this morning i did a last test-run with the tweaked Felder ISO-Cream profile:

yeah… at the top i thought it is in the cooling step already and opened the window – with ~3°C cold air from outside it dropped fast..
then i found it is in the middle of the reflow – sorry… and closed the window again – until it really switched to cooling..

the old left-over pcb i use for these is done now.. i comes from my LEDBoard_4x4_16bit project – and if i remember correctly i backed it with the assembled board in the oven multiple times back then..
now grilled it again ~4-7 times. it smells very bad – is super dark discolored.. i think that is ok with about ~12 times solder cycles..

and then started to assemble a simple board to really test the profile 🙂

then reflowed:

i added a paper-lid to have stable air inside..

reflow was successful 🙂
my profile is just a little bit to long for my right angle touch switches:

they melted away 🙁 – lesson learned – have a look in the datasheet and you know that they are very heat sensitive!

in general i have the feeling that my heating elements get a little bit to hot – the pcb also slightly discolored at on place…
so will keep an eye on this and improve it..

Open Points

• add housing
  • i would like to have class at the top for a good view what is happening inside..
• metal frame for heating-elements
• quite 5V fan with PWM control for cooling
• add second temperature sensor
  • contact-less melexis MLX90614ESF ??
• spring thing to hold board down
• way to fix sensor position on board
• more heating elements for bigger working area
  • switchable configuration for long or more square pcbs?!
• bigger power supply ?! (~750W)

for tuning i followed more or less the tutorial PID Without a PhD
from Tim Wescott
and the tutorial and video from PID Explained Team.

first i just checked with low temperatures of 20..40°C
as i went on and tested up to 260°C i noticed that the current did decrease. and the temperature did not increase any more.
i could see this in my graph as the heating got slower and slower with the rising temperature… (also the pid already saturated at the output..)

so i measured the resistance during the cool down of the heating elements to get some insights:
(4x in series → 48V/4=~12V/Module)

Temperature (°C)	Resistance (Ohm)	Current (A)	Power @48V (W)
255	40	1,19	57
250	39	1,21	58
240	38	1,23	59
230	36	1,26	60,5
220	38	1,26	60,3
200	34	1,35	64,8
100	26	1,8	88,6
80	24	2	96
60	22	2,18	104
40	20,9	2,3	110
25	19,5	2,46	118

result: the ~57W is not enough to get to more than 255°C…

i rearranged the Modules into 3-in-series connection.
this means ~16V/Module – and tested again:

Temperature (°C)	Resistance (Ohm)	Current (A)	Power @48V (W)
255	27,5	1,45	70
250	27,0	1,5	72
240	26,6	1,6	77
230	25,6	1,67	80
220	25,3	1,7	82
200	24,1	1,8	86
100	18,8	2,1	100
80	17,5	2,7	130
60	16,7
40	15,6
25	14,3

with this i found that i can go above 255°C.

i then tested the profile for the Felder ISO-Cream “Clear” and found that in the reflow stage the heat-up is a little to slow:

in the *my setup* picture is a temporary cardboard thing with a 80mm 12V fan (connected to 5V) to cool down faster between tests.
for the final setup i think i will buy 1 or two 5V and PWM capable fans….
and also exchange the *chamotte* ston with some metal frame.
this way i also can cool the bottom side..

so i again switch the configuration –
now i have a 2-in-series config: 24V/Module
CURRENTLY THIS TABLE IS ONLY CALCULATED VALUES!!

Temperature (°C)	Resistance (Ohm)	Current (A)	Power @48V (W)
255	20	2,4	115
250	19,5	2,46	118
240	19	2,52	121
230	18	2,67	128
220	17,5	2,74	132
200	17	2,82	135
100	13	3,69	177
80	12	4	192
60	11	4,36	209
40	10,45	4,59	220
25	9,75	4,92	236

i also tested this with the Felder profile:

this time the heat-up is fast enough! 🙂
the nice and working pid tuning i had for the 4-in-series arrangement is now out of tune…
so i will have to re-tune it to get less overshoot / swing.

while having a break i thought about the maximal power in this configuration –
and found that this way i only be able to power 2×2 modules with my 250W power supply.
for now i leave it this way. in the long run i hope with the other frame concept i get more heat to the pcb and less into the stone and this way be able to use the 3S config.

Tuning

after a day of mostly waiting til the system cooled down again
– one test cycle <=60°C needs 400s → 6:40min –
i just rebuild my hw mounting setup.

• 

• 

this way i can warm up quicker and cool down much quicker as i do not store heat in the stone. – at least that is what i hope..

hmmm – does not seem to change much..

i then tested the actual Felder Profile:

seems i have a working profile.
i will add a little more time for the prepare phase. so the pcb is really fully at the 50°C. at the top i have a little bit of a mis-match –
i saw on my temp sensor directly connected to the heating elements at the top ~265°C – so that is hot…
the pcb seems to increase its temperature resistance at higher temperatures… at the peak i have 230°C to 245°C error. and to the heating this results in ~35°C difference…

i will report when i solder the first real board. 😉

there was a response to my request in the forum..
and hw & sw basics done..

the idea came from this Applied Science Video:
Electroluminescent paint and multi-channel control circuit
21 Nov 2018 starting at 11:25

there is a link to amazon for a element – and it is not available to delivery to germany 🙁
so i went on with some help of friends and found

dimensions	voltage	power	current	resistance (guess)	power@ 6V	power@ 9V	power@ 12V	power@ 24V	link
70mm x 15mm	12V	70W	5,8A	2,06R	17W	39W	70W	280W	HALJIA 12V 70W Wired MCH Metal Ceramic Heating Plate Heating Element 70mm x 15mm
70mm x 15mm	24V	110W	4,6A	5,2R	7W	15,6W	27W	110W	Haljia 24V 110W Wired MCH Metal Ceramic Heating Plate Heating Element 70mm x 15mm
40mm x 40mm	12V	48W	4A	3R	12W	27W	24W	192W	Haljia 12 V48 W Wire MCH Metal Ceramic Heating Plate Heating Element 40 mm x 40 mm
40mm x 40mm	24V	96W	4	6R	24W	13,5W	48W	96W	Haljia 24 V96 W WIRED MCH Metal Ceramic Heating Plate Heating Element 40 mm x 40 mm

to get an idea of how much power i actually need i had a look at the small commercial IR-Heaters and Hot-Plates –
they all have about 800W:

180mm * 235mm = 42300 mm² = 423 cm²
a = 60mm * 60mm = 3600 mm² = 48 cm (1x4)
b = 60mm * 80mm = 4800 mm² = 48 cm (2x2)
c = 60mm * 90mm = 5400 mm² = 54 cm² (1x6)
d = 60mm * 120mm = 7200 mm² = 72 cm² (1x8)
e = 120mm * 60mm = 7200 mm² = 72 cm² (2x4)
f = 120mm * 90mm = 10800 mm² = 108 cm² (2x6)
g = 120mm * 120mm = 14400 mm² = 144 cm² (2x8)

423 cm² == 800W
1 cm² == x

x = 800W *   1cm² / 423cm² = 1,89W
a = 800W *  36cm² / 423cm² = ~68W (1x4) 
b = 800W *  48cm² / 423cm² = ~91W (2x2) 
c = 800W *  54cm² / 423cm² = ~102W (1x6)
d = 800W *  72cm² / 423cm² = ~136W (1x8)
e = 800W *  72cm² / 423cm² = ~136W (2x4)
f = 800W * 108cm² / 423cm² = ~204W (2x6)
g = 800W * 144cm² / 423cm² = ~272W (2x8)

30x40mm: ~23W/module
60x15mm: ~17W/module

then i calculated the resistance of the found element to check on what wattage i can do at what voltages:

U = R*I
P = U*I
→ P = U*(U/R)

(i added these *guesses* in the table above)

So I decided to go with the 70x15mm 24V model.
and will update here if i found how this works out..

and for the first test setup i will go with the concept
12V→ 27W / module
so definitive more then enough..

as power supply i will use a MeanWell GST280A48-C6P
(reichelt) with an fitting connector (reichelt)
to get a 5V for the controller i will go with a recom R-78HB50-05 (VIN: 9-72V)
and for switching the power to the heating elements i will use IRLB4030PBF – MOSFET N-LogL 100V 180A 370W 0,0043R TO220AB
and to drive this a BC 550C as mentioned in this nice article:
Schalten und Steuern mit Transistoren III – Mit MOSFETs höhere Ströme schalten

so when all the parts arrive i can go on.. with building.

for the Controller i plan to write it in CircuitPython and run it on an adafruit ItsyBitsyM4. and maybe later add an LCD – or use a PyBadge –
for now i just want to use the arduino serial plotter or similar with an second CDC-device enabled to log the progress and the flash-drive function of CircuitPython for a text-file with the temperature-profile.
i have written a request in the adafruit CircuitPython forum if there are any PID controller things out there…