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I've noticed that the burners of some or all of the newish-style electric stoves which have a flat top have a peculiar property. They do not seem to be capable of running at a constant low temperature, instead periodically coming on high for brief-to-longer periods of time. These stoves were not, as I understand it, discount priced. They are not induction stoves, but seem to have an element embedded in the material which forms that part of the stove top.

Is this an across-the-board trend in new stoves? Is it difficult to find electric stoves which work the "old way", supplying a constant temperature? Is there an advantage to the way these newer stoves work? It would seem to be difficult or impossible to cook a range of dishes on such appliances.

intuited
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    Just from an engineering point of view -- what you're calling 'binary' is often called a 'duty cycle'. (where the duty cycle can either describe the % time is spends on, or how long it stays on for a given time period) – Joe Mar 03 '11 at 13:21

3 Answers3

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The burners on essentially all electric stoves are binary in that they are either fully on, or fully off. It would be more expensive and less energy efficient to use electronics that continuously vary the current flow through an electric element, and this would make no significant difference in temperature behavior at the cooking surface. Instead, electric stoves use a bimetallic switch which is a relatively simple way to have an on-off pattern with variable on/off times. To create constant heat, all electric stoves use materials that are bad conductors of heat between the electric element and the cookware surface to buffer the huge temperature swings at the element and produce very steady heat at the cooking surface.

The difference you are seeing between electric coil heating elements and glass-ceramic cooktops is that in the electric coils there is an inner heating element, then a thick ceramic layer, followed by an outer layer of metal. The element itself is heated in a binary manner, but all you can observe is the heat after the buffering of the ceramic layer has made up for the large fluctuations at the element (i.e. the outer metal glowing fairly constantly once it's heated). In a glass-ceramic cooktop, since the buffer layer (the glass-ceramic surface) is translucent, you are seeing the actual element glow (often this is an infrared lamp instead of a resistive wire) so you are viewing the non-buffered heating pattern. If you had a clear coil, you'd see the same heating on/off patterns in a coil stove as you do in glass-ceramic.

Consequently, if you measure the surface temperature of a glass ceramic cooktop, you should see a fairly constant temperature.

timmyp
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    I don't think it'd actually be difficult (some power triacs should be able to do it, I believe), just more expensive & less efficient (more heat ejected into the room vs. the pot), and probably for no benefit since the temperature is already being averaged by thermal buffering. – derobert Mar 01 '11 at 17:08
  • Thanks for the explanation! I updated the answer to reflect this. – timmyp Mar 01 '11 at 18:19
  • This is quite informative. Is it possible that some newer stoves do not use adequate buffering? It seems that water boils harder when the elements/lamps/magic-red-circles are lit up. – intuited Mar 04 '11 at 05:26
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    That's possible. Another thing to consider is that since the surface is transparent, there is likely additional radiant heat when the element is on. I tried turning on my glass ceramic stove and almost immediately could feel heat a good 2-3 feet above the burner, so I think that pretty clearly means that there's a good bit of radiant heat that could cause the faster boil. It seems like they could easily solve this by just coating the bottom of the glass with black paint. I wonder if the additional radiant heat is desirable, or maybe the paint just wouldn't be aesthetically pleasing. – timmyp Mar 04 '11 at 18:35
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    @timmyp regarding the last sentence of your answer, I can dispute that. Easier than trying to hold a thermometer above a radiant heat source is to boil a little water. If you take a skillet that covers the entire element, and put in a 1/2 inch (or 1cm) of water, you will be able to clearly see the duty cycle. Once the water is boiling, you can see surging in the boiling activity. You will also be able to see the hot spots on your stovetop as well because some of the water will be boiling while some may be perfectly still. Even with cast iron (retains heat better) you can see the difference. – Escoce Apr 15 '16 at 15:16
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The "old school" type of cast iron hobplate, and also the type of glass ceramic hob directly derived from that design, controls power output, NOT temperature, although the more powerful types have a bimetallic switch to stop them from self-destructive OVERheating (somewhere above 300°C IIRC, this won't keep you from starting a grease fire and is likely not meant to). Such control is by employing more than one actual heating element inside the plate, and enabling only a select set of heating elements for a given setting, also taking advantage of series circuits to arrive at lower wattages. This is not stepless, usually such stoves will have 3 or 6 steps available (see http://www.herd.josefscholz.de/7Takt/4_und_7_Takt.html for all the electrical details - German language but comprehensive schematics).

So if you are looking for a "non-binary" stove, look for models (often inexpensive) that have fixed steps in their heat settings.

Actual Rheostats will never be used since they would themselves generate SIGNIFICANT waste heat when operating; the best thing to use for stepless power output control would be a TRIAC circuit similar to a light dimmer - such might be infrequently found because it is difficult/expensive to build (for a power handling approaching 2 kilowatts compared to a few ten to hundred watts in lighting!) at that power level without creating a lot of radio interference and power quality issues (light dimmers are notorious for that already).

The disadvantage of the old cast iron type is that it is very slow to react to control inputs, the advantage is that thin walled cookware can be used (allowing for very QUICK temperature control by taking it on and off the hob, or even using another, cold hobplate as a heat sink!) since the hobplate itself is a big thermal buffer and power output is indeed constant.

rackandboneman
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With reference to the first statement that electric heating elements used to hold a constant temperature compared to today's, clearly visible, on, off cycle of heat. Older manfgr's use to make controls with rheostats that allowed the user to adjust the flow of electricity thereby controlling the amount of electricity used to generate heat in the element. Compared to today's (cheap) method of cutting out the rheostat and through historical experimentation, the control uses "timed" on, off to generate different temperatures. It is possible to make an electrical element that uses simple on, off at the control knob and still maintain constant temperature in the element, but manfgr's seem to not have engineers smart enough to make them.

Practical Answers
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  • Or.... the buffering results in close enough to a constant temperature that they don't need to do anything beyond the on/off cycling. – Cascabel Apr 15 '16 at 15:07
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    I've never seen a stove with rheostatic controls (a rheostat being a variable resistor), however old electric stoves used to have heating elements made up of more than one segment. The control switch would turn the segments on in different combinations of series or parallel circuits, achieving a constant output at several fixed wattages. Because rheostats are resistors, they give off heat. If you used a rheostat to modulate a high wattage heating element, the rheostat itself would become very hot. – ElmerCat Apr 16 '16 at 22:20
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    Like the first guy, I am interested in an electric burner with consistent levels of heat range, so, we're not alone when demanding the return of heating elements made up of, as described by ElmerCat, segmented elements. Thanks for explaining that, Elmer! Someone thinks cycling elements on and off is an efficient method of electric heating, bah, if cycling on and off is so popular and efficient, gas cooking burners should be cycled on and off too. –  Apr 26 '16 at 14:07
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    Electricity is not gas -- it's more efficient to cycle electricity, as you can't restrict the 'flow' of electricity without adding some other load (something which consumes power, like a resistor). Pulsing the power can let you send only 30% power (as compared to being on full time) without having to dump 70% of the power elsewhere. – Joe Apr 26 '16 at 14:35
  • You can still buy old school electric stoves if you want to... @Joe multi-element systems do not need cycling. But yeah, a *rheostat* would be the worst choice. Either you cycle but do so much quicker (essentially in milliseconds - that is what a TRIAC circuit more or less does), or you would need an inverter or variable tranformer (too much hassle). – rackandboneman Dec 13 '16 at 10:39
  • @Jefromi you can sell them cheap stoves and heavy, awkward, expensive pans that way :) With some induction devices using slow cycling, and thin pots, you actually can get water to visibly boil periodically ;) – rackandboneman Dec 14 '16 at 11:54