Patent Document

CROSS-REFERENCE TO PRIOR APPLICATIONS 
     This application is the U.S. National Phase application under 35 U.S.C. §371 of International Application No. PCT/IB13/050020, filed on Jan. 2, 2013, which claims the benefit of U.S. Provisional Patent Application No. 61/583,707, filed on Jan. 6, 2012. These applications are hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an electrical device for compensating an effect of an electrical current of a load and a corresponding method for compensating an effect of an electrical current of a load, in particular an LED unit comprising one or more LEDs. Further, the present invention relates to a driver device for driving a load, in particular an LED unit having one or more LEDs. 
     BACKGROUND OF THE INVENTION 
     In the field LED drivers for offline applications such as retrofit lamps, solutions are demanded to cope with high efficiency, high power density, long lifetime, high power factor and low cost, among other relevant features. While practically all existing solutions compromise one or the other requirement, it is essential that the proposed driver circuits properly condition the form of the mains energy into the form required by the LEDs while keeping in compliance with present and future power mains regulations. In addition, it is required that the driver circuits comply with existing power adjusting means, e.g. dimmers or the like, so that the drivers can be used universally as a retrofit driver device including the LED units. 
     The driver circuits should comply with all kinds of dimmers and especially the drivers should comply with phase-cut dimmers, which are preferably used to regulate the mains power with low power loss. Those dimmers which are usually used to regulate the mains energy provided to a filament lamp need a low load impedance path for a timing circuit operation current to adjust the phase-cut timing. Alternatively to providing this path continuously, making and breaking that path for certain parts of the mains voltage cycle can also result in stable operation. The provision of this low impedance path has to be adjusted with respect to the zero crossing of the mains voltage. Further, to provide a proper timing circuit operation, a high impedance state of the load has to be provided since a load current of an LED unit usually decreased rapidly after a dimmer is switched on. During this high impedance phase a leakage current of the load influences the timing circuit operation and may cause an early switching of the dimmer. In the case that the load of the dimmer consists of multiple retrofit lamp in parallel, each having an individual leakage current, the total leakage current increases accordingly and may cause an unacceptable error of the timing circuit operation, limiting the dimming range. 
     WO 2011/073865 A1 discloses a driver device for a solid state lamp, wherein a current detector is connected to a rectifying unit and a charge buffer device is incorporated in the driver device. The charge buffer device is provided for generating a suitable drive current and the current detector is provided for driving a current generating unit for adjusting the drive current provided to the lamp. 
     This driver device is provided for adjusting the drive current as desired for an LED unit, however, this driver device does not prevent an error of the timing circuit caused by a leakage current of the LED unit. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an electrical device for compensating an effect of an electrical current of a load, a corresponding method for compensating an effect of an electrical current of a load and a driver device for driving a load, in particular an LED unit comprising one or more LEDs, providing compatibility of a dimmable load to different power supply units, in particular to phase-cut dimmers, to ensure a proper operation of the power supply unit with low technical effort. 
     According to one aspect of the present invention, an electrical device is provided for compensating an effect of an electrical current of a load, in particular an LED unit having one or more LEDs, comprising:
         a connection element for electrically connecting the electrical device to an external power source providing a supply voltage for powering the load,   a monitoring device for monitoring the electrical current of the load during a first time interval, and   a signal controller connected to the connection element for providing an electrical compensation signal to the connection element during a second time interval on the basis of the electrical current monitored by the monitoring device.       

     According to another aspect of the present invention, a driver device is provided for driving a load, in particular an LED unit having one or more LEDs, comprising:
         input terminals for receiving an input voltage from an external power source,   output terminals for providing a load current for powering the load,   a monitoring device connected to at least one of the input or output terminals for monitoring an electrical current during a first time interval, and   a signal controller connected to at least one of the input terminals or the output terminals for providing an electrical compensation signal to at least one of the input terminals or the output terminals during a second time interval on the basis of the electrical current monitored by the monitoring device.       

     According to still another aspect of the present invention a method is provided for compensating an effect of an electrical current of a load, in particular an LED unit comprising one or more LEDs, the method comprising the steps of:
         connecting an electrical device to an electrical power supply by means of a connection element,   monitoring the electrical current during a first time interval, and   providing an electrical compensation signal to the connection element during a second time interval on the basis of the electrical current monitored during the first time interval.       

     According to the invention, the monitoring device detects the electrical current or receives data corresponding to the electrical current or is provided to get the information regarding the electrical current in general in a different way. 
     Preferred embodiments of the invention are defined in the dependent claims. It shall be understood that the claimed method has similar and/or identical preferred embodiments as the claimed device and as defined in the dependent claims. 
     The present invention is based on the idea to provide an electrical device as an add-on device connectable to a power source and a dimmable load to provide compatibility of the load and the power supply including a dimmer device and to ensure the proper operation of the timing circuit of the dimmer device. To achieve the proper operation of the timing circuit, the electrical device controls the operation of the timing circuit by providing an electrical signal that influences the operation accordingly. Since an error of the timing circuit is usually caused by an electrical current, e.g. a leakage current occurring during a first time interval of a duty cycle of the supply voltage and causes an error after this time interval, the electrical current forming the root cause of the error of the timing circuit is monitored during the first time interval and the correction signal is provided during a second time interval to compensate the error. Hence, an error of the timing circuit of the power source or a connected dimmer can be corrected with low technical effort and the compatibility of the load to the dimmer device can be achieved. 
     Alternatively to full correction of the error, the error may be stabilized to a fixed value, such that it is only perceived as an offset in the control characteristics of the system, but does not change e.g. with different number of lamps per dimmer or from installation to installation. 
     According to a preferred embodiment, the electrical current is a leakage current of the load. This is a possibility to monitor the electrical parameter having the largest influence on the dimmer device operation. 
     According to a further preferred embodiment, the monitoring device comprises a measuring device for measuring the electrical current or a receiver for receiving a signal corresponding to the electrical current. This provides a simple solution to detect the electrical current or receive a corresponding signal with low technical effort. 
     In a preferred embodiment, the compensation signal is a charge current exchanged between the power supply and the electrical device to compensate the effect caused by the leakage current. This provides a simple solution to adjust a voltage of a timing capacitor of the dimmer timing circuit and to correct the error of the timing circuit caused by the leakage current. 
     In a preferred embodiment, the compensation signal is a voltage provided in series with the load. This is a simple solution to drive an additional current to charge or discharge the timing capacitor of the timing circuit to correct the error caused by the leakage current. 
     In a preferred embodiment, the signal controller comprises an impedance path forming a defined current path for providing the charge current during the second time interval. This is a simple solution to charge or discharge the timing capacitor of the timing circuit and to reduce the voltage at the timing capacitor caused by the leakage current. 
     According to a preferred embodiment, the signal controller comprises a resistor for changing a resistance of the impedance path to control the charge current during the second time interval. This is a simple solution to adjust the electrical charge of the timing capacitor of the timing circuit to a desired level to control the timing of the timing circuit by means of the electrical device. 
     According to a further embodiment, the signal controller is adapted to decrease the resistance of the impedance path continuously or stepwise during the second time interval. Hence, the charge stored in the timing capacitor can be adjusted precisely with low technical effort. 
     According to a further embodiment, the second time interval is adjusted to a zero crossing of the supply voltage such that the current path is provided before and after the zero crossing of the supply voltage. This is a simple possibility to adjust the voltage of the timing capacitor to a predefined level with low technical effort. 
     In a further preferred embodiment, a transition from the first to the second time interval is adjusted close to the zero crossing of the supply voltage, and preferably provided within a time frame of 2 ms around the zero crossing. This provides a further degree of freedom to adjust the accumulated charge of the timing capacitor. 
     According to a further preferred embodiment, the signal controller comprises a capacitor for providing the charge current during the second time interval, wherein the monitoring device is adapted to charge the capacitor during the first time interval. This is a simple and self-adjusting possibility to monitor the leakage current, to store the respective charge in the capacitor and to provide the stored charge during the second time to correct the error of the timing capacitor caused by the leakage current. Further, this is a simple solution to detect the leakage current individually independent of the attached load and to adjust the charge and the voltage of the timing capacitor accordingly. 
     In a preferred embodiment of the driver device, the driver device comprises a first current path and a second current path, wherein the first and the second current path form a part of a rectifier unit, wherein the first current path and second current path each comprises a monitoring device and a signal controller, wherein the monitoring devices are provided for monitoring the electrical current in the respective current path and the signal controller are provided for providing the electrical compensation signal. This is a simple solution to integrate the monitoring device and the signal controller in the driver device with low technical effort, since the respective current paths are provided for unipolar operation. 
     In a further preferred embodiment of the driver device at least one of the input terminals is connected to a voltage converter unit which is connected to the external power source, wherein the voltage converter includes a timing capacitor, and wherein the compensation signal is a charge current which is provided to the voltage converter to at least partially charge or discharge the timing capacitor. This provides an effective solution to adjust the error of the timing capacitor caused by the leakage current of the dimmable load. 
     As mentioned above, the present invention provides a simple and effective solution to adapt a dimmable load, in particular an LED unit comprising one or more LEDs, to a power source and to ensure the compatibility of the load to the power source including a dimmer device, wherein a timing circuit operation is not affected by the connected load and operates as desired. This is achieved by measuring an electrical signal, in particular a leakage current of the load and by providing a compensation signal, preferably a current exchanged with the dimmer device to compensate a charge which is accumulated in a timing capacitor of the timing circuit due to the leakage current of the load. Hence, a proper operation of the dimmer device can be achieved with low technical effort and can be integrated as a retrofit element to an existing power source including a dimmer device and, further, to an already existing dimmable load, in particular an LED unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. In the following drawings 
         FIG. 1  shows a schematic block diagram of a known dimmer device connected to an incandescent lamp, 
         FIG. 2  shows a diagram illustrating the voltage supplied by the dimmer device, 
         FIG. 3  shows a schematic block diagram of a first embodiment of the electrical device connected to an external power source and to a dimmable load, 
         FIG. 4  shows a second embodiment of the electrical device connected to an external power source and to a dimmable load, 
         FIG. 5  shows a timing diagram of the voltage provided by the dimmer device to explain the function of the electrical device, 
         FIG. 6  shows a schematic equivalent circuit diagram of one embodiment of the present invention, 
         FIG. 7  shows a detailed schematic block diagram of the electrical device of  FIG. 3 , 
         FIG. 8  shows a detailed schematic block diagram of a driver device connected to an external power source for driving a dimmable load. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a schematic block diagram of a dimmer device generally denoted by  10 . The dimmer device  10  is connected to an external voltage supply  12 , which is preferably mains, which provides a supply voltage V 10 . The dimmer device  10  provides a modified input voltage V 12  having a leading edge phase-cut and a load current I 1  to a load  14 . The load  14  may be an incandescent bulb lamp. 
     The dimmer device  10  comprises a triac  16  for connecting the external voltage supply  12  to the load  14 . Parallel to the triac a timing circuit  18  is connected. The timing circuit  18  comprises a timing capacitor  20 , a variable resistor  22  and a diac  24 , which is connected to the triac  16 . The voltage of the timing capacitor  20  is provided to the diac  24  which switches the triac  16 . When the charge of the timing capacitor  20  reaches a predefined level, the diac  24  is switched off and the supply voltage V 10  is provided to the load  14 . When the triac  16  is switched off, the supply voltage V 10  is provided to the timing circuit  18 . Hence, the timing capacitor  20  of the timing circuit  18  is charged up to a predefined voltage level, which switches the diac. As soon as the predefined voltage is reached, the triac  16  is switched on again and the timing capacitor  20  is discharged to a forward voltage of the diac  24 . 
     During a phase when the triac  16  is switched on, the voltage across the timer circuit  18  is close to zero and the timing capacitor  20  is not charged. The triac  16  connects the external voltage supply  12  to the load  14  until the current through the triac  16  and thus the load current I 1  is above a hold current of the triac  16 . Then the triac is switched off and the charging of the timing capacitor  20  starts again. 
     If the load  14  is an high power incandescent bulb lamp, the triac  16  keeps conducting until or just before the zero crossing of the input voltage V 10 . The impedance of the load  14  is low enough to ensure a high enough load current I 1  to ensure the conduction of the triac  16  up to the zero crossing. 
     If the load  14  is an LED unit a normal operation comparable to the operation with an incandescent bulb (incandescent-like operation) can be assured only if the triac current, i.e. the load current I 1  is larger than the hold current of the triac  16 . This can be achieved only for corresponding power levels (e.g. 40 W) having a respective load current I 1 . Most of the SSL retrofit lamps are operated below that level. Hence, it is inevitable to switch the triac  16  off before the zero crossing as described below. 
     In  FIG. 2 , a diagram of the input voltage V 12  provided by the dimmer device  10  is schematically shown. Each half cycle of the supply voltage V 10  (dashed line) comprises three different phases. The first phase, the off-phase T off , when the triac  16  is switched off and the input voltage V 12  is zero. The second phase is the on phase T on  following the off-phase T off , when the triac  16  is conducting and the input voltage V 12  (solid line) is basically identical with the supply voltage V 10 . After the on phase T on , a disconnection-phase T disc  is provided wherein the triac  16  is switched off. During this disconnection-phase T disc , the load impedance should be increased to avoid a charging of the timing capacitor  20  and to avoid an early switching of the diac  16 . During this disconnection-phase T disc , the impedance of the load  14  should be larger than the impedance of the timer circuit  18 . Preferably, the impedance of the load  14  during the disconnection-phase T disc  should be at least 2 MOhm. After a zero crossing t z , the off-phase T off  of the following half cycle of the supply voltage V 10  begins. During this off-phase T off , the impedance of the load  14  should be low to charge the timing capacitor  20  comparable to normal operation. Hence, the impedance of the load  14  has to be switched from the high impedance state to a low impedance state precisely at the zero crossing t z  of the supply voltage V 10 . 
     During the disconnection-phase T disc  an open circuit should be connected to the dimmer device  10 , however, since the connected load  14  has to monitor the input voltage V 12  in order to switch to the low impedance state during T off , a measurement circuit may be used across the input terminals of the load  14 . This measurement circuit will have an input current, referred here to as leakage current during the disconnection-phase T disc . This leakage current is also provided to the dimmer device  10  and charges the timing capacitor  20 . When the next off-phase T off  starts and the low impedance path is connected to the dimmer device  10 , the timing capacitor  20  comprises a not desired electrical charge or, in other words, the timing capacitor  20  is precharged. Hence, the charge of the timing capacitor  20  reaches the predefined voltage which switches the diac  24  at a different point in time during the following off-phase T off . An undesired altering of the switching time of the triac  16  results from the leakage current during the disconnection-phase T disc . In the case that one load  14  is connected to the dimmer device  10 , the altering of the switching point is usually small, however, if a plurality of loads  14  are connected in parallel to the dimmer device  10 , the switching point of the dimmer device  10  is strongly affected. 
     In  FIG. 3  an embodiment of an electrical device is schematically shown and generally denoted by  30 . The electrical device  30  is schematically shown integrated in an electrical circuit. The electrical device  30  is connected to the dimmer device  10  and receives the input voltage V 12  from the dimmer device  10 . The external voltage supply  12  provides the supply voltage V 10  to the dimmer device  10 . The electrical device  30  is also directly connected to the external voltage supply  12  or connected to neutral. The electrical device  30  is connected to a load  34  which is formed of a driver device for driving an LED  32 . The load current I 1  is provided from the dimmer device  10  through the electrical device  30  to the load  34  and the driver device provides a drive current to the LED  32 . The drive current may be different from the load current I 1 . The load  34  is also connected to the external voltage supply  12  or to neutral. A current I 2  is exchanged with the dimmer device  10 . The electrical device  30  adds a compensation current I 3  (of potentially variable amplitude and polarity) to the current I 2 , which is exchanged with the dimmer device  10  during certain time intervals to compensate a leakage current of the load  34  in at least one different point in time as described below. 
     As mentioned above, during the disconnection-phase T disc  the load  34  has a leakage current which is also provided to the dimmer device  10  and charges a timing capacitor  20 . To compensate the leakage current, the electrical device  30  provides a compensation current I 3  in addition to the current I 2  to the dimmer device  10  during the off-phase T off  or after the disconnection-phase T disc  has been terminated. 
     To provide the compensation current I 3 , the electrical device  30  measures the leakage current during the disconnection-phase T disc  and provides the compensation current I 3  after the disconnection-phase T disc . 
     In  FIG. 4  an alternative embodiment of the electrical device  30  is schematically shown and integrated in an electrical circuit. The electrical device  30  is connected to an electrical connection  36  connecting the dimmer device  10  to the driver device  34 . The load  34  is connected to the external power supply  12  or to a neutral. Since the electrical device  30  needs the value of the leakage current (by monitoring, evaluating, estimating, etc) occurring during the disconnection-phase T disc  the electrical device  30  is also connected to the load  34  and receives an electrical signal  38  corresponding to the leakage current during the disconnection-phase T disc . On the basis of the received leakage current information, the electrical device  30  exchanges the compensation current I 3  with the dimmer device  10  after the disconnection-phase T disc  has been terminated to compensate the leakage current. 
     The measurement of the leakage current and the exchange of the compensation current I 2  with the dimmer device  10  is provided in different ways as described in the following. 
       FIG. 5  shows a timing diagram of the input voltage V 12  provided by the dimmer device  10  for explaining the function of the electrical device  30  synchronized to the input voltage V 12 . 
     As described above, the zero crossing t z  of the supply voltage V 10  is detected by the electrical device  30  and the electrical device  30  switches from the high impedance disconnection-phase T disc  to a low impedance state, the off-state T off  to start the charging of the timing capacitor  20 . Since the residual voltage in the capacitor  20  has a different polarity than the final charging stage during the following charging period, initially the voltage across the timing capacitor  20  decreases. This is the intended operation. As mentioned above, the leakage current during the disconnection-phase T disc  increases the voltage across a timing capacitor  20 , so that the charging into the one direction starts at a too high level and will hence take longer than without the leakage current. To compensate the charge accumulated in the timing capacitor  20  by the leakage current, the electrical device  30  switches from the high impedance state to the low impedance state at t 1  slightly before the detected zero crossing t z . Since the input voltage V 12  at t 1  is lower than the voltage across the timing capacitor  20 , the timing capacitor  20  can be discharged earlier during a time interval T DC  and the decrease of the timing capacitor voltage starts earlier so that the error due to the leakage current can be compensated. The electrical device  30  determines the switching point t 1  dependent on the measured leakage current to compensate the effect of the leakage current accordingly. Since the possible shift of the switching point t 1  is limited due to the relation of the value of the supply voltage V 10  to the value of the (residual) voltage in the timing capacitor  20 , this compensation method is preferably used for single lamp systems which have a low leakage current. 
     Further, an intermediate resistance state can be introduced to stabilize an error to due to the leakage current. After detection of the zero crossing t z  the electrical device  30  switches to an intermediate resistance state by means of an intermediate resistance path during a time interval T IR . Hence, the charging of the timing capacitor  20  is reduced compared to the original low impedance state T off . After the intermediate resistance state interval T IR  the electrical device  30  switches to the low impedance state during the off-phase T off . This will delay the switching point of the dimmer device  10 . However, this delay is fully under control of the electrical device  30 , so the switching time when the triac  16  is switched on can be determined by the point in time t z  when the resistance is switched from the intermediate resistance state T IR  to the low impedance state T off . Hence, the switching point of the dimmer device is slightly delayed due to the slower charging of the timing capacitor  20 , however, the delay of the switching point of the dimmer device  10  can be determined by the electrical device  30  by determining the switching point t 2  switching from intermediate resistance state T IR  to the low impedance state T off . 
     Accordingly, the electrical device  30  detects that load current I 1  delivered from the dimmer device  10 . On the basis of the measured load current I 1  and the measured leakage current, the electrical device  30  can estimate the number of connected parallel load  14  (e.g. lamps) and shift the switching point t 2  closer to the zero crossing to compensate the shift of the switching point of the dimmer device  10  accordingly. 
     According to a preferred embodiment, the resistance of the intermediate resistance path of the electrical device  30  is decreased continuously during the intermediate resistance state interval T IR  e.g. by a programmable, voltage controlled current sink. 
     According to another embodiment, a capacitor is connected to the input terminal of the electrical device  30  during the disconnection-phase T disc . Any current through the dimmer device  10  during the disconnection-phase T disc  will flow through the timing capacitor  20  and will charge the timing capacitor  20  accordingly. This leakage current will also flow through the electrical device  30  and will at least partially charge the capacitor accordingly. In other words, the charge which is accumulated in the capacitor of the electrical device  30  during the disconnection-phase T disc  is related to the charge in the timing capacitor  20 . During the off-phase T off  after the zero crossing t z  the charge accumulated by the capacitor of the electrical device  30  will be provided as the compensation current I 3  to the dimmer device  10  and will compensate the charge accumulated in the timing capacitor  20  at least partially. Hence, the leakage current can be measured for each connected lamp and the compensation current I 2  can be provided to the dimmer device  10  accordingly. Hence, no separate measurement of the leakage current is necessary. The main benefit of this method is that multiple connected lamps are supported and the compensation current I 3  is adapted to the leakage current accordingly. 
       FIG. 6  shows a schematic diagram of one embodiment of the electrical device  30  simplified to single polarity operation during the disconnection phase T disc . The dimmer device  10  is connected to neutral and to the external power supply  12  and the electrical device  30  is connected to the dimmer device  10  and to the external voltage supply  12 . In  FIG. 6  the load  34  is not shown. The electrical device  30  comprises a sensing resistor  42  for sensing the input voltage V 12  connected in parallel to a diode  44  for simulating the switching from the disconnection-phase T disc  to the off-phase T off . The sensing resistor  42  also represents the components of the electrical device  30  and the load  34  which cause the leakage current I L . In series to the sensing resistor  42  and the diode  44  a parallel connection of a capacitor  46  and a Zener diode  48  is provided. The capacitor  46  is charged by the leakage current I L  during the disconnection-phase T disc . During the off-phase (not shown), the charge accumulated in the capacitor  46  is released and provided to the dimmer device  10 . The benefit of the circuit shown in  FIG. 6  is that no separate measurement is necessary and the charge accumulated in the capacitor  64  is provided to the dimmer device  10  accordingly. The leakage current I L  leads to the undesired charging of the timing capacitor  20 . When the capacitor  46  is discharged during the off-phase T off , the voltage across the timing capacitor  20  is reduced to the usual starting point of the charging procedure during the low impedance state T off . The capacitor  46  preferable has a capacity of 10 nF. The sensing resistor  42 , so the equivalent input impedance of the load  34  may have a resistance of 2 MOhm. 
     In  FIG. 7  an embodiment of the electrical device  30  is schematically shown for bipolar operation. The electrical device  30  is connected to the dimmer device  10 , to the load  34  and to neutral. The electrical device  30  comprises a capacitor  52  and a protection device  54  connected in parallel to the capacitor  52 . The electrical device  30  further comprises a low resistance path  56 , a variable resistance path  58  and a resistance path  60 . The electrical device  30  further comprises a first and a second switching element  62 ,  64  for connecting the components  54 - 60  of the electrical device  30  to an input terminal  66  and to output terminals  68 ,  70  of the electrical device  30 . The switching elements  62 ,  64  are preferably formed of semiconductor devices. To realize the different states during the intervals T on , T disc , T off , and T DC  the switching devices  62 ,  64  connects the low resistance path  56 , the variable resistance path  58  and/or the resistance path  60  to the input terminal  66  and one of the output terminals  68 ,  70 . For measuring the leakage current I L , the capacitor  52  can be connected to the input terminal  66  and the output terminal  68  in a first switching position to charge the capacitor  52  during the disconnection-phase T disc  and after the zero crossing t z , the polarity of the capacitor  52  is inverted by means of a second switch position  72  to provide the collected charge as the compensation current I 2  to the dimmer device  10 . 
     Hence, the different states described above can be provided by the electrical device  30  shown in  FIG. 6  to compensate the effect of the leakage current I L  in order to operate the dimmer device  10  as desired. 
     In  FIG. 8  a driver device  80  for driving a load  81  is schematically shown. The driver device  80  comprises two electrical devices  82 ,  82 ′ and a control unit  84  for controlling the electrical devices  82 ,  82 ′. 
     The driver device  80  comprises two input terminals  86 ,  88  connecting the driver device  80  to the voltage supply  12  and to the dimmer device  10 . The driver device  80  comprises two current paths  90 ,  92 , each comprising two diodes  94 ,  96  forming a rectifier unit. The electrical devices  82 ,  82 ′ are each incorporated in one of the current paths  90 ,  92  for measuring the leakage current I L  in the respective path  90 ,  92  and for providing the compensation current I 3 . The electrical devices  82 ,  82 ′ each comprises a capacitor  98  a low resistance path  100 , a variable resistance path  102  and a current source  104 . The electrical devices  82 ,  82 ′ each comprises a switching device  106  for connecting the components  98 - 104  to the respective current path  90 ,  92 . The control unit  84  is connected to each of the electrical devices  82 ,  82 ′ and receives a measurement signal  108  from each of the electrical devices  82 ,  82 ′. Dependent on the measurement signal  108 , the control unit  84  controls the switching devices  106  by means of a control signal  110  to connect the different components  98 - 104  to the respective current path  90 ,  92  to provide the compensation current I 2  to the dimmer device  10 . Hence, for each of the current path  90 ,  92  a unipolar operating electrical device  82 ,  82 ′ can be provided to measure a leakage current IL in the respective current path  90 ,  92  and to provide the respective compensation current I 3 . The control unit  84  may be adapted to measure the leakage current I L  in one of the current paths  90 ,  92  and to provide the compensation current I 3  to the same or the other current path  90 ,  92 . The switching devices  106  are preferably formed of semiconductor devices. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. 
     In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. 
     Any reference signs in the claims should not be construed as limiting the scope.

Technology Category: 5