Electric Grill - Explanation of the Circuit and Repair of Heat Controller

[spongioblast]

Hi

I received a table grill as "not working" as it barely reaches the temperature needed to melt the cheese. I tested it and it works fine initially for the first 10 minutes but then it seems the temperature sensitive circuit heated up too much switching the circuit mainly off. This results in the grill not getting hot enough anymore for the reminder of the session.

I opened the grill trying to figure out the circuit to fit my needs, but I happened to find something I did not expect / do not understand how it works. Seems something expands with heat so the circuit disconnects. Could somebody explain to me how this thing works and a recommended manipulation to up the operating temperature? My initial thoughts where to shorten the white stick by 0.5mm.

Notes:
Picture "top": The element with the arrow pointing towards is attached to the sensor as marked on picture "circuit 2". I assume it is used to transmit the heat down to the controller.
Video: turning the knob from low setting, to max setting, to low setting

Thanks!

[Armadillo]

Trying turning the knob to the max.
Then pull out the turning knob from the spindle.
Literally rotate it to the lowest position and insert back to the spindle, pointer is at lowest position.
then turn the knob to the max again.
This sets the white plunger deeper in.
Can you get the picture..... LOL... hope my English can make sense.

Edit: Someone took the knob out, for washing or whatever and insert back at the wrong position thus the grill is out of calibration. Common Error.!

[glarsson]

It's an old fashioned mechanical thermostat.

We start at the "arrow". The vertical metallic piece is constructed from two different metal layers (bi metal). The metals expand differently when their temperature changes. This creates bending forces on the bi metal piece. The bending force is transferred to the right via the horisontal day grey pin to the switch. The switch is designed like a micro switch and the curved part makes the contacts (brass color) snap quickly between open and close. The white pin to the right is connected to a user accessible knob used to set the temperature. By turning the knob the tension on the switch is changed and the balance between this force and the bending force on bi metal piece therefore setting the wanted temperature.

[floobydust]

No, this is one of those oscillating thermostats.

This thermostat cannot accurately know grill temperature, because it is very difficult to get grill heat (transfer) to the bi-metallic switch. You need metal to conduct the heat between grill-thermostat but you also need insulation (electrical) which does not transfer heat well.

So instead, I have seen some cookers/toasters use a scheme where the thermostat incorporates a tiny heater. I see that in your pic. Wait, is it open-circuit, I see a break?
The thermostat+tiny heater is supposed to match the grill's thermal characteristics, like it is modelling the grill+big heater thermal response time.
I'm not sure what the carbon-slug does, if it's a resistor for the tiny heater.

So there is positive feed-back, this entire thing oscillates and here it sounds like the tiny heater puts out too much not enough and after a few minutes the thermostat thinks the grill is very hot when it actually is not. I'm not sure, I don't have success finding the adjustment/calibration screw on these.

[tpowell1830]

This is a bi-metallic strip, basically 2 metals with different expansion rates, the current from the circuit causes it to heat up and and when the temperature hits the expansion point, the contacts will snap apart, causing the current to stop flowing until the bi-metal strips cool down, and then the cycle repeats. The white pin on the right in the first picture is a pin which puts varying pressure on the metallic strip that allows adjustment of the set point.

The adjuster may need to be cleaned as someone said, or it may have been assembled wrong after someone else cleaned it.