EP1363305A1

EP1363305A1 - Pressure switch

Info

Publication number: EP1363305A1
Application number: EP02425297A
Authority: EP
Inventors: Andrea Ambrosi
Original assignee: Bonferraro SpA
Current assignee: Bonferraro SpA
Priority date: 2002-05-15
Filing date: 2002-05-15
Publication date: 2003-11-19

Abstract

A pressure switch includes a container (1) closed by a cover (4) and inside which there is arranged a membrane (2), as well as a microswitch (5) mounted on the cover (4) so as to be activated upon rising of said membrane (2) on which there is also arranged a ballast (3) of ferromagnetic material which undergoes the attraction of one or more magnets (7) arranged on the cover (4). In this way the pressure switch retains the functionality of conventional pressure switches yet with a structure extremely simple to manufacture, which results in low manufacturing cost and high reliability, and makes simpler to adjust the intervention value and to set the restore value, allowing for an adjustment range of the latter which can even extend to zero.

Description

The present invention relates to pressure switches, and in particular to an improved pressure switch to be used in domestic appliances.
It is known that in modern domestic appliances (dishwashers, washing machines and the like) there is a tendency to minimizing the water consumption by introducing into the machine the minimum amount of water required for a correct operation thereof. Exactly due to the fact that the machine operates at the limit, it easily occurs during the washing step that the water level decreases below the minimum value required for a correct operation of the wash pump and/or of the heating element. This may occur, for example, due to the overturning of a container which becomes filled with water subtracted to the washing cycle in a dishwasher, or due to the formation of foam in a washing machine and so on.
The controlling and restoring of the water level in the tank are carried out by means of a pressure switch which opens and closes the water inflow electric valve. In practice, the electric valve is opened when the level decreases below the minimum sustenance value, and is closed just before the water reaches again the normal operating level. The anticipated closure takes into account a certain "inertia" of the system whereby the water continues to enter for some moments after the pressure switch has issued the command for closing the electric valve.
The drawback of this type of control is that repeated activations and deactivations of the system around the intervention value often occur, because when the pump starts again to operate at full running, and therefore with its normal flow rate, the water level in the tank decreases again below the intervention level. As a consequence several activations occur, each one with its own final "inertia" period, which globally imply an amount of water inflow greater than necessary and therefore an increase in the consumption.
This depends from the fact that normal pressure switches have a rather precise intervention value, but also have a restore value which is usually very close to the intervention value. In other words, the pressure switch closes the valve exactly at the required water level, but it is sufficient that said level decreases a little (even only 15-30 mm of water column less) for the pressure switch to re-open the valve to restore the water level.
A similar drawback occurs in case of failure of the primary pressure switch when the safety pressure switch must intervene. In fact the machine is usually provided with a second pressure switch to avoid that a failure of the primary pressure switch, e.g. the choking of the tube which allows the detection of the pressure in the air trap, causes a flooding due to the lack of intervention thereof.
In such an instance when the safety pressure switch detects an excessive water level in the tank it activates the drain pump but also in this case, similarly to the above-described activation of the wash pump, this causes a sudden drop in the pressure especially due to the fine-mesh filter on the tank bottom which hinders the passage of water. As a consequence, the initial pressure drop may reach 40-50 mm of water column, causing the stop of the drain pump and triggering therefore a series of activations and deactivations which jeopardize the operation of the safety pressure switch.
This makes necessary to use pressure switches with a restore value properly spaced from the intervention value, but such prior art pressure switches have the drawback of being quite complicated and expensive, requiring particularly sophisticated manufacturing techniques.
In fact these known pressure switches include a plurality of small resisting and biasing springs, strips, contacts and inlets for the passage of current since they make part of the electric circuit they control. Such a structure is quite delicate, in that an erroneous adjustment may cause the blocking of the pressure switch which in fact is usually sold with a factory setting that the user can not change.
Therefore the object of the present invention is to provide a pressure switch free from said drawbacks. This object is achieved by means of a pressure switch provided with magnetic means, preferably adjustable, to set the restore value. Other advantageous features of the present pressure switch are specified in the subsequent claims.
The main advantage of this pressure switch is exactly that of retaining the functionality of conventional pressure switches yet with a structure extremely simple to manufacture, which results in low manufacturing cost and high reliability.
Another advantage of the present pressure switch is that of making simpler to adjust the intervention value and to set the restore value, allowing for an adjustment range of the latter which can even extend to zero.
Further advantages and characteristics of the pressure switch according to the present invention will be clear to those skilled in the art from the following detailed description of an embodiment thereof, with reference to the only drawing annexed as fig. 1, wherein the pressure switch is shown in a diagrammatic vertical sectional view.
With reference to said figure, there is seen that the pressure switch according to the invention conventionally includes a container 1, which receives the water, inside which there is arranged a membrane 2 water-tightly secured by means of a cover 4.
The novel aspect of this pressure switch is the presence on cover 4 of one or more magnets 7 and a microswitch 5, which is activated by a rod 6 moved by membrane 2 passing through a corresponding hole in cover 4, as well as the presence of a ballast 3 of ferromagnetic material arranged between membrane 2 and rod 6.
As it is readily understood, magnets 7 attract ballast 3 and therefore oppose the action of its weight more and more as it gets close to cover 4. In other words, once the weight of ballast 3 has been selected according to the pressure established for the intervention value of the pressure switch, the latter will operate as a conventional pressure switch during the circuit closure phase (closure of the inflow valve or activation of the drain pump), but then a greater pressure drop will be necessary in order for the weight of ballast 3 to overcome the action of magnets 7 and cause the lowering of rod 6 and therefore the re-opening of the circuit.
In practice there is an "hysteresis" of the restore value which is greater than the pressure drop caused by the activation of the wash or drain pump, whereby the pressure switch re-opens the electric valve or stops the pump only when it is actually necessary.
It is obviously possible to change the restore value of the pressure switch by acting on the magnetic attraction force in different ways: it is possible to replace magnets 7 with other magnets of higher or lower intensity, or to change the number of magnets or simply to mount them in such a way that their distance from ballast 3 is adjustable (e.g. with a fine-pitch screw). It is even possible to use an electromagnet instead of permanent magnets, whereby the attraction force is adjustable as desired by changing the supply voltage.
It should be noted that by changing said force the restoring can take place at lower and lower pressure values, arriving at the limit of a force greater than the weight of ballast 3 so as to obtain a manual restoring pressure switch. This is clearly impossible to achieve with prior art pressure switches in which the width of the adjustment range of the difference between the intervention value and the restore value does not exceed 20% of said difference (e.g. 20 mm of water ± 4 mm).
It is clear that the above-described and illustrated embodiment of the pressure switch according to the invention is just an example susceptible of various modifications. In particular, the exact shape and arrangement of the elements may be somewhat changed, for example by providing the integration of rod 6 in ballast 3 or any other way to activate microswitch 5 when membrane 2 rises.

Claims

A pressure switch including a container (1) closed by a cover (4) and inside which there is arranged a membrane (2), characterized in that it further includes a microswitch (5) mounted on said cover (4) so as to be activated upon rising of said membrane (2), as well as a ballast (3) of ferromagnetic material arranged on the membrane (2) and one or more magnets (7) arranged on the cover (4).
A pressure switch according to claim 1, characterized in that said one or more magnets (7) are permanent magnets mounted in a removable way.
A pressure switch according to claim 1 or 2, characterized in that said one or more magnets (7) are permanent magnets mounted so as to make adjustable their distance from the ballast (3).
A pressure switch according to claim 1, characterized in that said one or more magnets (7) are electromagnets provided with a device for adjusting the supply voltage.
A pressure switch according to one or more of the preceding claims, characterized in that the microswitch (5) is activated by a rod (6) arranged on the ballast (3).
A pressure switch according to one or more of the preceding claims, characterized in that the membrane (2) is water-tightly secured between the container (1) and the cover (4).