Cooktops can be purchased as a range fully equipped with an oven and stovetop heating coil elements (Figure 1), or they can work as an independent appliance (Figure 2) fitted on a countertop.
The cooktop part of the stove applies direct heat to food. Instead of controlling from the ambient heat in the oven, it controls it with an "infinite switch" (AKA ""simmerstat") that determines how long the heat is applied to the heater coil before switching it off until it cools down and switches back on. This constitutes a complete cycle, and it keeps repeating itself. The time it stays on depends on the setting of the control knob.
How the Controller Works
Figure 3 shows the inside of the simmerstat with two sets of contactors that make contact as the dial is turned clockwise, letting the current go through a bi-metal strip that generates a small amount of heat. This causes the bi-metal (right contactor in Figure 4) to flex out and break contact.
As the control button is turned clockwise, it rotates a cam that applies an increasing amount of pressure, pushing the contacts together so when it reaches the max setting, the bi-metal cannot counteract it, causing the direct heat to remain turned on at maximum wattage until it's turned down.
With the dial turned down, the element is on for a few seconds until the bi-metal switches it off. Once the contacts are opened, with no more current flowing through, the bi-metal strip gradually cools down to the point where the contacts close again, initiating a new cycle to repeat itself. The average cooking temperature is therefore based on how long the cycle lasts.
Conventional vs Radiant Coils
Figures 5 & 6 show two different types of heater coils—the conventional coil and the radiant coil. The radiant coils were introduced to accommodate the ceramic glass cooktops with a concealed heat source beneath the glass top. The radiant coils are also complemented with a built-in “limiter” to protect against overheating by shutting down the power to the coil elements when they get too hot (Figure 6).
Troubleshooting an Overheating Coil
Safety first! Troubleshooting an element that always heats on high requires visual observations, so the power has to be on to perform some of the tests. However, before starting, anything sitting on the cooktop must be removed. At some point the elements will require to be switched on, so to avoid personal injury, it’s crucial that any parts of the body and clothing are kept away from any hot surfaces!
Step 1 - Stuck Contacts
A functional simmerstat operates by switching a heating coil on then off repeatedly while going through preset timed cycles. When turned on a low setting the coils should only stay on for a few seconds before going off. If any of the coils stay permanently red hot without ever flicking off, a defective simmerstat with its contacts fused together is at fault and needs to be replaced.
Step 2 - Defective Bi-Metal Strip
Also called an infinite switch, the simmerstat will be the likely culprit when a coil that stays too hot stays on for more than 2 1/2 minutes, especially after being turned down to a low setting. There is an easy and quick way to test the simmerstat by doing a comparison check since it is not the voltage getting cut down when turning down the heat, but rather the length of the time delay the switch stays on before going off for the cooling down of its bi-metal strip (Figure 4), so with that in mind, if this only happens with one of the cooktop’s coil, the following comparison should be performed:
2.1—There should be a second coil exactly like the defective one in the cooktop. Both coils should be simultaneously turned on Low. To avoid burn injuries, don't touch or get too close—this is an observation test only.
2.2—Observe the working coils getting red hot then switching themselves off, again and again. Normally, both coils should be cycling at about or close to the same rate. In this case, you can see that the defective coil probably operates at a much longer delay cycle rate. The simmerstat needs to be replaced as in the previous situation.
The previous testing may prove that the heating elements are working properly, so the coils are probably getting overheated by unknowing misuse. Before calling in a technician, make a few simple checks to determine if an inappropriate practice could be causing the problem.
Step 3 - Drip Pan Liners
Although they're designed to keep drip pans clean, aluminum foil liners can reflect back more heat at the coil above them, making it hotter and more difficult to adjust to the right cooking temperature. Even when these liners are designed to be energy-efficient, that's not their primary purpose.
Step 4 - Oversize Pots
A pot or pan should never exceed the size of the coil by more than one inch (25mm), as shown in Figures 7 and 8.
4.1—If a coil is too small, the edges of the pot may not get hot enough, which will affect the way heat is distributed across the bottom of the pot. Cooking elements often have two coils combined together, a small one in the center, and a larger on the outside to accommodate big pots (Figure 8).
4.2—If the large pot in Figure 8 was used with the smaller coil only, the excess overhang of the pot could “trap” the heat beneath it even as the coil goes through its cooling down cycle from the infinite bi-metallic switch, causing the next cycle to start before the coil cools to its intended temperature. It will therefore build up from that last temperature level every time it cycles, making it hotter and hotter each time.
Step 5 - The Oven Vent
With conventional coil stoves, if there's only one coil overheating, it might be from an oven’s vent opening placed right underneath that coil. If so, there's a possibility the heat venting out from the oven increases the cooking temperature of that coil, making it too hot. You can check this by turning off the range to let it cool down, then testing it again without turning the oven on. The owner’s manual should be checked at this point to verify and possibly adjust the vent properly.
Observing the range’s behavior and taking the proper corrective measures will restore your cooking results to their original standards.