Reflow Toaster Oven - Part 10 - Insulation

Yay! The thermal insulation blanket I ordered arrived today!

Cutting this stuff is fairly easy with a snap-off knife. The thickness is just about right (1 inch, 25mm) for the top.

Still have to get rid of the electrics here. That motor's gotta go as well.

Plenty of glass wool left. But I'll use that later, I'm not completely done yet.

Before insulating it I did a few test runs while logging the temperature. After insulating I repeated those tests.

Well, that's disappointing. A cold insulated oven (first run) actually performs worse than a pre-heated uninsulated oven :( It doesn't seem to matter much whether its insulated or not, weird...
But, what's more important is that the top of the oven no longer reaches 80 degrees C, but remains at a comfortable 35 degrees C. The uninsulated back of the oven on the other hand... Like I mentioned before, the back is just a thin piece of single layer sheet metal. It goes well beyond 200 degrees C; not something you'd want to touch. Also, the side of the oven where the electrics are stay below 40 degrees instead of 80. Not ideal yet but much better than before.

So what's next? I'd like to attach another piece of sheet metal to the back of the oven so I can add some insulation there. I need to strip all of the existing electrics and add some solid state relays. I also received the heat sinks for the relays this week, so I can finally make a more permanent installation. I'd like to install some thermocouples as well. I'm probably going to need a fan to cool the electronics, and I'd like a large slow fan in the oven to create a bit of airflow and help spread the heat more evenly.

Toaster Reflow Oven - Part 9 - It's raining components

In an attempt to find out whether or not the heat is spread evenly in the oven, I decided to attach multiple thermocouples to a large PCB and give it a go.
Fortunately I haven't been to the waste disposal center recently, so I still had a PCB from an old laptop in my junk drawer.

I tried to find some bare pads or large patches of solder on the board and stick the thermocouples in place with some aluminium tape.
Shortly after I turned the oven on, the outer two thermocouple came loose and detached from the board, crap. There seemed to be some temperature difference between the remaining two thermocouples; about 10 degrees Celsius. As the temperature approached 200 degrees C, some unfamiliar sounds came from the oven. Plonk... Plonk... I soon realized the solder on the bottom of the board was starting to melt and the larger components were starting to fall... Doh! Removing the hot board from the oven didn't do much good to it either...

Hmm, missing some bits here...

The CPU doesn't need those caps right? Right?

So that's one way to harvest a board for components.

Who wants some SMT electrolytic caps? Or some coils?

Or a whole bunch of ICs?

Some passives anyone?

So much for my first (accidental) reflow :)

Reflow Toaster Oven - Part 7 - Taking it up a notch

Top to bottom: clunky thermocouple in oven (181.2), thermocouple attached to pcb in oven(207.9), thermocouple in air in oven (210.1), thermocouple on back of SSR (51.8, yikes!).

I was trying to keep the heater at 300 deg C, check out how fast the temperature drops when the heater is turned off! And how long it takes for the element to heat up again. Let's have a closer look:

So what do we see here? The thermocouple registers 300+ deg C (blue line) and the Arduino turns the SSR off (red line). The temperature still rises a little bit to 301 degrees, but then after a second or two the temperature starts to drop. Two seconds after that the temperature drops below 300 and the SSR is turned back on. For the next 14 seconds, the temperature keeps dropping to 273, wow! Then it takes another 10 or so seconds until 300 deg has been reached again.
So what happened? Why does it drop so fast? I guess the main reason is the difference in temperature between the heater and the air around it. The air is almost 100 degrees C cooler than the heater. So even though the heating element has been turned off for just a few seconds, it takes quite some time compensate for the energy that has been lost.

So what do these tests tell us? I'm just guessing here, I didn't study physics or thermodynamics so my guess is as good as yours :) But I'm guessing that if we run this test for a longer period then the ratio between on/off (duty cycle) can also be used with a much shorter period (say, 1 second). So if on average the heater is turned on for 24 seconds and off for 4 seconds, then we have a duty cycle of 85% (24 out of 28 seconds on). Then my guess is that the heater can be kept at the same temperature by turning the heater on for 0.85 seconds and off for 0.15 seconds. But the result would probably be a much smoother temperature curve.

My second guess is that the duty cycle will need to be higher when the difference between the heater and the air around it is greater. So that means that in order to keep the temperature of the heater constant, the duty cycle needs to be slowly decreased as the temperature in the oven rises.

I really need to order more MAX6675's and some thermocouple connectors, being able to log only one thermocouple is annoying. I noted a few more numbers on paper during testing.
At heater=200, the air was 140 and the pcb 120.
At heater=250, the air was 170 and the pcb 150.
At heater=300, the air was 200 and the pcb 180.
After some time, the air was 205 and the pcb 190
After some more time, the air was 208 and the pcb 200
After some more time, the air was 209 and the pcb 205
Finally, the air was 210 and the pcb 208. By this time the SSR was well above 50 deg C so I turned everything off.

But as you can see, the air seems to reach a maximum temperature of about 210 deg C, and the pcb slowly catches up. The air will most likely never reach the same temperature as the heater, since it's in contact with the walls of the oven and transfers quite some of its heat to that. The PCB on the other hand is only in contact with the air (and the wire rack, but let's ignore that for now) so it will eventually reach the temperature of the air.

This brings us to the topic of insulation. If we can decrease the amount of heat that is lost, then the temperature of the air will follow that of the heater more closely. Now we can't prevent the air from heating up the walls of the oven, unless we add some sort of heat reflective coating to it (not a bad idea actually), but we can prevent the walls from transfering heat outside of the oven. By padding the walls with insulation we should be able to reduce losses quite a bit. I've ordered some thermal fiber glass insulation designed specifically for insulating commercial and industrial ovens. This stuff is different from normal glass wool that you'd use to insulate your house. Although they're both made of glass fibers, the bonding material used in normal glass wool will start to melt/burn at high temperatures. This oven rated insulation has a different type of bonding material which should be able to withstand temperatures of up to 500 degrees C. It'll be interesting to see the difference this stuff will make :)

Reflow Toaster Oven - Part 6 - PWM Fun

MAJOR WARNING NO 1: THIS COULD KILL YOU!

No, seriously, mains voltages can kill you, keep that in mind whenever you work with it. Having the cover off doesn't make it much safer, there's plenty of exposed contacts with 230V on them. That's lethal.

So now that we've cleared that up, let's have a look at what's happening here. I've replaced the thermostat with a solid state relay. There's a thermocouple connected to the heating element and plugged in to the Arduino. The Arduino monitors the temperature and turns the relay on when it's too cold, and off when it's too warm. The target temperature can be set using the serial interface. There's a bunch of other thermocouples in and around the oven to monitor the rest.

MAJOR WARNING NO 2: SOLID STATE RELAYS REQUIRE HEATSINKS!

I know it says "25Amp" on the cover, but if you check out the datasheets for these kind of relays then you'll notice they're only rated about 4A without the heatsink. That's why I've connected a thermocouple to the bottom of the SSR, so I can monitor its temperature (heatsinks are on their way). So on the thermometer I'm monitoring the following from top to bottom:
-oven temperature (large thermal mass probe)
-oven air temperature (small probe)
-oven top temperature
-SSR temperature

So after increasing the temperature a couple of times this is the result:

The blue line is the heater temperature. The red line is the target temperature and whether the element is on (high) or off (0). As you can see, lower temperatures require less frequent heating (duh) and it overshoots the target temperature sooner than with higher target temperatures.

This was to be expected, but it's good to see it in hard numbers. Next step is to also track the air temperature and see what kind of influence that has on the required dutycycle.

And yes, the SSR runs warm, 40 deg C and rising, target temperature is 250. Air temperature was 176 and the top of the oven was nearly 70 deg C.

Reflow Toaster Oven - Part 5 - Crispy Thermocouples

I like my thermocouples well done...

So that little blue piece of shrink tubing(?) doesn't seem to cope well with 200+ degrees C. Oh well.
Did a bit of testing tonight. I placed the four thermocouples in and around the oven:
- one attached to the heater
- one just floating in the middle of the oven
- one stuck through one of the vent holes on the side
- one attached to the top of the oven
And then I turned the oven on :) I recorded the whole thing and manually put all temperatures at 5 second intervals into a spreadsheet. Here's the result:

At one point the thermocouple on the heater fell off, that's when I turned the oven off. The interior was about 270 degrees C by then and kept rising for a short period. Then I opened the oven's door and the air temperature plummeted.The reason these readings differ so much from earlier profiling is because of the thermocouple I used. There small ones have very little mass, so they heat up much faster than the one I used earlier.
What worries me a bit are the green and purple lines. They reach about 80 degrees C, that means the oven is way too hot to put any electronic controls inside. I hope this is reduced significantly after I insulate everything.

I also played a bit with RaceRender, it can overlay various types of data on video images.

Nice :)

Reflow Toaster Oven - Part 3

Grounding issues

During the heat profiling I ran into some problems while trying to get stable results from the thermocouple. One of the problems was that the wire from the connector to the MAX6675 was way too long; over a meter. When I shortened this to 5 cm the readings were way more stable than before. Another issue arose when I put the thermocouple in the oven. When it touched any metal part of the oven then the readings would be all over the place. They'd jump about 10 degrees C up and down.

As it turns out, pretty much every metal part of the oven is grounded to mains earth. The thermocouple itself is shielded, but the shielding is electrically connected to the thermocouple. The thermocouple works by creating a really tiny voltage (in the order of microvolts) that rises when the temperature rises. The MAX6675 amplifies this voltage, reads it and performs some compensation. As you can imagine, it doesn't take much to throw off the readings completely. Connecting the thermocouple to mains earth is one of them...

I conducted a small experiment this afternoon to verify that this was in fact the issue. All I did was take a mains extension cord and touch the earth tab with the thermocouple.

I think the readings need no further explanation:

Averaging the values may make them a bit more useful; they're not completely random :)

Another thing I tested today was the temperature of the heating elements. The outside of the elements are connected to mains earth, so the readings aren't entirely accurate, but they should give a rough idea of the temperatures involved.

Yes, that's 400+ deg C before the thermostat cuts the power, yikes! Raw data can be found here. And 400+ deg looks a bit like this:

After the second time the thermostat cut the power I pulled the plug of the oven, disconnecting it's earth connection. You can see the signal immediately cleans up.

These grounding issues are annoying though. I need to have fairly fast and accurate temperature readings on the elements and oven, so I need to solve them some way or the other. I first thought about using kapton tape to electrically isolate the thermocouple from whatever it's connected to (while still maintaining thermal conductivity), but now that I've seen how hot these elements get I doubt that's going to be a good solution. Needs more research...

I need a different thermocouple, this one has too much thermal mass; it takes too much time to respond to temperature changes; the recorded temperature kept increasing even though the heating element just turned off. In this situation you want the thermocouple to have as little mass as possible so it will respond really fast to temperature changes. More eBay purchases coming up :)

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.