Engineering:Infinite switch
An infinite switch, simmerstat, energy regulator or infinite controller is a type of switch that allows variable power output of a heating element of an electric stove. It is called "infinite" because its average output is infinitely variable rather than being limited to a few switched levels. It uses a bi-metallic strip conductive connection across terminals that disconnects with increased temperature. As current passes through the bimetal connection, it will heat and deform, breaking the connection and turning off the power. After a short time, the bimetal will cool and reconnect. Infinite switches vary the average power delivered to a device by switching frequently between on and off states.[1] They may be used for situations that are not sensitive to such changes, such as the resistive heating elements in electric stoves and kilns.
History
An early switch operating by this principle was invented by Chester I. Hall of the General Electric Company, with a patent filed in 1921 and approved in 1924. Like the modern infinite switch, Hall's invention used a bi-metallic strip, heated by a constant current, to break a connection after a given period of time. It also used a rotatable cam attached to a knob to control duration. However, unlike its successors, Hall's invention was not self-resetting and would only stay on for a single cycle, after which a manual reset button could be pressed. According to the patent, the intended use was to control the timing of exposures in medical radiography.[2]
The infinite switch itself was described in a 1975 patent of George F. Esker Jr. and Otto J. Cousins of the Harper Wyman Company. This switch would reset itself as the bi-metallic strip cooled, provided a cycle of calibrated opening and closing. It also provided an indicator light that remained on as long as the knob was not in the closed position. This switch was designed for use in electric stoves.[3]
Technical details
A rotary hand control turns a shaft that is connected within the infinite switch to a cam and follower. At the off position both power line connections are interrupted. At all on positions the manual contacts connect one of the line terminals to the pilot lamp terminal and one of the terminals for the heating element being controlled. At the maximum heat position the cam follower applies sufficient force for the cycling contacts to remain closed at all times. At the other positions the cam follower applies less force allowing the cycling contacts to cycle. Initially, the cycling contacts close in all positions and a permanent magnet pulls them to remain closed. One of the two cycling contacts energizes the controlled heating element and the other energizes a very small electric heating element attached to a bimetallic strip. The force from the heated bimetallic strip increases as its temperature rises until it overcomes the combined force from the cam follower, an unheated bimetallic strip that compensates for ambient temperature, and the permanent magnet allowing the cycling contacts open which interrupts the electric current to both heating elements. Both the bimetallic strip and the "burner" begin to cool. Eventually the heated bimetallic strip will no longer have enough force to overcome the force from the cam follower and the unheated bimetallic strip. As the contacts approach each other the permanent magnet attracts the contact beam rapidly closing the contacts and the cycle repeats.[4]
Alternatives and similar devices
See also
References
- ↑ FCS Electrical Workmanship L4, Pearson South Africa, 2009, ISBN 177025370X, page 96
- ↑ Chester I. Hall, "Thermal responsive switch", US patent Expired 1487515A, issued 1924-3-18, assigned to General Electric Co
- ↑ & Otto J. Cousins"Infinite switch and indicator" US patent Expired 4052591A, issued 1977-10-04, assigned to Harper Wyman Co
- ↑ "This Is How The "INF" Infinite Control Operates". RobertShaw. http://www.appliance411.com/faq/how-infinite-heat-switch-works-C107.pdf.
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