EP2845215B1 - Electromagnetic relay with shorter switching time - Google Patents

Description

The invention relates to a switching arrangement with an electromagnetic relay, which has a relay coil and switching contacts, with a voltage source connected to the voltage source and with a drive circuit which is adapted to drive the relay coil by means of a voltage provided by the voltage source.

In electrical devices, electromagnetic relays are often used to perform controlled switching operations. Electromagnetic relays usually consist of a relay coil and at least one pair of electrical switching contacts. If the relay coil flows through an electric current, a magnetic field is generated around the relay coil, whereby - in so-called self-opening relay - a closing of the relay contacts is effected, so that a current flow through the relay contacts is possible. If the current flowing through the relay coil current is interrupted again, the movable part of the relay contacts, for example by means of a spring device, moved back to its original position, causing an opening of the relay contacts and interrupts the flow of current through them. For self-closing relays, the contacts are closed in the de-energized state of the relay coil and opened in the current-carrying state.

Electromagnetic relays are usually used where by means of a comparatively small control current from a drive circuit, a comparatively larger current in a switching circuit should be switched on or off, and / or where between the drive circuit and the switching circuit galvanic isolation is to be achieved. The electromagnetic relay in this case forms the galvanic decoupling of the drive circuit and the switching circuit.

Electromagnetic relays are used, for example, in electrical protective devices for monitoring electrical energy supply networks in order to trigger a tripping of an electrical circuit breaker in the event of a fault in the electrical energy supply network by closing the relay contacts of a so-called "command relay", thus interrupting the fault current. Another use of electromagnetic relays in protective devices is given in so-called binary outputs, where by switching on and off of relays binary communication signals with high signal level (binary "1") or low signal level (binary "0") can be generated.

In some applications, the requirement is placed on an electromagnetic relay, that in the case of a current flow through the relay coil has the shortest possible response time, so very quickly a switching action of the switching contacts of the relay is triggered. In addition to the shortest possible turn-on time, a fast turn-off can also be important. For example, these requirements are placed on relays used for binary outputs of electrical protection or control devices because such binary outputs are used to communicate information to other devices, e.g. additional protection or control devices are used, and the signal propagation time should be kept as short as possible. Therefore, the time from the activation of an electromagnetic relay to the final closing of its switching contacts must be as short as possible. Typical switch-on and switch-off times of electromagnetic relays are currently between about 5 and 15 milliseconds. Heretofore, in order to realize a short switching time of e.g. 7 to 8 milliseconds to relatively expensive relays with high-quality components are used.

For the realization of an electromagnetic relay with the shortest possible response time, it is also for example from the German patent application DE 102 03 682 A1 known, parallel to the switching contacts of the electromagnetic Relay to use a semiconductor switch, which has a very fast response time due to the lack of mechanical moving parts and can ensure the production of a current flow until the final closing of the switching contacts of the electromagnetic relay. Such a semiconductor switch must be designed in this case to be able to carry a comparatively high current, since the entire current of the switching circuit must flow until the closing of the switching contacts of the relay via the semiconductor switch.

Further disclosed DE 100 03 531 A1 a switching arrangement for switching an inductive load with movable parts, with an inductive load (2) having a coil with a voltage source connected to the coil and with a drive circuit (3, 4) for driving the coil by means of a Voltage source provided voltage is set up; wherein the drive circuit (3, 4) is adapted to apply an excessive turn-on voltage (Um) to the coil to turn on the inductive load (2), the turn-on voltage being greater than the nominal voltage (Un) of the inductive load; wherein the drive circuit (3, 4) is set up to apply a lower normal voltage (Uh) to the coil during the switch-on process of the inductive load instead of the switch-on voltage, compared to the switch-on voltage (Um), wherein to determine a switchover time, to which the switching to the normal voltage takes place, a coil current flowing through the coil is used; wherein in the current path of the coil, a current detecting means (1) is provided which is adapted to detect a coil current flowing through the coil and for outputting a coil current indicating current signal (6) to the drive circuit (3, 4); and wherein the drive circuit (3, 4) for determining the switching time (T1), at which the switching from the turn-on voltage (Um) to the normal voltage (Uh) takes place, the current signal (6).

The invention has for its object to provide a way with which a switching arrangement of the type mentioned in structurally simple structure with a shortest possible switching time can be operated.

This object is achieved by a switching arrangement according to claim 1.

The particular advantage of the invention is that a significantly shorter turn-on time can be achieved with the drive circuit described. For this purpose, the Erdfindung makes use of the knowledge that the magnetic field of the relay coil is built much faster than by applying the bare nominal voltage by the application of a significantly excessive turn-on - and an associated faster rising coil current. To the relay, in particular the switching contacts as a result of Relative armature moving moving magnetic field, not to damage by a too hard impact on its end position, the application of the turn-on voltage is limited in time. For this purpose, a switching time is provided, to which instead of the increased turn-on voltage, the normal voltage is applied to the relay coil, which is determined by using the coil current.

The value of the normal voltage is chosen such that a proper operation of the relay can be ensured without destroying components of the relay or wear above average. The normal voltage may correspond, for example, to the nominal voltage of the relay or be based thereon.

In the case of the switching arrangement according to the invention, it is provided that a current detection device is provided in the current path of the relay coil, which is set up for detecting a coil current flowing through the relay coil and for outputting a current signal indicating the coil current to the control device, and the drive circuit for determining the changeover time the switchover from the switch-on voltage to the normal voltage takes place, which uses the current signal.

In the switching arrangement according to the invention, the coil current flowing through the relay coil is determined by measurement. The drive circuit evaluates the current signal and can make depending on the current signal in a simple manner, the control of the relay coil with either the excessive turn-on voltage or a normal voltage. Since for the inventive switching arrangement except a voltage source with the ability to provide multiple output voltages, a simple current measurement and the appropriately designed drive circuit no structural elements must be provided, the switching arrangement is correspondingly easy to implement.

In the case of the switching arrangement according to the invention, it is provided that the drive circuit is set up to use the value of the coil current to determine the switchover time and to set the switchover time to the point in time at which the value of the coil current exceeds a first current threshold value.

This can be done using a relatively easy to monitoring criterion, namely the value of the coil current compared to a first threshold, a determination of the switching time are performed. Another advantage of this embodiment is that with a suitable definition of a maximum coil current damage to the relay coil can be excluded by a too high coil current.

A further advantageous embodiment of the second embodiment of the switching arrangement according to the invention is given by the fact that the drive circuit is adapted to apply after the switching time a voltage pulse with respect to the normal voltage reverse polarity to the relay coil and after the end of this voltage pulse, the application of the normal voltage to the relay coil continue. By this measure can be achieved that for switching on the relay, a higher inrush current can be selected without the relay armature is moved too violent against its final position, because by the negative voltage pulse targeted degradation of the magnetic field can take place and thus an end to the acceleration or - in the case of a polarized relay - Even a deceleration of the relay armature can be effected. By choosing a higher turn-on voltage, the movement phase of the relay armature can be shortened, as a result of this a stronger overall magnetic field is generated by the relay coil, which drives the relay armature. In addition, by the targeted delivery of the negative voltage pulse and a reduction of the contact bounce of the relay contacts can be achieved because the contacts meet gentler.

According to a further advantageous embodiment of the second embodiment of the switching arrangement according to the invention is also provided that the drive circuit is adapted to reduce after switching on the relay, the current flowing through the coil coil current to a minimum value, which is adapted to the relay contacts in the keep switched on position.

As a result, the power consumption of the relay can be reduced during the holding phase. The reduced coil current is to be chosen so that a reliable hold in the closed position can be ensured.

Specifically, it can be provided in this context that the drive circuit is adapted to make the reduction of the coil current after a predetermined waiting time after the start of switching on the relay.

By specifying the waiting time, it is intended to ensure that the switch-on process is completed in any case before the coil current is reduced for power reduction. The waiting time may e.g. be chosen at about 50 ms.

A further advantageous embodiment of the second embodiment of the switching arrangement according to the invention also provides that the drive circuit is adapted to apply a counter voltage provided by the voltage source to the relay coil to turn off the relay, the reverse voltage with respect to the normal voltage in reverse Polarity is present and in terms of their amount is greater than the rated voltage of the relay.

By applying the counter voltage, a faster degradation of the magnetic field can be achieved, so that the relay armature can be returned to its rest position. Particularly advantageous is the combination of the application of the counter voltage with a reduced coil current in the holding phase is considered because the reduced magnetic field, the magnetic field is already weaker before the actual turn-off, as it would be at a high-current coil current.

In this context, it can be concretely provided that the drive circuit is set up to terminate the application of the counter voltage after a predetermined switch-off period has elapsed, before the switch-off process has been completed. In this way, in turn, it can be achieved that the relay armature is not moved too violently in its rest position in the case of a polarity-reversed relay or unintentional reclosing is prevented in an unpoled relay.

The above object is also achieved by a method according to claim 7.

An advantageous embodiment of the method according to the invention provides that the switching time selected in such a way is that a maximum coil current is not exceeded.

In order to be able to achieve an accelerated switch-off, it is proposed according to a further advantageous embodiment of the method according to the invention that for switching off the relay a counter-voltage is applied to the relay coil, wherein the counter-voltage relative to the normal voltage is in reverse polarity and greater in terms of their amount as the rated voltage of the relay.

In this context, it can also be provided that, after a predetermined switch-off time has elapsed, the application of the countervoltage is terminated before the switch-off process has been completed.

With regard to the advantages of the method according to the invention and its embodiments, reference is made to the above explanations regarding the circuit arrangement according to the invention.

Figur 1

ein Diagramm zur Erläuterung der Verläufe von Strömen und Spannungen während des nicht beschleunigten Einschaltens eines Relais;

Figur 2

ein schematisches Blockschaltbild einer ersten Ausführungsform einer Schaltanordnung mit einem elektromagnetischen Relais;

Figur 3

ein Diagramm zur Erläuterung der Verläufe von Strömen und Spannungen während des beschleunigten Einschaltens des Relais gemäß der ersten Ausführungsform der Schaltanordnung;

Figur 4

ein schematisches Blockschaltbild einer zweiten Ausführungsform einer Schaltanordnung mit einem elektromagnetischen Relais;

Figur 5

ein erstes Diagramm zur Erläuterung der Verläufe von Strömen und Spannungen während des Einschaltens des Relais gemäß der zweiten Ausführungsform der Schaltanordnung;

Figur 6

ein zweites Diagramm zur Erläuterung der Verläufe von Strömen und Spannungen während des Einschaltens des Relais gemäß der zweiten Ausführungsform der Schaltanordnung; und

Figur 7

ein Diagramm zur Erläuterung der Verläufe von Strömen und Spannungen während des Ausschaltens des Relais gemäß der zweiten Ausführungsform der Schaltanordnung.

The invention will be explained in more detail below with reference to two exemplary embodiments. Show this

FIG. 1

a diagram illustrating the waveforms of currents and voltages during the non-accelerated switching on a relay;

FIG. 2

a schematic block diagram of a first embodiment of a switching arrangement with an electromagnetic relay;

FIG. 3

a diagram illustrating the waveforms of currents and voltages during the accelerated switching on of the relay according to the first embodiment of the switching arrangement;

FIG. 4

a schematic block diagram of a second embodiment of a switching arrangement with an electromagnetic relay;

FIG. 5

a first diagram for explaining the waveforms of currents and voltages during the switching of the relay according to the second embodiment of the switching arrangement;

FIG. 6

a second diagram for explaining the courses of currents and voltages during switching on of the relay according to the second embodiment of the switching arrangement; and

FIG. 7

a diagram for explaining the waveforms of currents and voltages during the switching off of the relay according to the second embodiment of the switching arrangement.

FIG. 1 shows a diagram with measured waveforms of currents and voltages in unaccelerated switching on a relay over time t. Specifically, the voltage waveform 11a indicates the coil voltage applied to the relay coil, the current waveform 11b indicates the coil current flowing through the coil, and the voltage waveform 11c indicates the load voltage applied to a load connected through the relay.

The following explanations are only examples of a relay of the type "NO" made, so a relay that closes its contacts on excitation of the relay coil on the output side. The described procedure can also be applied without problem to relays of the type "normally closed" or change-over relay.

At time t = 0, a coil voltage of the magnitude of a normal voltage U _N , which is oriented, for example, at the value of the nominal voltage indicated for the relay, is applied to the relay coil (coil voltage profile 11a). This causes an increase of the coil current (current waveform 11b) during a first period T ₁ , until after expiration of T _1, a maximum of the coil current is reached and the relay armature of the relay begins to move. The movement of the relay armature during a second period T ₂ , as can be seen from the voltage curve 11 c of the load voltage, leads to a closing of the relay contacts. Finally, after the expiration of the period T _2, the movement process of the relay armature is completed; the relay armature is in its end position and the relay contacts in its closed position.

In order to accelerate the switch-on operation of the relay which requires the entire time duration T ₁ + T ₂ , a comparatively higher switch-on voltage is now applied to the relay coil for switching on instead of the normal voltage. In order not to damage the relay or cause increased wear, the normal voltage is applied to the relay coil at a switching time instead of the excessive turn-on.

In the following two exemplary embodiments will be explained, with which an accelerated switching operation of a relay can be achieved.

This is in FIG. 2 in highly schematic representation, a first embodiment of a switching arrangement 20 is shown. The switching arrangement 20 comprises an electromagnetic relay 21 having a relay coil 21a, a relay armature 21b and schematically indicated relay contacts 21c for closing and interrupting a switching current path 22, not further described below, to which any electrical load can be connected. The switching arrangement further comprises a drive circuit 23, consisting of an energy storage device, which in the embodiment of the FIG. 2 only by way of example in the form of a capacitor device 24, as well as a switching device 25 comprising three individual switches 25a, 25b and 25c-in the present exemplary embodiment. The drive device 23 is designed to supply to the relay coil 21a either one by one in FIG. 2 not explicitly shown voltage source provided normal voltage U _N or a comparatively higher turn-on voltage U _E apply.

The functioning of in FIG. 2 shown switching arrangement 20 when switching on the relay 21 is based on FIG. 3 be explained in more detail. FIG. 3 FIG. 12 is a graph showing measured waveforms of currents and voltages when in accordance with the switching arrangement 20. FIG FIG. 2 accelerated turning on a relay over time t. Specifically, the voltage waveform 31a indicates the coil voltage applied to the relay coil, the current waveform 31b indicates the coil current flowing through the coil, and the voltage waveform 31c indicates the load voltage applied to a load connected through the relay.

Before switching on the relay at the time t = 0, the switching device 25 is in a rest position, in which the switch 25a is closed, while the switches 25b and 25c are opened. By the switch 25a, the capacitor device 24 is charged with the excessive turn-on voltage U _E. Since the switches 25b and 25c are opened, no voltage is applied to the relay coil 21, so that no coil current flows and the relay remains in its open position.

At the time t = 0, the switching device 25 is excited to switch to an active state. At this time, the switch 25a is opened while the switches 25b and 25c are closed. As a result, the starting voltage U _E prevailing in the capacitor device 24 is applied to the relay coil 21 a. Due to the higher in comparison to the normal voltage U _N turn-on voltage U _E a higher coil current is driven through the relay coil 21a in a relatively short time when it is at unaccelerated power (see. FIG. 1 ). As a result, a faster structure of the magnetic field is effected, so that after a significantly shorter period of time T ₁ already uses a movement of the coil armature 21b. After the expiration of the period T ₂ , the switch-on is completed and the relay contacts 21c are in their closed position.

Since the capacitor device 24 discharges during the period T ₁ via the relay coil 21a and thereby causes a comparatively high inrush current, the coil voltage continuously decreases as seen at the voltage waveform 31a, until the switching time t = t _U instead of the turn-on voltage U _E finally through the switch 25c applied to the relay coil 21a normal voltage U _N comes into play.

In the case of the switching arrangement according to this first exemplary embodiment, by means of a suitable selection of the values for the turn-on voltage U _E and the capacitance of the capacitor device 24, a course of the coil current during switching on which does not exceed a maximum permitted value can be achieved. By determining the amount of charge flowing through the relay coil 21a during the un-accelerated turn-on, which must be provided by the capacitor means 24 during the accelerated turn-on, can be given the boundary condition of the given maximum coil current, for example, by the relay manufacturer in the data sheet of the relay 21 , calculate the sought values for turn-on voltage U _E and capacitance of the capacitor device. The switching arrangement 20 according to FIG. 2 Consequently, it has the advantage that an accelerated switching on of the relay 21 is made possible solely by its design taking into account the maximum permitted coil current. For example, for a relay with a rated voltage of 5V, by choosing a turn-on voltage of about 24.5V and a capacitance of capacitor device 24 of about 2.2μF a significant reduction of the required switching time can be achieved.

In FIG. 4 is a schematic representation of a second embodiment of a switching arrangement 40 is shown. The switching arrangement 40 comprises an electromagnetic relay 41 with a relay coil 41a, a relay armature 41b and schematically indicated relay contacts 41c for closing and interrupting a switching-current path 42, not further described in the following, to which any electrical load can be connected. In addition, the switching arrangement 40 has a voltage source 43, which is set up to provide various voltages on the output side, a drive circuit 44, which is set up to drive the relay coil 41a with different voltages provided by the voltage source 43, and a current measuring device 45, which is intended for detection of the coil current flowing through the relay coil 11a and for supplying a corresponding current signal to the drive circuit. Optionally, the switching arrangement may also have a current limiting device 46 connected to the drive circuit 44, for example a controlled constant current source, in the current path of the relay coil 41a.

The drive circuit 44 is designed to ensure a comparatively fast switching on and off of the relay 41. For this purpose, the drive circuit 44 is applied to the relay coil 41a when switching briefly with an excessive turn-on voltage U _E and when turned off briefly with a reversed reverse voltage U _G. For holding the relay 41, a lower normal voltage U _N is otherwise used. The normal voltage U _N , the turn-on voltage U _E and the reverse voltage U _G are provided on the output side by the voltage source 43. The drive circuit 44 can switch between the individual voltages U _N , U _E and U _G by means of a changeover switch 47, which is indicated only by way of example.

The operation of the switching arrangement 40 according to the second embodiment will be explained below with the addition of FIGS. 5 and 6 for the switch-on and FIG. 7 be explained in more detail for the switch-off.

This shows FIG. 5 a diagram in which voltage or current waveforms over the time t are shown schematically. Specifically, the voltage waveform 51a indicates the coil voltage applied to the relay coil 41a, the current waveform 51b indicates the coil current flowing through the coil, and the voltage waveform 51c indicates the load voltage applied to a load connected through the relay 41.

The relay 41 is in an off state until time t = 0; Coil current and coil voltage have the value zero, the load on the contact side of the relay 41 is de-energized.

At the time t = 0, the drive circuit 44 is supplied with a switch-on command; Such a switch-on command may, for example, have been generated by a control device of an electrical protective device comprising the switching arrangement 40 for monitoring power supply networks, in order to cause the relay 41 to drive a circuit breaker connected to the protective device. In the case of the present switch-on command, the drive circuit 44 applies the switch-on voltage U _{E provided} as a coil voltage to the relay coil 41 a on the side of the voltage source 43; the voltage curve 51a shows a corresponding jump from 0 to U _E at t = 0. The turn-on voltage U _E is an excessive voltage compared to the rated voltage for which the relay 41 is designed by the manufacturer. In one possible embodiment, the rated voltage is 5V; the value of the turn-on voltage U _E is set to 20V in this embodiment. The comparatively high turn-on voltage U _E applied to the relay coil 41a drives a high inrush current through the coil 41a, so that the magnetic field of the relay coil 41a is faster than when the nominal voltage is applied builds. The relatively rapidly increasing coil current can be recognized from the current profile 51b within a time period T ₁ in which the field charging of the magnetic field takes place. During field charging, the relay armature 41b does not move, the switching contacts 41c remain in the open position. By applying the excessive turn-on voltage, the time period T ₁ can be compared with the un-accelerated switch-on (cf. FIG. 1 ), because the charging of the magnetic field by the corresponding higher coil current takes place faster.

Finally, if the magnetic field is sufficiently strong at the end of the period T ₁ , the relay armature 41 b moves during a subsequent period T ₂ and actuates the switching contacts 41 c of the relay 41. To relay relay 41b and switch contacts 41c not too fast to move to their final position and avoid due to the associated heavy impact increased wear of the relay 41, the excessive turn-on voltage U _{E must} be taken back in time and the value of a normal voltage U _N , their value, for example may correspond to the rated voltage of the relay or at least oriented to be reduced.

The switching time t _U , at which the switchover from the switch-on voltage U _E to the normal voltage U _{N is to} take place, is derived from the current profile 51b of the coil current. For this purpose, the coil current with the current measuring device 45, for example a current sensor, is measured and a current signal is generated, which is supplied to the drive circuit 44 and evaluated by the latter for determining the switchover time t _U.

As a possible criterion for determining the switching time t _U , the height of the current signal indicating the coil current can be selected. In this case, it can be determined that the switching time t _{U is set} as the time at which the current signal exceeds a specified threshold. Out FIG. 5 it can be seen that when reaching of the maximum permissible current I _max the switching time t _{U is} set.

Alternatively - in FIG. 5 However, not shown - can be selected as a criterion for determining the switching time t _U and the value of the slope of the coil current indicating current signal. As well as from the current 51b in FIG. 2 is recognizable, the slope of the current profile 51b decreases continuously with greater formation of the magnetic field and finally reaches the value zero at maximum saturation; the relay armature now starts to move. Therefore, it can be set as the criterion that the switching time t _{U is set} as the time at which the slope of the current signal falls below a predetermined threshold. This threshold should be set to detect the changeover timing in good time before reaching zero current slope. In order to determine the slope, during the switch-on process the current signal can be stored repeatedly and the slope calculated from two successive values of the current signal.

The running during the period T ₂ movement of the relay armature 41b and the switching contacts 41c takes place all the faster, the more the magnetic field was charged at the switching time t _U. A higher turn-on voltage U _E - and thus a higher coil current - causes a stronger charge and thus consequently a faster movement of the relay armature 41b.

After the switch contacts are finally closed after expiration of the period T ₂ , the magnetic field is maintained by the normal voltage U _N. In order to keep the relay coil 41a closed as low as possible, the drive circuit in the holding phase of the relay, the current limiting device 46 corresponding to the reduction of the coil current (not in the diagram in FIG. 5 shown). The reduced coil current must be selected so that a safe hold of the relay is ensured in the on state. The time of power reduction must be sufficiently long after the start of the switch-on, so that the switching on of the relay 41 is not hindered. For this purpose, the drive circuit, for example, after expiry of a waiting time that is significantly longer than the usual switch-on, cause the current limiting device 46 to reduce power. The waiting time can be selected, for example, at a value of about 50ms.

In order to be able to select the value of the turn-on voltage U _E - and thus of the coil current when switching on - as high as possible without damaging the relay armature 41b and switching contacts 41c by an excessive impact on their end position, a short negative voltage pulse can be used during the period T ₂ in order to discharge the magnetic field more quickly or (in the case of a polarized relay) to purposefully decelerate the movement of the relay armature 41b. This is exemplary in FIG. 6 shown. FIG. 6 shows a diagram in which voltage or current waveforms over the time t are shown schematically. Specifically, the voltage waveform 61a indicates the coil voltage applied to the relay coil 41a, the current waveform 61b indicates the coil current flowing through the coil, and the voltage waveform 61c indicates the load voltage applied to a load connected through the relay 41.

Within the period T ₂ , the negative voltage pulse 62 is delivered with. Time and magnitude of the negative voltage pulse must be chosen so that the movement of the relay armature 41b is not interrupted in order not to hinder the switch-on. For example, a counter voltage U _G provided by the voltage source can be used for the voltage pulse. After the end of the voltage pulse 62, the normal voltage U _N is again applied to the relay coil 41a.

During a further time period T ₃ lying within the period T ₂ , the switching contacts 41c of the relay 41 touch for the first time and briefly spring back, which is referred to as so-called "contact bounce". Within the period T ₃ is the first time the load voltage on the switching side of the relay 41 is constructed, which can be seen in the voltage curve 61c. Due to the contact bounce, on the one hand, the wear of the relay is increased and, on the other hand, the period T _{2 is} extended until the final closing of the switching contacts 41c. Therefore, the effect of contact bounce should be minimized. This also contributes to the negative voltage pulse 62, as this is the relay armature 41b and thus also the switching contacts 41c targeted the momentum of their movement is taken.

Currents and voltage curves during the switch-off process of the relay 41 are finally in FIG. 7 shown. Specifically, the voltage waveform 71a indicates the coil voltage applied to the relay coil 41a, the current waveform 71b indicates the coil current flowing through the coil, and the voltage waveform 71c indicates the load voltage applied to the load connected through the relay 41. Before the time t = 0 in FIG. 7 the relay 41 is held in its on position by applying the normal voltage U _N ; the current flow through the relay coil is constant and the load voltage is applied to the load.

At the time t = 0, a switch-off command is transmitted to the drive circuit 44. Thereupon, the drive circuit 44 causes, instead of the normal voltage U _N, a countervoltage U _G provided by the voltage source to be applied to the relay coil 41 a. This counter voltage accelerates the degradation of the magnetic field and thus reduces the time period T ₄ used for this purpose. The counter voltage can be selected in the present embodiment (rated voltage of the relay: 5V), for example, at a value of -12V.

Analogous to the switch-on voltage during the switch-on process, the duration of the application of the countervoltage U _G must also be limited in time in order to prevent the relay anchor from moving too quickly into its rest position. Therefore, the application of the back voltage U _G after a fixed off period, the for example, may be about 1ms, terminated and no voltage applied to the coil. The end of the off period is in FIG. 7 indicated with t = t _A. It is also advantageous for a fast and gentle switch-off process if during the holding phase of the relay 41 - as explained above - a reduced coil current is used, since in this case the magnetic field strength is already correspondingly reduced and can be reduced more rapidly.

If the magnetic field of the relay coil 41a then reaches a minimum value, the relay armature 41b can no longer be held in its switched-on position and begins to move in the direction of the rest position. The period of movement of the relay armature 41b to its final OFF position is in FIG. 7 indicated as T ₅ . During the period T ₅ breaks through the opening of the switching contacts 41c and the load voltage, which can be seen on the voltage curve 71c. By the movement of the relay armature 41b, a coil current is again induced in the relay coil, which degrades gradually again after reaching the rest position at the end of the period T ₅ . After the end of the period T ₅ , both the coil voltage and the load voltage are at a value of zero, the relay is in its off position.

Claims (10)

1. Switching arrangement (20, 40) having an electromagnetic relay (21, 41) which has a relay coil (21a, 41a) and switching contacts (21c, 41c), having a voltage source (43) which is connected to the relay coil (21a, 41a) and having a control circuit (23, 44) which is configured to control the relay coil (21a, 41a) by means of a voltage which is made available by the voltage source (43); wherein

   - the control circuit (23, 44) is configured to apply an elevated switch-on voltage to the relay coil (21a, 41a) in order to switch on the relay (21, 41), wherein the switch-on voltage is higher than the rated voltage of the relay (21, 41); and

   - the control circuit (23, 44) is configured to apply, instead of the switch-on voltage, a normal voltage to the relay coil (21a, 41a) during the switch-on process of the relay (21, 41), said normal voltage being made available by the voltage source (43) and being lower compared to the switch-on voltage, wherein in order to determine a switch-over time at which the switching over to the normal voltage takes place, a coil current which flows through the relay coil (21a, 41a) is used; and wherein

   - in the current path of the relay coil (41a) a current-detection device (45) is provided which is configured to detect a coil current flowing through the relay coil (41a) and to output a current signal, indicating the coil current, to the control circuit (44); and

   - the control circuit (44) uses the current signal to determine the switch-over time at which the switching over from the switch-on voltage to the normal voltage takes place, wherein

   - the control circuit (44) is configured to use the value of the coil current to determine the switch-over time and to define the switch-over time as that time at which the value of the coil current exceeds a first current threshold value.
2. Switching arrangement (40) according to Claim 1, characterized in that

   - the control circuit (44) is configured to apply, after the switch-over time, a voltage pulse with a reversed polarity in relation to the normal voltage to the relay coil (41a), and to continue the application of the normal voltage to the relay coil (41a) after the end of this voltage pulse.
3. Switching arrangement (40) according to Claim 1 or 2,
   characterized in that

   - the control circuit (44) is configured to reduce, after the switching on of the relay (41), the coil current flowing through the relay coil (41a) to a minimum value which is suitable for keeping the switching contacts (41c) in the switched-on position.
4. Switching arrangement (40) according to Claim 3, characterized in that

   - the control circuit (44) is configured to reduce the coil current after the expiry of a predefined waiting time after the beginning of the switching on of the relay (41).
5. Switching arrangement (40) according to one of Claims 1 to 4,
   characterized in that

   - the control circuit (44) is configured to apply, in order to switch off the relay (41), an opposing voltage, made available by the voltage source (43), to the relay coil (41a), wherein the opposing voltage is present with a reversed polarity in relation to the normal voltage and is higher, in terms of its absolute value, than the rated voltage of the relay (41).
6. Switching arrangement (40) according to Claim 5,
   characterized in that

   - the control circuit (44) is configured to end the application of the opposing voltage after the expiry of a predefined switch-off time period, before the switch-off process is terminated.
7. Method for controlling an electromagnetic relay (21, 41) using a switching arrangement according to Claim 1, which electromagnetic relay (21, 41) has a relay coil (21a, 41a) and switching contacts (21c, 41c), wherein the following steps are carried out in the method:

   - in order to switch on the relay (21, 41) a switch-on voltage is applied to the relay coil (21a, 41a), wherein the switch-on voltage is higher than the rated voltage of the relay (21, 41);

   - during the switch-on process of the relay (21, 41), instead of the switch-on voltage a normal voltage which is lower compared to the switch-on voltage is applied to the relay coil (21a, 41a), wherein a coil current flowing through the relay coil (21a, 41a) is used to determine the switch-over time at which the normal voltage is applied to the relay coil (21a, 41a); and wherein

   - the coil current flowing through the relay coil is detected and a current signal which indicates the coil current is output; and

   - the current signal is used to determine the switch-over time;
   characterized in that

   - the value of the coil current is used to determine the switch-over time, and the switch-over time is defined as that time at which the value of the coil current exceeds a first current threshold value.
8. Method according to Claim 7,
   characterized in that

   - the switch-over time is selected in such a way that a maximum coil current is not exceeded.
9. Method according to Claim 7 or 8,
   characterized in that

   - an opposing voltage is applied to the relay coil (21a, 41a) in order to switch off the relay (21, 41), wherein the opposing voltage is present with a reversed polarity in relation to the normal voltage and is higher, in terms of its absolute value, than the rated voltage of the relay (21, 41).
10. Method according to Claim 9,
    characterized in that

    - after the expiry of a predefined switch-off time period the application of the opposing voltage is ended before the switch-off process is terminated.

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