# How does this stove work?

This stove has not been heating well during the last week, so today we opened it. A rusted wire was to blame; we fixed it and the stove is as good as new now. But I do not quite understand its inner workings, hence this question.

I am attaching a photo and my take at a circuit scheme. I will also describe how it operates outwardly. I hope for an explanation of the function of its parts and an overview of the circuit.

simulate this circuit – Schematic created using CircuitLab

1. There is a handle on the side of the stove that rotates smoothly in the range of about 300 degrees. There are markings on it: "off" on 0 degrees (counter-clockwise limit) and then from "1" onwards with "5" at 300 degrees (clockwise limit).

2. There is an orange light source beside the handle. When the handle is at "off", it is never alight. Otherwise, it lightens and darkens alternately (in a discrete fashion) in intervals at the scale of a few minutes. The farther the handle is to "5", the longer the intervals of it shining and shorter the intervals of it darkening, but even at "5" it goes dark for about a minute every five minutes or so.

3. There is a large flat metal spiral on top of the stove that kitchenware is placed upon for heating. While cooking, it heats up, often so much as to shine dark red. It appears that the spiral heats up when the orange light is shining, and cools down when it is dark.

My idea is that there is a thermistor somewhere in the spiral that is joined in series with a potentiometer controlled by the handle, and the presence of the current in the spiral depends on the current through that potentiometer-thermistor series. But I cannot quite place this hypothetical thermistor-potentiometer series on the circuit scheme.

Here's how it works. It's actually simpler than you're making it out.

That dial that goes from 0-5 is a crude temperature controller. It contains a bi-metalic strip which pushes on some electrical contacts. When the temp is low, the strip pushes the contacts together which completes the circuit to the heater. Once the temperature increases, the strip bends away, breaking the contact and turning off the heater. When you turn the dial it moves the contacts slightly, increasing or decreasing the temperature at which this happens.

The light is in parallel with the heater, so you can see whether the element is on or not.

Those 2 wires that go into the surface are the heating element fyi. You're not going to find any active electronics or thermistors anywhere.

Edit: Side note: Make sure that wire that you replaced is rated for high temperatures. It's going to get HOT inside this cavity. You'll notice that all the other wires have high temperature fiber insulation.

• And I'll bet that they did not use the proper wire. There may be Darwin Award pending. – WhatRoughBeast Jun 6 '18 at 17:49
• @WhatRoughBeast Worry not. Only the end of the wire at S2 was rusted, so we removed about ½ inch from the wire and attached it back in place. – Ignat Insarov Jun 7 '18 at 3:50
• @IgnatInsarov - That's a relief. Sorry I doubted you. – WhatRoughBeast Jun 7 '18 at 16:50

I think what you have there is not a thermostat. Rather it is a "simmerstat" or "infinite switch", which is an electromechanical PWM controller with a relatively long timebase.

It does not measure temperature at all (or only has a weak sensitivity to temperature to be precise), it just turns on & off with variable duty cycle depending on the knob position.

Very simple and very reliable. Used in ovens and kilns for many, many years. After EMP knocks everything electronic out, it will still be clicking away.

• Yes, the switch cycles based entirely on self-heating of the bimetallic strip. It's completely open-loop, with no feedback from the temperature of the actual heating element on top. – Dave Tweed Jun 6 '18 at 17:16
• Good call, at first I thought it was a thermostat with poor thermal coupling but the simmerstat makes more sense. – John D Jun 6 '18 at 17:21