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  #1  
Old 05-13-2002, 06:48 PM
MHand MHand is offline
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Does my electric burner stove have a potentiometer or a thermostat?

I have a single-coil electric burner. You can plug it in and turn a knob from Low to Medium to High and it heats up a coil which is used for cooking.

I want to know if the knob controls a potentiometer (variable resistor) or a thermostat. That is, if I set it to Medium, will it regulate to some temperature, or will it regulate by drawing a fixed wattage.

I opened it up. The circuit seems to be just this "device" connected to the knob, in series with the wall and the heating element.

The "device" consists of two thin pieces of meta that don't quite touch each other. Turning the knob bends the metal strips further away from each other.

So -- is my burner temperature regulated or not?

As a second bonus question: If its a potentiometer, does that mean that when I have the burner on medium, the knob disappates as much heat as the heating element? Or, is there some way to make a voltage divider that doesn't "waste" energy?
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  #2  
Old 05-13-2002, 06:53 PM
Joey P Joey P is offline
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Those two pieces of metal are a bi-metal switch. The same way you make christmas lights flash. They bend at different rates under the same amount of electricity or heat. When one touches the other it shuts off the flow of electricity until it cools back off and they straighten out. By turning the knob it brings them farther apart and they take longer to touch, therefore letting the coil get hotter.
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  #3  
Old 05-13-2002, 06:56 PM
Joey P Joey P is offline
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oh yea, to answer your question... That would be a thermostat.
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  #4  
Old 05-13-2002, 07:20 PM
MHand MHand is offline
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Thank you JoeyP.

Is it the heat from the heating element that causes them to bend? Or, is it just the normal electric current through them that makes them bend?
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  #5  
Old 05-13-2002, 07:23 PM
MHand MHand is offline
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Also -- How does touching the two pieces of metal stop the current to the element?

Thank you for you help in understanding this.
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  #6  
Old 05-13-2002, 09:20 PM
kanicbird kanicbird is offline
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the touching most likely activates a relay which supplies power to the coil. THe would be internal of the switch or could be a sepperate unit.

some other things of note (doesn't look like it applies to your stove):
1 there is a 3rd option that the knob time slices the current cyclying the current full on or off.
2 AC devices don't need to dissipate heat but can play with converting power into electromagnetic fields and back - this allows for a ac dimmer that's not a resistor (actually it's an inductor) and idealy creates no heat - real world conditions differ somewhat.
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  #7  
Old 05-13-2002, 10:00 PM
Patty O'Furniture Patty O'Furniture is offline
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Burner coils are rated anywhere from 1200 to 3000 watts, so the controlling element is certainly not resistive. A 3KW burner at half its maximum current draw would still be drawing 6 or 7 amps. The limiting element, if it was a resistor (variable or not doesn't matter for this simple calculation) would have to be about 70 ohms to get the power drop to the load right. Using the power law (P = I×E) we get the limiting resistance's power drop at about 3.4KW. Since a 20 watt resistor is about the size of a cigar, I can't fathom how massive a 3KW resistor would be. Then there is the problem of making a variable version.

I suspect that an industrial triac is used. Triacs are excellent power control devices, and have very low resistance themselves so they don't have much of an I˛R loss. I've worked with triacs rated up to 25 amps, and I'm sure they go higher than that.
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  #8  
Old 05-13-2002, 10:01 PM
JimOfAllTrades JimOfAllTrades is offline
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Quote:
Those two pieces of metal are a bi-metal switch. The same way you make christmas lights flash. They bend at different rates under the same amount of electricity or heat. When one touches the other it shuts off the flow of electricity until it cools back off and they straighten out. By turning the knob it brings them farther apart and they take longer to touch, therefore letting the coil get hotter.

oh yea, to answer your question... That would be a thermostat.
Joey P got this right except the last sentence. This is not a thermostat in the normal sense. The heat that causes the bi-metal strip to bend and make or break the contact comes from current flowing through the strip, not from the temperature of the burner. So this is a current sensitive setting, not a temperature sensitive one.

Since the bi-metal strip is in series with the burner element, all of the current that flows through the burner also flows through the switch. So when the strip bends and breaks the circuit, the current stops flowing through the burner and the switch, the strip cools down, makes the connection again, and the cycle starts over.

So the bi-metal strip allows the current to flow for a certain amount of time, then breaks the current for a certain amount of time, and then lets it flow again. When the knob is at a low setting, the “off” part of the cycle is longer than the “on” part. As you turn the knob up, the “on” portion of the cycle gets longer, and the “off” porting gets shorter. So a low setting gets you a lower average current flow, while higher settings get you higher average current flows.

Note that there are electric burners that do work on genuine thermostats. They have a sensor of some kind (usually a filled system or thermocouple) right in the burner assembly, usually a small spring-loaded circle that the pan sits on when sitting on the burner. But these controls are different; they don’t have the bi-metal strip setup and will have the sensor attached, so they’re pretty easy to spot.

Ugly
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  #9  
Old 05-14-2002, 12:09 AM
justwannano justwannano is offline
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I just looked at an old milkhouse type heater I have. It uses the bimetalic strips like Joey explained. The manufacturer (Rival) chose to call the device a thermostat.
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  #10  
Old 05-14-2002, 01:13 AM
JimOfAllTrades JimOfAllTrades is offline
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Quote:
I just looked at an old milkhouse type heater I have. It uses the bimetalic strips like Joey explained. The manufacturer (Rival) chose to call the device a thermostat.
This is absolutely true, many manufacturers do call these types of controls thermostats, and they’re not entirely wrong. The controls are thermostats in that they regulate the temperature of the burner.

However, when most people think of a thermostat they’re thinking of a device that responds to changes in temperature of the system being controlled. The thermostat on the wall of your house or apartment is a good example of a device that responds to temperature, and is one practically everyone would be familiar with. A bi-metallic control doesn’t do this. It responds only to current flow in the system, and not to the temperature.

So while the manufacturers can (and often do) call these types of controls thermostats (they can call them left-handed-metric-prop-washers for that matter), it is somewhat of a misnomer.

And I think (although I could be wrong) that the OP was not asking what these thing are called, but rather how do they work? Do they work like a thermostat and sense temperature, or do they regulate current flow like a potentiometer? It turns out they control current flow (although not quite like a potentiometer does, but by cycling the current completely off and back on), and so are not a thermostat in the sense asked.

Ugly
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  #11  
Old 05-14-2002, 01:49 AM
scr4 scr4 is offline
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So do all 1200W (or whatever) of power flow through the bimetal? Or does it activate a relay?
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  #12  
Old 05-14-2002, 01:58 AM
JimOfAllTrades JimOfAllTrades is offline
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Quote:
Originally posted by scr4
So do all 1200W (or whatever) of power flow through the bimetal? Or does it activate a relay?
In the ones I'm familiar with, the bi-metal control is directly in series with the burner. However, most of my direct experience is almost 15 years old, so this may have changed. On my current electric stove, which is about 6 years old, it is still simply a bi-metal control directly in sreies with the burner element.

I'm inclined to think that most units are still this way, as it works well enough for the application, and it is simple and rugged technology. Relays would be too complex and costly, I would think, for very little gain in controlability (is that even a word?).

Ugly
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  #13  
Old 05-14-2002, 08:26 AM
Joey P Joey P is offline
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I just figured I would call it a thermostat (since it does indirectly control the temprature) as opposed to a Rheostat.
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  #14  
Old 05-14-2002, 08:27 AM
Crafter_Man Crafter_Man is offline
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Quote:
Originally posted by Attrayant
I suspect that an industrial triac is used. Triacs are excellent power control devices, and have very low resistance themselves so they don't have much of an I˛R loss. I've worked with triacs rated up to 25 amps, and I'm sure they go higher than that.
I dunno… just a WAG, but I would think triacs would not be used due to cost. Using a bimetallic control would be far cheaper, and they may actually be more reliable. Besides, triacs are typically used only when precise control is required (e.g. industrial ovens & controllers).

Again, just a WAG...
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  #15  
Old 05-14-2002, 08:34 AM
Crafter_Man Crafter_Man is offline
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BTW: If a bimetallic element is used to “control” the heater power, then I would call it an open-loop controller. In essence, it is really nothing more than a power controller triggered by an adjustable (but free-running) time base. By contrast, if a feedback transducer were used (e.g. thermocouple), I would call it a closed-loop controller.
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  #16  
Old 05-14-2002, 01:58 PM
casdave casdave is offline
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Hmm, plenty close but not quite there.....

Firstly, inside the heater controller unit there is both a bi-metallic strip and a small heater element.(Control heater)

When you turn on the electric burner, current passes through the bi-metallic strip and into the burner element, thus it gets hot.

The heater element inside the burner control also warms up, and as it does so it will cause the bi-metallic strip to bend, opening up contacts to the burner element.

The small heater element inside the burner controller runs in parallel to the main burner element, when the bi-metallic strip opens its contacts it cuts off current to both the main burner and the control heater.

The temperature of the control heater is not related directly to the main burner.

The bi-metallic strip is adjusted by the user and this determines how long it takes for the Control heater to warm and open the contacts.
The longer the contacts stay closed, the hotter the main burner gets.
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  #17  
Old 05-14-2002, 08:48 PM
MHand MHand is offline
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Thank you everyone for helping me with this. To summarize and make sure I understand:

The bimetalic strip is either current sensitive (joeyP, RJKUgly) or heat sensitive to heat from a control-heater (casdave), but is certainly not heat-sensitive to heat from the main burner element.

One thing I still don't understand is exactly how the bimetalic strips shut off current to the burner. It seems that when I turn the knob to "high", and the burner is not plugged in, the strips are bent separated. When the knob is on low or off, they are also separated, but much less.

So, that would imply that when the strips come close together, the current is shut off, but I don't understand that. How does moving strips close to each other, or even touching them to each other, shut off the current? It would seem that completing a circuit would, if anything, turn something on.
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  #18  
Old 05-16-2002, 09:27 AM
justwannano justwannano is offline
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Have you tried "How stuff works"?

It might be interesting to see what they have to say.

Like RK its been years since I have had to tear my range apart. I guess thats good. They are rugged and reliable.
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