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 :)