Reflow Oven Controller - Attiny 461-based

[SaabFAN]

Hi

I have build a controller for a toaster oven to turn it into a reflow oven.
I was inspired by this http://thomaspfeifer.net/backofen_smd_reflow.htm guy and more or less recreated his circuit and designed a PCB for it, which you can see in the attachments.
On the hardware-side the circuit has the capabilities to use i2c-connected H44780 LCDs (can be found on ebay for under 10€ with free shipping) and 4 buttons to control it.
Unfortunately, the ATtiny461 isn't supported really well in Arduino, so the advanced function pretty much went down the drain when I realized that a few hours ago. -.-
So I will probably resort to just coding the most basic control and then send it to the chip via the ISP-Connector with the 2 pins for i2c connected to 2 LEDs to show the mode (Preheat, Soak, Reflow, Finished) and using one button to start a cycle until the chip is supported by the Arduino libraries.
Or maybe one of you guys would like to have a crack at it. I currently don't have the time to adapt one of the ATtiny-libraries for Arduino (https://code.google.com/p/arduino-tiny/ & http://highlowtech.org/?p=1695), as I am occupied with finishing the project I did this little side-project for, as well as University.
As a little incentive, I'm offering 4 of my remaining 9 (bare) boards to the one who makes the Arduino IDE compatible with the Attiny 461 and 3 boards to anyone who then writes a software that allows the User to program custom temperature-profiles with the available interface/display-combination. These have to be open source and shared with the community of course

A few words about the circuit: It uses 2 Solid State Relays (S202S02 Opto-Triacs made by Sharp) to switch the 2 heater-banks inside the oven on and off. The SSRs are able to handle 8A, which is enough for a 1KW oven (standard seems to be between 900W and 1,2KW).
Temperatures are measured with a diode: Voltage-drop across the diode has a more or less linear dependency, which gives enough accuracy for this kind of application.
Power-Supply is provided by a 7805 that in turn is fed by any power-source within the limits of the IC (Battery, Transformer + Rectifier, dynamo for the guys who want to work out when doing electronics XD).
High Voltage-Path can be protected by a fuse on board if not already available in the casing.
If the controller is used externally, one of the SSRs can be left out of the circuit and the two free pins then used for something else.
User Interface can be made with a i2c-connected Display of some sort and the connectors provide enough pins for 4 GPIO-Ports and one Output (5 GPIO-Ports if the internal PullUp-Resistor of the Attiny is used).
Programming is done via the ISP.
For further information, see the attached schematic.

The used components should be readily available to everyone here: 0,1µF Ceramic Caps, 10K, 1K, 330/220Ohms Resistors (all in 0802-Package), 1 Axial 47µF Electrolytic Cap, 1 7805, some Pin-Headers, a few cables and a LED. The remaining parts: The SSRs, buttons, Display and the Attiny461 can be found on ebay for less than 15€ combined. Bringing the total system-cost to about 20€, including 1PCB and a short thermal resistant cable.
The total BOM comes down to 35€ - 40€, including the oven and adding some reserves for shipping etc.

Well... I hope you guys like this little device and hopefully someone can come up with a solution to use its full potential.
In the attached Zip-File you will find the schematics.

PS: I know, the traces for the high voltage-stuff have pretty small gaps. My father's Fluke says "OL" between them an the protective Ground Layer, but next time I will make the gap a lot bigger

Note: The ZIP-File contains the updated circuit and looks a lot different from the pictures.
The pictures have been left in there only to make the following conversation more understandable.

[wraper]

This board does not seem to be made right/safe on the mains side at all. Tiny gap between Live and neutral and normally you shouldn't do copper filling on the mains side at all. Also strange terminal block marked earth, while for sure it cannot be connected to the earth. SSRs must be mounted on the heatsink to handle rated current.

[SaabFAN]

Earth is for the 3rd cable, which is connected to protective ground (Schutzleiter in German) and the case of the device. It is supposed to ensure that any surface that can be touched is always grounded. It is left out on many new devices though, because the case of these things is made of non-conductive materials (plastics).
"Earth" is probably a bit misleading: It means "Erde" in german, which in turn can be translated as "Soil", "Ground", "Planet Earth", as well as the term for electrical ground.

Thx for the input on the mains-side. I haven't made anything with more than 12V yet, so this is a first. How would you connect the protective ground to the device? I put a ground plane there because I've seen a similar construction in a PC Power Supply: It had a big ground plane running across the entire device, which was connected to protective earth and the case, grounding the entire PC-case.

About the heatsinks: Good catch. I probably would have burned out the SSRs pretty quick (didn't read the datasheet and just assumed the original circuit was ok). Based on the datasheet, a small heatsink from an old amplifier plus a small fan should be enough.

[wraper]

Earth is for the 3rd cable, which is connected to protective ground (Schutzleiter in German) and the case of the device. It is supposed to ensure that any surface that can be touched is always grounded. It is left out on many new devices though, because the case of these things is made of non-conductive materials (plastics).

I know for what earth is for, I just don't get what the hell this connector is doing there. Of course I cannot see what on the bottom of the board but on top it seems that it is connected to the copper fill. If this is true, then connecting earth to that point is dangerous because it won't protect anything (there is really no point for earth being connected on that side) but waiting for a moment for a short appearing between the live and the earth.

[SaabFAN]

...but waiting for a moment for a short appearing between the live and the earth.

That was kind of the idea: If for some reason voltage from the mains "leaks" into the PCB and thereby creating the possibility to reach the case and anything I could touch, this copper-plane would short out the 220V, tripping the FI-Switch inside the Fuse-Box. Plunging the house into darkness, but keeping anything nasty away from me.

[wraper]

How would you connect the protective ground to the device?

First of all solid connection to the case if it is made of metal. Secondly good connection to the ground of low voltage side. And keep it on safe distance from the live parts (4-5mm at minimum, better 6-8 mm + slot). You might want to connect some few nF Y2 rated safety capacitors between the live wires and earth if concerned about interference. Actually that would be one of the very few reasons why you would want to keep the earth conductor close to the live conductors at all.

[wraper]

...but waiting for a moment for a short appearing between the live and the earth.

That was kind of the idea: If for some reason voltage from the mains "leaks" into the PCB and thereby creating the possibility to reach the case and anything I could touch, this copper-plane would short out the 220V, tripping the FI-Switch inside the Fuse-Box. Plunging the house into darkness, but keeping anything nasty away from me.

You do not want to something leak at all. It should only leak to earth only if it leaks to the low voltage side where it becomes dangerous. Now imagine:
1. What if the wiring in the house is crap and there actually is no earth connection or it is unreliable.
2. What if there is no reliable earth wire connection in the mains plug for some reason.
3. What if FI-Switch is faulty or replaced with some bolt or whatever that make things work.
3. Whatever that makes earth connection unreliable.

Now consider that you just made something that in some situations will artificially short live wires to the case of the device  . Earth is additional protection if the things go wrong somewhere. You must not increase the probability of the things going wrong in the first place.

[wraper]

Not sure if I'm really blind or you are really crazy, but are all of those points marked with red connected together  ?

[SaabFAN]

No. The ones marked "Earth" on the top of the picture are connected to the big copper plane, the other ones are connected to a dedicated trace.

[wraper]

Well, then I guess the gap is so small that I cannot see it on the picture. Anyway if you are planing to use this board, do not connect mains earth wire to the mains side/copper fill. Or it will be dangerous as hell. And if you are going to use holes on the mains side for PCB mounting, do so only with plastic standoffs, not metal, or again, it will be dangerous as hell.
You should take some SMPS pcb which have the earth connection and look there how thees things are done, before doing something like this again.

[Phoenix]

PS: I know, the traces for the high voltage-stuff have pretty small gaps. My father's Fluke says "OL" between them an the protective Ground Layer, but next time I will make the gap a lot bigger

This is not enough to demonstrate safety by any stretch of the imagination.

The only real way to test this is with an insulation resistance tester (commonly called a megger). They apply 500V or 1000V and record 10's of gigohms. If you get anything but max scale when using a megger on a circuit like this there is a problem.

[wraper]

Here is the bare minimum I would consider to be safe.

[SaabFAN]

Okay, based on your comments, I will not offer the boards anymore and instead scrap them and, until I get new ones, control the soldering in the oven with a thermometer and a switchable power cord.

Would this be a better design?

I have renamed the Schutzleiter to "PE"(for "Protective Earth"), as it seems that the naming caused some confusion. And also reduces the number of SSRs to 1, to include a fan-connector. This limits the max. input-voltage of the control-circuit to 12V, however.

[wraper]

Connect PE to the ground plane of the low voltage part. Otherwise there is no reason to keep this connector on the board at all.

[sunnyhighway]

Put the rating of the fuse on the silkscreen.

[SaabFAN]

Okay, thanks for the help to make this safer.
This is what it looks like now:


In the attachment you'll also find the new Eagle-Files in a zip-File if someone wants to play with those.
I still haven't found any lib to make the 461 work with Arduino, so I'll change the circuit to the 84 or even the 85. It'll require one more i2c-Expander for the input, but offers the advantage of using up to 8 keys instead of just 4.


Edit:
I have now added the Attiny85-Version as well.

[sunnyhighway]

Not sure how the silken marking for the S202S02 will work out. Might just work in a zero gravity environment. 

Guess you also ment 250V/6A instead of 220V/6A

And that 230V marking for the terminal. Is that AC or DC?

Be explicit and precise.

[SaabFAN]

Not sure how the silken marking for the S202S02 will work out. Might just work in a zero gravity environment. 

Guess you also ment 250V/6A instead of 220V/6A

And that 230V marking for the terminal. Is that AC or DC?

Be explicit and precise.

Idk what you mean with the silken marking for the S202S02, but about the rest you're right: 250V/6A and its AC.
*Updated schematics for the Attiny85-Version in the first post.*

EDIT: I think, I know what you mean: The silkscreen for the SSR is right across the gap.

[redshift]

Idk what you mean with the silken marking for the S202S02

I think sunnyhighway is saying that the silkscreen will be printing over the cutout(where there is no pcb to be printed on)

[SaabFAN]

I figured as much.
Well, that's something i can totally live with.
I've ordered a new batch of boards and will post here again once they arrived. For the time being I'm occupied with getting the PM3320A to work again anyway (which in turn MIGHT require the use of this little device)

[timofonic]

Nice prpject!

If I understand it, can this modify a regular electric oven into a reflow oven? What models are compatible?

[SaabFAN]

Basically every model is compatible. Unless the thermal switch inside the oven disables the heaters at a temperature below 230°C.
But even if it does, it is still possible to use the oven without modification to it: The Software has to be modified to give the oven a 100% Duty-Cycle instead of a controlled ramp after the soak-phase. That way the temperature will overshoot (a lot) and will reach the required temperature for the solder paste to melt.
Alternatively, you can also use a solder paste with a lower melting point.

If you install the controller into a dedicated case outside the oven (which I recommend), you don't even have to open the oven. Just plug it into the controller-box, set the temperature-profile and solder away

By the way, the current version uses an Attiny85 instead of the 461, because I can use the Arduino IDE and all the libraries available. For the user-interface, it requires a 2x16 LCD-Display with i2c-Interface and a i2c-Based Input-keyboard. I recommend the PCF8574 Chip. A heatsink for the Triac and a fan are also required.
An important note here: The controller requires at least 220V for a 1000W oven, otherwise the Opto-Triac has to carry too much current. And you also require a 7 to 28V DC-Power Supply. Any old transformer plus rectifier should do: 1 large cap and the 7805 should provide enough ripple-rejection. I'm using a 9V Battery for this job.

EDIT: Attached are the files and a Screenshot of the most recent Version of this device.

[moemoe]

I normally work with Eagle's net classes to ensure spacing between the high voltage parts. This works also fine with copper planes, see attached files. This also has the big advantage that the DRC warns you if you route traces too close to each other.

As a general guideline I'm using https://www.mikrocontroller.net/articles/Leiterbahnabst%C3%A4nde

3mm between L and N
6mm between L/N and Low-Voltage
6mm between PE and everything (not mentioned in the article, but should be save).

I don't see any good reason to connect PE with GND, I'd rather leave it floating. By referencing GND you remove all your galvanic isolation without any reason.

[free_electron]

You are going to use a diode as temp sensor. This diode will sit inside the oven right ?

What will you use as solder to connect to its wires ? It better be above the max oven temperature or the diode will desolder itself on the first cycle you run ....

[moemoe]

It better be above the max oven temperature or the diode will desolder itself on the first cycle you run ....

I'd recommend no solder at all, but a proper crimp.

[wraper]

Diode is a wrong type of temperature sensor first of all. And there is no guarantee it won't die after a few dozens of reflow cycles.

[wraper]

I normally work with Eagle's net classes to ensure spacing between the high voltage parts. This works also fine with copper planes, see attached files. This also has the big advantage that the DRC warns you if you route traces too close to each other.

As a general guideline I'm using https://www.mikrocontroller.net/articles/Leiterbahnabst%C3%A4nde

3mm between L and N
6mm between L/N and Low-Voltage
6mm between PE and everything (not mentioned in the article, but should be save).

I don't see any good reason to connect PE with GND, I'd rather leave it floating. By referencing GND you remove all your galvanic isolation without any reason.

What is the purpose of PE if you leave it unconnected? You are left without any protection it should provide. What isolation? for what reason GND should be floating against oven enclosure. And if you don't connect oven enclosure to PE too, you are an idiot. PE shouldn't be connected only if this is double insulated device which usually is not the case for high power devices.

[moemoe]

What is the purpose of PE if you leave it unconnected? You are left without any protection it should provide. What isolation? for what reason GND should be floating against oven enclosure. And if you don't connect oven enclosure to PE too, you are an idiot.

I never talked of leaving the oven enclosure unconnected.

But it's still legitimate to leave your GND floating. In fact, I don't know any power supply or lab supply that has GND referenced to anything, so all devices connected to them are also floating. Are all these designers idiots?

[fcb]

What is the purpose of PE if you leave it unconnected? You are left without any protection it should provide. What isolation? for what reason GND should be floating against oven enclosure. And if you don't connect oven enclosure to PE too, you are an idiot.

I never talked of leaving the oven enclosure unconnected.

But it's still legitimate to leave your GND floating. In fact, I don't know any power supply or lab supply that has GND referenced to anything, so all devices connected to them are also floating. Are all these designers idiots?

Huh?  All my bench supplies, and all those I have come across in my many years have the chassis connected to earth!  Please give the model number of the ones you have found that don't have an earthed chassis so I can be sure to avoid any labs with them in...

[moemoe]

I never talked of leaving the oven enclosure unconnected.

But it's still legitimate to leave your GND floating. In fact, I don't know any power supply or lab supply that has GND referenced to anything, so all devices connected to them are also floating. Are all these designers idiots?

Huh?  All my bench supplies, and all those I have come across in my many years have the chassis connected to earth!  Please give the model number of the ones you have found that don't have an earthed chassis so I can be sure to avoid any labs with them in...

It seems you're confusing GND/Enclosure/PE.

To clarify:
GND is the low voltage side's GND/0V
Enclosure is the metal outside, which should be always connected to PE, of course
PE is the green/yellow wire, or whatever color it has in your country, connected to "real" earth outside of the house/N in your distribution

[wraper]

I never talked of leaving the oven enclosure unconnected.

But it's still legitimate to leave your GND floating. In fact, I don't know any power supply or lab supply that has GND referenced to anything, so all devices connected to them are also floating. Are all these designers idiots?

In the lab supply negative terminal isn't connected to the ground because you can use the output in various ways. For example, connect multiple PSUs in series. There usually is earth terminal too with a metal plate included which you can attach between negative terminal and earth.
Laptop power supplies usually have negative contact connected to the earth. In the the PCs GND is connected to the PE too. Almost every oscilloscope excluding portable ones. If negative terminal isn't connected to the enclosure (PE) in the PSU itself, doesn't mean it shouldn't be grounded. You may want to make one grounding point somewhere else to avoid ground loops.

[moemoe]

Laptop power supplies usually have negative contact connected to the earth. In the the PCs GND is connected to the PE too. Almost every oscilloscope excluding portable ones. If negative terminal isn't connected to the enclosure (PE) in the PSU itself, doesn't mean it shouldn't be grounded. You may want to make one grounding point somewhere else to avoid ground loops.

I just measured all my Thinkpad Power supplies - none of them hast GND and PE connected.

On my battery charger (BC700), Bat- is not connected to PE (or, to be more precisely, the external power supply doesn't conenct them, as it has a Euro plug).

On my printer (Brother MFC), the Network/USB shields are not connected to PE.

[wraper]

I just measured all my Thinkpad Power supplies - none of them hast GND and PE connected.

On my battery charger (BC700), Bat- is not connected to PE (or, to be more precisely, the external power supply doesn't conenct them, as it has a Euro plug).

They are double insulated devices.

as it has a Euro plug

Europlugs are only designed for low-power (less than 2.5 A) Class II (double-insulated) devices that operate at normal room temperature and do not require a protective-earth connection.

[wraper]

On my printer (Brother MFC), the Network/USB shields are not connected to PE.

They likely are not connected to GND directly but through the capacitor and likely bleeding resistor too.

[eneuro]

Diode is a wrong type of temperature sensor first of all. And there is no guarantee it won't die after a few dozens of reflow cycles.

Anyway, f someone wants sense temperature at low cost why not? 

Accurate Temperature Sensing with an External P-N
Junction
http://cds.linear.com/docs/en/application-note/an137f.pdf

Diode-Based Temperature Measurement
http://www.ti.com/lit/an/sboa019/sboa019.pdf

However, I'm interested if there is some kind of diode testing in this project MPU software, to ensure we hav enot faluty diode, eg. at room temperature 25*C we expect on 1N4148 some voltage, so if this diode is damaged I;d like to see the massage on LCD or "SOS" diode health status blink to let us know tha we ahve take another 1N4148 and replace inside reflow oven 
So, there is no problem that a few cents 1N4148 fails, but some kind of management needed

Another thing is, how temperature sensing is implemented there? Using formulas mentioned in linked application notes or by experimental data?

I never tried this method of higher temperatures sensing, but I like this idea 
There is -2mV/C forward voltage drop mentioned in this fan controler based on 1N4148 Vf sensing:
Using 1N4148 As A Temperature Sensor
http://electronics.stackexchange.com/questions/80131/using-1n4148-as-a-temperature-sensor

I'm interested in using those cheapy 1N4148 as temperature sensors in my custom reflow owen based on hot iron ceramic heater, so I'm thinking to close such 1N4148 diode inside reflow oven and starting from room temperature ~23*C  made measurements of Vf and temperature up to lets say 100*C, than plot this data to see relationship between temperature as this Vf changes and make: linear or maybe better parabolic approximation to xtrapolate above 100*C up to 300*C.
How do you think, such approximation of real data and extrapolation to higher temperatures makes sense or we have to do all this "hard" math and use delta Vf measurements (using eg. two constant current sources) to be able to estimate sensed temperature as mentioned in linked above application notes? 


Of course I will try sense temperature inside kitchen oven using this calibrated 1N4148 temperature sensor and we'll se if I will get something which was preset at this oven temperature scale 

[wraper]

Anyway, f someone wants sense temperature at low cost why not?

Because one fried board/batch of boards very likely will cost more than a decent temperature sensor (+ coresponding circuit) like PT100 or thermocouple . It is just not wise idea (and waaay over spec) to use semiconductors at so high temperature.

[eneuro]

It is just not wise idea (and waaay over spec) to use semiconductors at so high temperature.

Semiconductors are used at such temperatures since you reflow them in ovens, maybe not so many times, so as I said probably we need to manage this and lets say find amount of reflows that will degrade its performace to the point that they will be useless
I'm thinking about something like arbitration logic in my solution-use three those cheapy 1N4148 diodes inside reflow oven, than connect to three MPU ADC pins and choose only two closest Vf voltages and skip third reading, so when one of those diodes fails during reflow it shouldn't affect this process.
When one of the diodes gives temperature readings far away from other two than fault diode blinks and output which diode is cause of this fault, so we know which one we need to replace...
I do not know if it will work, but I'll test it next week to see what happends later in kitchen oven

Update: If we're talking about specs we can find in absolute maximum rating for 1N4148
http://www.nxp.com/documents/data_sheet/1N4148_1N4448.pdf

Tstg storage temperature -65*C .. +200*C
Tj junction temperature 200*Cmax

So, yes 250*C is above its specs, but I'd like to see when (after how many made cakes in kitchen oven) this 1N4148 fails 

[free_electron]

It is just not wise idea (and waaay over spec) to use semiconductors at so high temperature.

Semiconductors are used at such temperatures since you reflow them in ovens,

and they undergo even higher temperature when they are being produced , BUT : THEY ARE NOT POWERED UP ! No electrons are flowing through them. !!!

you will create electron migration and other strange effects if you do this.

[wraper]

It is just not wise idea (and waaay over spec) to use semiconductors at so high temperature.

Semiconductors are used at such temperatures since you reflow them in ovens

And if you look in the datasheet for the soldering info, there will be something like 3-5 times for a few seconds at peak temperature.

Tstg storage temperature -65*C .. +200*C
Tj junction temperature 200*Cmax

Should be some special flavor of 1N4148 from NXP as in other datasheets Tj is 165 - 175^OC

[eneuro]

This diode will sit inside the oven right ?
What will you use as solder to connect to its wires ?

I want use this small custom PCB to hold diode in reflow oven


No need to solder anything


I've in my toolbox something like this for PCB and heatsinks assembly

[eneuro]

Tstg storage temperature -65*C .. +200*C
Tj junction temperature 200*Cmax

Should be some special flavor of 1N4148 from NXP as in other datasheets Tj is 165 - 175^OC

I'm more concerned for the moment about this maximum permissible continuous forward current as a function of ambient temperature   


Unfortunatelly, I can't see schematics of this reflow oven controller since there is no PDF version included (I use different PCB design tools), so I have no idea how much current is provided througth this diode 

Anyway, looking into another schematics attached below, I looks like while 10V zener was uses and there is at least 10k resistor from diode to gound, less than 1mA will flow, which seams reasonable since from NXP graph above for temperature ~175*C they mention ~25mA maximum permisible continuous forward current, so close to 200*C, lets say 199*C we get ~1mA from proportion.

However, maybe we could quite safelly use short pules of 1mA current at 200*C (and beyond) only when we want sense temperature, so hopefully maybe we could be still at quite safe  current levels, since probably pulse current allowed will be higher than continous 

[free_electron]

This diode will sit inside the oven right ?
What will you use as solder to connect to its wires ?

I want use this small custom PCB to hold diode in reflow oven


No need to solder anything


I've in my toolbox something like this for PCB and heatsinks assembly

not a good idea... poprivets are aluminum... not guaranteed a good electrical contact. plus you are probably creating four thermocouple junctions  as well as you have dissimilar metals... (cable -> board - board - diode cathode , diode  anode ->board - board -> cable.

i would try this out first before you develop this thing... my gut feeling say 'trouble'....  even if it works that diode die due to electromigration effects.

you are running the thing waaaay too hot. and powered up

[eneuro]

(cable -> board - board - diode cathode , diode  anode ->board - board -> cable.

I didn't show you how diode terminals will be connected with copper wires, so this what you wrote is only your guess, not mine 

So, I can tell you and reveal more details, that aluminium rivets shown on those pictures of course will not conduct temperature sensing current, but... will work as a metal glue, which will press 1N4148 ~0.6mm in diameter copper terminals with ring copper connectors together, so there will be copper to copper connection 

BTW: I was pretty sure, that someone will notice this you pointed out and what can I say, I've shown those aluminium rivets ... in the hope someone will try to find issue there 
I'm very happy, you noticed this and this maybe very usefull information-it could be a trap for young electronics players 

[free_electron]

and what will you use as wire ? it better be fiberglass insulated or it will melt ..

i am still convinced this will not work right and the sensor will die prematurely.

[eneuro]

We'll see what happends
I've made quick prototype PCBs, to be able install  those temperature "sensors" inside my small reflow oven and another one in kitchen oven:


I think, I'll connect such three sensors to ATTiny85 ADC's and make I2C interface on two other pins to be in final solution have I2C temperature sensor ready to connect to another MPU, so basicly this temperature sensor MPU will do nothing more than monitor those 1N4148 diodes, compute estimated temperature, do arbitration logic  and maybe store in MPU EEPROM something like temperature accumulated over time in 0.1 second steps, to be able estimate how much average heat those diodes experienced in their lifespan while used during reflows processes until one of them failed and we left with two good diodes, but need report fault condition, so I2C interface will have faulty bit for each of those diodes, estimeated temperature of each of them and at another adress final temperature after arbitration logic rules applied-this is information reflow oven main MPU will look for, but other PC i2C optoisolated master will log some performance data on disk to futher analysis at the same to see in real time on PC what is going on inside reflow oven 

I hope, that not all diodes will fail at the same time, so I will be able catch the moment when one of them fails 

[SaabFAN]

I'm using the 1N4148 as the temperature-sensing diode. It is connected to VCC via a 1K Resistor and the ADC. It will be connected with crimp-contacts and a 300°C rated cable. Or the pop-rivet method. Kinda like that idea
Idk how long the diode will last, but I'm not planning on soldering dozens of boards each day. And it is the easiest way I could find to measure temperatures that high without paying a fortune for the sensor.

I haven't really started writing the firmware for this thing yet, as I was able to manually solder a few boards with the oven (turning the heaters on and off with the switches on the oven, while monitoring the temp with a Multimeter and its thermocouple). Adding a sanity-check for the diode is a good idea btw. I will certainly do that. Not sure how exactly, though
To get a reasonable accurate temperature-reading, I'm implementing a calibration-scheme with fixed measurement-points at 25 or 50°C intervals. Values between these intervals are interpolated.
Completely frying any board should be prevented by the thermo-switch inside the oven, which turns it off at a temperature of about 250°C. And also by watching the whole thing when in operation.

About the interface: I'm planning to use these widly available 2-line 16 Character LCDs combined with a i2c-Port Expander,.
Input is provided by buttons or switches, whatever I can find in the basement, which are also connected to another i2c-Port Expander. This allows up to 8 buttons, which are read in regular intervals.

Attached is the schematic.

[fcb]

AFAIK no commercial or even hobby level reflow oven uses semiconductor junction temperature sensing - it seems really daft as thermocouples are cheap, come with long leads and are well characterized.

The diode is not designed for cycling at the temperatures your going to need - at least not for long.

[eneuro]

It will be connected with crimp-contacts and a 300°C rated cable. Or the pop-rivet method.

In some application pop-rivet method might be good, but in the case of my prototype reflow oven based on flat heater metal plate I realized that it will be easier assembly this temperature sensor like this

Of course instead of normal wires used there for temperature calibration only and to make quick test, in final design high temperature resistant wires should do the trick 

I've installed four such sensor in my reflow oven concept (based on flat iron heater element, but unsure if I will be able get decent temperatures, so in final design probably high temperature resistant concrete used in fireplace will be used instead of wood, but reflow control software will be the same)


Today realized, that I can disable ATTiny85 Reset pin and use it as  4th ADC, so I've four 1N4148 based temp sensors inside this oven (this temp sense MPU will have bootloader capable after restart to reflash using only one pin, so no need to use huge bloody few pins ISP connector and Reset pin no longer needed- so I have 4 ADCs available and two pins for software I2C slave implementation)

NOTE: Those orange and black pipes... are nitrogen inlet and outlet to replace oven atmosphere during reflow with nitrogen of course
I will need a few more ATTiny85 in SO8M package to controll everything, but buying more those MPUs t other projects too, they will be cheap, so it will be fun connect them together to one I2C network.

Highest priority now has my ATTiny85 custom 1 wire bootloader, since I've a few projects where I'd like to use it, so bootloader software development time right now
Can't wait to reflash this tiny monster with versatile, password protected 1 wire bootloader to be able disable Reset pin and implement i2C interface for those four sensors.

[timofonic]

Any news?

[SaabFAN]

I haven't found the time to install the controller into the oven. Partially, because manually controlling the temperature worked pretty well for several boards so far - switching the heaters on and off while monitoring the temperature with the Multimeter.

[mike_58]

Have a look at : https://github.com/0xPIT/reflowOvenController - seams more advanced and with TFT ...
http://www.ebay.com/itm/272074791359?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1558.l2649

[mike_58]

This diode will sit inside the oven right ?
What will you use as solder to connect to its wires ?

I want use this small custom PCB to hold diode in reflow oven


No need to solder anything

alternative solution:


I've in my toolbox something like this for PCB and heatsinks assembly

not a good idea... poprivets are aluminum... not guaranteed a good electrical contact. plus you are probably creating four thermocouple junctions  as well as you have dissimilar metals... (cable -> board - board - diode cathode , diode  anode ->board - board -> cable.

i would try this out first before you develop this thing... my gut feeling say 'trouble'....  even if it works that diode die due to electromigration effects.

you are running the thing waaaay too hot. and powered up

Cuper Rivets are avalible as well (Al, Stainless steel, coated,..)

[fcb]

AFAIK no commercial or even hobby level reflow oven uses semiconductor junction temperature sensing - it seems really daft as thermocouples are cheap, come with long leads and are well characterized.

The diode is not designed for cycling at the temperatures your going to need - at least not for long.

Does "not designed for" preclude the possibility that it could work? The OP may add something to the design of reflow temperature sensing.

I'm curious why 4 sensors are needed. It seems to suggest the possibility of uneven temperature. Which one will be used to control the reflow?

Of course your right - I'm not precluding the possibility it might work.

But I can't see any reason you would even try a diode sensor at those temperatures unless you needed to make an ultra cheap device, even then thermocouples will be easier to mount than riveting a diode onto a bit of insulator.

[wraper]

I haven't found the time to install the controller into the oven. Partially, because manually controlling the temperature worked pretty well for several boards so far - switching the heaters on and off while monitoring the temperature with the Multimeter.

Human PID?