Standard Kettle Circuit Diagram

In a domestic kettle the basic electrical components are:

• Switch
• Indicator light
• Heating element
• Thermostat (Steam)
• Thermostat (Boil Dry)

In first diagram on the bottom right is the power source; depending on the design of the kettle this will either be a power plug or some kind of cordless interface system like the Stric P69.

The arrow symbol represents the ground connection; usually this is a return path for the electric current or a physical connection to the Earth.

Next, is the resistor (left hand side) it is represented by a zigzag. In a kettle the heating element is the main resistor, which is located in the water reservoir. As the current passes through the resistor it is converted into heat, this is what boils the water. Heating elements are available in various shapes and sizes and can be customised built if required.

The steam thermostat is the switch that turns the kettle on and off. The thermostat is a bimetallic disc that changes shape depending on the temperature. In a domestic kettle the disc is roughly 1 cm by 1 cm and triggers at around 98ºC. The temperature at which a bimetallic disc will trigger depends on the size and thickness of the disc. I have found that in a domestic kettle when you press the ‘on’ button the steam thermostat is pushed down, which completes the circuit allowing the electricity to flow through the rest of the circuit. Once the water starts to boil the steam is directed through a ‘steam vent’ to the bimetallic disc. As the disc reaches the set temperature it expands and flips causing the circuit to be broken, this stops the flow of electricity to the heating element. The steam thermostat is reset by pressing the on/off button, which will pushes down the disc, and starts the whole process again.

In these circuit diagrams when the steam thermostat is ‘on’ the indicator light is also on, the circle just under the switch represents the light.

The boil dry thermostat is a safety mechanism that ensures the kettle switches off if there is no water. It is very similar to the steam thermostat however it is thicker which means the temperature required to triggers it is higher. If there is no water in the water reservoir the steam thermostat cannot be triggered so the heating element would continue heating, causing the kettle to be damaged or catch on fire. The boil dry thermostat is placed on or close to the heating element if the heating element becomes too hot the thermostat will triggered. Unlike the steam thermostat which can be reset with the on/off button the boil dry thermostat is almost impossible to get to, which means if it triggers you will need to buy a new kettle.

The second diagram is almost the same; the only different is the power source is shown as an alternating current. And the other difference is the manual mode selector, which it is represented by a simple switch.

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