My own reflow controller
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Author:  ivan141 [ Sat Sep 13, 2014 12:27 pm ]
Post subject:  My own reflow controller

Since I want to get back into electronics as a hobby I decided it would be great to have a reflow oven at hand.
Looking around the internet, it seems just about everybody and their mum has created their own controller, so why
shouldn't I join their ranks :lol2: .

Working from the following base wishes:
-No need for an onboard MCU, so just sticking a full arduino on there will do just fine.
-I don't know which control strategy is bestfor my intended oven, but since it's basically just
a halogen lamp, I decided to go for triac leading edge phase control, should keep the halogen cycle going.
-This is my first time dabbling with mains, so I'll stick a control output for a zero crossing SSR on there just in case.
-Minimal hardware user interface.. will probably run it connected to a pc with a GUI.

Already gathered the following bits:
-BT139/600 triacs
-MAX13855 thermocouple IC
-Cheap(ish) thermocouple

Still busy with the design of the thing.. but I'm pretty much done.
Got a bit too creative with the silkscreens, but that's part of the fun.

Killed some time doing up a logo.. let the old country inspire me since I'm on a short holiday in serbia to attend a wedding.
Gotta love old commie imagery.

Here's the chinese 'flavorwave-turbo' style oven I got.. they boast decent ramp-up speeds, especially with slight modification and
only cost 30 euro's delivered to my doorstep. If Mr.T says it's the best, I would be a fool not to buy it, right?

Author:  Fred [ Sun Sep 14, 2014 8:08 am ]
Post subject:  Re: My own reflow controller

Beautiful trace routing! :-)

Author:  ivan141 [ Sun Sep 14, 2014 11:51 am ]
Post subject:  Re: My own reflow controller

Thanks Fred, that's the result of me discovering the miter tool in Eagle.
Thought it might be fun to do curvy traces for a change.
It did help route the traces to the LCD and button connectors in a tighter space,
in an attempt to keep everything as far away from mains carrying traces as possible,
while still fitting the 10x10cm size constraint for cheap chinese production.

Word to the wise: don't do this unless you're absolutely sure you're satisfied with
the routing, it's really hard to correct things afterward.

Author:  Fred [ Sun Sep 14, 2014 12:50 pm ]
Post subject:  Re: My own reflow controller

Off topic: < this avatar of yours has never loaded for me. Instead I see the default alternative text "User avatar". It 404s in a browser, too. Fix! :-D

Author:  ivan141 [ Sun Sep 14, 2014 3:59 pm ]
Post subject:  Re: My own reflow controller

Yes.. hasn't worked since my parents changed provider a while ago. I'll see if I can fix it.

Author:  KLAS [ Mon Sep 15, 2014 9:01 pm ]
Post subject:  Re: My own reflow controller


i got such a "kitchen tool" for free and never know what to do with it.
not that i know what to do with a reflow oven, but you never know

Author:  ivan141 [ Fri Oct 17, 2014 8:55 pm ]
Post subject:  Re: My own reflow controller

Boards are finally in, soldering commences:

Nice quality for 30 dollars shipped (10 boards), I used Elecrow this time round and I'm happy with their service.

Silkscreen quality is more than decent for this pricerange:

The mains side of the board, it all looks so dinky IRL, but it 'should' be able to stand up to about 7 amps on the triac circuit according to my calculations.

Bottom of the board, I like to keep things simple there. Notice the bare copper on the mains traces.. the traces themselves are much bigger,
but I left a 5mm shortest route bare so I can solder some wire on there to reduce trace resistance (again, expect max 7 amps to pass here).

Some experimenting with logo's in copper.. came out ok-ish, would do it differently next time though.. copper would contrast better with the dark
red where there is no copper.

Stuck the rectifier, triac optocoupler, MAX31855 and my level shifting mosfets on.. now need to sift through the parts bin in the basement
to find all the correct resistors and capacitors etc for the rest of the board.

Author:  ivan141 [ Tue Oct 28, 2014 8:39 pm ]
Post subject:  Re: My own reflow controller

Finished initial assembly of the board, just need to find some thermal paste before I can fit the insulator for the triac.
Here it is running my first successful test of the thermocouple interface:

As is often the case, I didn't get the board right the first time round. There are a couple of areas that need improvement.
For starters my nifty logo should not be hidden by an arduino next time around.

In the more serious category I made the following fuckups:
-Mosfet bidirectional level shifter circuit incorrect. Dont know how this one slipped past me but I have 3 BSS138's on there for level shifting between
the 5v Arduino and the 3v3 MAX31855, and they are all 3 wrong. Had to do a little rework to fix this (luckily hidden from sight once arduino is installed).

The other (less serious) mistake is that I underestimated the dimensions of 2W THT resistors. I ordered Vitrohm ones which I thought from some specs
would be relatively compact, but I ended up having to do creative lead bending:

All this is academic since I really only need 1 to work as intended, I wasn't planning on making any money off these, but one cant help but hope when you
have a 10 board minimum order from the PCB fab.

Once the triac is properly isolated from the heatsink I will need to find my balls and commence testing with the zero crossing detection circuit. Might be a wise idea
to fab up some (wooden or plastic) enclosure first so I cant touch anything by chance.

Author:  ivan141 [ Thu Dec 25, 2014 12:32 am ]
Post subject:  Re: My own reflow controller

Finally picked this up where I left it a few months ago.

Since reworking the board I have done the following:
-Properly mounted the triac to the heatsink.

-Tested the MAX31855 with an actual thermocouple (works a treat).

-Tested the zero crossing detector (also works)
This one was a bit iffy.. I neglected to put this on a regular external interrupt pin so I had to dig up a library to use the pin change interrupts.
Did learn that those are really useful when you need more than 2 interrupt pins on your old ATMEGA328.
Performance is in the category: good enough. Due to the nature of the circuit, it will trigger a few hundred microsecs to early (or late depending
on what flank you trigger the interrupt from). But it's stable enough to work around this with a fixed offset.

-Tested the triac with an incandescent bulb.
I tried my hand at dimming a lightbulb with the triac. This is an easy feat, but I wanted to hand this all off to timers so it could work on interrupts.
Found a library for the 16 bit timer.. found out it sucked at what I needed it to do, rewrote the damn thing to work in CTC mode and added methods
that allow proper timer reset without having to reattach my interrupt all the time (in the end that was the only thing wrong with the original library).
Did learn a lot about timers in the process, and that's actually more important than the board doing what it is designed to do eventually.

-Wire up a button and LCD display board so I can test those interfaces.
-Reacquaint myself with PID control.

Author:  ivan141 [ Sat Feb 14, 2015 7:34 pm ]
Post subject:  Re: My own reflow controller

Again some minor process on this little distraction of an electronics project.

Having sussed the thermocouple interface and TRIAC phase control aspects out I set out to bring the 2 together into something a step closer to a reflow controller.
I coded up the MCU to dim a lightbulb as temperature readings increase, and it all works as you would expect.

The one thing messing with my head though is the massively unstable readings I get from the MAX31855K IC. I used the exact reference layout used by Maxim on their
eval board and I get a 5 deg. celcius variance on my signal. Tried with 2 different k-type thermocouples (one shielded, the other naked), and both show the same issue,
the shielded one just adds a big delay to the temperature response.

No amount of faffing about with the components / adding filtering capacitors seems to have a noticeable effect on this issue, so I have put a workaround in place.
A kalman filter was just the ticket. I have no scientific idea of what I'm doing, but the math works, and my output is now smooth as a babies bottom.
Here's a chart showing the difference:

Good enough for me.. next step: put my triac to the test and see if it will survive the oven's current draw at full blast for 10 minutes.
If anyone has any safety pointers to share on the matter I would be delighted to hear them. I've heard what big mosfets can do when
overheated (things went off like huge firecrackers in the testing oven of a former workplace of mine), and I wouldnt like to have that happen in my guest bedroom,
especially not with anyone around.

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