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.