Abstract:
A method for driving a discharge lamp ( 1 ) having lamp electrodes ( 2, 3 ), at least one of said electrodes ( 2 ) being implemented as a filament having two electrode terminals ( 2   a,    2   b ), comprises the following steps: during a first time interval (t 1 -t 2 ), generating a discharge lamp current (I L) in said discharge lamp ( 1 ); during a second time interval (t 2 -t 3 ), interrupting the discharge lamp current (I L); during both intervals (t 1 -t 3 ), passing an electrode heating current (I C) through said one electrode ( 2 ); wherein, during said first time interval (t 1 -t 2 ), the discharge lamp current magnitude (I L 1 ) is less than 90% of the nominal current magnitude (I NOM); and wherein the electrode heating current (I C) is set such that the hot resistance R H of said one electrode ( 2 ) is within 4.3 to 4.7 times the cold resistance R C; wherein during the second time interval, the electrode heating current is larger than during the first time interval.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates in general to a method and device for driving a discharge lamp, specifically a fluorescent lamp. 
       BACKGROUND OF THE INVENTION 
       [0002]    A fluorescent lamp comprises, in general, a transparent vessel, typically glass, usually of a tubular shape, having two electrodes disposed at opposite ends of such tube. The tube contains a specific gas atmosphere, typically comprising more than 50% Argon. In operation, an electrical power source is connected to the electrodes, such that a discharge is caused in said atmosphere. During discharge operation of the lamp, the voltage over said electrodes has a typical value, indicated as lamp voltage, and the current through the lamp has a typical value, indicated as lamp current. Although the lamp current can typically be controlled by a driving power source, or driver, a lamp has typical nominal operating voltage and operating current values which depend on lamp type. The lamp is capable of being continuously operated at nominal operational parameters, i.e. nominal voltage and nominal current, and under those nominal operational conditions the lamp generates a typical light intensity according to design specifications. 
         [0003]    There are situations where it is desirable that the fluorescent lamp is operated in a switched mode, in which the lamp is alternatively switched ON and OFF at a certain switching frequency. 
         [0004]    For instance, it may be desired that the light output is reduced, i.e. that the lamp is dimmed. When the lamp is operated in a switched mode, the lamp generates light only during the ON-periods and generates no light during the OFF-periods. The switching frequency is typically selected to be 100 Hz or more, in order to prevent undesirable flicker phenomena. Then, the human eye only observes an average light intensity which depends on the duty cycle, i.e. the ratio of the ON-periods with respect to the total switching period. In another example, a fluorescent lamp may be arranged in an array of multiple fluorescent lamps, and it may be desirable to alternatively switch the lamps on and off. An example of such application is a scanning backlight unit such as used for instance in an LCD television. 
         [0005]    For adequate lamp operation, the cathode should have a certain operational temperature in order to be able to emit electrons which carry the lamp current across the lamp atmosphere to the anode. If the cathode is too cold, it becomes difficult to re-ignite the lamp current for the next ON-period. Re-ignition may then require the use of high-voltage ignition pulses. 
         [0006]    On the other hand, if the cathode is too hot, there are consequences of heating the glass tube, heating the surroundings, and increasing the mercury pressure inside the tube. 
         [0007]    Further, the lamp electrode is provided with an emitter material, typically barium. During use, this emitter material is consumed, and this consumption depends strongly on the electrode temperature. The amount of emitter material is finite. Once the emitter material has been consumed completely, the lamp has reached the end of its lifetime. Thus, a high electrode temperature causes large consumption of emitter material which reduces the lifetime of the lamp. 
         [0008]    Thus, it is known that the operational temperature of the cathode should be within certain predefined margins. 
         [0009]    The cathode is heated by the lamp discharge current. When the lamp is operated in a switched mode to achieve dimming, the lamp discharge current can only heat the cathode during the ON periods of the lamp. When the duty cycle of the lamp current is reduced, the heat input to the cathode from the lamp discharge current is likewise reduced, resulting in a cooler electrode. 
         [0010]    In order to prevent this problem, it is known to provide additional heat to the cathode by passing an electric current through the cathode. In case of AC operation, both electrodes can act as cathode, so both electrodes are provided by electric heating means. 
         [0011]    The basic operation of such lamp system may be explained with reference to the schematic illustration of  FIG. 1 . An elongate lamp  1  has opposite electrodes  2  and  3  connected to a first voltage source  4 , providing a lamp voltage V L ; this voltage source will also be indicated as lamp power source. For the present explanation, it is assumed that the lamp power source  4  is a DC source, having a negative output terminal  4   a  and a positive output terminal  4   b . Lamp electrode  2  connected to the negative output terminal  4   a  is the cathode of the lamp; the opposite electrode  3  is the anode. The cathode  2  is implemented as a spiral filament having two terminals  2   a  and  2   b . The first cathode terminal  2   a  is connected to the negative power source terminal  4   a . A second voltage source  5  has its output terminals connected to the electrode terminals  2   a  and  2   b ; this second voltage source will also be indicated as electrode power source. Although the polarity of the second power source  5  is not essential, the second power source  5  will typically have its negative output terminal connected to the first electrode terminal  2   a  and will typically have its positive output terminal connected to the second electrode terminal  2   b.    
         [0012]    It is noted that it may be that the lamp  1  has a symmetrical design; in that case, the anode will be implemented as a spiral filament as well. 
         [0013]    A first controllable switch S 1  is arranged in series with the lamp  1  and lamp power source  4 . Further, a current limiting device C L  is arranged in series with the lamp  1  and lamp power source  4 . A second controllable switch S 2  is arranged in series with the lamp filament  2  and the electrode power source  5 . The switches S 1  and S 2  are controlled by a controller  10 . The controller  10  is designed to control the first switch S 1  to alternatively close and open.  FIG. 2  schematically illustrates the resulting lamp current I L . On time t 1 , the first switch S 1  is closed and the lamp current I L  flows with a nominal current value I NOM . On time t 2 , the first switch S 1  is opened, so that the lamp current is interrupted, illustrated as the lamp current I L  having a value 0. On time t 3 , the first switch S 1  is closed again, and the above is repeated. The time period from first time t 1  to third time t 3  is indicated as current period T. The duration from first time t 1  to second time t 2 , during which the lamp current is flowing, is indicated as ON-time t ON . A duty cycle Δ is defined as Δ=t ON /T. 
         [0014]    In order to provide heating of the cathode  2 , a heating current is applied to the cathode  2  by closing the second switch S 2 . 
         [0015]    In such cases where the lamp electrode is electrically heated, there is the problem of suitably setting the magnitude of the electrode heating current. This is already a problem when the lamp is operated in dimmed mode in order to achieve dimming, but this problem increases if the duty cycle of the lamp is varied in order to achieve variable dimming. 
         [0016]    The electrode heating current directly influences the temperature of the electrode. Thus, if the electrode current is too low, the electrode temperature may be inadequate for ignition. On the other hand, if the electrode current is too high, the electrode may be too hot. Further, higher electrode currents result in higher electrical losses (I 2 R). 
         [0017]    Further, the amount of emitter material per unit length of electrode has a certain maximum. In order to increase lifetime of the lamp, it is known to increase the electrode length. However, when the length of the electrode spiral is increased, also the resistance of the electrode increases, which in turn increases the electrical losses. 
         [0018]    If an optimum electrode current magnitude is used, so that the emitter consumption is reduced, the total amount of emitter material in the electrode can be reduced while maintaining a long lifetime, meaning that the length of the electrode can be reduced, resulting in a reduced resistance, hence reduced electrical losses. 
         [0019]    It is noted that Japanese patent application 1987-324854, published 28-6-1989 with publication number 1989-163998, discloses a driving circuit for an electric discharge lamp having electrode heating current, wherein the electrode heating power is switched on and off at a constant duty cycle, and wherein the lamp power is switched on and off at the same frequency. The lamp power is switched on only during the OFF period of the electrode heating power. The resulting lamp heating current I C  for such case is also shown in  FIG. 2 . The duty cycle of the lamp power is varied in order to dim the lamp. Thus, there is no electrode heating power applied when the lamp power is switched on. Further, there is a time period during which neither lamp power nor electrode heating power is applied. Although it is advantageous that electrode heating power is applied at least immediately before application of the lamp power, it has been found that this method of operation does not yield optimal conditions. 
       SUMMARY OF THE INVENTION 
       [0020]    According to an important aspect of the invention, a heating current is passed through the cathode during the OFF period of the lamp current as well as during the ON period of the lamp current. According to a further important aspect of the invention, the heating current during the OFF period of the lamp current is larger than heating current during the ON period of the lamp current; preferably, the heating current during the OFF period of the lamp current is equal to the summation of the lamp current and the heating current during the ON period. According to a further important aspect of the invention, the lamp current during the ON period is less than 90% of the nominal current, and the duty cycle is less than 70%. 
         [0021]    It has been found that, with such settings, the temperature distribution in the electrode is homogenous, and the cathode drop is relatively low, so that, in all dim conditions (i.e. all power settings) a very long lifetime of the lamp can be achieved. Or, it is possible to reduce the size of the electrode and thus reducing the electrical losses while maintaining the lifetime. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    These and other aspects, features and advantages of the present invention will be further explained by the following description with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which: 
           [0023]      FIG. 1  is a block diagram schematically illustrating the basic design of a lamp driver circuit in accordance with prior art; 
           [0024]      FIG. 2  is a timing diagram schematically illustrating the timing of some currents in the lamp operating circuit of  FIG. 1 ; 
           [0025]      FIG. 3  is a block diagram schematically illustrating a possible embodiment of a lamp driver circuit according to the present invention; 
           [0026]      FIG. 4  is a timing diagram schematically illustrating the timing of some signals in the driver circuit of  FIG. 3 ; 
           [0027]      FIG. 5  is a block diagram schematically illustrating another possible embodiment of a lamp driver circuit according to the present invention; 
           [0028]      FIG. 6  is a block diagram schematically illustrating a lamp driver circuit for driving a plurality of lamps; 
           [0029]      FIG. 7  is a timing diagram schematically illustrating the timing of some signals in the driver circuit of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]      FIG. 3  schematically shows a block diagram of a lamp driver device  100  for driving a fluorescent lamp  1 . As compared to the device illustrated in  FIG. 1 , the important differences are that the electrode power source  150  is a controllable power source, controlled by the controller  110 . 
         [0031]    More specifically, the controller  110  has a first output  111 , providing a first control signal S C1  for controlling the first switch S 1 . The controller  110  further has a second control output  112  providing a second control signal S C2  for controlling the second switch S 2 . The controller  110  has a third control output  113  providing a power control signal S CP  for controlling the electrode power source  150 . 
         [0032]    The electrode power source  150  has a first output terminal  151  connected directly to the lamp electrode  2 , and second and third output terminals  152  and  153  connected to input terminals a and b of the second controllable switch S 2 , respectively. The second controllable switch S 2  is of a type having an output terminal c which is either connected to the first input terminal a or to the second input terminal b, depending on the second control signal S C2  received from the controller  110 . The output terminal c of the second controllable switch S 2  is connected to the second electrode terminal  2   b . The electrode power source  150  further has a control input  154  receiving the power control signal S CP  from the controller  110 . 
         [0033]    In the embodiment illustrated, the controllable electrode power source  150  continuously provides two different output voltages V 2C  and V 2H  at its second and third output terminals  152  and  153 , respectively, where the second output voltage V 2H  at the third output terminal  153  is higher than the first output voltage V 2C  at the second output terminal  152 . Depending on the operative state of the second switch S 2 , the lamp voltage provided at the output c of the second switch S 2  is then either equal to the first output voltage V 2C  or equal to the second output voltage V 2H . As an alternative, it is possible that the controllable electrode power source  150  is of a type having only one output terminal directly connected to the lamp electrode  2   b , and that the power source  150  is controllable to provide a low output voltage V 2C  or a high output voltage V 2H  at this one output terminal. In that case, a separate second switch S 2  is no longer necessary, and the controller  110  does no longer have to provide the second control signal S C2  for this second switch. 
         [0034]    The controller  110  has a first sense input  116 , receiving a voltage sense input signal S V  representing the voltage at the output terminal c of the second switch S 2 , which therefore indicates the voltage drop over the lamp electrode  2 . The controller  110  has a second sense input  117 , receiving a current sense input signal S I  provided by a current sensor  118  associated with the connection from switch output terminal c to lamp electrode  2 . This current sensor  118  may be any suitable type, as will be clear to a person skilled in the art, so that it is not necessary here to further explain the details of the current sensor  118 . 
         [0035]    It is noted that the electrode power source  150  may be a voltage source, so that the resulting electrode current I C  is determined by the resistance of the lamp electrode  2 , but it is also possible that the electrode power source  150  is a current source, so that the electrode current I C  is determined by the power source  150  while the electrode voltage is determined by the electrode resistance. The phrase “power source” is used to cover both possibilities. 
         [0036]    With also reference to  FIG. 4 , which is a timing diagram showing the behavior of some signals as a function of time, the operation of the driver device  100  is as follows. 
         [0037]    On time t 1 , the controller  110  controls the first controllable switch S 1  to close, so that the lamp current I L  flows with a current magnitude I L1  lower than the nominal current value I NOM .  FIG. 4  illustrates this current as a constant current, but actually the current has a high-frequency component in the order of about 20-200 kHz; the current magnitude I L1  is the average value of this high-frequency current. 
         [0038]    Simultaneously, the controller  110  controls the second switch S 2  to switch to the operative condition where output terminal c is connected to the first input terminal a, indicated as first operative condition AC, resulting in the lamp electrode  2  receiving a low electrode voltage V 2C , as illustrated in  FIG. 4 . Also, the electrode current I C  will have a low current magnitude I CC , as also shown in  FIG. 4 . The electrode heating power can now be written as P CC =V 2C ·I CC . 
         [0039]    At time t 2 , the controller  110  controls the first switch S 1  to open, so that the lamp current is interrupted, and simultaneously the controller  110  generates the second control signal S C2  for the second switch S 2  to switch over to the second operative condition where the output terminal c is connected to the second input terminal b, indicated as second operative condition BC. As a result, the electrode voltage V C  is switched to the high voltage value V 2H , and the electrode current I C  is increased to the high current magnitude I CH . The electrode heating power can now be written as P CH =V 2H ·I CH . 
         [0040]    On time t 3 , the first switch S 1  is closed again, and the second switch S 2  is switched to its first operative state AC again. 
         [0041]    The time interval from t 1  to t 2  will be indicated as ON period, the time interval from t 2  to t 3  will be indicated as OFF period. It is noted that the applied electrode heating current is substantially constant during the ON period, and is also substantially constant during the OFF period. It is further noted that the applied electrode heating current and the applied lamp current are always switched substantially simultaneously. 
         [0042]    During the ON period, the heat input into the lamp electrode  2  is determined by the current magnitude I L1  of the lamp current I L  and the current magnitude I CC  of the electrode current I C . During the OFF period, the heat input into the lamp electrode  2  is determined by the current magnitude I CH  of the electrode current I C  (more specifically: the corresponding power I CH ×V 2H ). As a result of these three heat input contributions, the lamp electrode  2  takes a certain electrode temperature T, which is substantially constant over the current period t 1 -t 3 . The driver device is designed to operate such that the electrode temperature T is within a certain operational range. The controller  110  may be designed to monitor this electrode temperature on the basis of measuring the electrode resistance. 
         [0043]    It is known that the electrode resistance is influenced by the electrode temperature, so that the electrode resistance is a reliable indication of the electrode temperature. It has been found that the electrode temperature has a suitable operational value if the electrode resistance is about 4.7±0.4 times as high as the electrode resistance of the cold electrode (i.e. room temperature). Expressed in a formula: 
         [0000]      4.3≦ R   H   /R   C ≦5.1  (1) 
         [0000]    wherein R C  indicates the cold electrode resistance and
 
wherein R H  indicates the hot electrode resistance. The above range from 4.3 to 5.1 will be indicated as the operational range of the electrode resistance, while the value of 4.7 will be indicated as the optimal operational value of the electrode resistance.
 
         [0044]    As explained above, the controller  110  has three possible heat sources for the electrode to control, and the optimal operational value of the electrode resistance can be achieved with several settings of these three heat sources. However, the inventors have found that the specific settings of said three heat sources play an important role, and the present invention provides a set of rules for the settings of these three heat sources, as will be explained in the following. 
       1. The Lamp Current Magnitude 
       [0045]    It is possible to achieve the optimal operational value of the electrode resistance R H =4.7·R C  with a continuous lamp current, without electrode heating. The lamp current magnitude required for such operation is indicated as nominal current I NOM . According to a first aspect of the present invention, the setting of the lamp current magnitude I L1  during the ON period of the lamp is selected substantially lower than the nominal current I NOM . More particularly, the lamp current magnitude I L1  is preferably set according to the following formula: 
         [0000]        I   L1 ≦0.9× I   NOM   (2) 
       2. The Electrode Current Magnitude 
       [0046]    The remaining heat input required for achieving the desired temperature setting is provided by the (power of the) electrode heating current I CC  during the ON period and I CH  during the OFF period. In principle, the controller  110  has some freedom in selecting a combination of these current magnitudes. Preferably, these current magnitudes are selected such that the following formulas are satisfied: 
         [0000]        I   CC ≧0.1× I   NOM   (3A) 
         [0000]        I   CH   ≈I   CC   +I   L1   (3B) 
         [0000]    Formula 3B means that the overall current through the electrode is substantially constant in time. In an alternative approach, it would also be possible, in stead of formula 3B, to apply the following formula: 
         [0000]      I CH =I CC   (3C) 
         [0000]    indicating that the electrode heating current through the electrode is substantially constant in time. 
       3. The Duty Cycle 
       [0047]    The duty cycle may be varied within relative wide limits. It should be clear that, when the setting of the lamp current magnitude I L1  remains constant, the settings for the current magnitudes I CH  and I CC  may depend on the duty cycle. According to an important aspect of the invention, the duty cycle is set at a value more than 0% and less than 100%. Preferably, the duty cycle Δ is set in accordance with the following formula: 
         [0000]      5%≦Δ≦70%  (4A) 
         [0000]    Preferably, the operational range of the electrode resistance is adapted to the duty cycle Δ, such that the operational range decreases with decreasing duty cycle. When a width σ of the operational range is defined such that the operational range extends from 4.7−σ to 4.7+σ, the width σ of the operational range is preferably set according to the following formula: 
         [0000]      σ=0.166+0.33·Δ for 0.05≦Δ≦0.7  (4B) 
         [0000]    According to the present invention, it has been found that operating the lamp in accordance with the above formulas results in very good performance and reduction of the above-mentioned problems. The temperature distribution of the electrode is very homogenous, and the cathode drop is relatively low. Specifically, a very long lifetime is achieved for all duty cycles, which is a surprising result because in general the duty cycle dimming operation is considered as reducing the lifetime. The invention makes it possible to make lamps with smaller electrodes while maintaining or even improving the lifetime. Further, the invention makes it possible to manufacture one general lamp design, which can be operated as a high light output lamp or a low light output lamp, as desired, simply by changing the setting of the duty cycle and corresponding current settings. 
         [0048]    It is noted that the “cold” resistance R C  of the lamp electrode  2  is a fixed property of the lamp. In a typical application, the lamp and the controller/power supply are manufactured as a fixed combination, and in such cases the known value of the “cold” resistance R C  can be stored in a memory associated with the controller, indicated at  120 . Or, this value may be incorporated in the software of the controller. In cases where the controller/power supply and the lamp are manufactured separately, and are combined later, for instance by a user, the controller may have a measuring mode for measuring the “cold” resistance R C : without lamp current, a small measuring electrode current I M  is applied to the lamp electrode, and the resulting electrode voltage V M  is measured, so that the “cold” resistance R C  of the lamp electrode  2  can be calculated according to the following formula: 
         [0000]        R   C   =V   M   /I   M   (5) 
         [0000]    In cases where the “cold” resistance R C  varies in time, during the lifetime of the lamp, this will also be a characteristic of the lamp that is known in advance and can be stored in a controller memory. 
         [0049]    During lamp operation, the controller  110  may be designed to calculate the hot electrode resistance R H  during the OFF-periods according to the following formula: 
         [0000]        R   H   =V   2H   /I   CH   (6) 
         [0000]    However, also the hot electrode resistance R H  can be considered as a device property that is known in advance. More specifically, it is possible to determine in advance the characteristic of resistance R H  as a function of power input, and this characteristic can be stored in a memory. Then, during operation, the controller  110  does not need to actually measure the hot electrode resistance R H  but may suffice with selecting a power setting for the electrode heating current selected in accordance with the predetermined characteristic. In such cases, the detectors for measuring electrode voltage (S V ) and electrode current (S I ) and the corresponding input terminals  116  and  117  can be omitted. 
         [0050]    In the embodiment of  FIG. 3 , the driver device has two functionally separate power supplies, one for the lamp current and one for the electrode heating current, and a controller controlling the current magnitudes.  FIG. 5  schematically illustrates a simplified driver device  500 , having only one common power supply  4 . The lamp  1  has two electrode filaments  2 ,  3 , each having electrode terminals  2   a ,  2   b  and  3   a ,  3   b , respectively. The power supply  4  is connected to electrode terminals  2   a  and  3   a , with an electronic ballast  505  in series. The other electrode terminals  2   b  and  3   b  are coupled to a controllable switch  520 , controlled by a controller  510 , with an electronic load  530  connected in parallel to the switch  520 . The electronic ballast  505  takes care of providing the required lamp current, especially the combination of DC current level and HF current component. 
       On Period 
       [0051]    When the switch  520  is OPEN, the output voltage of the power source  4  and/or ballast  505  is available over the lamp  1 . In the lamp  1 , a lamp current I L  will flow. In the parallel load  530 , an electrode heating current I CC  will flow. The ballast  505  provides a supply current I S =I L +I CC . 
       Off Period 
       [0052]    When the switch  520  is CLOSED, the lamp is short-circuited, and the supply current I S  will flow through the electrodes  2 ,  3  and the switch  520  as electrode heating current I CH . In this design, the supply current I S  as provided by the electronic ballast  505  and the impedance of the load  530  are set to meet the above formulas. The controller  510  controls the duty cycle; variations in the duty cycle require no adaptations of the supply current I S . 
         [0053]      FIG. 6  schematically illustrates a driver device  200 , suitable for driving a plurality of lamps according to the principles of the present invention, based on the design of  FIG. 3 ; an alternative design based on the design of  FIG. 5  is also possible. In the illustration of  FIG. 6 , only three lamps L 1 , L 2 , L 3  are shown, but the invention is of course also applicable for an array of two lamps, or an array of four or more lamps. The lamps are all connected to the first power source V 1 , each lamp L 1 , L 2 , L 3  having a corresponding first controllable switch S 11 , S 21 , S 31  connected in series between the lamp anode  31 ,  32 ,  33  and the positive power rail  4   b . The driver device  200  has a controller  210 , which has control outputs  211 ,  221 ,  231  coupled to the first controllable switches S 11 , S 21 , S 31 , respectively. 
         [0054]    Each lamp L 1 , L 2 , L 3  has its cathode  21 ,  22 ,  23  connected to a controllable power source V 11 , V 21 , V 31 , respectively. Each of the electrode power sources can be considered to be equivalent to the combination of electrode power source  150  and second controllable switch S 2  discussed in the above with reference to  FIG. 3 . The controller  210  has control output terminals  212 ,  222 ,  232  coupled to control input of these electrode power sources V 11 , V 21 , V 31 , respectively. 
         [0055]    Further, the controller  210  has sense input terminals  213 ,  223 ,  233 , receiving information on the electrode voltage and electrode current, respectively, of the lamps L 1 , L 2 , L 3 , respectively. 
         [0056]    The operation of the driver device  200  is illustrated in  FIG. 7 . On time t 1 , the controller  210  closes the first controllable switch S 11  of the first lamp L 1 , while the controllable switches S 21  and S 31  of the second and third lamps are open. Thus, only the first lamp L 1  has a lamp current flowing. Also on time t 1 , the controller  210  instructs the first electrode power source V 11  to provide a low electrode heating current I C1L , so that the controller  210  is capable of measuring the hot electrode resistance R H1  of the first electrode  21  on the basis of the electrode voltage and electrode current information received at its sense input  213 , as explained in the above. 
         [0057]    On time t 2 , the controller  210  opens the first controllable switch S 11  and closes the second controllable switch S 21 , so that the first lamp L 1  goes to its OFF-state while the second lamp L 2  goes to its ON-state. Simultaneously, the controller  210  controls the second electrode power source V 21  to provide low electrode heating current I C2L , allowing the controller  210  to calculate the hot electrode resistance R H2  of the second lamp electrode  22 . Regarding the first lamp L 1 , the controller  210  controls the first electrode power source V 11  to provide high electrode heating current. 
         [0058]    On time t 3 , the controller  210  opens the controllable switch S 21  of the second lamp L 2 , so that this second lamp L 2  is switched to its OFF-state, and closes the controllable switch S 31  of the third lamp L 3 , so that this third lamp L 3  is switched to its ON-state. Simultaneously, the controller  210  controls the third electrode power source V 31  to provide low electrode heating current I C3L , allowing the controller  210  to calculate the hot electrode resistance R H3  of the third lamp electrode  23  on the basis of the voltage and current information received at its sense input  233 , and controls the second electrode power source V 21  to provide high electrode heating current I C2H . 
         [0059]    On time t 4 , the first lamp L 1  is switched ON again and the third lamp is switched OFF. Simultaneously, the controller  210  controls the third electrode power source V 31  to provide high electrode heating current I C3H , and controls the first electrode power source V 11  to provide low electrode heating current I C1L . 
         [0060]    It should be clear to a person skilled in the art that the present invention is not limited to the exemplary embodiments discussed above, but that several variations and modifications are possible within the protective scope of the invention as defined in the appending claims. 
         [0061]    In the above, the present invention has been explained with reference to block diagrams, which illustrate functional blocks of the device according to the present invention. It is to be understood that one or more of these functional blocks may be implemented in hardware, where the function of such functional block is performed by individual hardware components, but it is also possible that one or more of these functional blocks are implemented in software, so that the function of such functional block is performed by one or more program lines of a computer program or a programmable device such as a microprocessor, microcontroller, digital signal processor, etc.