Author Topic: Finally starting on this convection oven -> reflow oven conversion project  (Read 6746 times)

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Offline mindcrime

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Project Update 05-14-2021

I decided to punt on the door control stuff for a moment, and focus on the interactive control elements. So I rigged up a 12 button keypad and a little 20x4 LCD display, and got that working with the Arduino to the point that I can get an interrupt when a key is pressed on the keypad, read the current key, stick it in a buffer, and repeat until I get the command delimiter ('#').  Right now I'm envisioning the command protocol being pretty basic. Something like:

  • n# for n = 1 - 9 or so -  means "Run Profile n"
     
  • 99# - means "abort current run and reset"

That may literally be it. I'm not sure if there are any other commands I would even need. Also, I'll have the LCD displaying some basic information like the currently running profile, current temperature, elapsed time, etc.

Also, a random thought occurred to me while working on this: I've always meant to learn to work with those IR remote controls, so I'm now toying with adding the ability to control this thing with a universal remote.  8)

Some pics of the work in progress:











A little commentary on that last picture - the IC on the right hand side of the breadboard is a 74C922 decoder, which translates the keypad button presses into a 4 binary digit signal which is then sent to the μ-controller. The other IC is a 74HC04 quad inverter. It's there to push the OE pin of the 922 low when the DA pin goes high.

The code running on the μ-controller attaches an interrupt handler to the digital pin that DA (Data Available) is connected to, and when it fires it reads one binary digit from each of the four data bus lines, then converts that to its integer form. Inputs are pushed into a fixed length buffer until either the buffer max length is reached with no terminator encounter (in which case the buffer is cleared and the previous input is assumed to be invalid), or the code mapped to '#' is encountered.  Once we see a '#', a flag is set that tells the main loop "it's OK to read the input buffer and process it now".  The rest remains to be implemented, but what will come next is the "run profile n" stuff and the actual temperature profile control logic (TBD).

« Last Edit: May 15, 2021, 09:46:17 pm by mindcrime »
 

Offline coppercone2

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I had to buy fiber glass silicone wire (the same kind as in the oven) to make the power and fan control extensions to my control  box which was bolted to the side of the top chassis. I usex like a 10x10x5 box on the side, and the lid of the box I made from 3/16 thick preforated steel sheet, so the PID and SSR are sure to get good cooling (like old HP equipment, but bigger preforations since its a power device without a internal heatsink).

Don't miss the detail, match the same type of wire they have in the oven side cabinet to the wire you are leading out, it gets pretty hot. They used the same type of wire for everything in mine, including the low power fan, until it got to a distribution block inside of the metal box, where I switched to conventional wire, since that are does not get hot.

Because I was too cheap to get a big base plate, to get stiff/ridgid mechanical assembly, its built for service by a octopus (i.e. mirrors and right angle key to service some things), thankfully my PID comes in a sleeve so theoretically I can swap out every part without undoing the wires, so long nothing in the oven breaks, then I will be cursing hard, well maybe if it broke right after I made it, but its been a year so I think I won't be too upset doing a bit of difficult disassembly.
« Last Edit: May 15, 2021, 09:01:00 pm by coppercone2 »
 

Offline mindcrime

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I had to buy fiber glass silicone wire (the same kind as in the oven) to make the power and fan control extensions to my control  box which was bolted to the side of the top chassis. I usex like a 10x10x5 box on the side, and the lid of the box I made from 3/16 thick preforated steel sheet, so the PID and SSR are sure to get good cooling (like old HP equipment, but bigger preforations since its a power device without a internal heatsink).

Don't miss the detail, match the same type of wire they have in the oven side cabinet to the wire you are leading out, it gets pretty hot. They used the same type of wire for everything in mine, including the low power fan, until it got to a distribution block inside of the metal box, where I switched to conventional wire, since that are does not get hot.

Because I was too cheap to get a big base plate, to get stiff/ridgid mechanical assembly, its built for service by a octopus (i.e. mirrors and right angle key to service some things), thankfully my PID comes in a sleeve so theoretically I can swap out every part without undoing the wires, so long nothing in the oven breaks, then I will be cursing hard, well maybe if it broke right after I made it, but its been a year so I think I won't be too upset doing a bit of difficult disassembly.

I went with this stuff https://iseinc.com/_shop/16-awg-tggt-high-temperature-hook-up-wire.html for wire. And it will be routed inside an additional insulating sheath as well.

 

Offline coppercone2

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That is similar, but I went with color coding and 12awg for a premium lol,

you can put heat shrink on the ends of the wire going into the chassis with the correct color code
 

Offline mindcrime

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That is similar, but I went with color coding and 12awg for a premium lol,

you can put heat shrink on the ends of the wire going into the chassis with the correct color code

Yeah, I was actually looking for colored wire, but was having trouble finding the right combination of temperature range, price, availability, and available colors. Most of the colored stuff I found was only available in spools of 100' and were in "not stocked" status anyway. This stuff seemed like a decent enough compromise. I like your idea about tagging the ends with some heat shrink tubing. That will definitely help keep things straight.

Edit: I found some black high-temp wire for a reasonable price as well, so now I have two colors.  Should be sufficient. I think the only long runs of "extra" wire I will have will largely be to the door actuator.

What I might have to do, is work out a way to add an extra little heat shield or something, to protect the PCB, the micro, and the small amount of wiring that will live in the right-hand side of the case. There are already vent scallops on the back wall of the case there, so adding a fan for that section might be a good idea as well.
« Last Edit: May 21, 2021, 02:26:07 am by mindcrime »
 

Offline fourfathom

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Here is the profile I get with my un-modified two-element toaster oven, this time with some gentle fan-driven airflow during the cool-down portion.  Also, the suggested profile from the ChipQwik datasheet (Sn63/Pb37).  My time above 183 deg C is still a bit long (120 sec vs the suggested 90 sec), but it's been working well for me. 

You can see my heater on/off control on the chart as well as the temperature.  As I mentioned, I didn't see any reason for duty-cycle control of the heat, as full-on / full-off seems the best when trying for rapid heating and cooling.  My control parameters are shown on the left of the chart: 
For the preheat I turn on the heat elements. 
When the temperature reaches 100 C, I turn off the heat for 60 seconds and start the soak cycle.  The temperature continues to rise.
At the end of the 60 second soak phase the temperature is at 150 C and I turn the heat back on and start the reflow stage.
I turn off the heat when the temperature hits 215 C, and the temperature continues to increase, peaking at the desired 235 C.
When the temperature falls by one degree I sound the buzzer and I manually open the oven door.
Here, I have a small fan that blows ambient air into the oven chamber.  This speeds up the cooldown phase.
 

Offline mindcrime

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Hey @fourfathom, thanks, that helps a lot. I'm just  starting to get some temperature ramp up data from mine, so it will be nice to have something to compare against. 

I did a test run just now, where I had the micro turn the oven on, and ramp it all the way up to 250° C, then turn it off, all while spitting out the elapsed time and temperature every two seconds. I have the raw data, but haven't had time to graph it or do any math on it yet, to see how close to linear (or another shape) the curve is. I'll probably work on that this weekend, along with more testing.

This is with no additional insulation added, and with the outer cover off the case. Putting the cover back on and adding insulation would almost certainly give it a faster ramp up, but not sure if the extra insulation will prove necessary or not. Note: times are elapsed time since the oven was turned on, in milliseconds. So it takes about 112 seconds to reach 150° C, and about 290 seconds to reach 250° C.

Unfortunately I forgot the logging code in the branch after the oven is turned off, so I didn't get any data on the cool-down period. That will have to wait for this weekend.


Code: [Select]
MAX31856 thermocouple test
Thermocouple type: K Type
Turning OVEN on
6094,38.70
8095,38.74
10096,38.92
12097,39.48
14098,40.01
16099,40.76
18100,41.38
20101,42.32
22102,43.68
24103,44.92
26104,46.48
28105,48.45
30105,50.43
32107,52.88
34108,55.23
36109,56.72
38110,58.66
40110,60.77
42111,62.39
44113,64.33
46114,66.89
48115,69.15
50116,71.81
52117,74.73
54118,77.52
56119,80.40
58120,83.06
60121,85.89
62122,88.76
64122,91.73
66123,94.66
68124,97.58
70125,99.91
72126,102.75
74127,105.54
76128,108.15
78129,110.71
80130,113.55
82131,115.97
84132,118.54
86134,120.90
88135,123.23
90136,125.71
92137,128.09
94138,130.46
96139,133.08
98140,135.23
100141,137.30
102142,139.69
104143,142.20
106144,144.77
108145,147.09
110146,149.40
112147,151.77
114148,153.75
116149,155.80
118150,157.81
120151,160.20
122152,162.41
124153,164.43
126154,165.95
128155,167.55
130156,169.28
132157,171.14
134158,172.90
136159,174.64
138160,176.48
140161,178.12
142162,179.54
144163,180.73
146164,182.05
148166,183.74
150167,185.57
152168,187.45
154169,189.18
156170,190.81
158171,192.52
160172,194.12
162173,195.80
164174,196.96
166175,198.50
168176,199.80
170177,200.97
172178,201.88
174179,203.13
176180,204.23
178181,205.58
180182,206.45
182183,208.28
184184,209.75
186185,210.84
188186,212.09
190187,212.93
192188,214.21
194189,215.20
196190,215.73
198191,216.87
200192,218.16
202193,219.02
204194,220.09
206195,220.93
208196,221.87
210197,222.83
212199,223.62
214200,224.29
216201,224.83
218202,225.88
220203,226.89
222204,227.71
224205,229.41
226206,230.29
228207,230.56
230208,231.59
232209,232.15
234210,233.00
236211,233.88
238212,234.80
240213,235.68
242214,235.87
244215,236.47
246216,237.03
248217,237.43
250218,237.94
252219,238.37
254220,238.89
256221,239.70
258222,240.63
260223,241.34
262224,241.95
264225,242.40
266226,243.06
268227,243.29
270228,243.59
272229,244.30
274230,244.66
276231,244.95
278232,246.23
280233,247.23
282235,247.55
284236,247.79
286237,248.37
288238,249.31
290239,250.00
Turning OVEN off
 

Offline fourfathom

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Since I'm turning the heater on / off / on / off, I can't compare your ramp-up rate exactly, but if we look at the time it takes to go from 75C to 100C (my heater has probably stabilized at full-output during this interval), my oven takes 20 seconds, and yours takes 18 seconds.  The ramp from 175C to 220C  Takes mine 30 seconds, and yours 31 seconds.  And I'm just eyeballing this from my chart (I didn't have the logging turned on), I'd say we're pretty close.
 
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Offline mindcrime

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Did some more temperature profiling tonight. Not trying to match a published profile yet, just some "ramp to peak / ramp down" stuff, with the door both closed and open during the ramp down period. Captured all the data this time and got some decent plots. So far I'd say this looks encouraging. I don't think I'll have too much trouble getting this to match the various solder profiles to within the allowed variance.

[See attachment images below]

Also, here are some random pics of the testing in progress, as this all starts to come together. Soon I need to go ahead and start looking at how to mount the various components in the case (whichever ones are going in the case anyway), so I can button this up and do some testing with the cover on. I also need to integrate the keypad/LCD stuff I did, come up with some kind of enclosure for those parts, and figure out how to mount that. Right now I think I'm going to leave the door actuator stuff for the very end. It's pretty simple conceptually, and I need the outer case cover on when I mount the actuator anyway.











Also, if anybody is wondering "Why is there a piece of cloth under a toaster oven that's heating up to 250° C?" the answer is... there isn't. That grey thing is a carbon fiber welder's blanket.

[attachimg=3]

[attachimg=2]
« Last Edit: May 22, 2021, 05:00:06 am by mindcrime »
 
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Offline fourfathom

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Good progress!  I would be interested in the temperature ramp after you turn off the heater.  On mine, the temperature continues to rise for quite a while.
 

Offline mindcrime

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Good progress!  I would be interested in the temperature ramp after you turn off the heater.  On mine, the temperature continues to rise for quite a while.

Yes, I also see the temperature continue to rise at least briefly, after turning off the elements. Hence the overshoot which I think is visible even in the plots. Here's the raw data around that point in the process, with the point where the elements are turned off marked.

Code: [Select]
0,29.21
2,29.25
4,29.25
6,29.21
8,29.21
10,29.23
12,29.30
14,29.44
16,29.68
18,29.91
20,30.30
22,30.75
24,31.33
26,31.98
28,32.79
30,33.71
32,34.79
34,35.91
36,36.99
38,38.45
40,39.77
42,41.46
44,43.19
46,44.95
48,46.70
50,48.57
52,50.59
54,52.70
56,54.86
58,57.18
60,59.40
62,61.76
64,64.00
66,66.50
68,69.02
70,71.49
72,74.17
74,76.94
76,79.66
78,82.38
80,85.20
82,88.01
84,90.89
86,93.78
88,96.70
90,99.67
92,102.68
94,105.45
96,108.47
98,111.59
100,114.45
102,117.34
104,120.23
106,123.29
108,126.22
110,129.09
112,131.96
114,134.71
116,137.52
118,140.55
120,143.35
122,146.27
124,148.97
126,151.77
128,154.49
130,157.11
132,159.80
134,162.45
136,165.24
138,168.12
140,170.59
142,173.07
144,175.52
146,178.14
148,180.67
150,183.06
152,185.30
154,187.61
156,190.00
158,192.35
160,194.48
162,196.81
164,199.04
166,201.21
168,203.29
170,205.52
172,207.52
174,209.66
176,211.77
178,213.76
180,215.70
182,217.84
184,219.87
186,221.84
188,223.83
190,225.77
192,227.60
194,229.45
196,231.41
198,233.20
200,234.97
202,236.46
204,238.06
206,239.95
208,241.79
210,243.52
213,245.16
215,246.80
217,248.16
219,249.65
221,251.42 # elements turned off here, as soon as temp > 250.0
223,252.95
225,254.27
227,255.27
229,255.77
231,256.19
233,256.43
235,256.75
237,257.18
239,257.54
241,257.59
243,257.71 # temperature peaks here, 22 seconds after the elements are turned off
245,257.67 # and declines from here on
247,257.34
249,257.02
251,256.62
253,256.20
255,255.62
« Last Edit: May 22, 2021, 05:48:42 am by mindcrime »
 

Offline beanflying

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Unlikely it is the bead type thermocouple lagging so it is more just a case of some thermal inertia carrying you past what you are aiming for. In Coffee Roasting we have a different set of issues as the beans go exothermic during the roast but if we need to bring the ramp down a quick blip of air through the chamber is way faster than waiting for the Element and Metal/Bean mass to respond.

Is your code written to work from actual temperature data only or does it look at a Rate of Rise/Fall as well? RoR or some sort of Temperature average over a few seconds is sometimes a better thing to run from.
Coffee, Food, R/C and electronics nerd in no particular order. Also CNC wannabe, 3D printer and Laser Cutter Junkie and just don't mention my TEA addiction....
 

Offline fourfathom

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Interesting.  Your peak happens 22 seconds after the heater is turned off, and mine (215C to 232C) takes 45 seconds.  No doubt the absolute and ambient temperatures makes a difference, as does the fact that you have removed the sheet-metal oven cover.  I also see a peaking difference in my oven when I start with the thing at ambient (the first run), compared to when in between cycles I only let it cool to 50C or so.
 

Offline mindcrime

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Unlikely it is the bead type thermocouple lagging so it is more just a case of some thermal inertia carrying you past what you are aiming for. In Coffee Roasting we have a different set of issues as the beans go exothermic during the roast but if we need to bring the ramp down a quick blip of air through the chamber is way faster than waiting for the Element and Metal/Bean mass to respond.

Is your code written to work from actual temperature data only or does it look at a Rate of Rise/Fall as well? RoR or some sort of Temperature average over a few seconds is sometimes a better thing to run from.

The current code is only looking at the actual instantaneous temperature, but I haven't even started seriously trying to implement the code to actually track real temperature profiles yet. I just wanted to get some raw data to see if the oven can fundamentally heat up and cool down fast enough to accomplish the job at hand. My tentative plan is to implement PID control eventually.

The more I think about it though, PID may be overkill. I may be able to get away with just "P" (proportional) control based on the rate of rise / fall. The rate of rise, looking at the plots, looks close enough to linear that I think that might work. I'll take the data and run some linear regression analysis on it later, and see what that gives me.
« Last Edit: May 22, 2021, 06:14:41 am by mindcrime »
 

Offline coppercone2

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do you have the fan in your oven? It makes a big difference, even a small slow fan

BTW the way I wired mine is the fan is connected to its own switch, the modes still control which elements turn on (i.e. broil), the timer is left in place, so the mechanical timer actually disconnects it if you forget. If i have a thermocouple in the middle and I turn the oven fan on after it stabilizes it spikes
« Last Edit: May 22, 2021, 08:58:26 am by coppercone2 »
 

Offline fourfathom

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Here's a plot that shows rate of change (in degrees C per second) and temperature (in degrees C) as my toaster runs the full cycle.  Again, I turn off the heater at 100C , wait 60 sec then turn it back on, off again at 215C, then open the door right after the peak.  I didn't use the cool-down fan for this run.  As you can see, the rate is quite variable.  It takes a long time for the heating elements to come up to full heat or to cool off. 

I'm going to run this again without the "soak" shutdown, and set my off-point at 250C, just to see the behavior.  I'll bet we could model this as a loaded RC (or LC?) low-pass filter.  We've got a voltage/heat source, a heater thermal lag, a "capacitor" (thermal inertia), and a load (thermal leakage to outside ambient).

Again, I'm not seeing the value in using a proportional heater drive.  I do this when doing thermal testing of components and I want to maintain a fairly low temperature in the toaster, but for the reflow even 100% on / 100% off barely gets me to the desired thermal ramp rate.  Using anything less than 100% is only going to slow down the rate of change.  Perhaps if I added insulation a proportional control would make sense.
 
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Offline mindcrime

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Again, I'm not seeing the value in using a proportional heater drive.  I do this when doing thermal testing of components and I want to maintain a fairly low temperature in the toaster, but for the reflow even 100% on / 100% off barely gets me to the desired thermal ramp rate.  Using anything less than 100% is only going to slow down the rate of change.  Perhaps if I added insulation a proportional control would make sense.

I'm probably mis-using the term "proportional" here. What I mean is just that, since the ramp up rate (with the elements full on) seems to be close to linear, the temperature at time t is directly proportional to t, including the overshoot period after the elements go of, so I can achieve a particular value of temp by doing a simple multiplication of t to tell how long to run the elements. What I'm envisioning would, in effect, be almost identical to what you do, modulo any difference in the constant(s) involved. Maybe you have to run your elements for 100 seconds for the first period, and I have to do 95, or 105, something like that. But the basic idea is the same.

I'm definitely not proposing anything like PWM, or trying to modulate the power to the heating elements in any way other than "on" or "off".

 

Offline mindcrime

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do you have the fan in your oven? It makes a big difference, even a small slow fan

There's no fan in this oven by default. I could see adding one if it becomes necessary though.

In fact, I almost certainly will add at least one, to cool the case area where the electronics live. Not yet sure if it will be necessary to add anything to forcibly move air through the cooking chamber or not.

 

Offline coppercone2

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I thought there was a fan in the oven, based on your picture.

I see a motor and some kinda vents in the side of the oven in the opening pictures in your thread. What am I looking at?
https://lh3.googleusercontent.com/pw/ACtC-3en77W0POn-RK16h_JZ2_LsB2ruJMBqGtA-JGGH6liVDBJRKC-_dlcBIdCFZGUCJPs9bT6etZf53wocdq4lBrfRc5DzyTUNW0X6xj2Rws_dt5OxKHtBWoktv4rUaSdeQxlRyrjh598n_k2_PZgPNq31=w1244-h933-no?authuser=0

at the bottom, that looks like a circulation motor.
 

Offline mindcrime

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I thought there was a fan in the oven, based on your picture.

I see a motor and some kinda vents in the side of the oven in the opening pictures in your thread. What am I looking at?
https://lh3.googleusercontent.com/pw/ACtC-3en77W0POn-RK16h_JZ2_LsB2ruJMBqGtA-JGGH6liVDBJRKC-_dlcBIdCFZGUCJPs9bT6etZf53wocdq4lBrfRc5DzyTUNW0X6xj2Rws_dt5OxKHtBWoktv4rUaSdeQxlRyrjh598n_k2_PZgPNq31=w1244-h933-no?authuser=0

at the bottom, that looks like a circulation motor.

Ah, sorry, I misunderstood you. I thought you were talking about a fan to vent air to the outside, for forcible cooling. Yeah, I guess the fan that circulates air inside the chamber counts as a fan too. :-) I just wasn't even thinking about it.  I wasn't planning to do anything with it, except let it keep it's default behavior, which as far as I know, is to be on whenever the heating elements are on.

I guess I could rewire it to switch independently of the heating elements, if needed. In my head I was thinking more about adding a separate fan if I wound up needing more cooling.
« Last Edit: May 22, 2021, 09:15:51 pm by mindcrime »
 

Offline coppercone2

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Well you are not turning the fan on and off on the SSR are you? Thats why I put its own mechanical switch, for instance if I am preheating parts for soldering, welding, painting, curing epoxy at some random elevated temperature, etc, I keep the fan off. I might be useful to not have a fan going if you are just doing a warming cure of saying potting compound. You get more out of your oven IMO if you have the option to disable the fan, it seems unnecessary for some times, and you can experiment which thermal profile is better. And of course you can experiment to see if its better maybe to run the preheat more aggressively then turn the fan on sometime after to get a rapid temperature spike that you need to match a profile, or to turn it off when you are blowing out the oven during its cool down phase, since you don't really want to be circulating the air in there anymore, you want it to get out.

I think maybe your oven and beans oven are different because one has a fan and one does not, it changes the behavior, I know this for a fact. I thin its more efficent because you have less stratification (bunch of hot air near the top), so it might actually also increase element life when its running real hot because their not bathed in their own hot air. But if I am doing like a 1 hour varnish cure, I don't see the point of running the fan, or if I am heating a part to max before letting it cool down to spray paint.

The other thing I did is I glued (with jb weld) a stainless steel piece of sheet I bent into a small U shape, so that I can put a ceramic thermocouple connector in there, and rest it in the top in the little shelf, so I can disconnect the thermocouple and put a new one on, because then you can just unplug it near the ceiling of the oven and change the thermocouple if you damage it easily without taking it apart . I also cut small noodles out of ceramic tube (from a broken heating element I found in the trash) and glued them into the hole s for running the thermocouple wire to make sure it does not get cut (I made it smooth with a dremel diamond point).
« Last Edit: May 22, 2021, 11:00:39 pm by coppercone2 »
 

Offline mindcrime

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Well you are not turning the fan on and off on the SSR are you? Thats why I put its own mechanical switch, for instance if I am preheating parts for soldering, welding, painting, curing epoxy at some random elevated temperature, etc, I keep the fan off. I might be useful to not have a fan going if you are just doing a warming cure of saying potting compound. You get more out of your oven IMO if you have the option to disable the fan, it seems unnecessary for some times, and you can experiment which thermal profile is better.

You make a good point. I might just go ahead and add a separate switch for that fan at some point. I can see how it might be useful for certain applications, now that you mention it.
 

Offline coppercone2

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I thought to add a speed controller for the fan too, it should fit in my oversized chassis, but its already pretty weak as it is, and I am not sure how much it will like a controller since its already operating in pretty extreme conditions for a fan, I am not sure about those cheap AC motor speed control circuits and how they do with heat and what the effect on the coils is.
 

Offline mindcrime

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I thought to add a speed controller for the fan too, it should fit in my oversized chassis, but its already pretty weak as it is, and I am not sure how much it will like a controller since its already operating in pretty extreme conditions for a fan, I am not sure about those cheap AC motor speed control circuits and how they do with heat and what the effect on the coils is.

I hear ya. For me, I'm probably just being overly paranoid, but given that almost all of my electronics knowledge / experience (to the extent that I have any) involves working with low-voltage DC stuff, I'm reluctant to do much fiddling with the AC mains voltage level stuff in the oven.

I've mostly been trying to treat it all as a "black box" where the only thing I do is use the SSR to turn the power on and off, and everything "downstream" of the SSR says unmodified. The only other thing I'm doing (so far) that involves the AC side, is tapping into the AC cord to connect my step-down transformer which provides the low-voltage DC rails, and adding a rocker switch on the back of the chassis.
 

Offline fourfathom

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mindcrime, I finally noticed that your oven has an internal fan, and mine doesn't.  The different construction is probably why our heating and cooling ramps aren't identical.  As for messing with the AC voltage wiring, The only modification I've done to the oven is to drill a tiny hole in the back for the thermocouple lead.  I'm using an aluminum foil-wrap gasket to protect the lead and to seal the air leaks.  I have an external SSR in the middle of a short extension cord that the oven is plugged into.  The SSR, and my controller are all powered via the USB interface to the controller.  My controller has two buttons and a small display which lets me start and stop the reflow cycle, or initiate self-calibration cycles, but I usually run the controller from a PC program that I wrote.

Just for fun, I've been trying to model the thermal behavior of the oven, and I've come pretty close with an electrical equivalent in LTSpice.  I ran some tests with the heater at 10% and 30% duty-cycle to see what the steady-state chamber temperature was for a given heater power, and logged the door-closed cool-down cycle to find the chamber thermal mass (capacitance) and the chamber-to-ambient thermal leakage (resistance).  I also empirically added a second R-C delay to the circuit to model the heating/cooling lag in the heater element.  This gives me my overshoot.  The component values look strange because I've been playing with time-scaling and other normalizations.  Now the scale is 1 second = 1 second, and a heater voltage of 10,000V = 100% duty-cycle.  It comes pretty close to matching my oven, but if I ever want to use the model for anything practical I will need to dial it in a bit more.
 


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