connected to the top elements. The diode acts as a half wave rectifier when the elements are connected through the diode and provides a reduced power output setting. The oven also provides a bypass around the diode for full power output from the top heating elements. I’ll be retaining the heating elements and convection fan, the rest is getting gutted.
With the factory controls removed, it was time to start insulating the box. The goal here was to keep heat losses to a minimum for efficiency and provide for an even temperature distribution within the oven.
The single wall back panel was the first to be addressed.
I added a fairly heavy cross break to this panel to increase stiffness and provide just a little more room for insulation. It is held in place by the same screws that hold the back panel in place.
Providing further challenges to the insulation task was the single wall bottom panel of the oven. However there was a “bump out” similar to the back panel, and a crumb tray with a matching bump out. I cut the front lip off the crumb tray, flipped it over, drilled some holes and screwed it down with a bit of insulation in the resulting cavity.
Insulating the left side of the oven. In the first picture you can see the ends of the heating elements and the spring that holds the door closed. A little careful trimming and this side is done.
The right side insulation gets a little more involved since I need to work around the convection fan, and also leave room for installing the new controls in this space. I made up several insulation retaining bales using stainless filler rod.
I built the new control on a Parallax Propeller project board and sprinkled it with various goodies. Internal oven temperature is monitored with two k type thermocouples hanging on a piece of stainless wire just above the wire rack. Thermocouples are being read via the MAX31855 on Adafruit’s handy breakout board. A DS1620 also resides on the board to monitor the temperature inside the control enclosure space. The Propeller monitors the temperature and turns the case fans on and off to keep temps reasonable. The convection fan is also controlled by the microcontroller via a relay soldered onto the board.
Space is a little tight so things are shoehorned in, but it all fits. In the following picture you can see how its all laid out. Control board at top left, cooling fans at top right. Note the high tech Altoid tin insulation shield to keep the insulation from being pulled into the cooling fans. DC power supply at bottom right (white box) provides 12VDC for control power. The black relay zip tied to the power supply is a safety shutdown for the heaters. It is wired into a bimetallic thermostat buried under the insulation and will drop out in the case of a control system failure and thermal runaway. Heater current is switched via the solid state relay seen at bottom left.
User interface consists of a rocker switch at the bottom of the front panel for system power. The red button pulls the safety relay in when pressed, thereby “arming” the heaters. The relay is wired to self latch when this button is pressed, and stay latched unless a thermal runaway event trips the safety thermostat. The rest of the user interface is via a resistive touchpanel from 4D Systems.
The touchpanel has a lot of capabilities that I’m not currently using. The software running this oven is pretty basic at this point but it gets the job done. Temperature regulation is via PID and is working really good. Reflow curve parameters are user adjustable via on screen menus. At some point I’ll write an update and throw some more features in & make the display a bit prettier too. I’m planning to add in a graph of the temperature during reflow into the large open space in the center of the home screen shown above.
The insulation helps a lot too. The exterior still gets warm during repeated extended use, but it no longer has the ability to instantly sear flesh. As of the time of this writing this oven is still going strong. It has even assisted in a couple of Kydex holster making projects at this point.