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Finally starting on this convection oven -> reflow oven conversion project

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mindcrime:
I've had this Black and Decker convection oven sitting on a shelf for a month or more now, so tonight I finally decided to get started on the project. All (or at least most) of the parts I need have arrived now: I have an SSR for the mains power side, a couple of thermocouples and a thermocouple amplifier board to detect the temperature, and a linear actuator to open/close the door. I have plenty of microcontroller boards laying around, so I just need to decide what I want to use to run the thing.



The sacrificial lamb before the hacking begins.



Opened up and ready for surgery.



That red wire looks important.



The existing controls. Looks like I can mostly leave this stuff alone and just insert the SSR "upstream" so to speak. Worst case, if that timer doesn't have an "always on" position, I can bypass that.

Note to self: I should have powered this thing up and frobbed the controls a little before starting the surgery. That said, I still could since I haven't touched any of the wiring yet.

Edit: on second thought, I am going to need one or two more parts that I may not have lying around here already. I'll need an h-bridge motor driver so I can reverse the linear actuator. I'll also need a transformer to give me 12V that I can rectify to DC to use for the actuator. I may have that in the parts box already, not sure offhand.

And the datasheet for my SSR suggests adding a fast blowing fuse on the load side, so I'll probably have to pick up a fuse / fuse holder. Still, the major elements are all here.  :-+

fourfathom:
I have what looks like a similar oven I use for reflow.  It has heating elements above and below the racks.  I have a single sheet of aluminum in the middle position to hold my circuit board being reflowed.  I use a thermocouple attached to a chunk of circuit board, screwed to the aluminum tray.  I don't have a door actuator, but rely on me being there to open it during the cool-down cycle.

My controller is an arduino-style "ItsyBitsy M0", and it drives an optoisolated SSR between the wall power and the oven power cable.

My oven is un-modified, and I found that the heating and cooling rates with the heater 100% on and off were pretty close to the desired profile.  I have an auto-learning program that adjusts the on and off timing to hit my targets after a few calibration runs.  There is no fancy PID loop or proportional heat control, it's just on and off.  Here's the sequence:
Preheat: turn on the heat, wait until it reaches about 100 C.
Soak: Turn off the heater. The heat continues to rise, but slowly, almost flattening out after 60 seconds.
Reflow: Turn on the heater.  Temperature climbs.  Keep heat on until temperature reaches about 205 C (this value changes as a result of calibration runs)
Peaking: Turn off the heater.  The temperature continues to rise, peaking at about 230 C (my target for leaded solder)
Cooling:  Once the temperature drops a few degrees, buzz the alarm to tell me it's time to open the oven door.

I have the electronics in a small aluminum chassis, with a couple of buttons and a display, and I can control it with those.  I also have a USB serial-port interface which lets me control the thing from a program running on a PC -- this gives me nice charting and logging.  I also have duty-cycle heater control capability, and sometimes use that if I want to use the oven for medium-temperature thermal testing, but for soldering the on/off heater control works well.  Duty-cycle control would only slow down the heating ramp, and it's already barely fast enough.


mindcrime:

--- Quote from: fourfathom on April 28, 2021, 04:55:07 am ---I have what looks like a similar oven I use for reflow.  It has heating elements above and below the racks.  I have a single sheet of aluminum in the middle position to hold my circuit board being reflowed.  I use a thermocouple attached to a chunk of circuit board, screwed to the aluminum tray.  I don't have a door actuator, but rely on me being there to open it during the cool-down cycle.

--- End quote ---

I'm never going to run this thing when I'm not physically present, but the door actuator seemed prudent based on the experience a buddy of mine had with his oven. He found that his oven doesn't cool fast enough during the "cool" parts of the cycle, to track the temperature profile well. He could only get the cooling to be fast enough by opening the door.


--- Quote ---My controller is an arduino-style "ItsyBitsy M0", and it drives an optoisolated SSR between the wall power and the oven power cable.

--- End quote ---

Hmmm... opto-isolation, that's probably a good idea. I should do that. Glad you mentioned that.


--- Quote ---My oven is un-modified, and I found that the heating and cooling rates with the heater 100% on and off were pretty close to the desired profile.

--- End quote ---

Gotcha. Other than the door actuator (and adding thermocouples), I don't plan to modify the core of the oven itself. It's all about switching the power on and off as needed through the SSR.


--- Quote ---I have an auto-learning program that adjusts the on and off timing to hit my targets after a few calibration runs.  There is no fancy PID loop or proportional heat control, it's just on and off. 

--- End quote ---

I thought about going that route as well, but this is as much a didactic exercise for me as it is just a tool building exercise, and I've always wanted to learn more about control theory, so I think I'm going to go the PID route. Although once all the "pieces" are in place (thermocouples, door actuator, SSR), then I can tweak everything else through software, so I may experiment with different algorithms and approaches as I learn more about control theory.



--- Quote ---I have the electronics in a small aluminum chassis, with a couple of buttons and a display, and I can control it with those.  I also have a USB serial-port interface which lets me control the thing from a program running on a PC -- this gives me nice charting and logging.  I also have duty-cycle heater control capability, and sometimes use that if I want to use the oven for medium-temperature thermal testing, but for soldering the on/off heater control works well.  Duty-cycle control would only slow down the heating ramp, and it's already barely fast enough.

--- End quote ---

Sounds pretty close to what I'm planning. I definitely want to support data logging / control via a PC. I might use wifi for the connection instead of a USB cable though. Heck, I could even experiment with using Bluetooth, I suppose.

That said, I like the idea of having a display and some sort of input mechanism right on the oven... at the very least I'd like  a way to select which temperature profile to run, a "start" and "stop" button, and a display of the instantaneous temperature in the oven.

That's an interesting thought about using the oven for thermal testing, and possibly other uses, besides just for soldering.  I hadn't even considered it having any application besides soldering (and learning control theory stuff).

rstofer:
I used this controller.  I remounted the display in a metal box mounted above the oven.  Clean installation.

https://www.rocketscream.com/blog/product/tiny-reflow-controller/

That version is retired, the new version is:

https://www.rocketscream.com/blog/product/tiny-reflow-controller-v2/

I installed the Solid State Relay in the right hand sidewall.  I also added high temperature mat insulation in the various cavities.  Without the insulation, I couldn't get up to temperature fast enough.  McMaster-Carr carries it but I don't remember which variant I used:

https://www.mcmaster.com/insulation/insulation-for-equipment-ovens-and-furnaces/

My oven is based on the Black & Decker InfraWave toaster oven.

fourfathom:
Yes, if you leave the oven door closed after the end of the reflow stage you will never get close to a recommended cool-down rate.  I might eventually add a door-opener.  Another option is to add an air duct and a blower, but that probably adds other thermal complications.  With my beeper alarm the system works well enough for now.

I've been asked to write up my toaster reflow, and when I've collected the data will post more here.  I've also hacked into a "Reptile Incubator" Thermal Chamber (see eevblog video: ), modifying it and adding an external controller (much like my toaster-reflow project).  This does used a PID control loop, and I'm writing that one up as well.

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