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(11) | EP 0 170 528 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Electronic starter circuit for discharge lamps |
| (57) Starter circuits are disclosed for a fluorescent discharge tube (10) of the type
having a pair of heater cathodes between which the discharge is struck and which are
connectable to the mains supply (V.) through a ballast inductor (L). The starter circuit
(20) includes a thyristor (SCR) that conducts current through the ballast inductor
and heaters on half-cycles on one polarity. A gate control circuit (30) includes a
capacitor (C1)that receives a controlled (by C2, Z1) bias voltage increment in each such half-cycle to eventually terminate triggering
of the thyristor (SCR). Preferably an automatic switching arrangement (C3, Z2) changes the value of capacitance to give a lesser rate of increment upon a certain
bias level being reached to provide a two phase start operation that eventually times
out. Alternatively, the maximum bias may be clamped and a positive temperature coefficient
resistor (PTC) carrying the heater current terminates the second phase upon heating
to a predetermined resistance. The starter circuit (20) also includes a non-linear
dielectric element (NLDE) that cooperates with the inductor to generate striking voltage
for the tube and that is connected in an ignition circuit including a capacitor (Cs) connected to the thyristor (SCR) to charge and maintain the NLDE quiescent during
normal operation but to be discharged by the thyristor (SCR) during starting to allow
the NLDE to achieve saturation and coact with the ballast inductor (L). |
[0001] This invention relates to electronic starter circuits for discharge lamps having pre-heated cathodes and particularly for fluorescent discharge lamps.
[0002] Recent years have seen developments in electronic starter circuits for the conventional fluorescent discharge tube having a respective heater cathode at each end. The tube is connected to the a.c. mains supply such that on starting current is drawn through a series circuit comprising a ballast inductor, the two cathode heaters and an electronic starter circuit that is connected between the two cathode heaters. It is generally desirable to provide a fast heating of the cathodes. The starter circuit is arranged also to provide in conjunction with the ballast inductor, trigger voltages across the cathodes to cause the discharge tube to strike. Thereafter, the starter circuit should become effectively inoperative.
[0003] A relatively simple circuit for this purpose is described in patent specification G.B. 1223733 in which the electronic starter circuit utilises an SCR as the controlled element to control the initial heater current, the thyristor being rendered conductive in half-cycles of one polarity. When the tube strikes, it shunts the thyristor circuit and renders it inoperative. Another such circuit is disclosed in patent specification 1264397 which provides the additional facility of cutting-off the thyristor operation after an interval if the tube has not struck. This is done by incrementally charging a capacitor biasing the thyristor gate in each mains cycle so that the trigger point in successive cycles is advanced until the thyristor no longer triggers.
[0004] A further discussion of both these circuits may be found in patent specification GB 1602456 which goes on to disclose further thyristor control circuits in which a capacitor is incrementally charged over successive cycles to effectively bias the gate beyond the point of triggering if the tube does not strike. Specification GB 1602456 discloses a bias that advances at an essentially fixed rate with time, that is with respect to the number of mains cycles since the circuit was switched on.
[0005] In these various circuits, the voltage required to strike the tube is generated with the aid of the inductor, and possibly an auxiliary capacitor, at a time when the thyristor current drops to zero and the thyristor turns off.
[0006] Another development in fluorescent tube starter circuits has been the adoption of non-linear dielectric elements (NLDEs) based on piezo-electric materials. Such an element, also known as a voltage dependent capacitor, has the property of charging like a capacitor up to a certain voltage at which it saturates and charges no further. If such an element is used in conjunction with an inductor, the interruption upon reaching saturation of charging current supplied to the element through the inductor produces a voltage spike across the inductor which can be used to strike the tube. Various circuits employing NLDEs are described in patent specifications EP 0048137B and EP 0102183.
[0007] One aspect of the present invention is concerned with starter circuits in which an incrementing bias is applied to eventually terminate triggering of the starter if the lamp has not struck. A particular aspect is the provision of means to provide a time delay in the operation of the circuit on a fresh mains cycle.
[0008] If such a starter circuit is to be able to provide an adequate interval to allow starting under adverse conditions, the rate at which the bias increments may actually prevent full starting as early as could be done under normal conditions. Another aspect of the invention is to provide a circuit in which the available starting period is divided into two intervals having different operational characteristics. This is described hereinafter in terms of what are called dual rate or dual slope circuits. The dual rate or slope may arise from different rates of incrementing the bias, or from a phase with an incrementing bias followed by a phase with essentially a fixed bias that is timed out by some other means, such as the use of a positive temperature coefficient resistor heated by the starter current.
[0009] Another aspect of the invention is concerned with the use of NLDEs and in particular a circuit arrangement to use the device in generating high striking voltages during the starting of the lamp but to maintain the NLDE quiescent during normal running.
[0011] The invention and its practice will be described with reference to the accompanying drawings in which:
Fig. 1 shows a fluorescent tube circuit including a starter circuit in accord with the invention;
Fig. 2 shows another fluorescent tube circuit whose starter circuit includes a gate control circuit having a dual-rate characteristic;
Figs. 3 and 4 are graphs pertaining to the operation of the circuit of Fig. 2,and show trigger voltage and heater current respectively as a function of time;
Fig. 5 shows a modification of the gate control circuit shown in Fig. 2;
Fig. 6 shows another fluorescent tube circuit including an NLDE for the generation of the striking voltage for the tube;
Figs. 7A and 7B are graphical representations relating to the operation of the circuit of Fig. 6, and Fig. 7C shows typical waveforms for one cycle of operation;
Fig. 8 is a graphical illustration contrasting the single rate operation of the circuit of Fig. 1 with the dual rate operation of the circuits of Figs. 2 and 6;
Fig. 9 shows a modification of the circuit of Fig. 2 to include the use of a PTC resistor both for protection and to control one phase of the dual-rate triggering operation; and
Figs. 10 and 11 are graphs corresponding to Figs. 3 and 4 but illustrating the operation of the circuit of Fig. 9.
[0013] Referring to Fig. 1, a discharge lamp 10 is shown as being of the conventional fluorescent lamp type having a heater cathode at each end. The lamp is connected to the mains supply Vs applied between terminals T1, T2 through a ballast inductor L. A starter circuit 20 is connected in series with the inductor and the two heater cathodes between supply terminals T1, T2.
[0014] The starter circuit includes a controlled switching element such as a thyristor shown in this example as an SCR providing a half-wave control the current through the heater circuit. The gate control circuit of the SCR is denoted 30 and is responsive to the supply voltage as it appears across the tube to control the triggering of the SCR gate. The gate circuit comprises a resistor Rl which supplies current via diode D1 on positive half-cycles to a timing capacitor C1 in series with the SCR gate. The charging current also flows to a second capacitor C2 in parallel with the gate-cathode circuit of the SCR. Resistors R2 and R3 provide discharge paths for C2 and C1. The voltage developed across C2 is applied to the SCR gate through a zener diode Z1 andresistor R5 which defines a set threshold voltage that must be reached for the SCR to be triggered.
[0015] In addition to the gate control circuit 30, the starter circuit includes a series capacitor-resistor combination C47 R4 connected in parallel with the lamp 10 and co-operable with the inductor L to provide a striking voltage pulse across the lamp 10 as will be described below.
[0016] In operation of the starting circuit of Fig. 1, the charging of the timing capacitor C1 is controlled by a second capacitor C2. For the thyristor to trigger the voltage on C2 must be approximately equal to the voltage of zener diode Z1. As the current charging C2 must flow in C1, the voltage increment on C1 for each cycle is defined
. R2 is of a value low enough to discharge C2 whilst the thyristor is conducting so that C2 is discharged ready for the next charging cycle. R3 is a high value resistor which has little effect during the starting sequence but which discharges C1 when the starter becomes inactive on striking of the lamp, ready for the next start operation. The circuit of Fig. 2 provides a well-defined trigger threshold and a well-defined incrementing of the bias voltage on capacitor C1 which has to be overcome by the voltage applied to the starter circuit. Thus the trigger point for the SCR advances further in successive positive half-cycles. To obtain tube ignition, a high voltage pulse must also be applied following the pulse of heater current in each cycle. For tubes with an Argon gas fill, a sufficiently high negative peak of voltage can be obtained by means of C4 placed across the tube cathodes. When the thyristor SCR stops conducting, C4 forms a resonant circuit with the ballast inductor to give a voltage overswing having a peak value of approximately 600v. (R4 is placed in series with C4 to limit the discharge current when the thyristor switches on again.) The circuit of Fig. 1 is suitable, as shown, for starting Argon tubes up to 65w.
[0017] The resistor R1 is chosen not to significantly impede the desired charging of C1 and C2 but to provide with Cl/C2 a time delay of say 2mS to give time for the lamp to ionize and conduct before the thyristor again triggers. It will be appreciated that although the SCR is triggered in the positive half-cycle, the build-up of current is determined by the ballast inductor, reaching a peak at about the time of the next zero crossing and decaying during the following negative half-cycle, the SCR remaining conductive essentially until the current falls to zero. At this time the negative voltage acts on the resonant circuit provided by L, C4 to provide the striking voltage across the lamp.
[0018] The rate of increase of trigger voltage in the circuit of Fig. 1 is substantially constant, and will determine the maximum time for which the starter circuit can remain active before the bias voltage reaches a level (the peak voltage of the supply) at which the SCR is no longer rendered conductive. It will also determine the minimum time taken for the tube conduction to become fully established. This is because at first the voltage level on which the SCR is triggered is lower than the re-ignition voltage required to produce positive conduction in the tube immediately following a negative conduction period, that is conduction in a positive half-cycle following a negative half-cycle in which the tube is struck. Thus, even though the cathodes may be sufficiently heated and the high negative voltage is causing negative tube conduction on successive half cycles, the positive conduction and therefore full running of the tube which could take place is actually inhibited by the starter re-triggering the SCR until the required applied voltage at which triggering takes place rises, i.e. increments in the way described, to a value equal to the positive re-ignition voltage, typically 250V for a 58 watt tube.For the fastest possible start then, the rate of increase according to this consideration needs to be high enough to provide just enough cathode heating by the time the trigger voltage level reaches 250V in the example cited. However, the rate cannot be chosen using this criterion applied to normal conditions but only to worst case conditions. If the rate is too high, then under conditions of low mains voltage, low temperature, end of life tube etc. the trigger voltage would have risen to the peak supply voltage and the starter therefore rendered inactive before the cathodes are sufficiently heated to provide tube conduction. The rate must therefore be selected to be sufficiently low to cater for adverse conditions, thus compromising the preferred higher rate which would give the faster start under normal conditions. It will be understood that in accord with what has just been said the trigger voltage referred to herein is that value of the applied voltage at which the SCR is caused to trigger into conduction.
[0019] The circuit of Fig. 2 is intended to provide a balance between these requirements by providing a dual rate of control of the triggering of the SCR. The circuit is the same as that of Fig. 1 except that a series combination of a further capacitor C3 and a zener diode Z2 is placed in parallel with timing capacitor C1. Z2 acts as an automatic switch to place C3 in parallel with C1, and thereby increase, the timing capacitance, upon the voltage across C1 reaching the zener voltage of Z2. Operation is similar to the circuit of Fig. 1 except that the finite charge per cycle as governed by C2 now charges C1 until the voltage across C1 reaches the breakdown voltage of zener diode Z2. The charge then has to flow in C1 and C3 in parallel thus reducing the voltage bias increment occurring in each cycle in the ratio
The trigger voltage v. time characteristic therefore becomes as shown in Fig. 3 with the heater current characteristic as in Fig. 4. The trigger voltage at which the rate changes (VBK in Fig. 3) is chosen to be a value at which positive conduction in the tube can initiate.
[0020] The trigger voltage is the voltage at terminals T3, T4 at which the SCR is rendered conductive. The voltage rises eventually to Vs, the maximum of the supply voltage, i.e. the peak of a positive half-cycle, at which time the SCR can no longer be triggered. The heater current shown in Fig. 4 starts at a high level reducing relatively rapidly until the trigger voltage VBK is reached, whereupon with the reduced rate of trigger voltage increase, the heater current falls more slowly, thereby giving time for sufficient heating of the cathodes under adverse conditions before the starter is finally rendered inactive.
[0021] Fig. 5 shows a variation of the gate control circuit (the remaining circuitry is not repeated) for switching the extra capacitor C3 by means of Z2. In Fig. 5, C3 and C1 are in series at the outset, Z2 providing an open circuit switch. When the voltage on C3 reaches the breakdown voltage of Z2, it is clamped at the voltage of Z2 which becomes a closed switch for charging C1 so that the effective capacitance increases to that of C1.
[0022] The embodiments thus far described show the provision of a capacitor C4 to resonate with the ballast inductor L to generate the striking voltage across the tube. Thus voltage may be up to twice the supply peak voltage. For Krypton filled tubes however, a pulse of at least 1000v is normally required. This can be generated using a non-linear dielectric element (NLDE), also known as a voltage dependent capacitor. The characteristics of such devices are described in the aforementioned Specification EP 0102183. NLDEs are available from the TDK Electronics Corporation of Japan. As shown in the just-mentioned specification the NLDE has a saturating charge/voltage characteristic that shows hysteresis and for a time-varying voltage provides a current peak as the device charges to saturation positively or negatively. When the voltage applied to an NLDE reaches a certain threshold it takes current until a finite charge is stored at which point it becomes saturated and cannot sustain further current. Placed in series with a fluorescent lamp ballast choke, current flows in the choke until the NLDE is saturated. When the NLDE becomes saturated and cannot sustain the current now flowing in the choke, a large induced EMF is generated by the choke.
[0023] The use of an NLDE will be described with reference to the circuit shown in Fig. 6 which shows a starter circuit which includes a gate control circuit for the SCR similar to that of Fig. 2 but in which the resonating capacitor C4 is replaced by an NLDE in a circuit that is considered novel and inventive in its own right.
[0024] If the NLDE were placed directly across the ends of lamp 10 it would then cause high voltage generation both in the positive and negative half-cycles (thus interfering with the thyristor triggering arrangements) and also, when the tube is running, it would continue to generate high voltage pulses. The circuit of Fig. 6 avoids these disadvantages. The NLDE is connected across the tube via a resistor R6 for charging in one direction (positive polarity) and via a series capacitor and diode combination (C5,D2) in parallel with R6 for charging on the opposite polarity half-cycles. The junction of C51 D2 is connected to the SCR anode and an isolation diode D3 is inserted in the anode line. The resistor R6 ensures that the NLDE is set in the positive direction but that the current is not high enough to give a significant induced EMF. The negative feed to the NLDE is via the capacitor C5 which charges on the negative half-cycles. In the triggering (start) mode, the capacitor C5 does not greatly affect the negative current to the NLDE because each time the thyristor triggers it discharges C5 before taking current from the ballast via diode D3* Once the thyristor stops triggering however, charge builds up on C5 rendering the NLDE quiescent.
[0025] Fig. 6 therefore shows a complete dual-rate starter in which positive conduction is accompanied by high voltage generation in the negative half-cycle.
[0026] The heater current and high voltage pulse change with time as shown in Figs. 7A and 7B respectively. (The onset of high voltage pulses is delayed at first because the initial low trigger voltage is not sufficient to set the NLDE in the positive polarity therefore no negative transition occurs.) A diode D4 in Fig. 6 isolates the high negative pulse generated by the NLDE from the rest of the circuit.
[0028] The dash, full and dotted lines show the mains supply voltage, voltage across the lamp or tube, and the cathode heater current respectively. With appropriate choice of components this can be arranged to be appropriate for starting a tube completely, provided the cathodes are sufficiently heated. A typical time of 0.3 seconds (tl) to reach VBK is normally sufficient for this to happen. Should the heating not be sufficient because of adverse conditions, then the starter continues to be active giving the additional heating required.
[0029] The firing angle now changes only slowly to give effective longer heating, but eventually times out completely if the tube fails to strike. This latter feature is necessary in order to prevent ballast overheating and annoying flashing, both of which can occur if a tube is faulty.
[0030] Fig. 8 illustrates the comparison between the locus of trigger voltage levels for the dual slope system described compared with a single slope system. Times are typical for a 58 watt tube. The component values corresponding to the times and levels of Fig. 8 are shown in Fig. 6. To illustrate the earlier discussion of the problem with single rate starter circuits it will be noted that if the trigger voltage Vtrig of the single rate starter lies below VBK then even if the tube is capable of starting at a time less than t4 (Vtrig = VBK), then the tube will not fully ignite due to continual retriggering in the starter circuit. A typical value for t4 is 1.2 seconds, after which the tube ignites normally. The dual rate has the tube lit in under 0.3 seconds in normal operation. If starting fails the starter times out at t5 as compared with the shorter t6 in the single rate case.
[0031] It may prove desirable to provide some form of independent current sensing to cause cut-out under starter fault conditions. Such provision may use a positive temperature coefficient resistor (PTC).
[0032] A PTC can be added to the circuits of Figs. 1, 2 and 6, for example, by breaking the circuit at terminal T3 and inserting the PTC at that point. However, advantage can be taken of the inclusion of the PTC to replace the function of the second time constant in the dual rate circuits of Figs. 2 and 6. Fig. 9 shows a modified version of Fig. 2 in which the zener diode Z2 and capacitor C3 are omitted but further voltage clamping zener diode Z4 is provided as shown to define a limit VBK to the bias voltage developed across C1. In this case the voltage is not allowed to rise to V s (see Fig. 3) so that the circuit continues to trigger to provide reduced heater current in the second phase of operation until the PTC heats to a resistance value that cuts out starter operation entirely, there being insufficient voltage available to the starter circuit to produce triggering. This is shown in Figs. 10 and 11 where (compare Figs. 3 and 4 respectively) after the initial fast ramp-up of trigger voltage to VBK at t1, the triggering remains constant until the cut-out phase at t2' As the PTC cools, the voltage fed to the starter circuit itself slowly increases. However, the rate of increase is very slow so that each successive cycle of applied mains results in only a very small charging current in C1 to the extent that the voltage thereby caused at C2 is less than the voltage of Z1, so the thyristor is not triggered. The PTC therefore cools completely (re-sets) without the starter triggering and it is not until the power is removed (either by switching off or removing the tube) that C1 can discharge and allow the starter to operate next time power is applied. The PTC causes the circuit to remain inoperative for a period until at t3 the PTC has cooled sufficiently to allow a fresh start operation to commence.
1. A starter circuit for a discharge lamp of the kind having cathode heaters energizable
from an a.c. supply comprising:
conductors connectable to the respective cathode heaters,
a thyristor connected between the conductors to provide a controllable path for heater current,
a gate circuit for triggering said thyristor connected to receive the supply voltage applied across the cathodes,
said gate circuit including first and second capacitors interconnected for charging by the applied supply voltage, said first capacitor.being connected in series with the thyristor gate to acquire an increment of bias voltage each time the thyristor is triggered so as to cause triggering to be increasingly delayed in successive supply cycles in response to the increasing bias voltage established across the first capacitor,
said second capacitor being connected with means ensuring that the second capacitor receives a substantially fixed increment of charge in each supply cycle, and
the interconnection of the capacitors providing that the sharing of charge therebetween is such that the first capacitor acquires a fixed increment of bias voltage in each supply cycle.
2. A circuit as claimed in Claim 1 in which said means connected with the second capacitor
defines a predetermined voltage developed thereon in each supply cycle.
3. A starter circuit as claimed in Claim 2 in which said voltage defining means comprises
a Zener diode.
4. A starter circuit as claimed in Claim 3 in which the second capacitor is connected
across the gate and cathode of the thyristor and is connected to the gate through
the Zener diode.
5. A starter circuit as claimed in any one of Claims 1 to 4, in which one of said
first and second capacitors comprises two capacitor elements and voltage dependent
means for switching one of the two elements upon the first capacitor achieving a predetermined
bias voltage thereacross such'that thereafter a lesser voltage bias increment is acquired
by the first capacitor in each supply cycle.
6. A starter circuit as claimed in Claim 5 in which said first capacitor comprises
the two capacitor elements.
7. A starter circuit as claimed in Claim 6 in which said switching means comprises
a Zener diode and is connected in a series combination with one capacitor element
of the first capacitor, the other capacitor element being connected in parallel with
said series combination.
8. A starter circuit as claimed in Claim 6 in which said switching means comprises
a Zener diode and is connected in a parallel combination with one capacitor element
of the first capacitor, the other capacitor element being connected in series with
said parallel combination.
9. A starter circuit as claimed in any preceding claim for use with a discharge lamp
of the kind stated operable with a ballast choke connected in series between the lamp
heaters and the a.c. supply, the starter circuit further comprising means connected
between said conductors and cooperable with the ballast choke, when in circuit with
the lamp, to generate a voltage spike across the conductors to initiate discharge
in the lamp in response to the turning-off of the thyristor.
10. A starter circuit as claimed in Claim 9 in which the voltage spike generating
means comprises a non-linear dielectric element chargeable to saturation by current
maintained through the ballast choke upon said thyristor turning-off.
11. A starter circuit as claimed in Claim 10 in which said thyristor is operable to
be triggered only on supply half-cycles of a given polarity, and said non-linear dielectric
device is connected in a first circuit path between said conductors that enables the
device to be charged to saturation as aforesaid following triggering of said thyristor
in half cycle of said given polarity, and in a second circuit path that enables the
device to be discharged from the saturated condition in the next following half-cycle.
12. A starter circuit as claimed in Claim 11 in which the second circuit path includes
a capacitor, this capacitor being connected to said thyristor for discharge when the
thyristor turns on, and being dimensioned to prevent the generation of voltage spikes
during normal lamp discharge when the thyristor remains off.
13. A starter circuit as claimed in any one of Claims 9 to 12 comprising a resistor
in series in the charging path of the first and second capacitors to provide a time
delay therewith to allow initiation of discharge by a voltage spike prior to a further
triggering of said thyristor.
14. A starter circuit as claimed in any preceding claim further including a positive
temperature coefficient resistor connected in series with said thyristor.
15. A starter circuit for a discharge lamp connectable to an energising a.c. supply
and having heater cathodes between which the discharge is struck, and comprising a
thyristor connectable in series with the heater cathodes to control the supply of
heater current therethrough and a control circuit for the thyristor arranged to terminate
operation of the starting circuit after an interval if the lamp fails to strike, characterised
by the control circuit operating in two phases and including first means for incrementing
the voltage at which the thyristor is rendered conductive in successive half-cycles
of the supply in a first phase, and second means for maintaining said voltage achieved
at the end of the first phase, or for incrementing said voltage at a lesser rate,
for a further period constituting said second phase.
16. A starter circuit as claimed in Claim 15 in which said first means comprises capacitance
on which a bias voltage is developed and incremented in successive half-cycles of
said first phase, and said second means responsive to the bias voltage reaching a
predetermined value to change the value of said capacitance such that the bias value
increments at a lesser rate in the second phase.
17. A starter circuit as claimed in Claim 16 in which triggering of the thyristor
ceases when the bias voltage reaches a predetermined value at which time the second
phase terminates.
18. A starter circuit as claimed in Claim 15 in which said first means comprises capacitance
on which a bias voltage is developed and incremented in successive half-cycles of
said first phase, and said second means comprises means for limiting the maximum value
of said bias voltage to a value at which the thyristor continues to be rendered conductive
in successive half-cycles and a positive temperature coefficient resistor connected
to carry the cathode heater current and to terminate said second phase upon the resistor
reaching a predetermined temperature.
19. A starter circuit as claimed in Claim 18 in which said capacitance is so arranged
that it is discharged at the end of said second phase and the thyristor is not triggered
into conduction again until said resistor has substantially cooled.
20. A starter circuit as claimed in any one of Claims 15 to 19 for a discharge lamp
having an inductor in series with its heater cathodes in the energising circuit, the
starter circuit further comprising an NLDE in an ignition circuit connectable across
the heater cathodes in parallel with the lamp, the ignition circuit comprising two
parallel paths connected to the NLDE, one path including a circuit element providing
current flow to the NLDE in half-cycles of one polarity, the other path comprising
a series capacitor and means providing current flow from the NLDE into the capacitor
in half-cycles of the opposite polarity whereby during the normal operation of the
lamp the capacitor achieves a bias voltage that maintains the NLDE in a substantially
quiescent state, and the capacitor in said other path being connected to the thyristor
for discharge when the thyristor is rendered conductive whereby during said first
and second phases the NLDE is coactable with such a series inductor to generate voltage
spikes to strike the lamp.
21. A starter circuit as claimed in Claim 20 in which said one path comprises resistance
and said means in the other path comprises a unidirectionally conductive device.
22. A starter circuit for a discharge lamp having heater cathodes between which the
discharge is struck and which are connectable in a series circuit with an inductor,
the starter circuit including a thyristor connectable in said series circuit to control
the heater current, and an ignition circuit connectable between said cathodes in parallel
with the lamp and including an NLDE to cooperate with the inductor to generate high
voltage to strike the lamp, the ignition circuit comprising two parallel paths connected
to the NLDE, one path including a circuit element providing current flow to the NLDE
in half-cycles of one polarity, the other path comprising a series capacitor and means
providing current flow from the NLDE into the capacitor in half-cycles of the opposite
polarity whereby during the normal operation of the lamp the capacitor achieves a
bias voltage that maintains the NLDE in a substantially quiescent state, and the capacitor
in said other path being connected to the thyristor for discharge when the thyristor
is rendered conductive whereby during active operation of the starter circuit, the
NLDE is coactable with such a series inductor to generate voltage spikes to strike
the lamp.
23. A starter circuit as claimed in Claim 22 in which said one path comprises resistance
and said means in the other path comprises a unidirectionally conductive device.