ELECTRONIC HOUSEHOLD APPLIANCE HAVING AN ELECTRONIC PROGRAM UNIT WITH AN INTERFACE DEVICE FOR CONTROLLING HIGHPOWER COMPONENTS

Abstract: An electric household appliance comprises an electronic program unit (24) adapted to control the operation of at least one electric high-power component (5) through an interface device comprising a controlled low-power circuit breaker (14) and at least one electric circuit breaker (10) connected in the energizing circuit (7,8) of the high-power component. A PCT (13) is connected in said energizing circuit in series with said controlled circuit breaker (14) and mounted in heat exchange relationship to a bi-metallic element (16) cooperating with the electric circuit breaker (10) for switching it between a first and a second operative position.

Description: The present invention relates to an electric household appliance such as a dishwasher, a laundry washing machine, a cooking oven or the like of the type having an electronic program unit for controlling the operation of the electric appliance. As generally known, such electronic program units usually comprise a microprocessor operating at low current value and thus requiring suitable interface devices enabling it to control high-power electrical compnents of the appliance such as an electric resistance heater. Commonly used interface devices include controlled power curcuit breakers such as TRIAC or SCR, although these components are undesirably expensive and only relatively reliable. It is therefore sometimes preferred to employ electromechanic relays under the control of low-power TRIAC or SCR components, which are much more reliable in operation, but also this solution appears unsatisfactory due to the complicated construction resulting therefrom, to the increased space requirements, and to the possibility of sticking of the electric contacts of the relay. It is therefore an object of the invention to provide an electric household appliance of the type having an electronic program unit capable of controlling the operation of high-power components through an interface device of simple construction, reliable operation and low cost. According to the invention this object is attained in an electric household appliance having an electronic program unit for controlling at least one electric high-power component through an interface device comprising at least one controlled low-power circuit breaker controlled by an output of the program unit, at least one electric circuit breaker connected in the energizing circuit of the respective high-power component and at least one thermistor in heat-exchange relationship to at least one bi-metallic element cooperating with said electric circuit breaker for switching it between a first and a second operative position. An electric household appliance of this type is characterized according to the invention in that said thermistor is arranged in series with said controlled circuit breaker and said high-power component is arranged in series with said electric circuit breaker, both series arrangements being connected across the terminals of said energizing circuit. The interface device is of a particularly simple construction and is preferably integrated in other devices, for instance, safety devices, provided in the electric household appliance according to the invention. The characteristics and advantages of the invention will become evident from the following description, given by way of example with reference to the accompanying drawings, wherein: fig. 1 shows a diagrammatic view of the principal components of an electrical household appliance for controlling a power circuit by means of an electronic program unit through an interface device according to an embodiment of the invention, fig. 2 shows a sectional view of a preferred embodiment of one of the components shown in fig. 1, and, figs. 3 and 4 show a modification of the component of fig. 2 in two different operative positions. With reference to fig. 1, the electric household appliance according to the invneiton comprises an electronic program unit generally indicated at 24 and operable to control the operation of the various low-power and high-power electric components of the appliance. For simplicity's sake, fig. 1 depicts only one of the high-power components of the appliance, in this case an electric resistance heater 5 of a washing machine. Heater 5 is secured to a wall 6 of the washing tub of the machine and adapted to be connected to the terminals 7,8 of an electric power supply through a main power switch 9 and an electric circuit breaker 10 connected in series therewith comprising a fixed contact 11 and a normally closed movable contact 12. As also shown in fig. 2, movable contact 12 of circuit breaker 10 is connected to one terminal of a thermistor 13, preferably of the positive temperature coefficient type (PCT), the other terminal of which is connected to terminal 7 through a controlled low-power circuit breaker 14. Circuit breaker 14 may for instance be a TRIAC of the type TLC 381 S adapted to a maximum current value of about 1 A. The control electrode of circuit breaker 14 is connected to a control output terminal 15 or program unit 24 for the supply of low-current control signals. Thermistor 13 and TRIAC 145 are thus connected in series between terminals 7 and 8 in parallel to the series-­ connencted resistance heater 5 and circuit breaker 10, and through main power switch 9. Thermistor 13 is mounted in heat-transmitting contact with a bi-metallic element 16 mechanically coupled to movable contact 12 of circuit breaker 10 through an actuating plunger 17 of an insulating material. In particular, bi-metallic element 16 is adapted to be deformed on attaining a predetermined temperature so as to switch circuit breaker 10 from its normally closed state to its open state. With the aim of a reduction of the number of components and their integration, circuit breaker 10 preferably forms part of another per se known component already provided in the electric household appliance. As shown for instance in figs 1 and 2, circuit breaker 10 may be part of a thermostatic overheat protection device comprising a further bi-metallic element 18 mounted in heat-transmitting contact with tub wall 6 in proximity to resistance heater 5. In the same manner as bi-metallic element 6, bi-metallic element 18 is mechanically coupled to movable contact 12 of circuit breaker 10 by means of an actuating plunger 19 of an insulating material extending substantially parallel to plunger 17. Bi-metallic element 18 is likewise adapted to be deformed as the predetermined safety temperature is attained for switching curcuit breaker 10 from its nomally closed state to the open state. Finally, a control input terminal 20 of electronic program unit 24 is connected in a per se known manner to a sensor 21 adapted to detect the internal temperature of tub 6. Operation of the appliance is started by closing main power switch 9 for energizing resistance heater 5. At the same time program unit 24 produces a signal at output terminal 15 for rendering TRIAC 14 conductive, whereby thermistor 13, which is originally cold and has therefore a low resistance value, is rapidly heated by the current flowing therethrough, and transmits the thus generated heat to bi-metallic element 16. This results in element 16 being deformed in a short time of about 3 seconds, so as to open circuit breaker 10 by means of plunger 17. as a result, the electric power supply to heater 5, thermistor 13 and TRIAC 14 is interrupted after a negligibly short period of time during which heater 5 had been energized, while the resistance of thermistor 13 rises with its temperature, until it acts as an open switch. Susequently, resistance heater 5 is in the state of being energized for heating the liquid contained in tub 6 after bi-metallic element 16 and thermistor 13 have cooled down. the cooling-down of the these two elements 13 and 16, and the resultant re-deformation of bi-metallic element 16 requires a relatively long time in the order of several tens of seconds, whereby the operation of the appliance is not, however, unduly influenced. When program unit 24 by the action of sensor 21 determines that the predetermined internal temperature of tub 6 has been attained, it generates in a per se known manner a control signal at output terminal 15 for rendering TRIAC 14 conductive. As described above, the resultant rapid heating of thermistor 13 and thus bi-metallic element 16 causes circuit breaker 10 to be opened for de-energizing resistance heater 5. In the case that overheating occurs within tub 6 or in the vicintiy of thermistor 13, bi-metallic element 18, which is suitably calibrated to be deformed at a higher temperature than bimetallic element 16, is deformed so as to open circuit breaker 10 by the action of plunger 19 for de-energizing heater 5. In this context it is to be noted that the mechanical forces exerted by bi-metallic elements 16 and 18 through the respective plungers 17 and 19 on circuit breaker 10 is sufficient to overcome the tendency of electric contacts 11, 12 of the circuit breaker to "stick". The above description clearly shows the operational reliability of the electric household appliance according to the invention, which is provided with an interface device for the control of power-operated components, including elements 10 to 14 and 16, 17. These elements are readily assembled, require little space, are inexpensive and are readily integrated in a unitary structure as shown in fig. 2. It is also to be noted that the only electronic component of the interface device is a controlled low-power circuit breaker 14 which is known to be highly reliable. The electric household appliance under consideration may, of course, be of any type other than the one described, and may undergo various modifications within the scope of the invention. The electronic program unit 24 may thus be designed to generate the control signal at its output terminal 15 at any other time, and the interface device may be designed in the manner shown in figs 3 and 4, wherein identical components are designated by the same reference numerals as above. According to this modification, contact 11 of electric circuit breaker 10 is resiliently deformed to the open state as shown in fig. 3, but is normally retained in the closed state of circuit breaker 10 by the action of actuating plunger 19 (fig. 4). Plunger 19 cooperates with a per se known safety button 22 itself biased by a resilient element 23 towards a projecting position (fig. 3). Likewise in a per se known manner, safety button 22 is adapted to cooperate with a loading door of the electric appliance. In particular, when the door is closed, safety button 22 is depressed as shown in fig. 4, while with the door open, resilient member 23 maintains safety button 22 in its projecting position shown in fig. 3. As a result, circuit breaker 10 is opened not only when thermistor 13 and bi-­ metallic element 16 (acting on contact 12) are heated, but also when the door of the electric appliance is opened. That is because, in this case, contact 11 is permitted to assume the resiliently deformed configuration shown in fig. 3 for opening the circuit breaker even when thermistor 13 and bi-­ metallic element 16 are below the predetermined operating temperature. As also shown in figs. 3 and 4, the main components of the interface device can also be fully integrated in an electric appliance having additional electric circuit breakers similar to the one indicated at 10. A suitable support structure may thus carry an additional circuit breaker 25 having contacts 26 and 28 and mounted parallel to circuit breaker 10 thereabove or below. In this case, a further insulating actuating plunger 27 is freely slidable in a guide bore 29 for transmitting the movements of bi-metallic element 16 to normally closed contact 28 via actuating plunger 17 and movable contact 12. Circuit breaker 25 thus operates simultaneously with circuit breaker 10 and my suitably be employed for controlling the energization of another high-power component of the electric appliance or for interrupting the connection to the other terminal of electric resistance heater 5.

Claims: 1. An electric household appliance having an electronic program unit for controlling at least one electric high-power component through an interface device including at least one controlled low-power circuit breaker controlled by said program unit, at least one electric circuit breaker connected in the energizing circuit of said high-power component, and at least one thermistor (13) disposed in heat exchange relationship to at least one bi-metallic element (16) cooperating with said electric circuit breaker (10) for switching it between a first and a second operative position, characterized in that said thermistor (13) is arranged in a series with said controlled circuit breaker (14) and said high-power component (5) is arranged in series with said electric circuit breaker (10), both series arrangements being connected across the terminals (7,8) of said energizing circuit. 2. An electric household appliance according to claim 1, in which said high-power component comprises a resistance heater for heating a treatment chamber, characterized in that a further bi-metallic element (18) is disposed in heat exchange relationship to said chamber (6) and adapted to cooperate with said electric circuit breaker (10) for switching it between said first and second operative positions. 3. An electric household appliance according to claim 1, in which said electric circuit breaker is of the normally closed type in its first operative position and includes a first and a second electric contact, characterized in that said first electric contact (12) cooperates with said bi-metallic element (16), while actuator means (22,23,19) associated to a loading door of the appliance cooperate with said second electric contact (11) for permitting it to assume a position opening said electric circuit breaker (10) when said door is in the open position.

Inventors: MILOCCO, CLAUDIO
Applicants: INDUSTRIE ZANUSSI S.P.A.
Application year: 1987
Main classification: G05D23
IPC codes: G05D23, H01H37, H05B1, H01H61