Abstract:
A circuit arrangement suitable for operating a high-pressure discharge lamp (80) in conjunction with a controlled current limiter (6) by means of a control signal which is at least composed of the sum of a lamp-voltage-dependent signal part and a lamp-current-dependent signal part. The invention, the absolute value of the lamp-current-dependent signal part is chosen to be smaller than the absolute value of the lamp-voltage-dependent signal part. The circuit arrangement provides a rapid control, which keeps the lamp voltage substantially constant.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a circuit arrangement for operating a high-pressure discharge lamp in conjunction with a controlled current limiter by means of a switching signal produced in the circuit arrangement and resulting from at least a first comparison of a lamp-dependent control signal S with a reference signal. The control signal S is at least composed of a summation of a lamp-voltage-dependent part and a lamp-current-dependent part. The invention further relates to a device provided with the circuit arrangement and to a lamp provided with the circuit arrangement. 
     A circuit arrangement of the kind mentioned in the opening paragraph is known from U.K. Patent Specification 1,167,920. 
     The known circuit arrangement is connected to two thyristors arranged in parallel with opposite polarities as a controlled current limiter. A coil operative as a current stabilization ballast is connected in series with the thyristors. The anti-parallel connected thyristors may be replaced by a triac. However, it is alternatively possible that the combination of thyristors and current stabilization ballast be replaced as a whole by a controlled current limiter. 
     It is common practice for high-pressure discharge lamps to be operated with an alternating voltage or with a pulsatory direct voltage. The power at which the lamp is operated is to be understood here to mean the power averaged over a time which is long as compared with the period of the alternating voltage and the pulse voltage, respectively. An average lamp voltage and current, respectively, may be formed by averaging in time the absolute value of the lamp voltage and lamp current, respectively. Another way in which an average lamp voltage and lamp current, respectively, may be formed is by the root of the time average of the square of the lamp voltage and current, respectively, the so-called R.M.S. value. In each period of the alternating voltage, the actual lamp voltage waveform will include a time period of comparatively very low voltage value, a re-ignition peak voltage and a time period having a comparatively high and approximately constant voltage value. The comparatively high approximately constant value is known under the designation of plateau voltage and its time duration corresponds to the time duration in which a discharge arc occurs. 
     In the known circuit arrangement, a high-pressure discharge lamp can be operated at a substantially constant power. For this purpose, at a nominal value of the lamp current and a nominal value of the lamp voltage and lamp-current-dependent part of the control signal is chosen to be equally as large as the lamp-voltage-dependent part. For a lamp with a work-point in the proximity of the nominal values of the average lamp voltage and the average lamp current, the control signal thus summed forms a very close approximation to a control procedure according to the product of lamp voltage and lamp current. A circuit arrangement in which signals are subjected to an addition can be practically realized in a considerably simpler manner than a circuit arrangement in which a multiplication of signals is effected. 
     High-pressure discharge lamps, more particularly high-pressure sodium discharge lamps, form very efficient light sources which are frequently used. A general phenomenon, especially of high-pressure sodium discharge lamps, is that during the lamp life time the lamp voltage varies. This influences not only the power consumed by the lamp and the intensity of the luminous flux emitted by the lamp, but also, as has been found, the color temperature T c  of the light emitted by the lamp. 
     SUMMARY OF THE INVENTION 
     The invention has for an object to provide a circuit arrangement suitable for operating a high-pressure discharge lamp in which the average lamp voltage is kept substantially constant. According to the invention, for this purpose a circuit arrangement of the kind mentioned in the opening paragraph is characterized in that the summation satisfies the relation ##EQU1## where I Ia  is the current through the lamp in A, 
     I Ia ,n is the nominal lamp current in A, 
     V Ia  is the voltage across the lamp in V, 
     V Ia ,n is the nominal lamp voltage in V, 
     β is constant, and 
     C is a proportionality constant expressed in V. 
     The nominal lamp current and voltage, respectively, are the nominal values of the average lamp current and lamp voltage, respectively. The current through the lamp may be the instantaneous lamp current. However, it is also possible for the satisfactory operation of the circuit arrangement to use the average lamp current. Likewise, the instantaneous lamp voltage may be used as the voltage across the lamp, but the average lamp voltage may also be utilized. For the average lamp voltage and lamp current, respectively, the R.M.S. value, as well as the value of averaging the absolute value, may be chosen. Although a difference may occur between these values, this difference does not detrimentally affect the satisfactory operation of the circuit arrangement. Preferably, the factor β satisfies the relation 0.1&lt;β&lt;0.5. When the average lamp voltage is kept substantially constant, it is achieved on the one hand that the life time is lengthened and on the other hand that the colour temperature T c  remains highly constant. Furthermore, the use of the circuit arrangement leads to a reduction of the spread in lamp properties between individual lamps of the same type. 
     In lamps with sodium as a filling constituent, the colour temperature T c  of the emitted radiation is related to the pressure of the sodium in the discharge vessel of the lamp. In the case of an excess filling of the discharge vessel, the sodium pressure is determined by the temperature of the sodium present in excess. The filling of the discharge vessel of high-pressure sodium discharge lamps generally consists of a sodium-mercury amalgam and a rare gas. The composition and the temperature of the amalgam are then of important factors for the lamp voltage because the latter is a function of the relative Na and Hg pressure. So long as the amalgam composition does not change due to disappearance of sodium, it is possible by keeping the average lamp voltage constant to also keep the Na pressure constant. 
     A property of at least high-pressure sodium discharge lamps is that with an abrupt variation of the average lamp current the average lamp voltage varies abruptly with an opposite polarity and then varies gradually with the same polarity as that of the current variation until a stable work-point associated with the varying lamp current is attained. A control technique is which a control signal is only dependent upon the lamp voltage requires in such a case a comparatively long time constant (of the order of a few tens of seconds) of the controlling process to obtain a stable control, as a result of which the quantity to be controlled, i.e. the lamp voltage, will be subjected to comparatively large variations. Besides, it is very objectionable when a time constant of a few tens of seconds is required in a circuit arrangement. 
     When now a fraction having a polarity corresponding to the polarity of the current variation is added to the control signal, the required time constant of the controlling process can be shortened, as a result of which the control of the lamp voltage can be effected much more rapidly and the relevant circuit arrangement can be considerably simplified. According to the invention, the fraction chosen is ##EQU2## preferably, β is then chosen so that it holds for the control signal that ##EQU3## where ΔI is an abrupt variation in the lamp current and 
     ΔS is an abrupt variation in the control signal S as a result of ΔI. 
     The control operation can then take place substantially instantaneously. This has the further advantage that the circuit arrangement can be simpler and such a choice of β then reduces the cost. When the value of ΔS/CΔI is kept small and hence the value of β is also kept small, it is achieved that the control is mainly based on the lamp voltage, which yields the optimum result for keeping constant the color temperature T c . 
     Lamp experiments have shown that a β of at least 0.1 is required to obtain a time constant of the controlling process which is at most 1 s. 
     In an embodiment of the circuit arrangement according to the invention, the switching signal is also the result of a second comparison of a sawtooth-shaped signal with an auxiliary signal proportional to the control signal S and a direct voltage signal is added to the sawtooth-shaped signal. An advantage of this preferred embodiment is that due to the choice of the value of the added direct voltage signal, the control range of the circuit arrangement can be adjusted in a comparatively simple manner. 
     A preferred embodiment of the circuit arrangement comprises a part for forming the sawtooth-shaped signal and this part comprises a first series-combination of a first semiconductor element with a diode characteristic, a capacitor shuntable by a switch, and a first resistor, while a junction of capacitor and first resistor is connected to a first input of the operational amplifier intended to carry out the second comparison. The first semiconductor element with the diode characteristic ensures in a very simple manner that a direct voltage signal is added to the sawtooth-shaped signal. The term &#34;diode characteristic&#34; in this description and the claims includes a characteristic of a Zener diode. 
     In a further preferred embodiment of the circuit arrangement, a second series-combination comprising a first semiconductor element with a Zener characteristic and a second resistor is connected parallel to the first series-combination and a junction of the first semiconductor element with the Zener characteristic and the second resistor is connected to a second input of the operational amplifier, this input serving as a connection for the auxiliary signal. This embodiment has the advantage that due to the semiconductor element with the Zener characteristic the value of the signal at the second input is always smaller than the maximum attainable value of the sawtooth-shaped signal. 
     In another preferred embodiment of the circuit arrangement according to the invention, the circuit arrangement comprises a voltage divider circuit which, when the lamp is connected, is arranged electrically parallel to the lamp and of which a first part serves to obtain the lamp voltage-dependent part of the control signal S. This first part is shunted by at least a second semiconductor element with a diode characteristic. 
     In a further embodiment, which is suitable for operation of the lamp with an alternating voltage, the first part of the voltage divider circuit is shunted by a second and a third semiconductor element with a Zener characteristic and connected with opposite polarities. 
     The preferred embodiments described have the great advantage that due to mutual adaptation of the voltage division in the voltage divider circuit and diode forward voltage or Zener voltage of the semiconductor elements, substantially only the plateau voltage of the lamp voltage contributes to the lamp-voltage-dependent part of the control signal S. As a result, β can also be chosen to be smaller, as experiments have shown. 
     It is achieved with the use of two semiconductor elements with opposite polarities that during both polarity parts of the alternating voltage supply the lamp-voltage-dependent part of the control signal is formed in the same manner. This prevents the lamp from flickering. This is advantageous, especially for comparatively low frequencies (50 Hz) of the alternating voltage. The use of semiconductor elements with a Zener characteristic then has the advantage that the influence of the ambient temperature on the operation of the circuit arrangement is greatly reduced. 
     The circuit arrangement may be constructed as a separate device. Preferably, the circuit arrangement is joined with the controlled current limiter to form a single device. It is also conceivable that the circuit arrangement is joined with both the controlled current limiter and a current stabilization ballast to form a sing device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     An embodiment of a circuit arrangement according to the invention will be described more fully with reference to accompanying drawing. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the drawing, a first connection terminal 1 is connected through a stabilization ballast 2 to a lamp connection terminal 3. Another lamp connection terminal 4 is connected via a resistor 5 to a main electrode 6a of a controlled current limiter 6 constructed as a triac. Another main electrode 6b of the triac 6 is connected via a coil 74 to a second connection terminal 7. The lamp connection terminal 3 is connected through a series-combination of a resistor 8, a resistor 9a and a resistor 9b to the lamp connection terminal 4. 
     A junction between resistors 9a and 9b is connected through a capacitor 10 and a resistor 11 to a positive input 12 of a first operational amplifier 13. A negative input 14 of the first operational amplifier 13 is connected via a resistor 15 and a capacitor 16 to the main electrode 6a of the triac 6. The capacitor 16 is shunted by a series-combination of a Zener diode 17 and a diode 17a connected with opposite polarities. 
     An output 18 of the first operational amplifier 13 is connected via a diode 19 to the negative input 14. A resistor 20 is connected at one end to the input 14 and is connected at another end on the one hand via a diode 21 to the output 18 of the first operational amplifier 13 and on the other hand via a resistor 24 to a negative input 22 of a second operational amplifier 23. A positive input 25 of the second operational amplifier 23 is connected to the positive input 12 of the first operational amplifier 13. An output 26 of the second operational amplifier 23 is connected through a resistor 27 to the negative input 22. 
     At the same time, the output 26 is connected via a resistor 28 to a negative input 29 of a third operational amplifier 30. A positive input 31 of the third operational amplifier 30 is connected to an adjustable tapping 32 on a potentiometer 33. The potentiometer 33 is connected on the one hand to the resistor 15 and on the other hand to the main electrode 6a of the triac 6. The op-amp 30 operates as a first comparator for comparing a reference signal at input 31 with a signal at input 29 determined by the control signal S. 
     An output 34 of the third operational amplifier 30 is connected on the one hand via a capacitor 35 to the negative input 29 and on the other hand via a resistor 83 to a positive input 36 of a fourth operational amplifier 37. The positive input 36 of the fourth operational amplifier 37 is also connected via a Zener diode 82 to the main electrode 6a of the triac 6. The op-amp 37 functions as a second comparator. An output 38 of the fourth operational amplifier is connected via a resistor 39 to a base 70 of a transistor 71. The base 70 is also connected through a resistor 72 to a common lead 73, from which (in a manner not shown) the operational amplifiers (13,23,30,37) are supplied. The transistor 71 is connected on the one hand to the lead 73 and on the other hand via a resistor 39a to a control electrode 40 of the triac 6. 
     A negative input 41 of the fourth operational amplifier 37 is connected on the one hand via a capacitor 42 in series with a stabistor 81 to the main electrode 6a of the triac and on the other hand via a resistor 43 in series with a resistor 45 to the lead 73. The positive input 12 of the first operational amplifier 13 is connected via a resistor 44 and the resistor 45 to the lead 73. The capacitor 16, the potentiometer 33 and the resistor 15 are also connected via the resistor 45 to the lead 73. In turn, the lead 73 is connected through a parallel combination constituted by a Zener diode 46 and a capacitor 47 to the main electrode 6a of the triac 6. The junction 44a is also connected on the one hand via a resistor 84 to the positive input 36 of the amplifier 37 and on the other hand via a resistor 49 to a photosensitive transitor 50, which in turn is connected to the main electrode 6a of the triac 6. The photosensitive transistor 50 constitutes, together with a light-emitting diode 58, an optocoupler 50-58. The photosensitive transistor 50 is shunted by a capacitor 51. At the same time, the photosensitive transistor 50 is connected to the base 52 of a transistor 53, which shunts the capacitor 42. 
     The triac 6 and the coil 74 are shunted by a parallel-combination, a first branch of which is formed by a capacitor 55 and a second branch by a series-combination of a resistor 56, a rectifier bridge 57, a Zener diode 48 and a diode 75. The polarities of the Zener diode 48 and the diode 75 are opposite to each other. The rectifier bridge 57 comprises the diodes 57a, 57b, 57c and 57d. 
     Rectifier terminals 75e and 57f of the rectifier bridge 57 are connected to each other through the light-emitting diode 58. At the same time, the rectifier bridge 57 is connected via the diode 76 to the lead 73. The connection terminal 1 is connected via a resistor 59, a capacitor 60 and a diode 61 to the main electrode 6a. At the same time, the connection terminal 1 is connected via the resistor 59, the capacitor 60 and the diode 62 to the lead 73. The diode 61 is shunted by a capacitor 77 and a capacitor 78 is connected to the connection terminals 1 and 7. The resistors 9a and 9b are shunted by a series-combination of a Zener diode 65 and a Zener diode 66 having opposite polarities. A discharge lamp 80 is connected between the lamp connection terminals 3 and 4. For starting the lamp 80, the latter may be provided with an internal starter. Alternatively, an external starter may be provided which is preferably connected between the lamp connection terminals 3 and 4. The circuit arrangement shown is suitable for operating a high-pressure discharge lamp from an alternating voltage supply source. The operation of the circuit arrangement can be explained as follows. The instantaneous alternating voltage across the resistor 9b constitutes the lamp-voltage-dependent part of the control signal S and the instantaneous alternating voltage across the resistor 5 constituted the lamp-current-dependent part. Thus, in this embodiment of the circuit arrangement, the instantaneous values of the lamp current and the lamp voltage, respectively, are used for the current through the lamp I Ia  and the voltage across the lamp V Ia , respectively. The summation of these alternating voltages, thus constituting the control signal S, is applied via the capacitors 16 and 10 to the input terminals 14 and 12 of the operational amplifier 13. The size ratio of the resistors 5 and the voltage divider circuit 8, 9a, 9b then determines the values of β on the one hand and CI i  Ia,n and CV i  Ia,n on the other hand. The circuit of operational amplifiers 13 and 23 forms from the alternating voltage control signal S at the inputs 12 and 14 a rectified signal at the input 29 of the operational amplifier 30. In the operational amplifier 30, this rectified signal is integrated on the one hand and is compared on the other hand with the direct voltage at the input 31 originating from the adjustable tapping 32 on the potentiometer 33. This integration means the averaging of |S|  and thus the averaging of the absolute values of the current through the lamp and the voltage across the lamp. The integration is effected with a time constant which is determined by the resistor 28 and the capacitor 35. The time constant is chosen to be large compared with the time duration per half cycle of the alternating voltage in which the triac 6 is non-conducting. A time constant of the order of the half cycle of the alternating voltage is then to be preferred. Due to the integration, the possibility of flickering of the lamp is reduced. The direct voltage originating from the adjustable tapping 32 on the potentiometer 33 serves as a reference signal and is fixed during adjustment of the circuit arrangement by adjusting the potentiometer 33. This adjustment further ensures that the influence on the switching signal due to differences between individual elements of the circuit arrangement is greatly reduced. The said differences are mainly due to a spread in the values of the components used in the circuit arrangement. An auxiliary signal, which is thus obtained at the output 34 and is proportional to the control signal S, is compared in the operational amplifier 37 as a second comparison with a sawtooth-shaped signal in such a manner that a low voltage is applied to the output 38 of the operational amplifier 37 as long as the auxiliary signal is larger than the sawtooth-shaped signal, while in any other case a high voltage is applied. Thus, the operational amplifier 37 constitutes the operational amplifier intended for carrying out the second comparison with 41 as a first input and 36 as a second input, the latter serving as a connection for the auxiliary signal. The input 41 is connected to a junction of the capacitor 42 and the resistor 43, which form part of a first series-combination of a part of the circuit arrangement for forming a sawtooth-shaped signal. The stabistor 81 is then a first semiconductor element with diode characteristic of the first series-combination, and the resistor 43 the first resistor. For the capacitor 42, which is shuntable by a switch, the transistor 53 serves as the shunting switch. The optocoupler 58-50 and the first series-combination of the transistor 53 and the capacitor 51 together constitute the part of the circuit arrangement for forming the sawtooth-shaped signal. 
     A second series-combination connected parallel to the first series-combination comprises the Zener diode 82 as the first semiconductor element with the Zener characteristic and the resistor 84 as the second resistor. A junction between the Zener diode 82 and the resistor 84 is connected, as described, to the positive input 36 of the operational 71 becomes conductive and the triac 6 is rendered conductive via the control electrode 40 of the triac. The triac 6 will be rendered non-conducting when at the end of each half cycle of the alternating voltage, the current through the triac has fallen to a value near zero. The voltage at the output 38 thus constitutes the switching signal produced in the circuit arrangement. 
     In the non-conducting state of the triac 6, the circuit comprising the resistor 56, the rectifier bridge 57, the Zener diode 48 and the diode 75 forms a shunt in a half cycle of the alternating supply voltage, as a result of which a so-called keep-alive current is maintained through the lamp 80. In the next half cycle of the alternating voltage, the keep-alive current flows through the circuit 46, 47, 76, 57 and 56. The keep-alive current ensures that ionization in the lamp is maintained during the non-conducting state of the triac 6, which improves the re-ignition of the lamp when the triac 6 becomes conducting. The keep-alive current further results in that the light emitting diode 58 emits light, so that the photosensitive transistor 50 is conducting and hence the transistor 53 is non-conducting. The capacitor 42 will then be charged via the stabistor 81, as a result of which the value of the voltage at the input 41 of the operational amplifier 37 increases. When the voltage at the input 41 becomes equal to the voltage at the input 36 of the amplifier 37, the triac 6 becomes conducting via the circuit 38, 39, 71, 39a and 40. However, as soon as the triac 6 is conducting, no current will flow any longer through the light-emitting diode 58, which results in a conducting state of the transistor 53, so that the capacitor 42 is discharged abruptly and the value of the voltage at the input 41 decreases abruptly. As a result, the sawtooth-shaped signal is obtained at the input 41. 
     By means of the circuit 59, 60, 62, 46 and 47, a direct voltage is formed between the main electrode 6a and the conductor 73 and this voltage provides, in a manner not shown, the supply voltage for the operational amplifiers 13, 23, 30 and 37. Via the resistor 45, the Zener diode 17 and the diode 17a, the adjustment point of the transistors 50 and 53 and the adjustment point of the operational amplifiers is determined. The circuit elements 55, 74, 78 and 77 ensure that radio-interference is suppressed. Furthermore, the coil 74 serves together with the capacitors 78 and 55 to ensure that the circuit arrangement is insensitive to any interference pulses originating from the alternating-voltage supply source. 
     The Zener diodes 65 and 66 ensure that the lamp-voltage-dependent part of the control signal S is mainly influenced by the plateau voltage of the lamp. 
     The combination of the Zener diode 48 and the diode 75 with opposite polarities ensures together with the diode 76 and the Zener diode 46 that the keep-alive current has the same value in each half cycle of the alternating voltage supply and moreover that the sawtooth-shaped signal at the input 41 is not dependent upon the polarity of the alternating voltage. 
     The stabistor 81 ensures that a direct voltage signal is added to the sawtooth-shaped signal at the input 41. The resistors 83, 84 ensure that the required voltage for satisfactory operation is present at the input 36 of the operational amplifier 37. It is achieved with the Zener diode 82 that the voltage at the input 36 has a smaller value than the maximum attainable value of the sawtooth-shaped signal at the input 41. 
     In order to prevent any overload of the resistor 5, the latter may be shunted by two diodes with opposite polarities. 
     A circuit arrangement of the kind described and suitable for operating a 50 W high-pressure sodium lamp of 200 V, 50 Hz, was proportioned as follows. 
     
         ______________________________________resistor 8           220    kOhmresistor 9a          15     kresistor 9b          2.7    kresistor 5           0.56   Ohmresistor 15          59     kresistor 11          10     kresistor 20          59     kresistor 24          59     kresistor 27          118    kresistor 28          100    kresistor 39          10     kresistor 39a         910    Ohmresistor 43          16     kresistor 44          59     kresistor 45          5.6    kresistor 49          16     kresistor 56          4.7    kresistor 59          820    Ohmresistor 72          10     kresistor 83          56     kresistor 84          10     kpotentiometer 33     4.7    kOhmcapacitor 10         0.1    μFcapacitor 16         15     μFcapacitor 35         0.1    μFcapacitor 42         0.1    μFcapacitor 47         15     μFcapacitor 51         0.1    μFcapacitor 55         0.068  μFcapacitor 60         0.1    μFcapacitor 77         2.2    nFcapacitor 78         0.033  μFzenerdiode 17 type BZX 79 B5V6 trademark Philipszenerdiode 46 type BZX 79 C15 trademark Philipszenerdiode 48 type BZX 79 C15 trademark Philipszenerdiode 65 type BZX 79 B6V2 trademark Philipszenerdiode 66 type BZX 79 B6V2 trademark Philipszenerdiode 82 type BZX 79 B5V6 trademark Philipsdiode 17a type BAV 20 trademark Philipsdiode 19 type BAV 20 trademark Philipsdiode 21 type BAV 20 trademark Philipsdiode 62 type BAV 18 trademark Philipsdiode 61 type BAV 18 trademark Philipsdiode 75 type BAV 20 trademark Philipsdiode 76 type BAV 20 trademark Philipsdiode 75a type BAV 20 trademark Philipsdiode 57b type BAV 20 trademark Philipsdiode 57c type BAV 20 trademark Philipsdiode 57d type BAV 20 trademark Philipsstabistor 81    type BZV 1V5 trademark Philips;light-emitting diode 58           together opto-couplerphotosensitive transistor 50           CNX 35, trademark Philips;operational amplifier 13operational amplifier 23           together IC LM 224,           trademark Signetics;operational amplifier 30operational amplifier 37transistor 53   BC 558transistor 71   BC 337coil 2 type HP 80 W/220 V-50 Hz, trademark Philips;coil 74 1.25 mH-1.6 A, Company Eichoff BV10520triac 6 type BT 136-600 E, trademark Philips.______________________________________ 
    
     A 50 W high-pressure sodium lamp was operated by the circuit arrangement thus proportioned. The lamp had a discharge vessel which had a construction as known from U.S. Pat. No. 4,475,061. The electrode gap was 16.6 mm, which during operation corresponded to a nominal lamp voltage V Ia ,n of 90 V and a nominal lamp current I Ia ,n of 760 mA. 
     The filling of the discharge vessel consisted of 10 mg of mercury-sodium amalgam containing 23% by weight of Na and xenon at a pressure of 53.3 kPa at 300° K. The color temperature T c  of the radiation emitted by the lamp was 2500° K. 
     The luminous efficacy with 100 operating hours is 50 lm/W. The value of β is 0.4. 
     During operation of a 30 W high-pressure sodium discharge lamp, the resistor 5 in the circuit arrangement is increased in value to 1 Q. At a nominal lamp voltage V Ia ,n of 90 V and a nominal lamp current I Ia ,n of 470 mA, this corresponds to a value of β of about 0.3. For this 30 W lamp, have determined what experiments what is the smallest value of β is which satisfied the relation ##EQU4## This is found to amount to 0.26 in the case where the plateau voltage mainly influences the lamp-voltage-dependent part of the control signal S. When also the re-ignition peak as a whole influences the control signal S, the required β is found to amount to about 0.4. 
     For a comparable lamp having a power of about 30 W, experiments determined the minimum value of β with different numbers of operating hours so as to satisfy the relation ##EQU5## The values found are as follows: 
     
         ______________________________________ 100 operating hours   β = 0.201000 operating hours   β = 0.122000 operating hours   β = 0.173000 operating hours   β = 0.20.______________________________________ 
    
     For the aforementioned 30 W lamp, with β=0.3 the influence of an abrupt variation of the alternating voltage supply has been determined at the average lamp voltage, the color temperature T c  and the coordinates of the color point. The abrupt variations are 10% with respect to the nominal supply voltage of 220 V. The results are indicated in Table I during operations with the circuit arrangement and in Table II during operation without the circuit arrangement. 
     
                       TABLE I______________________________________Supply alternating voltage (V)             198      220      242average lamp voltage (V)             102.3    104.8    105.6color temperature T.sub.c (K)             2470     2493     2498coordinates of the color point             x.483    .481     .480             y.419    .419     .418.______________________________________ 
    
     
                       TABLE II______________________________________Supply voltage (V)             198      220      242average lamp voltage (V)             72.1     88.9     113.7lamp power (W)    24.9     31       43.9color temperature T.sub.c (K)             2205     2453     2980coordinates of the color point             x.515    .481     .436             y.430    .419     .402______________________________________ 
    
     The values of the average lamp voltage indicated in Table I are comparatively high due to the strongly increased re-ignition peak with the use of the circuit arrangement as compared with the operation of the lamp without the circuit arrangement. The indicated lamp voltage values are measured according to the R.M.S. principle. However, it is remarkable that a variation of 10% in the supply voltage with the use of the circuit arrangement results in a variation of the average lamp voltage of not more than about 2%. Without the use of the circuit arrangement, on the contrary, a variation in the average lamp voltage up to even 28% is obtained. 
     Two 30 W lamps of the same type as described above are operated in the same manner without the use of the circuit arrangement described. The most important results are: 
     
         ______________________________________             lamp 1 lamp 2______________________________________Average lamp voltage (V)               79.8     88.9Color temperature T.sub.c (K)               2309     2453Coordinate of the color point               x.502    .485               y.426    .420______________________________________ 
    
     With a corresponding operation with the use of the circuit arrangement described, the results are: 
     
         ______________________________________             lamp 1 lamp 2______________________________________Average lamp voltage (V)               101.3    104.8color temperature T.sub.c (K)               2470     2493coordinates of the color point               x.483    .481               y.419    .419.______________________________________