Reflow Toaster Oven - Part 2

Having an oven is one thing, knowing what it's capable of is just as important. In order to accurately control the temperature it's not enough to simply turn the heating element on when it's too cold and off when it's warm enough. The heating element has quite a bit of thermal mass, it will stay warm for quite some time even when you turn it off, which means that it will keep on heating its environment, so the temperature in the oven may still rise even though the heating element has been turned off.

If I want my reflow oven to be accurate, I need more data on it. How fast does it heat up? How fast does it loose heat? What is the maximum temperature? How fast can it reach that temperature? The only way to get answers is by simply testing it.

So just stick a probe in the oven, connect it to an Arduino, turn the oven on and start logging!

The probe is a K-type thermocouple I got from eBay for a few bucks.

It's connected to a MAX6675 Cold-Junction-Compensated Thermocouple-to-Digital converter which converts the minuscule voltage generated by the thermocouple to a digital value. The MAX is connected to an Arduino which simply polls it about every second and prints the value on the console.

The first value is seconds since the last reset, the second one the temperature in degrees Celsius. I simply copied the output from the console to a file, named it .csv and opened it in Excel. The result is a nice little graph.

Just a few notes here. You can see a little bulge around 460 seconds. The thermostat of the oven is connected directly to the heating element. Around 440 seconds the heating element aparently reached 280 degrees C (which is what the thermostat was set to). It took a bit of time for the heating element to cool down, but it did so by warming up the rest of the oven. Around 530 seconds the element turns on again and the temperature starts to rise "rapidly" again.

When the temperature reached 245 degrees C, the wire of the probe got so hot that it was starting to melt the handle on the oven door it was touching. So that's when I turned the oven off and set the door ajar. I have no clue what the little hickup around 706 seconds is.

The time spent above liquidus (217 deg C) is 220 seconds, which is well above what's recommended (60-120 seconds). The time until peak temperature is 641 seconds, which is also well above the recommended maximum of 8 minutes.

Long story short: it's too slow, needs thermal insulation.

Reflow Toaster Oven - Part 1

Converting a toaster oven to a reflow oven is something that's been on my to-do list for quite some time now. Hand soldering really tiny SMT components is a PITA, having a reflow oven would make things a lot easier.

So this afternoon I finally got my hands on a suitable used oven :) A Tefal Activys. I think this one was also sold under the Moulinex brand. I guess it costs around 100 EUR new.

It's got the right size, has a large door, two large heating elements and no complicated electronics. It's pretty clean for a used oven, doesn't seem to be used that much. And best of all, I only paid 20 EUR for it :)

 

As some crazy ozzy bloke would say: "Don't turn it on, take it apart!"

The cover is held in place with a bunch of self-tappers, it's just sheet metal, in fact, the whole oven is made of sheet metal, sheesh.

Nothing much on the left side, just the spring for the door. There's a bit of room between the cover and inner panel so I can put some insulation in between it to improve the oven's performance.

More sheet metal on the top. Even less room for insulation, oh well. The back panel is just a single piece of sheet metal, that's going to get pretty darn hot when this thing turns on...

And here's what makes this oven tick, quite literally. No digital electronics at all. Plain and simple. Let's have a closer look.

This is the motor for rotating dead birds, that's gonna go, but the hole may be handy to route a thermocouple through.

A very crude thermostat. It's relies on a bi-metal. It's screwed almost directly to the bottom heating element.

And the temperature knob on the front is connected to it like this, expect the temperature to be off by at least 30 deg C...

And this is what makes it tick, it's a mechanical timer, with a nice ring at T=0 :) Partially shown below it is the mode selector (grill/oven/roast) which selects which heating element to use.

Another thermostat, probably a safety switch to prevent it from overheating. As you can see, the top heating element is held in place with a spring, you can pull the element down for easy cleaning of the interior.

The bottom element can be moved as well but is not spring loaded, it simply relies on gravity.

And a closeup of the knobs. 280 deg C? Probably a bit optimistic :)

2900W? Let's test that! With just the top heating element on my meter shows around 1300W. With both elements on it uses a whopping 2500W, still a bit short of 2900W but plenty nonetheless. This thing heats up pretty fast as it is already, but I'll need to do some more testing to get the actual figures. And as noted before, the back of the oven get's HOT! Not good. I'll probably add an extra panel at the back (either on the inside or outside) so I can add some extra insulation...

So much for now, stay tuned for more :)