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Simmerstats: The genius old tech that controls your stovetop

May 03, 2024
This is my stove. It's a glass-top electric radiant stove and if you've ever used one of these, you may have noticed something interesting about its behavior. The heating elements start to glow quickly once turned on and you can feel the intense heat emanating from them immediately. but when you lower the control knob to, say, medium power, the element simply turns off. It doesn't continue working but at half power, it's simply turned off. But then, after a short while, it comes on again... and then goes off again. This pulsating behavior is repeated incessantly. What is causing that?
simmerstats the genius old tech that controls your stovetop
The answer is this funky component called the infinite switch, also known by the much, much better name Simmerstat. It may not seem like much, but each heating element on the stove is connected to one of these things, and the

simmerstats

are responsible for that pulse. Let me show you: through the magic of buying two of them and a few other things, I've built this little box so I can control anything with a simmerstat. I'll plug in a lamp so it's obvious what it's doing, plus a mystery charge into the other outlet that isn't important right now.
simmerstats the genius old tech that controls your stovetop

More Interesting Facts About,

simmerstats the genius old tech that controls your stovetop...

Let me limit myself to one control knob. That is better. Set on high, the simmerstat doesn't interrupt the power flow through it at all, but when you take it off the highest power setting, it eventually cuts the power. However, the interruption is quite brief. Finally, the power returns. These periodic interruptions will repeat indefinitely, but as you turn the control knob clockwise, each interruption increases in duration. Once it reaches the medium setting, it will eventually stabilize to a point where it will spend approximately the same amount of time on and off. And then the trend will continue: if you go further, you only trade in short pulses and those pulses get further and further apart.
simmerstats the genius old tech that controls your stovetop
The reason simmerstat behaves like this is because... This is the only option. Electrically, the heating elements of a conventional electric stove are simply resistors, a very simple component. But those resistances are enormous! Each of these stoves is capable of producing at least 1,200 watts, with the largest producing three kilowatts. That's great for boiling water, but too much power for more delicate cooking tasks like, oh, what a good example, oh. To simmer. To allow that, we have to domesticate them and reduce their energy production. But how would you do that? You might think about adding a second resistor in series with the stove element, maybe even a variable resistor. but at these power levels, that theoretical resistance would have to be gigantic and, by restricting current flow, would generate a lot of its own heat, which is a waste of energy and would require cooling.
simmerstats the genius old tech that controls your stovetop
Therefore, you may want to use a variable transformer, such as a variac, to produce a variety of voltages to choose from: reducing the voltage reduces the power consumed by those heating elements. But we have a similar problem there: to handle that much power, that variable transformer would have to be quite large and therefore quite expensive, and remember that you will need four. So, to enable precise control over the power output of heating elements with minimal energy losses and component costs, the simmerstat was born. This device produces any arbitrary power level by repeatedly turning its load on and off for varying periods of time, a method known as duty cycle control.
However, before I explain what's going on in here, I want to point out that these things are by no means a recent development. In fact, the

tech

nology is quite old. Coil stoves dating back to the 1940s use these exact same

controls

to modulate their power output. In fact, the

tech

nology just turned 100 years old. The first place I found it described was in this 1924 patent for Chester I Hall's invention assigned to General Electric. Coil-type heating elements like this are also just giant resistors, so, as with the glass-top stoves that came later, the simmerstat was the most cost-effective way to regulate their output.
That means that coil stoves also exhibit this pulsating behavior. However, you usually don't realize this is happening with a coil stove because you can't see it. The heat produced by these coils is generated by a thin wire element embedded in the center of the hollow metal tube that actually forms the coil. But the space between the heat-producing wire and the walls of the tube is filled with a sand-like material. Sand fills the tube so it can be bent into different shapes while keeping the wire inside centered, which prevents you from receiving electric shocks. Which is pretty good.
But to get the heat produced by the wire element out of the tube and into the cookware, it has to reach the outside surface, which means it also has to heat all the sand, which makes up a lot of thermal mass. . The result is that it takes a long time for these elements to heat up enough to visibly glow, so the pulsating behavior of the simmerstat is visibly obscured. However, the elements under a glass cooktop are very, very different. If I explained it to you, would that be a tangent? Well, it's not directly related to

simmerstats

, so I guess so, but that's a shame.
I bought this so we have to discuss it (I spent it). This set of heating elements is what is actually found under the glass of many glass-top electric radiant stoves. It's incredibly simple: just a flat disc of heat-resistant support material that houses a very long zigzag piece of nichrome wire. Actually, two: This is a size-selectable burner like this one on my home stove, made up of a six-inch inner section and an outer ring to fill it to the full 9-inch diameter for larger cookware. Not all of them are made exactly like this, sometimes the heating elements are structured a little differently, but they all work the same way: when voltage is applied across the nichrome wires, they heat up very quickly to the point of glowing brightly. .
That speed is the main functional advantage of a glass-top stove: almost instantaneous heat production from cold. But you'll notice that outdoors this looks very bright and orange, like the heating elements in a toaster. That's because, well, that's pretty much what they are! But, as you say, under the glass of a stove, when they are in operation, they appear a deep cherry red color. Why is that? Well, this glass is actually a special ceramic material with very peculiar characteristics. It is deliberately very bad at conducting thermal energy through itself, keeping other areas of the stove cool to the touch even when the parts next to it are hot enough to melt lead.
But the glass allows infrared radiation to pass through almost unhindered. That's what allows the radiant heat produced by the under-glass heating elements to reach

your

cookware and why you can feel the intense heat coming from them immediately once they're turned on. The deep red color seen through the glass is the result of filtering: the glass is opaque to almost all wavelengths of visible light, making it appear black to our eyes. But since infrared light passes well, the near-infrared frequencies at the very edge of the visible spectrum will escape this filtering and you will be able to see them.
Pretty wild, right? And if you've ever noticed the probe running through the center of one of these burners and wondered what it's for, well, actually, two things: This is a sensing probe for two thermostatic switches in this little remote cabinet. The first of those switches is very sensitive and closes its contacts in the presence of minimal heat to illuminate the hot surface indicator (or indicators) to warn you that the cooktop is still hot. Count

your

blessings if you get a separate light for each burner, most of the time they are all connected in parallel so any one of them can turn on a single warning light.
Those cost reducers who cut costs. But the other switch here is connected in series with the heating element. Remember that this flat disk of searing heat is trapped under the glass, and while that glass is infrared transparent, it is not perfectly transparent, so it will absorb some thermal energy and get quite hot. Additionally, even if it were perfectly infrared transparent, there will be a cookware on top of the glass that will reflect some of the thermal energy back down and into the element. Therefore, tremendous heat builds up in the tiny air space between the glass and the bottom of this infernal frisbee.
To prevent the glass from getting too hot, the temperature probe will open a safety switch to cut power to the heating element and prevent everything from melting. It usually resets within a few seconds, but will cut power again if necessary to enforce a high temperature safety limit. This usually only occurs if the burner has been in continuous use at full power for several minutes, such as when bringing water to a boil. Anyway, this video is supposed to be about simmerstat. But before I speak, no, I'm joking. I previously said that these modulate the power output of the stove burners by controlling the duty cycle.
If that doesn't mean anything to you, it's just a way of saying "the percentage of time spent averaging." Imagine you have a 1200 watt burner but you only need 600 watts of power for whatever specific cooking task you are doing. Well, if you run that burner at a 50% duty cycle, perhaps running it for 5 seconds out of every 10 second period, then although the actual heat output will alternate between zero and 1200W, over time the heat output Effective heat is the maximum power multiplied by the duty cycle; in this case, 1200 X 0.5, or 600 watts. Those of you who are more digitally minded might be thinking about pulse width modulation right now.
Duty cycle control and PWM are quite similar in regards to their net effect of producing a lower average output power by repeatedly turning a... whatever, but the two terms technically describe different things. . Some might choose to argue with me on this, but PWM doesn't necessarily attempt to modulate the power output of anything. In fact, it can be used as a signaling protocol for quite complex tasks. Servomotors, for example, are sometimes controlled using PWM signaling. The duration of the pulses they receive encodes a position that they must assume. When you use PWM simply to modulate power output, say when you use a PWM dimmer to dim DC-powered LEDs, strictly speaking, that's still duty cycle control: the apparent brightness of the LEDs is determined by the percentage of time.
They spend in and out, which is literally another way of saying their duty cycle. Pulse width modulation in that context is simply a modern means of achieving duty cycle control, with the added benefit of high switching frequencies giving it some functional advantages. But since simmerstat is literally a 100-year-old technology, there's nothing modern here. However, it is absolutely a fully functional and capable duty cycle controller. How the hell could that be? Well, let's look inside the simmerstat to see what's going on. That... that's less clear than you think. First, because we're going to turn this off now.
Because we are dealing with the US split phase electrical system here, in the 240V circuit used for control there are two hot wires and no neutral. The heating elements of a stove are connected between lines one and two like any other 240V device. You can learn more about that in this video if you want. Due to this fact, this device has two switch contacts, here and here, so that it can open both sides of the circuit when turned off and completely isolate the heating element from the voltage. That makes the simmerstat essentially a single-pole, two-pole switch.
Now the left side of the simmerstat doesn't actually matter at all for your power modulation purpose. That's actually just an extra isolation point that breaks line 1, and only opens when the control knob is in the off position. In all other positions it is closed. Oh, and this extra piece of copper here is used to send power to a pilot light to indicate that a stove burner is on. That's why the terminal on the back is labeled P and why they didn't bother giving it a proper contact surface - it only sends a small amount of current to a neon indicator likethis.
However, you'll notice that the right side of the simmerstat features a much beefier pair of switch contacts, one of which is attached to a wide copper bar. This is the contact that opens and closes periodically to modulate the power output. Uh, to avoid confusion, remember that you only have to break one side of a circuit to stop the flow of current. When the other switch is closed but this one is open, the heating element will still have a potential of 120V from line 1, but there is no complete circuit to line 2 for power to actually flow through it.
Except the pilot light, well the pilot light only works at 120V and the other side is connected to neutral so it stays on no matter which burner on the stove is running. I'm really sorry that our power system is strange. Just… ignore everything to the left. It doesn't matter. Moving on, if we look at the back of the copper bar that houses our main switch contact, we'll see something that looks an awful lot like a bimetallic strip. Because it is. These little heroes appear in the darkest places. Why are you here? Well, this copper bar carries the current that flows to the heating element of the stove when the switch contact is closed and power flows.
And there is a small amount of electrical resistance across the copper bar - it's too small to measure with a multimeter, but it's enough to produce some heat when the 10 or 11 amps drawn by the stove burner flow through. through it. Bimetallic strips warp when they change temperature, and since there is one attached to this copper bar that we just set up that heats up when current flows through it, the bimetallic strip will start to bend when that happens. Unfortunately, I can't demonstrate this in the circuit. You'll see why in a moment. However, what I can do is use this little heat gun and show you what happens when the support bar gets hot.
It's quite subtle, so watch carefully. When hot, the copper bar bends so that the bottom switch contact goes deeper into the body of the simmerstat and away from its partner above. Now, think about what that means. If the copper bar heats up every time the switch is closed and power flows, and that heat causes the bar to bend in such a way that the switch opens, then the switch doesn't want to stay closed. Every time it closes, it gets hot and the bimetallic strip tries to open it. In practice, it looks like this. From below we can't see the switch contacts, but we can see the copper bar moving back and forth slightly.
Every time the switch is closed and power flows to the load, the copper bar begins to heat up due to internal resistance. We can see this quite clearly with the thermal camera. As that happens, the bimetallic strip bends the bar so that the bottom contact begins to move closer to the chamber and away from its partner. Eventually the strip bends enough to break the circuit, so current stops flowing. However, once it does, the bar starts to cool down quickly. That causes it to reverse course and begin to approach the other contact of the switch, then they touch, current can flow again, the bar heats up, and the cycle repeats.
And now, let's see if you make a technological connection. I am using a lamp now connected to the simmerstat. And that lamp, due to the slow fire, flickers. I've covered another piece of technology we used to make lamps blink in the past. That piece of technology is placed in series with the lamps it is meant to light. The current flowing through it would cause a switch inside it to open briefly and then close repeatedly. I am referring, of course, to the turn signal. These old-school thermal turn signals also work thanks to a bimetallic strip. That strip will deform when heated and open or close a switch (depending on design details) that repeatedly applies and removes power to the incandescent lamps that make up the turn signals to make them flash and therefore more visible.
That behavior is certainly much faster than how simmerstat works right now, but it's not really any different, is it? It's the same, only accelerated. And in essence, this simmerstat is really just an oversized turn signal capable of handling up to 11 amps of current at 240 V or 2600 W. But the simmerstat is a turn signal with a twist...literally. The turn signal has a fixed duty cycle and behavior, at least when controlling the same load with the same voltage. But the simmerstat can adjust its duty cycle according to the position of the control knob. Because? And how? You know, in Britain they call kitchen burners hobs.
Does that mean it's a kitchen knob? Anyway, when we were looking at this from above, you may have noticed that the switch contacts were not close to each other. They are clearly activated in such a way that by default they are in the open position, so what closes those switches in the first place? To find out, let's switch to Cam Cam. ♫ Offenbach comes out of nowhere ♫ It's this camera! I don't know how I live with myself either. The simmerstat control knob is attached by a shaft to this plastic cam covered in a large amount of grease.
When assembled, the cam presses down on these protrusions once it is rotated out of the off position, and that is what closes the switch contacts. You'll notice that the chamber has two sections with different profiles: the inner section presses and closes the switch to the left of the simmerstat, which remember, doesn't matter. It's just a safety switch. But the outer section, activated with a bimetallic switch, has a very subtle ramp built into it. Can you see that? This is the off position which does not press the switch at all. But just to the left of that position, the camera profile becomes very high.
That means you push the switch down pretty far. But then there's a pretty steep drop-off before the camera very subtly gets thinner and thinner until it reaches the off position. Can you imagine why that might be? Vote now on your phones. What that variable cam profile actually does when you turn the knob is change the rest position of the two switch contacts on the bimetallic switch. You can see from below that when I turn it, the copper bar and bimetal strip move up and down very slightly. This may seem quite inconsequential, but that is actually the key to this entire device.
But explaining why... is complicated. I've been stuck on this script for a while because, although this component is incredibly simple, there are three connected concepts working together here to make this what it is, and that makes it difficult to explain without getting stuck in a loop. But I will try. Let's review that footage from earlier. Here, the low heat was set to medium and I let it stabilize. In this condition the switch closes for about 5 seconds before opening and then remains open for about 5 seconds and this repeats. This is how we achieve a 50% duty cycle. But why precisely does the switch open and close with such predictability?
To find out, let's take a look at the thermal camera again. You will notice that in this configuration, the contact support bar appears to peak at around 100 degrees Celsius and then the temperature begins to drop. Then we see it steadily bottoming out near 78 degrees Celsius and starting to rise again. This tells us that the switch actually opens and closes based on the temperature of the support bar and its bimetallic strip. Simple enough, but why? Why does the circuit open and close at those specific temperatures? Well, that's because of this cam and how far it pushes down on the top switch contact.
Remember that the bottom switch contact is pulled back into the simmerstat body as it heats up. The distance that contact actually moves is a function of the temperature of the bimetallic strip on the support bar. And through the position of the cam and its profile, we have chosen to place the top switch contact at some specific point along that deflection path. In this position, the contacts are forced to stay together until the copper bar reaches 100 degrees Celsius. At that precise temperature, the deflection of the rod is sufficient to open the switch. Actually, the main function of this device... is a thermostat.
By turning the knob, we decide how hot we want to allow the copper rod to get before it turns off the stove burner. So the last piece of the puzzle is how that choice becomes a consistent work cycle. Because remember, the purpose of this device is not to maintain a specific temperature like an oven thermostat, but rather to maintain a specific duty cycle and therefore the power output of the stove burners. Yet somehow we are doing that with what is apparently a thermostat. Well, this is where I hope it all fits together. Remember that the copper bar inside is both a thermostatic switch AND a heater.
Whenever the switch is closed, it dissipates a constant amount of energy and that generates a constant amount of heat inside the bar. And like all things that produce heat, with a constant power output, how much heat is actually heated and therefore how far the rod will deflect is a function of how long the heater runs in a given period. And now physics becomes our friend. Let's say I change the control position to medium-high. What that will actually do is push down further on the top switch contact to force the two contacts to stay together until the bottom support bar has reached 130 degrees.
So, let's do that. The temperature of the bar, since energy is now flowing through it, is rising. But you will notice that the rate of temperature change slows down as it continues to increase. What is happening here is a result of the fact that the rod is approaching the limit of how hot it can get before the heat it gains through its internal resistance matches the heat leaving through the radiation to the surrounding air. And as we approach that limit, heat gain slows down significantly. This means that it will take longer to reach the new target temperature of 130 degrees, which in turn means that the switch contacts will remain closed for a longer period of time before opening again.
And what's more, the temperature difference between the bar and the air around it is greater when it's hotter, meaning that once it stops heating up, it will lose thermal energy faster than before. This shortens the time the switch remains open before closing again, although this effect is minor compared to lengthening the on time. But what about going in the other direction? What happens when you turn the knob to, say, medium-low? Well, with the knob at the nine o'clock position, the cam just lightly presses the switch. You're pushing so gently that the bar only needs to reach about 60 degrees Celsius before bending enough to open the switch.
Since it is much closer to room temperature than 130 or even just 100 degrees, it takes very little time for it to reach that temperature when the current passing through it heats it up. And you'll notice that the switch only closes again when the bar drops to about 40 degrees Celsius, but it takes a long time for the bar to cool down to that temperature, so the pulses it sends are short and infrequent. Do you see how this all fits together? We are actually controlling this circuit based on the temperature of the heater inside it. When properly calibrated, it can be used as a duty cycle controller.
Because to heat that heater, the heater itself must run at a longer duty cycle, so its average temperature becomes an effective indicator of the duty cycle needed to reach that temperature. And that's incredibly fortuitous. Simply calibrate the bimetallic bar to dissipate the correct amount of heat when the circuit is passing current, give the cam that presses the switch a nice subtle ramp to allow it to break the circuit at a specific temperature of that bar, and you can produce any cycle of work you need with incredibly crude technology. But it gets even better! Because the switching action occurs as a function of the temperature of the bimetallic strip, a really useful memory effect occurs here.
The main annoyance of using a conventional electric stove is the reaction time. The thermal mass of the materials involved retains heat for a time, whether it is the coil of a coil stove or the glass of a glass countertop. This is useful because it helps soften the effect of the pulsating behavior, but it also means that the stoveIt takes a long time to react to a change in power level. The simmerstat can't solve that problem, but consider what happens when you change the power level: if, for example, you were on medium-low heat but needed to go to medium-high heat, then turning the knob is simply changing the temperature that must reach the internal heater before the power goes out.
And if on the medium-low setting the heater was holding, say, 80 degrees on average, then when you change the setting it has to go up to 130 degrees before the element shuts off. That means it will produce a very long output pulse and help the burner reach its new target as quickly as possible. The same thing happens when you turn down the power: if it's already hot enough, turning the knob will open the switch and it won't close again until the heater has turned down to the new target, which will take a while. I first noticed this behavior of my glass stove and assumed it was being accomplished logically, but no!
Is that how it works! Now I've left something important out. You may be wondering how the simmerstat can keep the element on at full power when it is on high. Well, that's what that really high point on the camera profile was for. That actually jams the switch, so it will never open no matter how hot the internal bar is. Pretty crude, huh? Except…the rawness doesn't end there. This particular simmerstat only works properly with fairly large heating elements that draw between 8.9 and 11 amps for which it is rated. This is because how quickly the copper bar heats up depends on the amount of current passing through it.
If you try to control a load outside that range, things get out of control. As luck would have it, this 1100 watt hotplate draws about 9 amps at 120V, so this simmerstat works fine with it. The hot plate has been my mystery burden throughout the video. But if I plug in this smaller cooktop that only draws 900 watts, the duty cycles this is supposed to produce get out of control. And with a load much less than that, it never interrupts power. You may also have noticed that the way this changes the load is terrible! Ideally, switches have a fast action to reduce arcing when interrupting electrical loads.
This guy just doesn't do it. The contacts barely move despite switching 10 amps, so occasionally quite unpleasant arcing occurs. It's not so bad because they are only used to switch resistive loads, and overextending the switch when the cam is pressed down produces a wiping effect that helps clear the contacts of carbon and debris buildup. By the way, that also happens inside the switches of an electromechanical pinball machine and yes, part three is coming, I haven't forgotten, hold your horses! I just thought a script like this would be faster. Why did I think that? I don't know. It never works!
Anyway, before I end this video I want to make sure to say that not all simmerstats will work exactly like these. For example, the thermostats on the stove in my house? Three of the four cannot use the current flowing through them to heat the bimetallic strip inside because they have selectable size elements. One even has three sizes. I suspect the main functionality of these simmerstats is exactly the same as this simple one, but there is probably a dedicated resistor with its own path back to neutral that produces the heat for the bimetallic strip, that way the duty cycles are consistent regardless of the load you are controlling at a given time.
I also think it could be controlling a relay or, another possibility, the knob could be controlling a variable resistor that produces different amounts of heat to open a limit switch. I think that might be the case because they have a definite click as they turn on and off, something this basic model doesn't have. But that's just a guess; I'm not going to break this down. I need it. And by the way, I built this box for this video, but I've been wanting to build something like this for a while specifically because of these cheap power boards.
If you've ever used one, you'll have realized that they are impossible to control! This is because this knob does not control a simmer state, it is just a plain old thermostat. Please note that it only clicks when you are far from the off position. You can make this work for what you need, but it's incredibly difficult because you don't even know where it measures the temperature and it will change depending on the cookware you're using. It's a little easier to control this style of cooktop. where the heating element is embedded in a metal disc and its average temperature means something, but this style of coil is completely impossible to control with a thermostat.
I'm sure it's much cheaper than a proper Simmerstat. I mean, these things cost like 15 bucks. But it's incredibly annoying and makes these only useful for boiling water. I thought I'd end up building an Arduino-based controller or something, but it turns out you can put a simmerstat in a handy box and that's it! I added the lights because I like it. Red is the pilot light and yellow indicates power is flowing. Uh... I won't show you how I made that work. And one last point: one could argue that we really shouldn't continue using them. I mean, if you buy an induction cooktop, it won't have them, but don't talk to me about the touch

controls

they usually have.
Hey, note to appliance designers: no one wants that! Just use knobs. Touch controls on a stove would be like taking the turn signals off a car and forcing people to adjust to strange buttons for no good reason. Anyway, what I mean by this is that the simmerstat works very well with coil stoves, since these heating elements retain a lot of heat, but with glass lids, where the radiant heat is transmitted immediately, the switching frequency It is possibly too slow. I don't usually notice this, and to be honest, I'm not sure it really matters, but when I fry some vegetables in a thinner cookware, I can tell when the element is working and when it's not.
The sizzling gets a little louder when it's on and if there's water at the bottom of the pot, it bubbles louder. Again, I'm honestly not sure how much that really matters, but I'm not a good enough cook to pretend my opinion is good. Still, with modern high-power solid-state switching components, stove burners could, in theory, be controlled as finely as a dimmer switch provides. That would cost more, of course, but at this point I'm not sure how much it would cost. I mean, induction cooktops have some wild power switching circuits and aren't very expensive anymore. Now, the main thing I would be concerned about in this hypothetical case is the noise.
These guys make a pretty noticeable hum when turned on, and adding some high-frequency switching to that mix could turn that hum into a strange sound. But hey, even a 1Hz switching frequency would be a big improvement over the simmering burns on my stove at home. Something to think about. OK. Well that's it! Believe. I didn't imagine this script would get so out of control. I mean, it's literally just a bimetallic strip in a box with a knob. But at this point I don't know why I'm surprised. Here, how about we try November in April? I think I'll do something really simple before the month is out.
Can I rise to the challenge? Find out in the next… whatever. Whenever it happens. Bye bye. ♫ obediently smooth jazz ♫ I'll plug in a lamp so what he's doing is obvious besides the eahhhh The response- The interruption is pretty brief, though. Finally, the power returns. This is going to be harder to time than I thought it was going to be. Once you reach the medium setting, now... oh right, that's what's supposed to happen. These periodic interruptions will repeat indefinitely, but as you continue to control the rotary knob clockwise. I previously said that these modulate the power output of the stove burners (thud), that was loud!
But to maintain a specissssffsfsfsfssfs You've heard of a flash in the pan, what about a flash under the pan? I think that joke needs a little more time in the oven, right? At best, half-hearted. But hey, at least I made a meal out of it. The soup is ready!

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