Patent Publication Number: US-3875459-A

Title: Arrangement for igniting and supplying a discharge lamp

Description:
United States Patent Remery et aI.  
 ARRANGEMENT FOR IGNITING AND SUPPLYING A DISCHARGE LAMP Inventors: Michel Remery; Claude Mothiron, both of Evreux, France U.S. Phillips Corporation, New York, NY.  
 Filed: May 7, 1973 Appl. No.: 357,622  
 Assignee:  
 Foreign Application Priority Data May 9, I972 France 72.I652l Dec. 6. 1972 France..... 72.43394 Mar. 21. I973 France 73.l0l49 US. Cl 315/205, 3l5/DIG. 5, 3l5/DIG. 7, 3I5/l03, 3l5/I05 Int. Cl. HOSb 37/00 Field of Search 3I5/DIG. 5, DIG. 7, 10], 315/103, I05, I07, 205, 289  
 References Cited UNITED STATES PATENTS 2/I972 Koyama et al. BIS/I00 T X [451 Apr. 1, 1975 3,705,329 I2/l972 Vogeli 3l5/l06 X 3,720,861 3/l973 Kahanic.......... 3lS/l07 X Primary E.raminerRudolph V., Rolinec Assistant Examiner-Lawrence .l. Dahl Attorney, Agent, or Firm-Frank R. Trifari; Bernard Franzblau [57] ABSTRACT The invention relates to an electronic starter for igniting a discharge lamp provided with preheatable electrodes.  
 According to the invention the starter includes a transistor which is rendered conducting and non-conducting at a frequency of 25 KHz at least just before the ignition of the lampv This control of the transistor is preceded by a time interval during which the electrodes receive a preheating current,  
 26 Claims, 8 Drawing Figures ARRANGEMENT FOR IGNITING AND SUPPLYING A DISCHARGE LAMP The invention relates to an arrangement for igniting and supplying a discharge lamp which is provided with preheatable electrodes, the arrangement being equipped with two input terminals which are intended to be connected to an alternating voltage source whose frequency is lower than 100 Hz. The two input terminals during ignition of the lamp are connected together by a series arrangement of at least a stabilizing inductance and the lamp. The ends of the lamp electrodes remote from the alternating voltage source are connected together by a starter including a semiconductor switch which is closed during the first part of the ignition procedure of the lamp and which is open in the operating condition of the lamp.  
  A discharge lamp which is provided with preheatable electrodes is, for example, a low-pressure mercury vapour discharge lamp.  
  To explain the above-mentioned expression ignition procedure of the lamp the following is to be noted. The ignition procedure commences at the instant when the arrangement is connected to the alternating voltage source and ends with the definitive ignition of the lamp.  
  A known arrangement of the kind mentioned above is described, for example, in French Pat. No. 2,041 ,024. In this known arrangement the semiconductor switch consists of a thyristor or a triac which during the ignition procedure of the discharge lamp is rendered conducting in each suitable half cycle of the alternating voltage source so that a preheating current can then flow through the lamp electrodes until the lamp ignites.  
  The reliability of starting the discharge lamp in this known arrangement is greater than when using a glow discharge starter provided with movable contacts and which has been known for a much longer time, but this advantage is offset by the fact that the thyristor or the triac is always rendered non-conducting at the instant when the electric current flowing therethrough is substantially zero. The latter means that, in an arrangement according to the said French Pat. No. 2,041,024, a voltage peak enhancing the ignition of the lamp cannot be generated with the aid of the stabilizing inductance arranged in series with the lamp. The stabilizing inductance, which may or may not be combined with a capacitor, is alternatively referred to as the ballast.  
  Due to the absence of this voltage peak, the discharge lamp in the known arrangement must to be ignited at a voltage level which is at most equal to the peak value of the voltage of the alternating voltage source. This requirement is difficult to meet, certainly as regards older discharge lamps, so that the ignition then becomes uncertain. As a result, a ballast is then often used which comprises a leakage transformer. A drawback of such a transformer is, however, that it makes an ignition and supply arrangement for a discharge lamp expensive.  
  An object of the invention is to provide an electronic starter for a discharge lamp in which, with the aid of a simple stabilizing inductance electric peak voltages can be generated across the lamp for the purpose of igniting this lamp, which peak voltages are larger than the peak value of the voltage of the alternating voltage supply source.  
  According to the invention an arrangement for igniting and supplying a discharge lamp of the type provided with preheatable electrodes, and including two input terminals intended to be connected to an alternating voltage source whose frequency is lower than Hz, in which the two input terminals during ignition of the lamp are connected together by a series arrangement of at least a stabilizing inductance and the lamp, and in which the ends of the lamp electrodes remote from the alternating voltage source are connected together by a starter including a semiconductor switch which is closed during the first part of the ignition procedure of the lamp and which is open in the operating condition of the lamp is characterized in that the semiconductor switch is a transistor and that a control circuit connected to the base of the transistor is present which includes a branch which is switched on at least during the last part of the ignition procedure of the lamp, said branch having such a short time constant that the transistor is then rendered conducting and non-conducting at least several times during a half cycle of the alternating voltage source.  
  An advantage of an arrangement according to the invention is that the starter does not have moving contact parts and that high peak voltages which quickly succeed each other can be generated across the lamp by controlling the transistor, so that the lamp is ignited in a reliable manner.  
  It is feasible that in an arrangement according to the invention two or more discharge lamps provided with preheatable electrodes are present, which lamps are ignited, for example, one after the other with the aid of the same transistor starter.  
  The control circuit of the transistor is connected, for example, to its own voltage source, for example, a battery incorporated in the starter.  
  ln a preferred embodiment of an arrangement according to the invention the part of the control circuit of the transistor having the short time constant includes a breakdown element connected to the base of the transistor and is additionally provided with a series arrangement of a first resistor and a first capacitor, the end of the breakdown element remote from the transistor base being connected to a point between the first resistor and the first capacitor, the said series arrangement associated with the control circuit being shunted by the transistor. The combination of at least the breakdown element and the first capacitor is shunted by a second resistor and the connection between the lamp and the first capacitor includes a diode whose pass direction corresponds to that of the transistor.  
  An advantage of the preferred embodiment is that the control circuit of the transistor is supplied from the alternating voltage source so that a battery in the starter is not necessary.  
  A further advantage of this preferred embodiment is that for realizing the voltage peaks across the lamp the transistor is rendered quickly conducting and nonconducting with the aid of a simple control circuit of this transistor. The first capacitor is quickly charged through the diode and the first resistor and, after reaching the threshold voltage of the breakdown element, it is quickly discharged again.  
  In a further preferred embodiment of an arrangement according to the invention the time constant of the said branch of the control circuit is so short that during the last part of the ignition procedure of the lamp the quotient of the frequency of the current flowing through the transistor on the one hand and the frequency of the alte rnating voltage source on the other hand is between and 10,000 and preferably between and 2,000.  
  An advantage of this preferred embodiment is that due to the relatively high frequency with which the transistor is rendered conducting and non-conducting, the frequency of the peak voltages is high so that the lamp is ignited quickly. This is so because a peak voltage is immediately available for ignition as soon as the lamp is in condition to be ignited (electrodes brought to a sufficient temperature).  
  In a further preferred embodiment according to the invention the breakdown voltage of the breakdown element is between the effective value of the voltage of the alternating voltage source and the operating voltage of the lamp.  
  An advantage of this arrangement is that no further auxiliary arrangement is required to render the starter inactive after ignition of the lamp.  
  In a further particular embodiment of an arrangement according to the invention the starter is provided with a second transistor whose base is connected to a branch of a second timing circuit whose time constant is so long that this second timing circuit changes the conducting state of the second transistor only after at least. 1 second, the branch of the second timing circuit being connected in parallel with the branch of the first timing circuit, the main electrode circuit of the second transistor being electrically coupled to the first timing circuit in such a manner that due to the said change of the conducting state of the second transistor a control voltage is applied on the base of the first transistor bringing the first transistor to its non-conducting state.  
  An advantage of this particular embodiment is that even when the discharge lamp is defective the starter occurs rendered inactive after some time. This is be cause the rapid change of control voltages across the base of the first transistor is interrupted after some time with the aid of the second transistor. When the first transistor is brought to its non-conducting state the result is that the starter is then rendered inactive, apart from a small current flowing through the control circuit. This is an advantage because it gives rise to less radio interference.  
  in the latter embodiment the second transistor preferably shunts the first capacitor.  
  In a further special embodiment of an arrangement according to the invention the starter is provided with a rectifier bridge whose input terminals are connected to the ends of the lamp electrodes remote from the alternating voltage source and the main electrodes of the first transistor are connected to the output terminals of this bridge.  
  An advantage of this embodiment is that the transis tor of the starter may be used both in the even and odd half cycles of the alternating voltage source.  
  When the starter is provided with two transistors, in accordance with a previously mentioned special embodiment, the branch of the second timing circuit in cludes a series arrangement of a third resistor and a second capacitor, while the base of the second transistor is connected through a fourth resistor to the connection between the third resistor and the second capacitor.  
  An advantage of this embodiment is that the time circuit of the second transistor is very simple. This is important because the entire starter is preferably accommodated in the smallest possible space.  
  In the latter starter the second capacitor is preferably shunted by a fifth resistor. An advantage thereof is that when switching off the arrangement the second capacitor can be discharged across this fifth resistor. As a result the arrangement is soon ready for a subsequent ignition of the discharge lamp.  
  In a further preferred embodiment according to the invention the first capacitor is shunted by a sixth resistor and the first and/or the sixth resistor is a resistor having a large temperature coefficient so that after the flow of an electric current through these resistors, and when the lamp is not ignited, the voltage across the first capacitor is lower than the threshold voltage of the breakdown element.  
  An advantage of this preferred embodiment is that even without the use of a second transistor the starter is rendered inactive both when the discharge lamp is ignited and when the lamp fails to ignite. This is because a current flowing through these series-arranged resistors (first and sixth resistors) changes the voltage division of these resistors to such an extent that, even in the case of a voltage across the series arrangement corresponding to a failing lamp, the voltage across the sixth resistor (thus across the first capacitor) remains below the threshold voltage of the breakdown element so that the first transistor is brought to its cut-off state. This starter then has one transistor only.  
  if the sixth resistor is the resistor having a large temperature coefficient, this resistor has a negative temperature coefficient.  
  It is feasible that during the entire ignition procedure of the lamp the transistor is rendered conducting and non-conducting at a relatively high frequency. it is likewise feasible that the transistor remains conducting during the first part of the ignition procedure of the lamp (so as to bring about a quick heating of the lamp electrodes) and only thereafter starts to switch on and off. The peak voltages are then only generated when the lamp electrodes have already gained temperature. Generally this is advantageous for extending the lifetime of the lamp. In fact ignition on two cold electrodes causes rapid ageing of these electrodes and blackening of the lamp envelope.  
  A special embodiment of an arrangement according to the invention which insures that the lamp will ignite on hot electrodes, which arrangement is provided with the previously mentioned second resistor in the control circuit of the first transistor, is equipped in such a manner that the main electrodes of a further transistor (third transistor) also form part of the series arrangement of the first resistor and the first capacitor connected in parallel with the main electrode circuit of the first transistor. The base of this further transistor is connected to a timing circuit whose time constant is so long that this further transistor is rendered conducting only after several periods have elapsed after the alternating voltage source has been switched on.  
  An advantage of this arrangement is that charging of the first capacitor in the control circuit of the first transistor is blocked in the first instance by the nonconducting further transistor. As a result the control of the first transistor is mainly determined by the voltage division of the first and the second resistor in combination with the breakdown element. Its value can be adjusted in such a manner that the first transistor is substantially always conducting. As a result a satisfactory preheating of the lamp electrodes is obtained in a simple manner.  
  The latter special embodiment of an arrangement according to the invention may be further improved by shunting the first capacitor with a further resistor having a negative temperature coefficient. An advantage thereof is that this starter automatically stops again. This means that the starter is automatically rendered inactive after some time.  
  The special embodiment of an arrangement according to the invention which is provided with a further transistor may also be improved by providing the control circuit of this further transistor with at least a series arrangement of a second diode, a seventh resistor and a third capacitor, one end of this series arrangement being connected to a point between one of the input terminals of the arrangement and the stabilizing inductance connected thereto.  
  An advantage of an arrangement according to the latter improvement is that during the short-circuit state of the first transistor a control voltage for the further transistor can be generated in a simple manner.  
  It is of course also feasible that during the shortcircuit state of the first transistor a control voltage is generated for the further transistor with the aid of the voltage derived from another part of the currentconveying main electrode circuit of the first transistor. For example, with the aid of an auxiliary control transformer connected the main electrode circuit.  
  The invention will be further described with reference to the accompanying drawing.  
  FIG. I shows a circuit diagram of an arrangement according to the invention;  
  FIG. 2 shows the voltage as a function of time between the electrodes of the lamp of FIG. 1 just after switching on the arrangement;  
  FIG. 3 shows the voltage as a function of time between the electrodes of the lamp of FIG. 1 right after ignition of this lamp;  
  FIG. 4 shows the voltage as a function of time between the electrodes of the lamp of FIG. 1, some time after ignition of this lamp;  
  FIG. 5 shows a second arrangement according to the invention;  
  FIG. 6 shows a third arrangement according to the invention;  
  FIG. 7 shows a fourth arrangement according to the invention; and  
  FIG. 8 shows the voltage as a function of time between the electrodes of the lamp of FIG. 7 during the ignition proecedure as well as shortly thereafter.  
  In FIG. 1, input terminals 1 and 2 of the arrangement are intended to be connected to a supply source of, for example, 220 Volt, 50 Hz. The terminal 1 is connected through a stabilizing inductor 6 to an electrode 3 of a low-pressure mercury vapour discharge lamp 5. The input terminal 2 is connected to an electrode 4 of the said discharge lamp 5. The electrodes 3 and 4 of the lamp 5 are of the preheatable type.  
  The ends of the electrodes 3 and 4 remote from the input terminals 1 and 2 are connected to the input terminals 7 and 8 ofa diode bridge (diodes 9, 10, ll, 12). An output terminal 13 of this diode bridge is connected to a conductor 15. Due to the arrangement of the diodes this conductor has a positive potential. An output terminal 14 of the diode bridge is connected to a conductor 16. Due to the arrangement of the diodes this conductor 16 has a negative potential.  
  A first transistor 17 (ignition transistor) of the npntype is arranged between the conductors l5 and 16, the collector being connected to the positive conductor 15 and the emitter being connected to the negative conductor 16. The base of the transistor 17 is connected to a resistor 18. The other end of this resistor 18 is connected to the conductor 16.  
  The control circuit of the transistor 17 consists of a timing circuit which is constituted, inter alia, by a series arrangement of a first resistor 19 and a first capacitor 20. This series arrangement is arranged between the conductors 15 and 16. The junction of the resistor 19 and the capacitor 20 is connected through a breakdown element 21, formed as a diac, to the base of the transistor 17.  
  One main electrode (the collector) of a second transistor 22 switch-off transistor) likewise of the npn-type is connected to the junction of resistor 19 and the capacitor 20. The emitter of transistor 22 is connected to the conductor 16.  
  A second timing circuit connected to the transistor 22 consists of a series arrangement of a resistor 23 and a capacitor 24. This series arrangement is connected between the conductors 15 and 16. A junction between the resistor 23 and the capacitor 24 is connected through a further resistor 25 to the base of the transistor 22. In addition the capacitor 24 is shunted by a discharge resistor 26.  
  The operation of the arrangement of FIG. 1 is as follows. The instant when the terminals 1 and 2 are connected to the alternating voltage source, the lamp 5 initially remains extinguished. A direct voltage (fullwave rectification) is generated through the inductor 6 and the diode bridge 9 to 12 between the conductors 15 and 16.  
  Initially the transistor 17 is cut off. The capacitor 20 starts to charge through the resistor 19 until the voltage across the capacitor has reached the value of the breakdown voltage of the diac 21. The diac then breaks down and the base of transistor 17 receives a positive voltage relative to its emitter. This transistor therefore becomes conducting and a preheating current starts to flow through the electrodes 3 and 4 of the lamp 5. This current flows through the circuit 1, 6, 3, 9, 13, 17, 14, 12, 8, 4 and 2. A similar current also flows in the reverse direction from terminal 2 through electrode 4, terminal 8, diode 11, transistor 17, terminal 14, diode l0, terminal 7, electrode 3, inductor 6 to the terminal 1. In the situation where the diac 21 conveys current the capacitor 20 is discharged partly through the resistor 18. As a result the diac 21 will revert to its nonconducting state shortly thereafter. Subsequently, the transistor 17 also blocks the current again. The situation is then repeated again. This means that the capacitor 20 starts to charge through the resistor 19. In the manner described the transistor 17 is successively rendered very quickly conducting and non-conducting. The currents flowing through the electrodes 3 and 4 during the conducting period of the transistor 17 heat these electrodes to a temperature at which the lamp can ignite.  
  At a given instant the voltage peaks which are generated at the instant when the transistor 17 becomes nonconducting and which are generated with the aid of the inductor 6 will be sufficiently large to ignite the lamp 5. If the lamp ignites the control of the transistor 17 will be maintained for some time. The voltage peaks between the lamp electrodes 3 and 4 generated thereby are then, however, considerably smaller because the current flowing through the inductor 6 is no longer completely interrupted.  
  Some time after ignition of the lamp the transistor 22 is rendered conducting because the timing circuit 23-24 has a slightly longer time constant. When the capacitor 24 has obtained such a charge that the transistor 22 begins to conduct, this causes a short circuit of the capacitor 20. As a result the transistor 17 permanently returns to its non-conducting state. During the operating condition of the lamp 5 the capacitor 24 retains its voltage.  
  In order to obtain a satisfactory reignition of the lamp 5 after this lamp is switched off, the capacitor 24 of the arrangement of FIG. 1 is shunted by a resistor 26. In fact. if the arrangement of FIG. 1 is switched off, the capacitor 24 will be discharged across the resistor 26. The arrangement is then ready for the lamp 5 to be switched on again. The presence of the transistor 22 and the associated control circuits are important so as to avoid radio-interference of the arrangement as much as possible.  
  An advantage of the arrangement of FIG. 1 is that if the lamp 5 were defective, the transistor 22 would still become conducting after some time and would thereby render the starter inactive.  
  In FIGS. 2 to 4 the reference E shows the AC line voltage as a function of time. Only one period of the supply voltage has been shown. In FIG. 2 the vertical lines show the peak voltages which are generated between the lamp electrodes 3 and 4 with the aid of transistor 17. In one embodiment the peak voltage of these peaks could rise to 700 Volts.  
  FIG. 3 shows the situation after the ignition of the lamp, but before the transistor 22 has become conducting. it can be seen that the vertical lines which also indicate the peak voltages have become considerably smaller in this case.  
  In FIG. 4 the solid line shows the voltage between the electrodes 3 and 4 as a function of time and this is the normal operating voltage of the lamp.  
  In one embodiment the lamp 5 was a low-pressure mercury vapour discharge lamp of 40 Watts provided with a fluorescent layer. The inductance of the ballast 6 was approximately I Henry. The resistor 18 had a value of approximately I kOhm. The resistor 19 had a value of approximately I5 kOhm. The capacitance of the capacitor 20 was approximately Nanofarad. The resistor 23 had a value of approximately l MOhm and the capacitor 24 had a capacitance of approximately 100 micro Farad. The resistors 25 and 26 had a value of approximately 27 k-Ohm. The frequency of the voltage peaks was approximately 25 kHz.  
  Dependent on the input voltage between the terminals l and 2 the lamp was ignited within approximately one-third to one half second. The transistor 22 was rendered conducting approximately 5 to 8 seconds after a voltage had been applied to the arrangement.  
  The arrangement shown in FIG. 5 largely conforms to that of FIG. 1. Corresponding parts therefore have the same reference numerals. A difference in the arrangement of FIG. 5 relative to that of FIG. I is that the second transistor 22 is absent in FIG. 5. This is offset by the fact that in the arrangement of FIG. 5 a resistor 30 has been added which shunts the capacitor 20. The resistor 30 has a large negative temperature coefficient (NTC).  
  The operation of the arrangement of FIG. 5 is, in the first instance, the same as that of FIG. 1. Here too the starter stops some time after a voltage is applied between the terminals 1 and 2 because in the case of FIG. 5, the current flowing through the resistor 30 decreases the resistive value of this resistor so that the potential divider 19, 30 ultimately acquires a new value such that the voltage present across the resistor 30, and thus across the capacitor 20, is lower than the breakdown voltage of the diac 21. The initial resistive value of the NTC resistor 30 was considerably higher so that initially the breakdown value of the diac 21 was achieved. This is the initial situation in which as described with reference to FIG. 1, the transistor 17 is switched on and off at a high frequency.  
  It is feasible to replace the NTC resistor 30 by an ordinary resistor, i.e. a resistor without a large temperature coefficient. If in that case the voltage division between 19 and this resistor is chosen such that the breakdown voltage of the diac 21 is achieved when the lamp is not ignited, but is not achieved at the lower operating voltage of the lamp 5, the starter in this arrangement will also be rendered inactive when the lamp is ignited.  
  A drawback of this solution is, however, that the above-described starter does not stop if the lamp 5 fails to ignite for some reason or other.  
  In the modification of FIG. 6 an arrangement is shown which resembles that of FIG. 5. Also in this case corresponding components have the same reference numerals. The difference over the arrangement of FIG. 5 is that in FIG. 6 a resistor 31 having a positive temperature characteristic is present. This resistor is connected in series with the capacitor 20. The capacitor 20 is shunted by an ordinary resistor 32. The operation of the circuit of FIG. 6 is similar to that of FIG. 5. In the case of FIG. 6 the resistor 31 initially has a very low resistive value so that the threshold voltage of the diac 2 1 is achieved across the capacitor 20. However, if an electric current flows for some time through the resistor 31, its resistance will increase considerably so that a fairly large voltage is present across the resistor resulting in the voltage across the capacitor 20 no longer being able to achieve the threshold voltage of the diac 21. The starter than also stops again.  
  Also in the modification of FIG. 6 the starter will stop both when the lamp 5 is ignited and when the lamp fails to ignite for some reason or other.  
  When using the arrangements of FIGS. 5 and 6, careful attention should be paid to the temperature conditions of the luminaires accommodating these starters because the ambient temperature, as well as the temperature in a luminaire, of course also exert an influence on the resistive values of the relevant resistors having a large temperature coefficient.  
  The modification of FIG. 7 also shows conformity with the circuit of FIG. 1. A difference is, however, the position of the transistor 35 as well as the introduction of the diode 36 and the connection of the control circuit of the transistor 35, which consists of the diode 40, the resistor 39 and the capacitor 37, while the capacitor 37 is shunted by a resistor 38. It can be seen in FIG. 7 that this control circuit, notably the end which is connected to the diode 40, is connected to a point between the connection terminal 1 and the stabilizing inductor 6. Other components of the arrangement of FIG. 7 which correspond to those of FIG. 1 again have the same reference numerals. FIG. 7 shows that, as in FIG. 5, a resistor 30 having a negative temperature coeffcient shunting the capacitor 20 is used.  
  The operation of the circuit of FIG. 7 slightly deviates from that of the previous Figures. In fact, if the terminals I and 2 of FIG. 7 are connected to an alternating voltage source, for example, 220 Volt, 50 Hz, the capacitor 37 will in the first instance be uncharged so that the transistor 35 is cut off. This means that the capacitor 20 cannot be charged. On the other hand this means that the direct voltage between the conductors I and 16 appears across a potential divider consisting of the series arrangement of, inter alia, the resistors 19 and 18. Together with the threshold value of the diac 21 this voltage divider is chosen such that the diac 21 breaks down early in each half cycle of the voltage across terminals 1 and 2. This means that in the first group of half cycles of the voltage between the terminals l and 2 the transistor 17 is substantially always conducting. Consequently, during the first part of the ignition procedure the electrodes 3 and 4 of the lamp 5 are substantially permanently preheated. Only after the capacitor 37 has obtained a given charge via the diode 40 and the resistor 39 does the voltage across the base of the transistor 35 become high enough to trigger the transistor 35 into conduction. Then a situation is obtained which can be compared with the situation of the starter of FIG. 1. In that case the capacitor will start to charge through the resistor 19 and the quick non conductance and conductance of transistor 17 wil! be realized in a manner corresponding to that in the circuit of FIG. 1. However, because the electrodes 3 and 4 have already received a given preheating in the case of FIG. 7, the number of times when peaks have to be generated between the electrodes 3 and 4 will be much smaller than in the case of FIG. 1. When the lamp ignites, the starter will stop a short time thereafter due to the action of the NTC resistor 30. The starter will of course also stop when the lamp 5 fails to ignite for some reason or other. An advantage of the circuit of FIG. 7 is that the risk of ignition of the lamp 5 on cold electrodes is substantially excluded in this case. This leads to a considerably longer lifetime of the lamp 5. A further advantage is that only a limited number of peaks need be generated to ignite the lamp. This likewise means that in this modification the duration of radiointerference pulses being transmitted is very short.  
  In FIG. 8 E denotes the voltage between the terminals l and 2 as a function of time. The arrangement of FIG. 7 is switched on at time 0. During the timeinterval which is denoted by t the transistor 35 is denoted by t the transistor 35 is non-conducting so that during that time-interval the electrodes 3 and 4 of FIG. 7 are substantially permanently preheated. At the end of the time interval the transistor 35 becomes conducting and (in this case) voltage peaks are generated with the aid of the transistor 17 and the inductor 6 during two half cycles (interval t of the AC voltage between terminals 1 and 2. After the time interval the voltage between the electrodes 3 and 4 follows the solid line variation in the right-hand section of FIG. 8. Actually a number of small peaks would be generated right after the time interval 1 between the electrodes 3 and 4, in a similar manner to that indicated in FIG. 3. If, however, the NTC resistor 30 has stopped the starter, the voltage shown in the extreme right-hand part of FIG. 8 is obtained.  
  In one embodiment the capacitance of the capacitor 37 was approximately 22 microfarad. The resistive value of the resistor 38 was approximately 22 kOhm and the resistive value of the resistor 39 was approximately 330 kohm. The preheating period of the electrodes 3 and 4 was adjusted at, for example, half a second.  
  In each of the given embodiments the starter section could be accommodated in a space of only a few cubic What is claimed is:  
  1. An ignition circuit for a discharge lamp having preheatable electrodes comprising, a pair of input terminals adapted to be connected to a low frequency alter nating voltage supply source, means connecting a stabilizing inductor in series with the lamp across said input terminals, a starter circuit for the lamp electrodes including means connecting the ends of the lamp electrodes remote from the input terminals together via a transistor switch, a control circuit coupled to the input terminals and connected to the base of the transistor and including means for closing the transistor switch during a first part of the ignition procedure of the lamp and which opens said transistor switch in the operation condition of the lamp, said control circuit including a branch circuit which is energized at least during the last part of the ignition procedure of the lamp, said branch circuit having timing circuit means with a short time constant relative to the period of the alternating supply voltage so that the transistor is rendered conducting and non-conducting at least several times during a half cycle of the alternating voltage.  
  2. An ignition circuit as claimed in claim 1, wherein the control circuit includes a breakdown element and the timing circuit means includes a series arrangement of a first resistor and a first capacitor, means connecting the breakdown element between the transistor base and a point between the first resistor and the first capacitor, means connecting said series arrangement in shunt with the transistor switch, a second resistor connected in shunt across the combination of at least the breakdown element and the first capacitor and the connection between the lamp and the first capacitor including a diode whose pass direction corresponds to that of the transistor.  
  3. An ignition circuit as claimed in claim 1, characterized in that the time constant of the timing circuit means of the control circuit is so short that during the last part of the ignition procedure of the lamp the quotient of the frequency of the current flowing through the transistor and the frequency of the alternating voltage is between IO and l0,000.  
  4. An ignition circuit as claimed in claim 2, characterized in that the breakdown voltage of the breakdown element is between the effective value of the voltage of the alternating voltage source and the operating voltage of the lamp.  
  5. An ignition circuit as claimed in claim 1 wherein the starter circuit further comprises a second transistor having a base electrode connected to a branch of a second timing circuit whose time constant is longer than the time constant of the first timing circuit means, said second timing circuit being connected to control the conducting state of the second transistor, means connecting the branch of the second timing circuit in parallel with the branch of the first timing circuit, means connecting the main electrode circuit of the second transistor to the first timing circuit so that a change in conduction of the second transistor applies a control voltage to the base of the first transistor to bring the first transistor into its cut-off state.  
  6. An ignition circuit as claimed in claim 1 wherein the starter circuit is provided with a rectifier bridge whose input terminals are connected to the ends of the lamp electrodes remote from the input terminals, and means connecting the main electrodes of the first transistor to the output terminals of said rectifier bridge.  
  7. An ignition circuit as claimed in claim 5 wherein said first timing circuit means includes a series arrangement of a first resistor and a first capacitor and said main electrode circuit connecting means connects the second transistor in shunt with the first capacitor.  
  8. An ignition circuit as claimed in claim 5, wherein the first timing circuit means includes a series arrangement of a first resistor and a first capacitor and a breakdown element connected between the first transistor base and the junction between the first resistor and the first capacitor, means connecting said series arrangement in shunt with the transistor switch, a second resistor connected in shunt across the combination of the breakdown element and the first capacitor, and the branch of the second timing circuit includes a series arrangement of a third resistor and a second capacitor, and means connecting the base of the second transistor through a fourth resistor to the junction between the third resistor and the second capacitor.  
  9. An ignition circuit as claimed in claiim 8, characterized in that the second capacitor is shunted by a fifth resistor.  
  10. An ignition circuit as claimed in claim 1 wherein the first timing circuit means includes a series arrangement of a first resistor and a first capacitor connected in shunt with the transistor switch and a breakdown element connected between the base of the transistor switch and the junction between the first resistor and the first capacitor, and the first capacitor is shunted by a second resistor having a large negative temperature coefficient chosen so that before the lamp is ignited, the voltage across the first capacitor is lower than the threshold voltage of the breakdown element.  
  ll. An ignition circuit as claimed in claim 1 wherein the first timing circuit means includes a series arrangement of a first resistor and a first capacitor connected in shunt with the transistor switch and a breakdown element connected between the base of the transistor switch and the junction between the first resistor and the first capacitor, and further comprising a further transistor connected in series with the series arrangement of the first resistor and its first capacitor in shunt with the main electrode circuit of the transistor switch, means connecting the base of said further transistor to a second timing circuit whose time constant is substantially longer than the time constant of the first timing circuit means so that the further transistor is triggered into conduction after a delay of at least one period of the alternating supply voltage.  
  12. An ignition circuit as claimed in claim 11, characterized in that the first capacitor is shunted by a further resistor having a negative temperature coefficient.  
  13. An ignition circuit as claimed in claim 11, further cmprising acontrol circuit for the further transistor including a series arrangement of a diode, a second resistor and a second capacitor, and means connecting one end of said series arrangement to a point between one of the input terminals and the stabilizing inductor.  
  14. An ignition circuit as claimed in claim 1 wherein the first timing circuit means includes a series arrangement of a first resistor and a first capacitor connected in shunt across the transistor switch and a breakdown element connected between the control electrode of the transistor switch and the junction between the first resistor and the first capacitor, and said starter circuit further comprises a diode bridge with input terminals connected to respective lamp electrodes and output terminals to which the transistor switch is connected.  
  15. A supply circuit for an electric discharge lamp having first and second preheatable electrodes comprising, a pair of input terminals for applying a low frequency AC voltage to the supply circuit, a ballast impedance, a controlled semiconductor switch, means connecting the ballast impedance and the semiconductor switch in series circuit with said first and second lamp electrodes across the input terminals to form a heating circuit for the electrodes, a control circuit coupled to the input terminals and having an output terminal connected to a control electrode of said semiconductor switch to control the conductive state thereof, said control circuit including a first timing circuit for applying a time varying switching voltage to said output terminal, the time constant of said first timing circuit being substantially shorter than a half period of the AC voltage whereby the switching voltage turns the semiconductor switch on and off a plurality of times during a half cycle of the AC voltage, said control circuit further comprising means responsive to the application of an AC voltage to said input terminals for holding the semiconductor switch in the cut-off state via said output terminal and commencing at a given time subsequent to the application of AC voltage to the input terminals, which given time is sufficient to allow the ignition of the discharge lamp.  
  16. A supply circuit as claimed in claim 15 further comprising a full wave rectifier circuit connected between the lamp electrodes and the semiconductor switch so that an AC heating circuit flows through the lamp electrodes when the semiconductor switch is closed during the ignition period, said rectifier circuit being connected to supply DC operating voltages to at least a part of the control circuit and to the semiconductor switch.  
  17. A supply circuit as claimed in claim 15 wherein said first timing circuit includes a capacitor, said supply circuit further comprising, a voltage breakdown element coupling said first timing circuit to the control electrode of said semiconductor switch, and a discharge circuit for the capacitor that includes said breakdown element.  
  18. A supply circuit as claimed in claim 15 wherein said holding means of the control circuit further comprises, a second controlled semiconductor switch coupled to the control electrode of the first semiconductor switch to prevent conduction thereof when the second semiconductor switch is triggered into conduction, and a second timing circuit coupled to the control electrode of the second semiconductor switch to apply thereto a time varying switching voltage, said second timing circuit having a longer time constant than the first timing circuit so that the second semiconductor switch is triggered into conduction several cycles of the AC voltage subsequent to the first conduction period of the first semiconductor switch during an ignition cycle.  
  19. A supply circuit as claimed in claim 18 wherein said first timing circuit includes a capacitor coupled to the control electrode of the first semiconductor switch via a voltage breakdown element, and said second semiconductor switch is connected in shunt with said capacitor to limit the capacitor voltage during the conduction period of the second semiconductor switch.  
  20. A supply circuit as claimed in claim wherein said first timing circuit includes an RC circuit with a capacitor coupled to the control electrode of the first semiconductor switch via a switching device controlled by the capacitor voltage, and said holding means includes a negative temperature coefficient resistor connected in shunt with the capacitor and chosen so that after a given heating period its resistance attains a value to limit the capacitor voltage to a value below the switching voltage of said switching device.  
  21. A supply circuit as claimed in claim 15 wherein said first timing circuit includes an RC circuit with a capacitor coupled to the control electrode of the first semiconductor switch via a switching device controlled by the capacitor voltage and a positive temperature coefficient resistor connected in series with the capacitor.  
  22. A supply circuit as claimed in claim 15 wherein said first timing circuit includes an RC circuit with at capacitor coupled to the control electrode of the semiconductor and switch via a switching device, said holding means comprises. a transistor connected in series with said RC circuit, a second timing circuit coupled to the control electrode of the transistor to apply thereto a time varying switching voltage, said second timing circuit having a longer time constant than the first timing circuit, the transistor being triggered into conduction at a time subsequent to that of the semiconductor switch which time is sufficient to heat the electrodes and ignite the lamp.  
  23. A supply circuit as claimed in claim 22 further comprising a negative temperature coefficient resistor connected in parallel with the timing capacitor.  
  24. A supply circuit as claimed in claim 22 wherein said second timing circuit comprises a diode, a resistor and a second capacitor connected in series circuit across the input terminals.  
  25. A supply circuit as claimed in claim l5 wherein said control circuit is connected so as to energize and operate said first timingcircuit during both half cycles of the AC voltage thereby to switch said semiconductor switch independently of the polarity of the AC voltage, and wherein the time constant of the first timing circuit is chosen so as to trigger the semiconductor switch at a frequqency between 10 and 10,000 times the frequency of the AC voltage.  
  26. A supply circuit as claimed in claim 15 wherein the time constant of the first timing circuit is chosen so as to trigger the semiconductor switch at frequency between 20 and 2000 times the frequency of the AC voltage.