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.


Bi-metallic Strip

August 10, 2009

A bi-metallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated, usually steel and copper, or in some cases brass instead of copper. The strips are joined together throughout their length either riveting, brazing or welding. The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its normal temperature. The metal with the higher coefficient of thermal expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled.

The sideways displacement of the strip is much larger than the small lengthways expansion in either of the two metals. This effect is used in a range of mechanical and electrical devices. In some applications the bi-metal strip is used in the flat form. In others, it is wrapped into a coil for compactness. The greater length of the coiled version gives improved sensitivity.

A bi-metallic coil from a thermometer reacts to the heat from a lighter, by uncoiling and then coiling back up when the lighter is removed.

Curvature of a Bimetallic Beam:

\kappa = \frac{6 E_1 E_2 (h_1 + h_2)h_1 h_2 \epsilon }{E_1^2 h_1^4 + 4 E_1 E_2 h_1^3 h_2 + 6 E_1 E_2 h_1^2 h_2^2 + 4 E_1 E_2 h_2^3 h_1 + E_2^2 h_2^4}

Where E1 and h1 are the Young’s Modulus and height of Material One and E2 and h2 are the Young’s Modulus and height of Material Two. ε is the misfit strain, calculated by:

\epsilon = (\alpha_1-\alpha_2) \Delta T \,

Where α1 is the Coefficient of Thermal Expansion of Material One and α2 is the Coefficient of Thermal Expansion of Material Two. ΔT is the current temperature minus the reference temperature (the temperature where the beam has no flexure).


Strix P69 Cordless System

August 10, 2009

The Stric P69 is a one piece, low cost cordless interface system incorporating contacts designed for robustness and reliability. It is designed to interface directly with the universally recognised Strix R72 and Strix R3 controls, and to allow the use of cosmetically co-ordinated base mouldings.



  • One piece cordless system that connects directly to standard Strix controls
  • Compatible with Strix R72 and Strix R3 controls allowing simple, robust conversions to cordless appliances
  • Assembled into power base from one side allowing simplified and fast assembly times
  • Small intrusion into appliance rear cover minimising design modification for cordless variant
  • Allows the use of colour co-ordinated cosmetic cover

Functional Description
The p69 comprises a single skeleton contact housing designed to be incorporated within the confines of the cordless base mouldings. It contains high quality conductors terminated with 4.8mm x 0.8mm male tab connextors to allow for ease of assembly. The entry ports into the contact housing are deisnged to exceed all electrical safety regulations but noonetheless both they are the contacts within are shrouded by a smooth shutter. The earth pin slides the shutter open as teh appliance is placed on the base. The pins of the R7 and the R3 control are then allowed to penetrate the housing and make electrical contact. The components are so arranged that the earth pin will always contact first and break contatct last, circuit continuity always being made and broken by the line pin. It follows that by fitting a P69 compatible Strix R72 and Strix R3 control and by simply changing the back cover somponent, the appliance can be convewrted from corded to cordless. The whole assembly is designed to minimise water ingress and to ensure electrical creepage and clearance path cannot be bridged.


The Strix R72 is a kettle or jud steam control incorporating self-resetting protection against accidental dry switch-on of the appliance or boiling dry.

Additionally, a ‘Thermal Fuse’ gives protection against dangerous over-heating in the unlikely event of the dry switch-on protector failing.



  • Three levels of appliance protection
  • Proven reliability minimise the need for appliance end of line testing.
  • Specifically designed for low installed cost and simple assembly operation.
  • Many variants allow maximum appliance fesign flexibility.
  • World-wide approvals facilitate entry to the international market place.
  • Designed to interface directly with the Stix Cordless Systems P69 and P75

Functional Description:
The control is designed for incorporation with the cosmetic cover of the kettle or jug, its tip lever interfacing with the switching arrangement of the manufacturer’s appliacne. The control comprises two single-pole switches, one in the line supply to the element and the other in the neutral. The neutral contacts are opened by the steam thermostat which also causes the trip lever to latch into the “Off” position. The silver line contacts are operated by the dry switch-on protector which responds to the heat conducted from the element should the element over-heat due to the lack of water in the appliance. The steam thermostat is operated by steam ducted into the control via a small hole near the top of the vessel or tube arrangement within th eappliance handle/cover/ This switches the appliance off when boiling occurs. The Thermal Fuse acts as an ultimate safety device to open the neutral contacts. This is a “one shot” device. (The appliance requires servicing once the thermal fuse has operated).


Parts of a Kettle

August 9, 2009

To get a better understanding of the inner workings of a kettle i disassembled a generic electric kettle. 


Parts of a Kettle:

1. Power cable with Strix P69 Cordless System
2. Base moulding
3. Water reservoir
4. Handle with lid mechanism
5. Heating Element
6. Removable filter
7. Bottom of base moulding
8. Lid release mechanism
9. Lid release button
10. Lid release mechanism
11. Rubber seal
12. Strix R72 kettle control, switch and thermal fuse
13. LED
14. Bi-metallic disk

Tom Metcalfe

August 9, 2009

The Coffee Maker Process

Take 50 green beans. Roast for 7 minutes for a medium roast, 9 minutes for a dark roast. Cool beans. Whilst grinding the cooled beans heat the water until air bubbles begin to pop. Place ground beans into water. Stir and brew for 4 minutes. Then pour and enjoy a cup of fresh perfect coffee.

Design Philosophy

To achieve genuine sustainability, products must not be temporary or transient. “My philosophy is to create beautiful pieces of elegant simplicity – which not only have longevity but true lasting value to the user. My Coffee Maker is designed with this in mind. It’s a multi-sensory product that is finished in appropriate materials that should endure whilst being enjoyable to use.”


Kávovar by Kamil Kurka

August 9, 2009

I tried to design coffee maker which is easy to use and maintain. My aim was to use smooth simple surfaces and only few buttons you really need. The whole shape of the coffee maker is based on spherical surfaces and their intersection, whereas the result evokes “sliced apple”. Contrast graphic applied on the body surface emphasize contours and control buttons.

The contrast surface can be done in luxury black & white glossy finish or in a range of colors that gives it modern and fresh look.

Metal plate equipped drip tray, attached to the body is removable for better cleaning.

On the front panel there are power, volume and froth control button. There is a release button for the drip tray and coffee capsule drawer on the side of the coffee maker. Water reservoir is accessible from the top of the machine.

This concept was hand sketched on the paper and then modeled in Rhinoceros, rendered in Vray.

I would be glad if I could continue on this project in cooperation with an manufacturer.