GB2097179A - Starting discharge lamps - Google Patents

Abstract

In a high pressure discharge lamp with an incorporated starter 16 (Fig. 5) connected parallel to an arc tube 13 and adapted to generate high voltage starting pulses, a micro-gap element 17 is further connected parallel to the arc tube, so as to protect the lamp from excessively high peak pulse voltage levels. The micro-gap element 17, Fig. 6, comprises, in a gas tight enclosure 22, a dielectric rod 27 having an electroconductive ceramic layer 25, e.g. of SnO2, Nb2O3, MoO3 or WO3, divided into zones by annular gaps 26 of width not exceeding 200 mu m so that the element 17 has a starting voltage of 500-1500v with a delay of less than 1 mu sec. The desired value of a resistor 18 in series with the element 17 is discussed and a number of alternative circuit arrangements incorporating one or two of the micro-gap elements are disclosed. <IMAGE>

Description

SPECIFICATION High pressure discharge lamp with incorporated starter BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an improvement in a starter for starting a high pressure discharge lamp such as a high pressure sodium lamp.

Description of the Prior Art In order to facilitate the starting of a high pressure discharge lamp of a high starting voltage difficult to start with the ordinary commercial line voltage, such as the high pressure sodium lamp, there is already known a starting circuit consisting of a serial connection of a resistor for example an incandescent filament and a bimetal switch and connected parallel to an arc tube of the high pressure discharge lamp which is connected to an AC power source through an inductive stabilizer such as a choke coil. The high pressure discharge lamp incorporating such starting circuit together with the arc tube in an outer bulb is becoming rapidly popular, since such lamp can be used in combination with the existing stabilizer without the use of an independent starting device.

The function of such starting circuit, as already disclosed in the Japanese Patent Application Laid-Open No. 51-67174, will be briefly explained in the following in order to facilitate the understanding of the present invention.

Referring to Fig. 1, upon start of power supply from an AC power source 2, a current supplied through a stabilizer 1 to a starting circuit 6 heats a bimetal switch 5, thus opening said switch. The sudden current interruption by the opening of said bimetal switch 5 induces a high pulse voltage in the stabilizer 1 by the inverse electromotive force. With the heating current being cut off by said opening, the bimetal switch 5 is subsequently closed again to repeat the above-mentioned procedure. The pulses of an appropriate number thus generated by the repeated opening and closing of the bimetal switch 5 are applied to an arc tube 3 together with the power supply voltage to start the high pressure discharge lamp.As the function of the discharge lamp becomes stabilized after its start, the arc tube increases the radiating heat to maintain said bimetal switch 5 in the open state. In the above-explained circuit, a current-limiting resistor 4 is inserted in the starting circuit for the following purposes.

Firstly, without said resistor 4, the current in the starting circuit 6 will become as almost large as the shortcircuiting current of the stabilizer 1 to induce an extremely high pulse voltage exceeding several kilovolts when the bimetal switch 5 is opened, thus destructing the insulation of the stabilizer or other circuits.

Secondly, even if the discharge in the arc tube is successfully induced by one of plural pulse voltages generated by the repeated opening and closing of the bimetal switch 5, the discharge will be extinguished by the diversion of the discharge current to the starting circuit upon re-closing of the bimetal switch 5 if said reistor is not present in the starting circuit.

Thus, in such known curcuit, the resistor 4 in the starting circuit 6 is so appropriate selected as to regulate the pulse voltage and to prevent the undesirable extinguishing of the arc tube.

Such known starting system is however defective in unstable function when powered by a commercial AC power supply because of the fluctuation in the instantaneous current at the opening of the bimetal switch, in the opening speed thereof and in the arc state between the contacts of the bimetal switch.

As an example, Fig. 2 shows the distribution of peak pulse voltages obatained in 1 50 re pealed operations of the bimetal switch 5 in a circuit shown in Fig. 1, employing and AC power source 2 of 200 V; a stabilizer choke coil 1 of ca. 1 20 mH; a high pressure sodium arc tube 3 of 360 W; and an incandescent filament resistor 4 of a cold resistance of ca.

200 S2. As shown in Fig. 2, the pulse voltages show considerable fluctuation ranging from ca. 1 KV to ca. 5KV. In such known system, a certain fluctuation in the pulse voltage is unavoidable in order to ensure satisfactory starting performance, thus leading occasionally to the appearance of a very high pulse voltage. Although such high pulse voltage can be avoided by an appropriate adjustment of the resistance of said resistor 4 in the starting circuit 6 to displace said voltage distribution toward a lower value, such adjustment will inevitably lower the starting probability of the discharge lamp.Stated otherwise, such known system is inevitably associated with the appearance of unnecessarily high pulse voltages in order to maintain a certain desired level of starting probabilty, and various methods have been proposed to eliminate such pulse voltages.

As an example, the Japanese Patent Application Laid-Open No. 53-103668 proposes, for absorbing unnecessarily high pulse voltage, to connect a pulse-absorbing discharge tube starting at a voltage higher than the starting voltage of %R&commat; the arc tube, parallel to said arc tube or to the bimetal switch in the starting circuit In practice, however, it is difficult to find an appropriate discharge tube capable of absorbing the pulse voltage in response to a pulse width as short as ca. 1 ,usec. generated by the function of the bimetal switch in the starting circuit, and such method is found to be unable to resolve the unstable function resulting from the fluctuation of the pulse voltage. As a method eliminating the fluctuation in the pulse voltage, the Japanese Patent Application Laid-Open No.

55-143771 proposes to enclose a gas for reducing the pulse voltage generated by the function of the bimetal switch 5 in the outer bulb 7 incorporating the starting circuit 6 shown in Fig. 1. This method is to suppress the generation of unnecessarily high voltages, and it is confirmed that the generated pulse voltage, with reduced high frequency components and increased pulse duration, exhibits stable starting performance despite of the generally reduced peak voltage. However such a method is still defective in the difficulty of manufacture as a determined amount of gas has to be enclosed in the outer bulb, and in the difficulty of maintaining stable performance as gaseous impurities are emitted from the arc tube and the support structure thereof during use.

SUMMARY OF THE INVENTION The object of the present invention, reached through detailed examination of the relationship between the starting condition of high pressure discharge lamp and the wave form of the pulse voltage, is to provide a starter capable of generating pulse voltages not having excessively high peak values without deteriorating the starting performance of the discharge lamp, thereby ensuring rapid and stable start thereof.

The above-mentioned object can be achieved according to the present invention in the following manner. As will be apparent from Fig. 3 showing the relationship between the breakdown or starting voltage of a 360 W high pressure sodium lamp and the half-peak width of the applied pulse voltage, the lamp requires a higher starting voltage at a smaller pulse width. Conversely, the use of a larger pulse duration allows to start the discharge lamp with a lower pulse voltage, thus alleviating the effect to the stabilizer and the related circuitry. In the conventional starter as explained in the foregoing, the generated voltage is composed, as shown in Fig. 4, principally of sawtooth-shaped pulses (a) with short durations and with relatively high absolute values, followed by sinusoidal pulses (b) with a broader form.Said sawtooth pulses (a) can only start the discharge lamp at 4-5 KV or higher due to the limited duration thereof as short as 2 Else. or less, and often remain unabsorbed even after the start of the discharge lamp. In this manner the conventional starter is inevitably associated with the high pulse voltages. On the other hand the present invention is to provide a novel starter capable of waveform conversion for suppressing the above-mentioned sawtooth pulses, reducing the pulse peak voltages and broadening the pulse duration, thereby ensuring easy and stable start of the discharge lamp.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a circuit diagram of a conventional starter for a high pressure discharge lamp; Figure 2 is a chart showing the distibution of peak pulse voltages in the starter shown in Fig, 1; Figure 3 is a chart showing the relationship between the starting votage for the discharge lamp and the half-peak width of the pulse voltage in the starter shown in Fig. 1; Figure 4 is a waveform chart showing pulse voltages generated in a conventional starter; Figure 5 is a circuit diagram of a starter embodying the present invention; Figure 6 is a cross-sectional view showing a pulse-controlling micro-gap element; Figure 7 is a waveform chart showing pulse voltages generated by the starter of the present invention shown in Fig. 5;; Figure 8 is a chart showing the distribution of peak pulse voltages generated by the starter of the present invention shown in Fig. 5; Figures 9 and 10 are circuit diagrams showing other embodiments of the starter of the present invention; Figure 11 is a chart showing the relationship between the peak residual voltage and the serial resistance; Figure 12 is a waveform chart showing residual voltages of high-voltage pulses; and Figures 13, 14 and 15 are circuit diagrams showing other embodiments of the present invention.

DESCRIPTION OF THE PREFERRED EMBODI MENTS Now reference is made to Fig. 5 showing an embodiment of the starter of the present invention for a high pressure discharge lamp, wherein an AC power source 11, a stabilizer 1 2 and an arc tube of the high pressure discharge lamp are serially connected to constitute a discharge holding circuit of said discharge lamp.A resistor 14 such as an incandescent filament and a bimetal switch 1 5 constitute a serial starting circuit 1 6 connected parallel to said arc tube 1 3. According to the present invention, the starter explained above is further provided, parallel to said arc tube 1 3, with a pulse voltage control circuit 1 9 composed of a resistor 1 8 and a microgap element 1 7 which functions at a voltage lower than the starting voltage of the discharge lamp but higher than the secondary load4ree voltage of the stabilizer and is responsive to high-frequency pulses.

The structure of the above-mentioned micro-gap element will be explained in the following. As already disclosed for example in the Japeanese Patent Applications Laid-Open Nos. 51-97740, 52-6956 and 55-128283, said micro-gap element 17 has a basic structure shown in Fig. 6, wherein an insulating rod 27 of a material having a relative dielectric constant larger than 1 such as mullite, forstelite alumina or steatite, is surfacially provided with an electro-conductive ceramic layer 25 for example of SnO2, Nb2O3, MoO2 or W02 divided into plural zones by annular gaps of a width not exceeding 200 !tom. The end zones of thus divided conductive ceramic layer 25 are equipped with metal caps 23, 24 for constituting electrodes.The above-mentioned structure is housed in an airtight container 22 filled with a rare or inert gas, and metal lead wires 21 connected to said metal caps are guided to the exterior through said container. Upon application of a surge voltage between the electrodes of such micro-gap element, the generated electric field is at first concentrated in the gaps 26 of the conductive ceramic layer 25, thus generating primary discharge in said gaps. The electrons released by said primary discharge collide with the surrounding gas to cause ionization thereof, and the new electrons released at said ionization further ionize said gas. Said ionization of gas proceeds rapidly by the repetition of the above-mentioned phenomenon to eventually cause insulation breakdown of the gas, thus initiating a secondary gas discharge between the electrodes.Said secondary discharge is extremely rapid, with little delay in time. Said secondary gas discharge shifts from glow discharge to arc discharge with the increase of surge current, but is principally composed of a surfacial discharge along the conductive ceramic layer. Said surfacial discharge, particularly marked in the initial phase of the secondary gas discharge, further accelerates said secondary discharge, thus reducing the delay of the discharge.The aboveexplained micro-gap element based on twostep discharge are characterized in having a smaller dimension, for example 6 mm in diameter and 21 mm in length for a starting voltage of 3 KV in comparison with the varistor or similar conventional elements, showing an extremely stable performance even in a high-temperature situation exceeding 1 00 C, and being capable of effectively absorbing rapid voltage pulses by the shorter delay in discharge than in the conventional discharge gaps. Furthermore it shows a satisfactory interrupting performance against the continued flow of electric current through the micro-gap element from the power source after the start of discharge in said element, a stable discharge starting voltage, absence of shortcircuit state after the insulation breakdown and a small limiting voltage.

The serial circuit of such micro-gap element 1 7 and a resistor 18, connected parallel to the arc tube 1 3 and the starting circuit 16, generates pulse voltages as shown in Fig. 7, in which the sawtooth part (a) is significantly decreased and the sinusoidal part (b) is transformed to a waveform of damped osciallation with a broadened duration, whereby the pulse energy can be more efectively utilized in the starting of the discharge lamp although the peak voltage Vp is reduced.The broadening of the pulse duration is attributable to a change from the osciallating condition to the damping codition caused by the increase in the resistance component in the equivalent LCR circuit in the stabilizer, and the decrease of the wastooth part is presumably attributable to the reduced discharge between the contact of the bimetal switch at the opening thereof, since the peak pulse voltage is reduced by the presence of said resistor 1 8.

Due to the reduced peak pulse voltage and the absence of uncontrollable discharge process in the contacts of the bimetal switch, the starter of the present invention is capable of significantly reducing the fluctuation in the height and form of the pulse voltages. As will be apparent form Fig. 8 showing the distribution of the peak pulse voltages, unnecessarily high pulse voltages in the range of 4 to 5 KV are entirely absent. Consequently the safety for the stabilizer and the related circuitry is remarkably improved, and the starting performance is still ensured by the broadened pulse duration. As already explained in relation to Fig. 3 showing the relationship between the starting voltage and the half-peak width of applied pulse voltage in a 360 W high pressure sodium lamp, the starting of the lamp is facilitated by a larger pulse duration.

The resistance of the resistor 1 8 in the circuit 1 9 is determined in consideration of the distribution of the peak pulse voltages and the starting characteristic of the discharge lamp. A larger resitance leads to an oscillating pulse form with increased high-frequency components, a higher pulse peak and a narrower pulse duration, and vice versa. In case of starting a high pressure sodium lamp of 180-980 W with a AC commercial power supply of 200 V through a stabilizer choke coil of 50-500 mH, said resistance is selected within a range from 1 to 10 k.

The performance of the starter of the present invention will also be understood from the comparison of Figs. 2 and 8. Assuming that the discharge lamp can be started at 2.5 KV, the probability of obtaining a pulse exceeding 2.5 KV in the conventional starter is 0.5 as determined in Fig. 2 by the ratio of the number of pulse exceeding 2.5 KV to that of entire pulses. Said probability is also 0.5 in the starter of the present invention. However the maximum pulse voltage is 3.8 KV in the latter whereas it is 5 KV in the former. Stated differently, the starter of the present invention is capable of reducing the maximum pulse voltage for a given starting performance, or of improving the starting performance for a given maximum pulse voltage.

As explained in the foregoing, the present invention is characterized by the presence of a micro-gap element 17, functioning at a voltage lower than the starting voltage of the discharge lamp but higher than the secondary loadfree voltage of the stabilizer 12 and responsive to high frequency pulses, in serial connection with the resistor 1 8 in the circuit 19.

If the resistor 1 8 is connected parallel to the arc tube 1 8 without such micro-gap ele ment 17, a constant current will be generated in said resistor 1 8 during the loadfree state of the stabilizer 1 2 and at the starting of the discharge lamp. In such case, for a resistance of 3 KS2 for example, said resistor 1 8 will consume a power W = V2/R = 2002/3 x 103 = 13.3 watts.

A resistor of such high power is practically scarcely acceptable for use in the outer bulb of the discharge lamp or even in an external casing.

The starting voltage of said micro-gap element 1 7 should be sufficiently high in order not to be activated by the secondary loadfree voltage of the stabilizer 1 2 and should not exceed the starting voltage of the discharge lamp. Said starting voltage of the micro-gap element is generally selected within a range ca. 500 to 1 500 v, since an excessively high starting voltage reduces the opportunity of waveform conversion in cooperation with the resistor 1 8 aimed at in the present invention.

Also the delay of discharge in the micro-gap element should not exceed 1 JLsec. in order to be responsive to the pulse voltages involving high frequency components generated by the function of the bimetal switch 1 5. Examination of various elements has revealed that a micro-gap element supplied by Mitsubishi Mining and Cement Co., Ltd. under a trade name DSA Surge Absorber is suitable for the purpose.It is sufficiently responsive to the high frequency components as the insulation breakdown caused by the concentrated electric field ensures a delay of discharge less than 0.1 !use. According to the present invention, the resistor 1 8 can be of a small dimension, for example of a nominal power of 1 KW or less, since the micro-gap element having a starting voltage of 500 to 1 500 V is not activated by the secondary loadfree voltage of the stabilizer. At the same time said resistor 1 8 should preferably by provided with a pulse voltage-proof in the order of 6 KV or higher.

These requirements can be satisfied by a carbon film resistor 1/2 to 1 W, which can be incorporated in the outer bulb of the discharge lamp because of the relatively small temperature dependence of resistance and the stability in a high-temperature condition in vacuum or in a gaseous atmosphere. The aforementioned micro-gap element can also be incorporated in the outer bulb of the discharge lamp.

In the foregoing description, the resistor 14 in the starting circuit 1 6 and the resistor 1 8 in the circuit 1 9 are respectively exemplified by an incandescant filament and a carbon film resistor, but said resistors can be satisfactorily replaced for example by metal thick-layer resistors formed on a ceramic substrate.Also said pulse voltage controlling circuit 1 9 may be connected parallel to the bimetal switch 15 in the starting circuit 16 as shown in Fig. 9, or may be placed, eventually in combination with the starting circuit 16, outside the discharge lamp as shown in Fig. 1 0. Furthermore, in case said circuits are placed outside the discharge lamps, at least one of said resistors 14 and 1 8 can be composed of a variable resistor in order to regulate the peak pulse voltage.

As another feature of the present invention, it is also possible to prevent the generation of excessively high pulse voltages by selecting, for the micro-gap element connected parallel to the arc tube, a starting voltage higher than mentioned in the foregoing.

The serial resitor 1 8 connected to the micro-gap element 1 7 is effective for preventing continued current in said element, but should not exceed a certain resistance in order to retain the surge voltage absorbing ability of the circuit. Experimentally the reistance R of said resistor 1 8 should be so selected that

does not exceed 3, wherein V0 is the secondary loadfree voltage of the stabilizer 1 2 and L is the equivalent serial inductance thereof, which is for example 1 22 mH for a discharge lamp of 400 W. Otherwise a continuous current is generated in the micro-gap element 17, thus reducing the service life thereof.On the other hand, the absorbing ability of the circuit for high voltage pulses decreases rapidly if said resitance exceeds 500 S2 and becomes almost nil at a resistance exceeding 5 kS1, as shown in Fig.

11, wherein the ordinate indicates the residual voltage remaining across the serial circuit of the micro-gap element and the resistor.

Such phenomenon observed at a higher resistance is presumably due to a fact that the discharge current after the activation of the micro-gap element at a predetermined voltage causes a significant voltage drop in the serial resistor, thus generating a voltage exceeding the starting voltage of the micro-gap element across the starting circuit to hinder the surge voltage absorbing effect. Because of these reasons the resistance of said serial resistor should be selected within a range not causing the continued current in the micro-gap element and not exceeding 5 kS for retaining the surge voltage absorbing ability.

In the experiments conducted by the present inventors, with a serial resistor 1 8 of 50 g in the circuit shown in Fig. 5, the micro-gap element 1 7 experienced certain change in performance and started to show continued current after applications of high voltage pulses by 500 times. On the other hand, with a serial resistor 1 8 of 500 S2, no such change could be observed even after applications of ca. 10,000 times. Fig. 1 2 shows the waveform of the residual high voltage pulses observed with a micro-gap element of a nominal starting voltage of 1.5 KV.

Fig. 1 3 shows another embodiment of the present invention, in which two micro-gap elements 17, and 172 are serially connected to ensure the effect of preventing the continued current. Even in such case, the presence of a serial resistor 1 8 in the order of 100 S2 extends the service life of said microgap elements to further ensure the protection against the continued current.

Fig. 14 shows still another embodiment of the present invention. The high-voltage pulse generating starter to be employed in the discharge lamp of the present invention can assume various forms, but can be most practically composed of a serial connection of a current-limiting resistor 14 and a thermal switch 15, wherin said resitor 1 4 is usually composed of an incandescent filament or a metal thick-film resistor formed on a ceramic substrate.In case of such starter structure, the micro-gap element 1 7 can be connected parallel to said thermal switch 15, or, stated otherwise, parallel to the arc tube 1 3 through said resistor 14, thereby omitting the serial resistor shown in Fig. 5 or 1 3. Also Fig. 1 5 shows still another embodiment employing two micro-gap elements 1 7t and 172 and constituting a variation to the embodiment shown in Fig. 14.

In the foregoing embodiments, the interior of the outer bulb 7 surrounding the arc tube is preferably maintained in a substantially vaccum condition because of the reason to be explained later, and in this connection the present invention is most effectively applicable to the high pressure sodium lamp in which the outer bulb is generally maintained in vaccum state. In the micro-gap element of the structure shown in Fig. 6, the expiration of service life or functional failure ordinarily takes place by the leakage of the rare or inert gas filled in the container to the outside. With such gas leakage, the micro-gap element shows an elevated starting voltage, thus losing the function as a surge-absorbing element.

However, in case such micro-gap element is incorporated in the outer bulb of the discharge lamp, the gas leaking from the microgap element fills the outer bulb. The gas leakage from the micro-gap element of a gas filling pressure ordinarily in the order of 500 Torr into the outer bulb maintained under high vacuum usually in the order of 10-7 Torr brings the internal pressure of said outer bulb up to 10-' to 1 Torrdepending on the dimension of said outer buld, thus creating a state of quite easily inducing a discharge.

Thus, even if pulse voltages are generated by the uncontrolled operation of the contacts of the thermal switch in such gaseous atmosphere, the discharge induced in said gaseous atmosphere dissipates the energy of said pulses, thus avoiding the high pulse voltages.

In this manner the above-explained structure not only prevents the damage to the external circuit by the absorption of excessively high pulse voltages with the micro-gap element during the normal function state, but also provides additional safety even when the micro-gap element is broken, as the gas leaking from said element forms a discharge-inducing space in the outer bulb, absorbing the pulse energy generated by the starter through the discharge in said outer bulb.

In the foregoing embodiments, the present inventor made intentional leakage from a micro-gap element of a gas filling pressure of 500 Torr placed in a vacuum outer bulb of a high pressure sodium lamp and measured the pulse voltages generated in such condition. As the result, the pulse voltages showed a mean value of ca. 1.5 KV did not exceed 2 KV.

As explained in the foregoing, the present invention is characterized by the use of the micro-gap element, which is smaller than varistor or other known elements and usable under high temperature and is therefore extremely adapted for use in a high-efficiency, high-temperature discharge lamp such as the high pressure sodium lamp. Also the faster response of such micro-gap element in comparison with the conventional discharge gap allows effective absorption of rapid high-voltage pulses. In addition, the incorporation of such micro-gap element in the vaccum outer bulb of the discharge lamp provides an additional safety for preventing abnormally high pulse voltages even when said element is damaged. In this manner the present invention is capable of providing a practically useful discharge lamp.

The present invention is further characterized, in a high pressure discharge lamp having a starting circuit which consists of a serial connection of a resistor and a bimetal switch and is connected parallel to an arc tube of the discharge lamp, by a serial circuit which consists of a resistor a micro-gap element capable of functioning at a voltage higher than the secondary loadfree voltage of said stabilizer and responsive to high frequency pulses and which is connected parallel to said arc tube, to the bimetal switch or to said stabilizer. The use of a serial circuit consisting of a resistor and a switching element such as a discharge gap and connected parallel to the bimetal switch is already known for example in a starter disclosed in the Japanese Patent Application Laid-Open No. 56-4198.Said known starter however does not aim at the conversion of pulse wave form but merely at the use of a pulse absorbing serial circuit consisting of a resistor and a discharge gap functioning at a voltage higher than the starting voltage of the discharge lamp, instead of a resistor serially connected with the bimetal switch in the starting circuit. Also said known starter is not provided with a resistor in the starting circuit.

In such known starter the discharge lamp, after it is started by a pulse voltage generated by the funciton of the bimetal switch, may be extinguished again when said bimetal switch is closed again. Also since the bimetal switch interrupts a large current close to the secondary shortcircuit current of the stabilizer, there are generated extremely rapid pulse voltages of a significant energy, which can be scarecely controlled to an appropriate level in the presence of a pulse voltage absorbing circuit.

In this manner the technology of the present invention provides advantages not achievable in the known technologies and is therefore clearly distinguishable from the known technologies.

The advantages of the present invention are summarized in the following.

As already explained in the foregoing, the pulse voltage generated by the starter of the present invention has a broadened pulse width, which allows to reduce the maximum pulse voltage without losing the ability for starting the discharge lamp. Thus lowered maximum pulse the related contributes to the safety of the stabilizer and related circuitry.

Also said pulse voltage, being capable in waveform and containing little sawtooth pulses, is capable of efficiently and stably starting the discharge lamp. Furthermore, the starting circuit and the pulse voltage controlling circuit can be easily incorporated together with the arc tube in the outer bulb of the discharge since the former is a simple twoterminal circuit composed of a resistor and a bimetal switch while the latter is also a simple two-terminal circuit composed of a resistor and a micro-gap element, and since temperature-durable components can be selected in said circuits. Furthermore the freedom in the designing of the starter is increased since the pulse voltage level can be arbitrarily regulated by the suitable selection of the resistances in the starting circuit and in the pulse voltage controlling circuit.

Furthermore the starter of the present invention tends to show an increased number of functions of the bimetal switch for an unidentified reason, so that the starting of the discharge lamp is facilitated by such increased number of pulse voltages.

As detailedly explained in the foregoing, the starter of the present invention for a high pressure discharge lamp is industrially useful because of various advatages over the conventional starters.

It will be appreciated from the above that an important, independent aspect of the invention is the use, as a surge arrester or switch in the starter circuit of a discharge lamp, of an element having conductive layers separated by a narrow gap such that a preliminary discharge across the gap upon application of a voltage to the element causes a secondary discharge between main electrodes of the element.

British Patent Specification No 1,523,375 describes a micro-gap element.

Claims (21)

1. A high pressure discharge lamp incorporating in an outer bulb thereof a starter for applying high voltage pulses to an arc tube, comprising at least a micro-gap element capable of functioning at a predetermined voltage or higher, and connected parallel to said arc tube.

2. A starter-incorporting high pressure discharge lamp according to Claim 1, wherein said micro-gap element comprises an insulating member of a high relative dielectric constant; a conductive ceramic layer formed on said insulating member and divided into plural zones by stripe-shaped gaps not exceeding a determined width; metal wires connected to the end ones of said plural zones to constitute electrodes; and air tight container housing the above mentioned structure and filled with a rare or inert gas.

3. A starter-incorporating high pressure discharge lamp according to Claim 1 or 2, wherein said micro-gap element is connected parallel to said arc tube through a serial resistor of a low resistance selected within a determined range.

4. A starter-incorporating high pressure discharge lamp according to Claim 3, wherein reistance R of said serial resistor is so selected that a value

does not exceed 3, wherein V0 is the secondary loadfree voltage of an inductance stabilizer.

5. A starter-incorporting high pressure discharge lamp according to Claim 3 or 4, wherein resistance of said serial resistor is so selected as not to exceed 5 uke2, particularly 500 a.

6. A starter-incorporating high pressure discharge lamp according to Claim 1, 2 or 3, comprising two micro-gap elements mutually connected in series.

7. A starter-incorporating high pressure discharge lamp according to Claim 1, wherein said starter is composed of a serial circuit of current-limiting resistor and a thermal switch.

8. A starter-incorporating high pressure discharge lamp according to Claim 7, wherein said current-limiting resistor is composed of an incandescent filament or a thick-film metal resistor on a ceramic substrate.

9. A starter-incorporating high pressure discharge lamp according to Claim 7 or 8, wherein said current-limiting resistor is connected serially to said micro-gap element and parallel to the arc tube.

10. A starter-incorporating high pressure discharge lamp according to Claim 1 or 9, comprising two micro-gap elements mutually connected in series.

11. A starter-incorporating high pressure discharge lamp according to either one of Claims 1 to 10, wherein the interior of the outer bulb housing said arc tube in maintained in a substantially vacuum state, and the contacts of the thermal switch of the starter and said micro-gap element are positioned in said outer bulb.

1 2. A starter-incorporating high pressure discharge lamp according to either one of claims 1 to 11, wherein said micro-gap element is adapted to absorb high voltage pulses exceeding a determined level.

1 3. A starter-incorporating high pressure discharge lamp according to either one of Claims 1 to 11, wherein said micro-gap element functions as a switch to be turned on at a determined voltage.

1 4. A starter for a high pressure discharge lamp having a serial starting circuit consisting of a resistor and a bimetal switch and connected parallel to an arc tube of the discharge lamp connected serially with an inductive stabilizer, comprising a pulse voltage controlling circuit consisting of a serial connection of a resistor and a micro-gap element capable of functioning at a voltage lower than the starting voltage of the discharge lamp but higher than the secondary loadfree voltage of said stabilizer and responsive to high frequency pulses, said pulse voltage controlling circuit being connected parallel to said starting circuit, or to the bimetal switch therein, or to said stabilizer.

1 5. A starter for a high discharge lamp according to Claim 14, wherein said resistor in the starting circuit is composed of an incandescent filament, and said resistor in the pulse voltage controlling circuit is composed of a carbon film resistor.

1 6. A starter for a high pressure discharge lamp according to Claim 14, wherein said resistors in the starting circuit and in the pulse voltage controlling circuit are both composed of thick-film metal resistors.

1 7. A starter for a high pressure discharge lamp according to Claim 14, wherein said starting circuit and said pulse voltage controlling circuit are both incorporated in an outer bulb of the discharge lamp incorporating an arc tube.

1 8. A starter for a high pressure discharge lamp according to Claim 14, wherein at least one of the resistors in said starting circuit and in said pulse voltage controlling circuit is composed of a variable resistor.

1 9. A starter for a high pressure discharge lamp according to Claim 14, wherein the micro-gap element in the pulse voltage controlling circuit is adapted to function as a switch.

20. A starter for a high pressure discharge lamp according to Claim 14, wherein the high pressure discharge lamp serially connected with the stabilizer is a high pressure sodium lamp.

21. A starter for a high pressure discharge lamp, substantially as herein described with reference to any of the accompanying drawings.