Kettle automatic switch. Scheme, description

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Kettle automatic switch. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches

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Modern electric kettles automatically disconnect from the mains when water boils in them. A similar service can be introduced into a conventional electric kettle, if it is supplemented with a simple device, which is described in this article.

As a water boiling sensor, a simple device is used (Fig. 1), consisting of a flexible tube and a temperature-sensitive element placed inside it on one side. The other side of the tube is inserted into the hole in the lid of the kettle so that heated air and steam get into it. At the moment of boiling water, their flow inside the tube increases, which is fixed by a temperature-sensitive element.

Kettle automatic switch

A germanium diode VD1 (Fig. 2) with its strong dependence of the reverse current on the heating temperature was used as a temperature-sensitive element. When the diode case is cold, its reverse current is small and the trigger, assembled on transistors VT1 - VT3, is in its initial state, in which the transistor VT1 is closed, and VT2 and VT3 are open. The open state of the transistor VT3 triggers the triac VS1, as a result of which the heating element of the kettle is connected to the network. When water boils, the temperature inside the tube rises, which leads to an increase in the reverse current of the diode VD1, as a result, the transistor VT1 opens and the trigger switches to the opposite state - the transistor VT1 is open, and VT2 and VT3 are closed. This leads to locking the triac VS1 and disconnecting the kettle from the network.

Kettle automatic switch

To turn it on again, turn off the device for 5...10 s and turn it on again. Capacitor C1 suppresses possible interference in the temperature sensor circuit, and also ensures that the trigger is set correctly when the power is turned on.

The sensor is made of a flexible thick-walled tube of insulating material, it has a length of 120...150 mm and an inner diameter of 6...8 mm. One side of the tube is inserted into a slightly enlarged hole in the lid of the kettle, designed to release steam. The VD1 diode with pre-soldered wires for sealing is dipped into epoxy resin and, after it hardens, is fixed inside the tube at a distance of 15 ... 20 mm from its opposite side (for example, glued to the wall). It is desirable to twist the conductors coming from the diode. The diameter of the tube must ensure the free passage of steam through the tube when the diode is installed in it. There are other options for installing the sensor, but you need to make sure that it works only after the water has completely boiled (it is controlled by a change in the reverse current of the diode).

Diode VD1 must be germanium, for example, from the D2 or D9 series with any letter index. The permissible load power is determined by the triac VS1 used - for a device according to the above diagram, if the triac is installed on a radiator, it is 1100 watts. If necessary, you can use a more powerful triac, while you will probably have to select the resistor R4 in accordance with the triac control current, and also replace the VT3 transistor with a more powerful one. Resistors and capacitors can be of any type.

The device is assembled in a small case made of insulating material. When adjusting it, it is necessary, by smoothly moving the slider of the resistor R1 from the lowest (according to the diagram) position, to achieve the activation of the load. After that, the resistor slider is slightly moved in the opposite direction. It is also advisable to check the current through the zener diode VD2, which, with the transistor VT3 open, should be 25 ... 35 mA. Finally, the response threshold is adjusted by the moment the load is turned off when the water boils.

The water level during operation must ensure that the openings connecting the main tank of the kettle with its spout are blocked.

It must be remembered that all elements of the device are under mains voltage, so the elements of the sensor circuit and the device as a whole must have reliable insulation. It is also necessary to observe safety precautions when setting up the device.

Author: V.Zaitsev, St. Petersburg

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