SparkFun Forums

[sparky]

I agree - fancy talk. But it's fun.

And right now, using a toaster is the last thing on my mind. We are so slammed with assembly, I don't have the time to develop/modify a toaster oven. We use what works. And that's the skillet - for us at least.

-Nathan

[MGP]

Well, I tried the "skillet method" last night on a fairly complicated prototype board and it worked fine.

A few problems I ran into:

1. My skillet bottom isn't exactly flat so the heating wasn't as even on the "low" spots. I measured the surface temperature on a dry run before I reflowed my board and was surprised that the heating inside the circle formed by heating element was actually pretty consistent.

2. I didn't do such a great job hand pasting the board so I had some touch up work to do. I need to find a cheap and easy way of getting stencils made. I may try the mylar stencils that Sparky and his guys at Sparkfun use. I've got inexpensive sources for all the other parts of the protyping chain now -- except stencils.

I also was digging through my toaster oven stuff this morning and found that it came with metal "deflectors" that mount over the heating elements. I may have to install them and try again with them installed. Since I have a convection oven, I'll bet they will help even out the heating inside the oven.

[FredS]

I do extensive surface mount work. The biggest problems are the new microwave chipl like the JEDEC MO-220. These have surface mount connections which are entirely under the chip, o that hand soldering with an iron is impossible.
I built a small hot-plate from a 2X2X.1 sheet of aluminium with 2ea 25 watt 30 Ohm power resistors in parallel attached to the bottom of the plate. This is then supported by 4 stand offs on a wood base. I apply solder paste with an acupuncture needle, put on the parts, and watch the process under a microscope while I apply 40 VDC. (40 X 40 / 15 = approx 100 watts.) This heats to soldering temp in about 2 minutes, and the MO-220 IC's self center and bond very nicely.
Clearly, there are some dangers, like forgetting to turn the hot plate off, but otherwise it works well.

[dpaton]

I also was digging through my toaster oven stuff this morning and found that it came with metal "deflectors" that mount over the heating elements. I may have to install them and try again with them installed. Since I have a convection oven, I'll bet they will help even out the heating inside the oven.

How'd that go MGP? I'm curious...

-dave

[sparky]

Nice work! I'd love to see some pictures of a plate like that!

We were contimplating building a profile controller for the skillet with a PIC, relay, LCD, thermocouple, etc. The PIC would PWM the skillet and monitor the temperature of the plate, etc.

Ben's solution was a $3 egg timer.

-Nathan

[bfmitch]

A little background. We have two toaster oven reflow ovens running. They are made from Oster toaster ovens with all the built in electrical controls bypassed.

120VAC Connection:

For those of you worried about that lethal 120VAC, here is how I solved the problem for my toaster oven controllers.

I bought a surplus SSR from All Electronics. I made a heavy, grounded extension cord terminating in a rounded/square outlet box. I bolted the SSR in the bottom of the box on one side and mounted a regular 2 outlet gang on the other side.

The SSR is hooked up so that it controls just 1 of the outlets. The other outlet is always hot. The always hot outlet is used for the wall wart that powers the controller. The "switched" outlet is used for the toaster oven. You have to have an outlet gang that allows the two outlets to be separated to do it this way. Most of them have a connecting tab you can cut.

Take the digital inputs to the SSR and run them to a plug on the box. I used a 3.5mm audio jack mounted in the cover.

I had to make an outlet cover. I bought one that had a place for a switch on one side and dual outlets on the other. I filled in the switch with some epoxy and then drilled my hole for the audio jack to mount.

On the controller side all you have do do is have a matching jack with a cable to run to the outlet box. Any kind of cable and jacks you have handy will do. You could hardwire it, but being able to disconnect it easily is a plus.

When you're done you'll just need to use 1 wall outlet to power the whole thing and the 120VAC is in a grounded box where it belongs.


Burning Down the House:

One of our controllers lost it's mind one day and just kept heating. It turned out that it was due to a connection that had gone bad where the thermocouple connects to the PC board. The PIC read it as a low temperature and just kept the oven running.

It's bad enough that it turned 4 boards, with components, into black, crunchy, crispy critters but the real problem is that it could have started a fire. It took half a day to air the place out.

If you build one of these oven controllers, make sure you implement a watchdog timer. Set the trip point for a bit longer than the longest heating state you have. If the controller doesn't get to the next state within that prescribed time, the watchdog time isn't touched before the trip time. If the watchdog timer trips the SSR is shut off.

[dpaton]

I have a plan for a very simple thermal cutout, with an old-school thermal breaker right in the oven between the line and the coils. An opto will sense when it gets tripped and activate a fail-safe routine to keep things like this from happening. I've had thermocouples go bad a few times, and the results can indeed be disasterous.

I still haven't figured out a foolproof way to PWM the elements however. I'm working on an industrial light dimmer here at work that might be able to donate some topologies, but the dimming board (fets, driver, filtering, etc) will need some uprating and modification to handle the load. If I ever get the hardware built (I just bought a house so things are moving slowly on th ehobby front) I'll be posting details.

-dave

[bfmitch]

I should have considered the failure modes a bit more completely right up front and had the watchdog timer in there from day one. At least I have it set so that the reset state of the PIC has the bit controlling the SSR turning the relay off.

The thermal breaker, right in the oven, is an excellent idea; even if it isn't monitored by the controller. In my case that breaker would have to be able to handle 120VAC since that's all there is at the oven itself.

[dpaton]

My plan is to put it on the outside of one of the 'inside' oven walls, and have it inline with the AC line feeding the relay. That way, if the oven overheats, it not only turns off the coils, but the power to the relay, effectively de-energizing all of the 120VAC parts of the circuit at once. Also, it won't be in the direct heat of the coils. If things get too warm, there's a distinct chance plastics could soften, and I prefer to take the power away completely if that's even a remote possibility.

I'm also thinking about a Robin Hood/Friar Tuck scheme, where a small, cheap micro would piggyback onto the system and serve as an active check for the main microcontroller, in both watchdog and monitor capacitites. That will no doubt add a lot of complexity to the project, and is thus at the very bottom of my list, behind the differential heater drive and the active fan control, but it will definitely make the project close to bulletproof.

-dave

[3dOptics]

Would it be possible to use a OptoIsolator instead of mechanical/solidstate relays?

[Kuroi Kenjin]

I can answer this one. For the AC line connections... no. Opto-isolators don't handle much current, and the Bipolar transistor (BJT) cannot do reverse blocking (or at least not more than a few volts before breaking). Unless you know of a really really beafy opto (like made out of an Insolated Gate Bipolar Transistor (IGBT) or what not), I'd stick to a relay.
The one I'm using on mine is a L90-5W ( 20A 277VAC, 20A 30VDC). I think Sparky was thinking about using it, but then re-evaluated his life and opted for something beefier. I'm gonna take some safety precautions, and just take the manufacturer on his word, since I'm a broke college grad who wouldn't mind seeing things blow up. If it survives and seems ok, then I just saved a buck or two. It's really small. There are pics of it, and the over worked protoboard on my website. (Projects-> Reflow Oven).

[Darth]

I had concerns about the ramp up/down times as well as the uneven heat of the benchtop oven so I converted a 2 slice bread toaster. I removed the middle element , added a couple of 'u' channels for a piece of al mesh to hold the boards then added some feet for the toaster to lie on it's side. I made up a small stainless fan which is attached to the rear (bottom) of the toaster for cooling. I did cheat on the control by buying a cal controls process controller to control the heating and cooling profiles (NZ$450). Its easy to program, has failsafe modes and you can enter multiple profiles. With it's small volume and low thermal mass it can heat to 225 degrees C in 75 seconds. I only make small boards so it does the job nicely.
A picture can be found at: http://202.0.54.135/ToyBox/SMDToasterOv ... fault.aspx

[RCT]

I know there are some people out there who just have to build it for themselves. So this post is not for them.
I have been using the "Temptell Reflower" to turn a toaster oven into a reflower for about 6 months. It works very well and you don't have to modify the oven at all. You simply plug the oven into the unit and the internal relay will handle up to 1800 watts. All aspects of the reflow cycle from the drying ramp to the reflow cycle are easy to control.
For $245.00 for the unit and the software it's hardly worth building your own device.
Good luck

[larosta]

Is there anyone who has a VB controller written for the Spark Fun Reflow oven controller that wouls be willing to share their source code ?

[fordp]

A TRIAC Based Lighting Dimmer can easily control an oven, and yes PID is the way to go.

I have lots of experience in Dimmers

I would suggest using and Opto Triac and something like a BTA40.

You can control it using Phase Angle control and using a zero crossing detector on the capture and the Opto TRIAC on the compare will work a treat.

Be careful with the mains guys and to be legal you will need a choke to control the rise time of the TRIAC.