EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: darrenb on May 04, 2012, 09:36:50 pm
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Hi,
Just wondering how my electric kettle knows when the water has boiled and to turn off? What kind of sensor or mechanism does it use?
Here in Australia it's known as a kettle but it may be known as a jug or water boiler or something else elsewhere. Anyway you put some water in it, plug it in to the wall turn it on and it boils water. How the ? does it sense when the water has boiled and know to turn itself off?
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When the water boils steam flows along a tube and heats up a temperature sensor like a bimetallic strip, which then trips off the power.
(If the tube gets clogged up with scale the steam may not reach the sensor and the kettle will gradually take longer and longer to switch off.)
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The bimetallic strip is explained here
http://en.wikipedia.org/wiki/Bimetallic_strip (http://en.wikipedia.org/wiki/Bimetallic_strip)
David.
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As with most appliances it must also have a failsafe system. That is typically a thermal fuse (a fuse set to blow at a specific temperature). The contact points can fuse together making it so that the heating element never turns off. In this case the thermal fuse will blow after the kettle has boiled dry. The boiling water keeps the fuse temperature below the trip point, when dry the temp rises it blows and your house has not burnt down.
Thermal fuses are often crimped into place because soldering them will kill them.
...mike
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Thank you.