EP0689687A1

EP0689687A1 - Method and apparatus for interrupting current supply to an electric load

Info

Publication number: EP0689687A1
Application number: EP93906642A
Authority: EP
Inventors: Erkki Telamo
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-03-19
Filing date: 1993-03-19
Publication date: 1996-01-03
Also published as: WO1994022065A1

Abstract

The invention relates to a method and apparatus for interrupting current supply to an electric load (L), such as an electric cooker. In the method the power supply (I) to the load (L) is interrupted by a switching means (1) controlled by a timer (5) on the basis of a preset time limit. In the method of the invention, the flowing of the current (I; Iv) to the load is observed by means of a current transformer (2); the timer (5) is activated when the current supply (I) begins, and the current supply is interrupted by opening the switching means (1, k1) after the preset time period has elapsed; the flowing of the leakage current (Iv) through a shunt circuit (9) arranged in parallel with the switching means (1) is observed after the load current supply (I) has been interrupted, in order to detect a disconnection proper; and after the disconnection the switching means (1) is closed in order to facilitate normal current supply to the load.

Description

METHOD AND APPARATUS FOR INTERRUPTING CURRENT SUPPLY TO AN ELECTRIC LOAD

The invention relates to a method for inter- rupting current supply to an electric load, defined in the introductory section of patent claim 1.

The invention also relates to an apparatus for realizing the method, defined in the introductory section of patent claim 2. In the prior art there are known methods and apparatuses for interrupting current supply to an elec¬ tric load, particularly to an electric device such as an electric cooker or the like, wherein the electric device is provided with a timer-controlled switching means, and a time limit is set manually to the said switching means. The switch is opened, and thus the current supply into the load is interrupted after the preset period of time has elapsed, unless the power supply is already cut off by using a switching means provided in connection with the load.

A drawback with the known method and appara¬ tus is that the time limit must be set always when the load is meant to be connected to the mains or to some other source of electricity. Moreover, the timer must be manually reset to zero before it is used again. This leads to the risk that the timer might not be used every time when the load is connected to a current supply. Thus the significance of the timer in control¬ ling the load is remarkably weakened. An object of the invention is to provide a novel and improved method for interrupting current supply to an electric load, and an apparatus according to this method. The purpose of the invention is to eliminate the above mentioned problems and to introduce a novel, automatically operated control system particu¬ larly for controlling electric devices that are used in homes, such as electric cookers. The method of the invention is characterized by the novel features enlisted in the appended patent claim 1.

In the method of the invention for interrup- ting current supply to an electric load, such as an electric cooker, in which method the current supply to the load is interrupted by using a switching means, controlled by a timer, on the basis of a preset time limit, unless the current supply is already interrupted by disconnecting the load altogether; in the said met¬ hod, the flow of current to the load is observed, the timer is activated when the current supply to the load begins, and the current supply is interrupted by ope¬ ning the switch after the preset time limit has elap- sed. According to the invention, the method further includes observation of how leakage current flows thro¬ ugh a shunt circuit arranged in parallel with the swit¬ ching means after the load current supply has been interrupted, in order to detect whether a disconnection proper has taken place; and after the disconnection, the switch is closed in order to allow normal current supply to the load.

The apparatus of the invention is characteri¬ zed by the novel features enlisted on the patent claim 2.

An advantage of the invention is that it provides a simple and reliable method for interrupting current supply to an electric load. Preparatory measu¬ res are not necessarily needed, but the control is carried out automatically. This is an extremely impor¬ tant advantage particularly with electric cookers, for instance in households where the cooker is used by old or handicapped people or children. Thus the cooker cannot get overheated, and serious consequences, parti- cularly fires, can be avoided.

The invention is explained below in more detail with reference to the appended drawing, which is a block diagram of an apparatus according to the pre¬ sent invention.

In connection with the electric load L, there is provided a switching means K, by intermediation whereof the load L can be connected to the mains in normal fashion via the fuse S. The load L is for ins¬ tance an electric cooker, one or several hotplates of the cooker, and/or an oven. The apparatus of the inven¬ tion is installed in between the load L and the fuse S, in series with the load L.

The apparatus of then invention comprises a relay 1 or a corresponding switching means, including at least one switch kl, a shunt circuit 9, a current transformer 2, an amplifier 3, a holding circuit 4, a timer 5 and a control circuit 8 of the relay. The shunt circuit 9 is coupled in parallel with the relay 1. The primary circuit 2¹ of the current transformer 2 is coup¬ led in series with the relay 1 and the shunt circuit 9. The secondary circuit of the current transformer 2 is coupled to the amplifier 3, and this further to the holding circuit 4. The holding circuit 4 is connected to the timer 5. The timer 5 in turn is connected to the relay control circuit 8, and this to the relay 1 in order to control the switch kl. The current transformer 2 is used to detect the current of the primary circuit 2¹, flowing via the fuse S from the power source, such as the mains, to the load L. The current is either normal load current I passing through the relay 1, or leakage current I_V<<I passing through the shunt circuit 9. By means of the amplifier 3, the current signal I' of the secondary circuit 2² is amplified and fed to the holding circuit 4. As long as even a slight current signal is detected, the output 4¹ of the holding circuit 4 contains a logi- cal "1" or corresponding signal which informs that current a I, ' IV flows from the mains to the load L. If the current transformer 2 does not detect any current signal at all,- the output 4¹ of the holding circuit 4 contains a logical "0" or corresponding signal, which informs that current is not supplied from the mains to the load L. By using these logical signals "1" and "0", the timer circuit 5 is controlled, and further the relay 1 and its switch kl are controlled, through the control circuit 8 of the relay.

In its simplest form, the shunt circuit 9 is either a condensator or a resistor. The purpose of the shunt circuit 9 is to provide a current path past the relay 1, when the current supply through the relay is prevented. Thus a slight current, i.e. a leakage cur¬ rent I , flows through the shunt circuit 9.

The timer 5 includes a clock circuit 6 and a calculator 7. A predetermined time limit is set in the timer 5, and in this case also in the calculator 7 - meaning the number of clock pulses corresponding to the said time limit. The holding circuit 4 is coupled at the output 4¹ to the first input 7¹ of the calculator 7. Respectively, the clock circuit 6 is coupled to the second input 7² of the calculator 7, wherethrough the clock pulses are fed into the calculator. On the basis of the control signal received from the holding circuit 4, which is either a logical one "1" or zero "0", the calculator either calculates or does not calculate the clock pulses coming from the clock circuit 6.

In addition to the relay 1, the control cir¬ cuit 8 of the relay 1 is connected to the clock circuit 6. The control circuit 8 stops the clock circuit 6 or cuts its connection to the calculator 7 when the switch kl of the relay 1 is opened.

The apparatus of the invention is operated as follows. At start, the switch kl of the relay 1 is closed. When the load L is connected, by means of the switch, to the power source via the fuse S, the infor¬ mation of the supply of current I to the load L is obtained through the current transformer 2, from its primary circuit 2 to the secondary circuit 2², and further via the amplifier 3 to the holding circuit 4, as was explained above. From the output 4 there is now received a control signal "1", which is fed to the input 7¹ of the calculator 7. The clock pulses of the clock circuit 6 are now fed via the input 7² to the calculator 7. A suitable time limit, for instance two hours, is preset in the calculator 7. When this time limit has elapsed, the calculator 7 informs the control circuit 8 to this effect, for instance by giving a pulse, and as a consequence the control circuit 8 is activated and directs the switch kl of the relay 1 to the open position. Now the supply of load current I from the current source via the fuse S to the load is interrupted.

The control circuit 8 of the relay also gives a signal to the clock circuit 6, and as a consequence the clock circuit 6 is stopped, or alternatively the flow of clock pulses to the input 7² of the calculator 7 is interrupted by means of a switch.

However, a leakage current I_v flows to the load L via the shunt circuit 9. The leakage current I_v is detected through the current transformer 2 in simi¬ lar fashion as the load current I proper, as was ex- plained above. The amplifier 3 amplifies the current signal received from the current transformer 2, and sends a message to the holding circuit 4. The output signal "1" remains in the output 4¹ of the holding circuit 4. However, the calculator 7 does not receive clock pulses from the clock circuit 6. Consequently changes do not take place in the mode of the calculator 7.

When the switch K connected to the load L is opened and the load is properly disconnected from the source of electricity, the supply of the leakage cur¬ rent I_v to the load L is stopped. Accordingly, the current transformer 2 does not send a current signal to the amplifier 3, in which case the signal fed from the amplifier 3 to the holding circuit 4 is roughly zero, or in general below the preset treshold value. As a consequence, the signal in the output 4¹ of the holding circuit 4 is changed to mode "0". Now the calculator 7 is reset to zero and the message of the resetting is received for instance as a control pulse in the control circuit 8, which further controls the relay 1, closing switch kl. Thus the relay 1 is returned to its starting position. At the same time the control circuit 8 also sends a message to the clock circuit 5, which is acti¬ vated or reconnected to the input 7² of the calculator 7. Now the apparatus is again ready for operation as soon as the current is switched to the load L by means of the switch K.

In connection with the timer 5, there can also be provided a multiplier for multiplying the pre¬ set time period. In the embodiment illustrated in the drawing, the multiplier 10 is connected to the input 7 of the calculator 7. Furthermore, the multiplier 10 is provided with a connected control key 11, and by pres¬ sing this key the multiplier gives a pulse to the cal¬ culator, thus doubling the preset time. If the control key is pressed several times, for instance four times, the preset time is multiplied, in this case by four. The use of the multiplier is useful when an electric load must for one reason or another be connected to the electric device for a longer duration than normally. A possible application is an electric oven. Baking times may sometimes extend to several hours.

In the above specification the invention is explained mainly with reference to one preferred embo¬ diment only. However, it is apparent that the invention can be realized in many different ways within the scope of the inventional idea defined in the appended patent claims. The different sections connected to the appara¬ tus can be compiled of known electronic components, which is obvious for anyone skilled in the art. For example the inputs 7¹ and 7² of the calculator 7 are connected to the inputs of suitable logical gate cir¬ cuits, and from their outputs there is received the desired control signal for the calculating unit proper of the calculator. The apparatus of the invention can also be used in connection with a three-phase load. In that case the current transformer 2 is most advanta¬ geously arranged in connection with all three phases, and the switches of the relay are respectively connec¬ ted to each phase, in order to be able to completely interrupt the current supply from the mains or other corresponding source of electricity as was described above.

Claims

PATENT CLAIMS

1. A method for interrupting current supply to an electric load (L) such as an electric cooker, in which method the current supply (I) to the load (L) is interrupted by a switching means (1) controlled by a timer (5) on the basis of a preset time limit, unless the current supply has already been interrupted by disconnecting the load proper, and in which method the supply of the current (I) to the load is observed, the timer (5) is activated when current supply (I) to the load (L) begins, and the current supply (I) is inter¬ rupted by opening the switching means (1, kl) after the preset time period has elapsed, c h a r a c t e r i- z e in that in the said method there is further observed the supply of leakage current (I ) through a shunt circuit (9) arranged in parallel with the swit¬ ching means (1) after the load current supply (I) has been interrupted, in order to detect a disconnection proper; and after the disconnection, the switching means (1) is closed in order to facilitate normal power supply to the load.

2. An apparatus for interrupting current supply to an electric load (L) such as an electric cooker, provided with a switch (K), the said apparatus comprising at least one relay (1), which is coupled in series with the load and the switch, a control circuit (8) of the relay, a timer (5) for controlling the relay according to the preset time limit, and means (3, 4) for activating the timer (5) when the load current supply (I) to the load begins, which current supply is interrupted by means of the relay (1) and the timer (5) after the preset time period has elapsed, c h a r a¬ c t e r i z e d in that the apparatus also comprises - a current transformer (2), with a primary circuit (2¹) coupled in series with the relay (1) and the load (L) , and a secondary circuit (2 ) which is coupled to the means (3, 4) for activating the timer;

- a shunt circuit (9), such as a condensator or a resistor, which is coupled in parallel with the relay

(i); - means (3, 4) coupled to the secondary circuit (2²) of the current transformer (2) for observing the leakage current (I_v) flowing through the shunt circuit (9) after the load current supply (I) is interrupted, in order to detect the opening of the switch (K) of the load (L), whereafter the relay (1) is closed and the timer (5) is set to its initial position in order to facilitate normal current supply to the load.

3. An apparatus according to claim 2, c h a r a c t e r i z e d in that the means for acti- vating the timer (5) when the current supply (I) to the load begins, and the means for observing the leakage current (I_v) flowing through the shunt circuit (9) comprise an amplifier (3) and a holding circuit (4) coupled thereto.

4. An apparatus according to claim 3, c h a r a c t e r i z e d in that the timer (5) comp¬ rises a clock circuit (6) and a calculator (7), and that a digital value corresponding to the predetermined time limit is set in the said calculator, and that to the first input (7^X) of the said calculator there is coupled the output (4¹) of the holding circuit (4), and to the second input (7 >)of the said calculator there is coupled the clock circuit (6), and that the calculator is controlled on the basis of the control signal "1" or "0" received from the holding circuit (4).

5. An apparatus according to claim 4, c h a r a c t e r i z e d in that in addition to the relay (1), the control circuit (8) of the relay (1) is connected to the clock circuit (6) in order to inter- rupt the operation thereof, when the switch (kl) of the relay (1) is opened.

6. An apparatus according to any of the claims 2 - 5, c h a r a c t e r i z e d in that the timer (5) is provided with a multiplier (10) in order to multiply the preset time period.