The present invention relates to a switch circuit for an electric tool, for instance a circular saw, right-angle grinder, fretsaw or a drill, with a motor and operating means for selectively connecting the motor to a power source, wherein the motor comprises at least one field winding and an anchor. It is otherwise noted that the present invention also relates to such a tool to the extent that a switch circuit according to the present invention is embodied therein. Many possibilities and solutions have been realized in the art to bring a motor to a rapid stop when it is switched off, for instance by releasing an on/off switch. Release of the operating means, such as such a switch, of a switch circuit must result, for safety reasons, in the moving parts of the electric tool, of which the switch circuit forms part during use, being brought to a rapid stop.

The proposed solutions and/or measures, to the extent they are known in the art, are all inadequate and result in large, complex and costly circuits intended to bring the motor to a rapid stop, and therefore also the moving parts such as a drill, a saw-blade and so on. The invention has for its object to provide an improved switch circuit, with the particular object of being able to bring about rapid stopping or braking of the motor, in a simple design which is preferably also cost-saving compared to previously proposed solutions and measures, wherein a switch circuit according to the present invention is distinguished in that the switch circuit comprises a loop for closing selectively in correspondence with the operating means and comprising

the field winding and the anchor, wherein the operating means comprise switch means for switching between closing of the loop, wherein the motor is disconnected from the power source, and connecting of the motor to the power source.

When the operating means are operated by a user to deactivate the tool, and more particularly the motor thereof, the switch means are actuated to close the loop. The motor is here therefore disconnected from the power source and further driving of the motor does not take place. By closing the loop a current will also begin to run therethrough, whereby the motor is braked quickly and effectively. The configuration provided for this purpose according to the present invention is outstanding in simplicity and elegance and in these respects is therefore far superior to previously proposed solutions and measures for braking the motor. In a preferred embodiment the loop comprises two conductors, each of which are arranged between the anchor and the field winding in a closed state of the loop. A current braking the motor can thus begin to run through the loop when the loop is closed by the switch means, this current coming from the field winding or also from other components of the motor. It is however noted here that loops consisting of fewer than two conductors, i.e. one conductor, are also possible within the scope of the present invention.

The loop preferably comprises a resistance in a loop section which, when the motor is connected to the power source, is disconnected by the switch means. Such a resistance or other electrical component can serve to dimension particularly the loop of the switch circuit, without this electrical component then having any influence on the electric power to be provided to the motor in operative state, since such an electrical

component is then disconnected. Such an electrical component therefore only has an effect in a situation where the loop which brakes the motor is closed.

In a preferred embodiment the switch means can comprise at least two switches for selective closing of the loop. Although this produces an acceptable increase in the complexity of the switch circuit according to the present invention, such an embodiment is still always preferred to a non-limitative method because the switch circuit is kept compact and well-ordered and can be particularly favourable in combination with an embodiment where the loop comprises two conductors which are arranged, also in the closed state of the loop, between the anchor and the field winding. In another preferred embodiment, the switch circuit according to the present invention preferably comprises a current or voltage limiter, such as a diode or a thyristor, having parallel thereover a short-circuit switch to be closed selectively in correspondence with the operating means. The limiter and the short-circuit switch, which are arranged in parallel to each other, are then preferably together in series with the motor. A very advantageous influence can hereby be exerted on the switching characteristics. Similar considerations apply when a rectifier is applied which can likewise be embodied as an SCR-ELO or a thyristor with a short- circuit switch parallel thereover. In a preferred embodiment thereof, when the motor is set into operation the short-circuit switch can exhibit a delay for short- circuiting the current or voltage limiter relative to the connection of the motor to the power source with the switch means. When the motor is set into operation, the drive current for the motor then runs through the current or voltage limiter so as to form a starting current limitation and not suddenly load the motor with

the full maximum drive current or voltage. Once the delay has ended, the short-circuit switch can be closed, whereafter the full current, which has previously run through the current or voltage limiter, can flow along the short-circuiting means. Only then is the motor loaded with the full drive current or voltage. It is further the case that when the motor is deactivated the switch means can exhibit a delay for opening or disconnecting the loop relative to opening of the short- circuit switch. Particularly with a rectifier the advantage is hereby gained that for a temporary short period during the preselected delay a direct current will run through the switch circuit having therein, among other parts, field windings of the motor, whereby the lamination core of the motor is magnetized in one direction. When the loop which brakes the motor is then closed and the voltage source disconnected from the motor, as a result of the magnetization of the lamination core that is still present a current will begin to flow through the motor to cause rapid braking of the motor. The effect of the present invention is thus reinforced to a considerable extent.

The present invention will be elucidated on the basis of the description hereinbelow of several non- limitative embodiments thereof, which are shown in the accompanying drawings and in which the same or similar components are designated with the same reference numerals, and in which: fig. 1 shows a partly cut-away side view of an electric hand drill as possible embodiment of the present invention with a switch circuit therein; fig. 2A and 2B each show schematically the same switch circuit according to the present invention in two different states; and

fig. 3 shows a graph of the switch-on characteristics of the switch circuit according to the present invention shown in fig. 2A and 2B.

Fig. 1 shows a hand drill 1. Hand drill 1 comprises a housing 2 with the usual components therein, such as a motor 3 and an operating switch 4 as embodiment of operating means for selective connection of motor 3 to an electric power source, i.e. via a cord 5. This takes place via a switch circuit 6 to be described hereinbelow and a possible embodiment of which is shown fig. 2A and fig. 2B.

Figures 2A and 2B show the same switch circuit in two different states thereof. Switch circuit 6 is incorporated in motor 7, which motor 7 comprises an anchor 8 and a first field winding 9 and a second field winding 10. Such a configuration of a motor 7 is usual, but the invention lies in the provision of switch circuit 6.

Switch circuit 6 comprises a first switch 11 and a second switch 12, in addition to a first conductor 13 and a second conductor 14. First switch 11 and second switch 12 can be correspondingly switched using operating switch 4, wherein the switching characteristics of switches 11, 12 is indicated schematically with double arrows A.

In the state of switch circuit 6 shown in fig. 2A, first switch 11 and second switch 12 are brought into a position corresponding to connection of motor 7 to a power source (not shown) . This therefore shows the normal operation of motor 7.

First switch 11 and a second switch 12 have however been placed very recently in the shown positions thereof relative to the state shown in fig. 2A in order to switch on the motor 7. A current has here just begun to flow through the first field winding 9, anchor 8, the

second field winding 10 and then subsequently via second switch 12 to a current or voltage limiter designed as thyristor 15. This thyristor 15 ensures that, when the electric hand drill 1 is switched on at time tl in fig. 3, a current lower than the maximum is allowed through thyristor 15, whereby the current through the first field winding 9, anchor 8 and the second field winding 10 also remains limited. A limitation of the starting current is thus realized. Switch circuit 6 also comprises a short-circuit switch 16 which is parallel over thyristor 15 and can switch into the positions indicated with double arrow B. Shortly after reaching the state shown in fig. 2A, i.e. after a delay which suffices to bring about an effective and efficient limitation of the starting current, short- circuit switch 16 is closed at time t2 in fig. 3. The maximum current or voltage through or over motor 7 then becomes possible and motor 7 begins normal operation at a maximum load. When a user wishes to end operation of the motor and uses operating switch 4 in fig. 1 accordingly, the intention is to stop motor 7 quickly. Operation of the operating switch 4 for the purpose of stopping motor 7 results first in short-circuit switch 16 being opened. As a consequence hereof a current once again runs through thyristor 15, which serves not only as current or voltage limiter but also as rectifier. A direct current of relatively low value thus begins to run through the first field winding 9, anchor 8 and the second field winding 10, whereby the lamination core of the motor is magnetized in one direction. After a short delay relative to opening of short-circuit switch 16, first switch 11 and second switch 12 are also brought into the positions thereof shown in fig. 2B to disconnect motor 7 from the power source, which is not

shown, and to bring about a loop formed by anchor 8, first conductor 13, the second field winding 10 and second conductor 14. The magnetic field in the second field winding 10 results in the state of switch circuit 6 shown in fig. 2B in a current through the thus formed loop, which current has a braking effect on motor 7.

It is thus found that thyristor 15, in combination with short-circuit switch 16, advantageously fulfils a dual function which is favourable in both the switch-on characteristics and switch-off characteristics of motor 7.

It is noted that a resistance 17 is arranged in the second conductor 14. It is hereby possible to bring about a good adjustment of the components and conductors forming part of the loop which is formed in the state of switch circuit 6 shown in fig. 2. Optimal switch-off characteristics and braking effect of motor 7 can thus be hereby realized. However, as shown with a broken line, a switch circuit can also be provided without a resistance 17 therein as part of second conductor 14, and it is noted that electrical components other than a resistance can also be applied.

It is thus the case that diverse alternative and additional embodiments are possible, and will occur to the skilled person, particularly after perusal of the foregoing description in which reference is made to the accompanying figures. Such alternative or additional embodiments must then not be deemed as embodiments of the present invention if they vary considerably from the definitions of the scope of protection for the present invention as stated in the appended claims, or from the spirit thereof.