I need to control a Toaster Oven to use as a Reflow Oven.

Where do I start? What will/would be the best temperature sensing method.

What do you do if you used a thermocouple just stick it through the door jamb?

Would it be possible to make a optical heat sensor, would that be practical?

I do not know the wattage of the oven but lets say it pulled 15 amps how would I control that with the Nerdkit?

I found this site selling Breakout Board kits

I am amazed at the price of breakout boards. But they have these kits that have everything one needs to do reflow soldering including the stainless steel solder mask and leaded solder syringes along with the PCB (printed circuit board). So I ordered a couple of kits to learn reflow soldering.

My soldering station is/has a reflow hot air gun I have seen people using the hot air gun to do reflow soldering but I'd like to put together a oven controller.

I'd appreciate links to your recommendations.

Ralph

You could use an electric frying pan heat it until it flows then turn it off. I haven't tried it myself, but I've watched several video's of it being done. Seems simple enough. Here is a link to a Sparkfun tutorial using different methods for reflow including a skillet.

With the amount of SMD stuff I've been messing with, I've thought of this myself. I've also seen projects reflow ovens from toaster ovens. People control the heating element with either relay's or Triacs.

I have a nice toaster oven I just can not get immediately to it to check the wattage/amperage requirements.

Yeah definitely because of the SMD/SMT devices. I signed up for the sample program from Texas Instruments (TI) and all of their sample devices are SMD. I got some 10 pin ic that are hardly a 1/8" x 1/8". Definitely need reflow soldering for them.

It's to bad that the SMD/SMT devices sort of crimp the idea of prototyping (experimenting) you are sota committed to your design when you have to make a printed circuit board using SMD profiles.

So what type of relay or triac for 110v AC 15amps am I looking at?

Also what temperature sensor?

Ralph

You could probably use something like the BTA20 20 amp triac. You would have to make sure there is plenty of heat sink for the triac. As for a temp sensor, you may need to get into an industrial k type thermocouple and appropriate amplifier.

Rick

I got some BTA20 triacs off ebay and Maxim has the thermocouple amplifiers which I ordered through their sample program.

I definitely am going to put this together.

Anybody have sources for the thermocouples there seems to be a big variety of prices. Of course I do not know what exactly I should be looking for.

What is this Profile that people talk about when talking about building reflow ovens.

I was thinking turn the oven on measure the temp turn the oven off at a certain temp(?).

But apparently it is more complicated than that.

Ralph

Now how would I control the oven, should/could I jut use ON/OFF or could I modulate the AC using PWM?

I have the suggested AC control TRIACS ordered through ebay.

I will be using a MOC3020 - MOC3023 OPTOCOUPLERS/OPTOISOLATORS again as suggested by Rick, man what would I do without your help and the help of others here in the Nerdkit forum. I cannot believe I am even venturing to do something like reflow soldering and building a oven control system to boot.

It just seems as if having a modulated AC would make for a more interesting project.

Well in looking at the MOC3023 datasheet it appears the MOC is a ON/OFF device so there would not be any direct benefit to using PWM.

I could modulate the AC just using a simple pin high/low pin on/off timing sequence. Or would that be controlled by PWM (shows you what I know)?

The control flow I am picturing is something like this (please correct/enlighten me with beter methods of control). This is a work in progress so it will be changing but it is nice to lay it out and hopefully receive feed back.

Turn heaters on.
      If temp < wrmupTmpSet stay ON else turn heater OFF (I think turning the current down instead of off would be better)
      If temp > wrmupTmpSet turn OFF (turn down)
      if wrmupTime == wrmupTime Start preHeatTime timer
      if preHeatTime == preHeatTime then boost heat to soldrTmpSet start soldrTime timer
      if soldrTemp < soldrTempSet stay on
      if  soldrTemp >= soldrTempSet then turn ALL off, beep buzzer flash LED.

So I will need to preset:
      wrmupTmpSet
      wrmupTime
      preHeatTime
      soldrTempSet

Well does that cover it? That was just off the top of my head I'll be thinking this through further on.

PWM is used to control the speed of motors using a mcu, I assume using PWM to modulate the current.

Would using PWM be feasible I could use different components.

Would there be any advantage to using PWM? I know switching the AC on and off will cause "noise" that will have to be dealt with.

Just modulating the current would be relatively quite.

Are heater coils considered a inductive load?

Thanks for the help.

Would someone venture to sketch out a schematic for me (I have to learn Eagle).

Ralph

oops I should have started this tread in the projects forum.

You can do PWM though the optocoupler. However, it isn't as easy as just setting a PWM channel on the micro to drive it. Since you are driving an AC line, you can only turn on the TRIAC at certain points in the Phase of the AC signal then off during the same cycle. So in order to time this to the AC of your house, you have to have what is called a zero cross circuit that will send a pulse to the micro at each passing of the sin wave across the zero volt line. This can be done by coupling a small voltage (6.3v) transformer and sending it's output into a zero cross optocoupler. Then take that pulse and feed it into the micro for phase control.

For an oven, PWM would not be needed but could be used if desired. I was messing around with Christmas lighting a couple of years ago and had built a circuit like that to dim the lights.

Rick

Ralph,

How many projects can you juggle at once?? You are the man! I am seriously jealous of all the stuff you have going on. These days I barely have time to think anymore, much less build anything. Anyhow.. a couple suggestions for you, as alway, trying to help out and put some of my useless knowledge to use. Take the good ideas, ignore the less than usefull ones..

Thermocouples.. K type will work fine, though it might be a hair cheaper to use J or T. I always prefer the 1/8" MgO insulated ungrounded type, they are a slight bit slower but they are so much more robust and you really need to work to fry one, unlike a bare tip or ring washer type. Omega engineering is the "standard" but you pay for it and their lead times can get silly(once waited 12 weeks for a generic TC). For 90% of my TC needs, I use cleveland electric labs. Decent prices, though no on-the-shelf TC's, they are all made to order, usually in a couple weeks. If I need a TC right away, McMaster usually works in a pinch, though their sizes are kinda limited and prices a little high. Though, usually all of their prices are lower than through a hobby place like sparkfun, pololu, and so on. If you brave and somewhat low on funds, you can always buy some TC wire, twist about an 1" of the ends together into a nice tight twist, a quick tack weld on the tip and end of the twist to hold it together and you have a rough TC that should work but won't be 100% as reliable as some of the others.

I would strongly suggest you plan some sort of "tuning" into your software for the hotplate controller. If you just do a <300C = power on, >300C power off, you risk badly overshooting and frying everything(where did my blue smoke go??). A lot of temperature controllers out there have tuning and ramping build into their controls, somewhat hidden away from the user but still there. So for example if you are shooting to run the skillet at 300C.. Perhaps set up your software so when temp is <100C "duty cycle" is >50%, when temp is >100C<240C "duty cycle" is 40%, and when temp is >240<285C "duty cycle" is 30%, and when temp is >285C "duty cycle" is 20%.. You would need to play around with the levels, because it will take a certain "duty cycle" to hold 300C with no chips in the skillet. I would also strongly suggest doing multiple "dry runs" to make sure you are happy with how it runs before you toss some hapless chip in.

hope that helps!

Thanks Rick and Doug, I can always count on you guys.

Doug remember I am unemployed, I have lots of time on my hands, when I am not diligently sending out my resume.

I also have a very very tolerant wife, that has not been demanding much of me over the winter, now that Spring is spring out she is mentioning some "honeydos" that will take more of my time, plus I have a bunch of outdoor projects I can not wait to get going on.

I actually do get tired of setting at my workspace for 10 hours a day every day (I really actually don't do that any more it just seems like I do and I certainly used to, I'd have a cigarette hanging out of my mouth (5 packs a day and a cup of coffee at my side (2 - 3 pots a day)) I have always been a bit obsessive.

This reflow oven is not a priority and is probable more more of a curiosity now, especially once I found I could do SMT/SMD soldering on a TSSOP .65mm device using my hot air reflow gun.

It really seems like I should do some method of modulation using a duty cycle schema so that implies PWM, or at least to my limited understanding.

Of course I don't know how todo any of it, yet.

I just received my TRIACS from China. I might hitch one up to a light bulb just to see it working and to get a better grasp on the concept of what I am trying to do.

Ralph

And just in case anyone wants to build there very own "zero cross circuit" here is what ATmel has to say in a Application Note

If some one understands this I sure would like a explanation, in reading the ATmel note I fail to see what sets the power level/brightness. Or where it is set. The zero crossing I can sorta understand but totally miss how one sets the expected power levels.

Ralph

What sets the brightness is the on duty cycle. You use the zero cross to trip an interrupt, then turn on the triac at a certain time of the on duty cycle. So lets say you want 50%, you would start the process at zero cross, wait 1/4 wave, then turn on the triac. At each zero cross, you would do this. If you wanted 100%, you would turn on the triac right after the zero cross. That way it is on during the full cycle. Personally, even though the Atmel note shows getting a zero cross pulse with a voltage divider off a bridge thru a transistor, I prefer the optocoupled option using a device like the H11AA1 or equivalent. This ensures isolation and makes me feel better :)

Rick

Interesting, do you have a schematic? Or a link to a example?

Ralph

This site gives some good info on how AC dimmers work. There is a sample dimmer circuit with an optocoupler. The SSR dimmer circuit can be built with less components. The one I built was just an optocoupler, a resistor, and a triac. The key is the timing of when to turn on the triac. A triac will conduct until the zero cross so if you turn it on, then off within the same cycle, it will not go off until zero cross.

Rick

So a triac is is not used as on/off switch.

It actually cycles with the cycle of the AC sine wave. So a full on it is actually off as much as it is it on.

To get less than full on you do not leave it on for a full half cycle.

So if you only wanted a third power (brightness) you would only leave it on for ...

Is a AC sine wave cycle 0 +180˚ 0 -180˚ 0?

So for 1/3 brightness the elapse would be 60˚?

I better check out your referenced site to get my head screwed on straight and stop guessing.

Thanks, another reference.

Ralph

I'd translate it more like 0 - 90 - 180 - 270 - 0 where 0 and 180 are at zero cross and 90 would be + swing and 270 - swing. The triac just like AC has no power at zero cross, but can be turned on during the entire AC swing as long as the gate is biased. ( I think that's the correct term)

So if in an SSR, you hold the optocoupler on, the triac would provide full power. If you are trying to reduce power, you shut off the triac and turn it on at some point during the on phase. So for 50% you would have the triac off during zero to 89 then on 90 to 180 off 180 to 269, on 270 to zero. The triac (depending on type) can be switched on during the conducting portion of the AC phase. It cannot be turned on at zero cross because there is no current flowing at that point.

At least that is my understanding. Please someone correct me if I'm wrong. I don't want to mislead.

Rick

Rick, where does 270 come from? I would expect 360 not 270.

Ralph

Degrees in a circle. Each quadrant is at 90 degree incrments so 0, 90, 180, 270, 360(0). Zero cross is at 0 and 180. Full power is at 90 and 270. To trigger the triac, you have to turn it on during a portion of the phase that has power. By timing this with the zero-cross (which happens twice per full cycle) you will generate a PWM signal synchronized to the frequency of your AC line.

Rick

Of course, this all only matters IF you want to create a PWM signal to control things. You could just as easily pulse the heating element in the oven in 500ms or greater pulses to "regulate" the temperature which would require no zero cross circuit.

Oh, so than the TRIAC would function as a ON/OFF switch, ON say for 1 minute or until a certain temperature is reached, possible 1 minute and then of for ten seconds on for ten seconds of for ten seconds etc.

The only way the "zero cross" comes into play is if I want to use it but there are other methods.

Wow, this is really great, do you need to now these things because of your work or just from your curiosity?

Ralph

Actually, no I don't do any of this for work. The "work" project just came about because I opened my big mouth and told the owner of the company I work for that I thought an issue they had could be solved with microcontrollers... And I got volunteered. I did the whole project on my own time with him financing the parts. When all was said and done, he had less than $500 invested in enough parts to build replacement boards for industrial joysticks that cost him over $2000 each when he purchased them. So He was happy, that was why to help compensate me for my time, he bought me the $300+ weller soldering station and gave me an old 50Mhz Digital storage Tektronix scope he had.

My real job where I work is using CAM software to take 3D CAD solid models and create programs and setup sheets for the CNC machining centers. If you are curious, go to Aeromet Industries scroll down the page to the Ryazan Lathe and you'll see a picture of me next to a big metal cutting lathe. Electronics has always just been a love of mine. I went to DeVry university for EET program for one semester around 27 years ago but that was it.

As for the Triacs, a couple years ago, I became fascinated with animated Christmas lighting so I started figuring out how they worked. That was when I built a 4 channel dimmer with an ATMega8 programmed in bascom avr. If I hadn't had the oscilloscope at that time, I would probably have never gotten it to work :D. I learned quite a bit about basic dimmer circuits with Triacs though.

Rick

Hey Rick, your work reminds me of my job as cad/cam analyst at General Electric many years ago. Same type of business generating CNC programs from 3D CAD drawings and I must say it was one of the most interesting and fun jobs I have had. Back in those days we were still using punched paper tape to move the CNC program to the machine and one of the projects I participated in was automating that process using etherNet. Thanks for reminding me of the fun I had. :-)

We actually have an old G&L VTL that we bought from GE that had an old GE Tape control on it. The owner and his son retrofit it with a more modern Control. Now we actually do a little work for GE on that machine making parts for Gas Turbine Generators. Small World :)

Rick