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EP 0 663 789 B1 |
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EUROPEAN PATENT SPECIFICATION |
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Mention of the grant of the patent: |
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15.12.1999 Bulletin 1999/50 |
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Date of filing: 22.07.1994 |
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International Patent Classification (IPC)6: H05B 41/29 |
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Preheat start-up device for fluorescent lamps
Vorheizstartschaltung für Leuchtstofflampen
Circuit d'amorçage, avec préchauffage, de lampes fluorescentes
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Designated Contracting States: |
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AT BE CH DE DK ES FR GB GR IE IT LI LU MC NL PT SE |
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Priority: |
18.01.1994 CN 94202144
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Date of publication of application: |
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19.07.1995 Bulletin 1995/29 |
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Proprietor: MASS TECHNOLOGY (H.K.) LTD. |
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Tsim Shatsui,
Kowloon (HK) |
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Inventor: |
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- Foo, Onn Fah
Tsim Sha Tsui,
Kowloon (HK)
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Representative: Archer, Philip Bruce et al |
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Urquhart-Dykes & Lord
European Patent Attorneys
New Priestgate House
57 Priestgate Peterborough
Cambridgeshire PE1 1JX Peterborough
Cambridgeshire PE1 1JX (GB) |
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Note: Within nine months from the publication of the mention of the grant of the European
patent, any person may give notice to the European Patent Office of opposition to
the European patent
granted. Notice of opposition shall be filed in a written reasoned statement. It shall
not be deemed to
have been filed until the opposition fee has been paid. (Art. 99(1) European Patent
Convention).
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[0001] This invention relates to a discharge lamp preheating start-up device comprising
a voltage-sensitive switching element and a temperature sensitive resistance having
a positive temperature coefficient, and a method of producing such a device.
[0002] More particularly, the invention relates to a pre-heating start-up device for electronic
ballast fluorescent lamps, and a method of producing same.
[0003] Fluorescent lamps have been widely used as an efficient lighting device and require,
during normal operation, accessories such as current limiting means and start-up means.
Current limiting means (ballast) frequently used are electronic and lamps using such
current limiting means are known as high frequency lamps. These have the advantage
of a low voltage start-up. The start-up means used as the operating switch in such
lamps are the so-called "jumping bulb" glow starter. This is a monodirectional glow
heat sensitive switch having a pair of head closing metallic electrodes sealed into
a glass shell filled with an inert gas. The normal operative gas discharging voltage
of the filament at the two ends of the fluorescent tube can be reached to cause glow
of the light tube. This method of starting glow is known as high voltage quick cold
start-up under normal temperature and does not involve any apparent pre-heating process
of the filaments of the light tube during the starting procedure. Accordingly the
light tube can only be started under high voltage conditions together with an impulse
of high current (approximately 2.5 times the operating current of the lamp). Such
high voltage quick cold starting may cause direct damage of the filament and the electron
powder and may also cause the "rectifying effect" of asymmetric electron emitting,
both affecting the life of the light tube.
[0004] More recently, fluorescent lamps have been developed which are gas charging lamps
of the cathode preheating type, which are hot starting lamps. In order to prolong
the life of a light tube it is important to "hot start" the light tube in preheating
conditions, that is, the tube is lit at a relatively low open line voltage (such as
at 300 to 400 volts instantaneous peak starting voltage) after a period of preheating
the filaments. This is advantageous to prevent the filament and its superficial electron
emitting material from "spurting out", whereby, the life of the light tube can be
prolonged.
[0005] Heat sensitive resistors are relatively commonly adopted in making preheating type
glow starters. This is a device having a positive temperature coefficient (PTC), which
has been described in detail in US patent number 3,444,399. PTC thermistors have temperature
sensitive characteristics and are directly used in electronic circuits to compensate
for voltage change and to provide a stable current passing through the circuit load.
Chinese Utility Model Patent number 92219487 discloses utilising a PTC in fluorescent
lamp glow preheating. The fluorescent lamp starting preheater disclosed therein is
a thermistor having a positive temperature coefficient and Curie point or switching
temperature. The two terminals of the preheater are coupled to the two thermals of
the main oscillatory circuit of the electronic ballast serially connected to the two
filaments of the fluorescent lamp, respectively, as shown in Fig 1 hereof. It is necessary
to match the parameters and volume of the PTC with the electronic parameters of the
light tube when the PTC is utilised as a preheating device and this is difficult to
achieve. In addition, the PTC itself also consumes power, its temperature rise is
high and its starting time is relatively long (about three seconds).
[0006] A discharge lamp preheating start-up device comprising a voltage-sensitive resistance
having a positive temperature coefficient is described in WO-A-93 00784. The resistance
is interconnected in series with the bidirectionally operating breakdown voltage-sensitive
switching element. The resistance and the bidirectionally operational breakdown element
are thermally coupled. The purpose is to provide simple circuit for operating a discharge
lamp which facilitates the substantially constant ignition of the lamp.
[0007] It is an object of the present invention to provide a fluorescent lamp preheating
starter having both the characteristics of positive temperature resistance and the
function of a voltage switch and/or to provide a fluorescent lamp preheating starter
having suitable start-up time and easy matching with the electric parameters of the
light tube; and/or to provide a fluorescent lamp preheating starter having low power
consumption per se and low temperature rise; and/or to provide improvements generally.
[0008] According to the invention there is provided a preheating starter-up device for electronic
ballast fluorescent lamps and a method of producing same as claimed in the accompanying
claims.
[0009] In an embodiment of the invention there is provided a preheating start-up device
for electronic ballast fluorescent lamps constructed in the following manner. Two
materials are used. One (referred-to below as the "V" layer) of which comprises more
than 80% zinc oxide with minor additions of oxides of bismuth, antimony, cobalt and
manganese. The other one (referred-to below as the "T" layer) of which comprises a
mixture of oxides comprising more than 70% by weight of a barium and strontium and
titanium complex oxide. These materials are mixed, respectively, and compressed to
form circular sheets of 4-7mm diameter, and shaped by sintering. Each of the two surfaces
of each sheet is then coated with a conductive layer of silver, thereafter, the two
sheets obtained are hot bonded into an integrated body to form a chip. In the chip,
the portion comprising a complex oxide of (inter-alia) barium is designated the "T"
layer, while the portion mainly comprising zinc oxide is designated the "V" layer.
Connecting leads are led out from the surfaces of the two layers, respectively, and
together they form a fluorescent lamp automatic preheating/switch device, referred
to as a VT device. The VT device is serially connected into the fluorescent lighting
circuit. The above-mentioned chip of the VT device is then sealed into a coating of
burn - resistant epoxy powder with is integrated by heating to form a complete device.
The fluorescent lamp automatic preheating/switching device thus formed is serially
connected in the loop of the filament of the fluorescent lamp after the parallel connection
of the two leads with a resonant capacitor.
[0010] The "V" layer in the VT device behaves as a voltage sensitive switching resistor,
it operates as a voltage sensitive switch and increases the initial current, speeds
up the variation of resistance of the "T" layer and improves the starting speed of
the light tube. The "T" layer in the VT device behaves as a PTC, which functions to
delay the rise of the voltage in the light tubes circuit and prevents the exceeded
current from passing through the "V" layer.
[0011] The selection of materials constituting the VT device and the ranges of its electric
parameters can be varied accordingly depending on the features of the lamps with which
they are used.
[0012] In use, voltage is applied to the two terminals of a fluorescent lamp. As the cold
state conducting voltage of the light tube is higher than 600V, when the conducting
voltage of said VT device is only approximately 150V, the VT device is first made
conductive at a relatively low voltage, thus, the two terminals of the VT device lower
the peak voltage.
[0013] Since the voltage at the two terminals of the light tube is maintained only at the
voltage value of the voltage sensitive switch of the VT device, that is approximately
180V, therefore the lamp is not conductive. During this period, the current in the
whole circuit passing through the VT device (approximately 80 mA) and the charging
and discharging currents of the resonant capacitor C heat the filaments, and at the
same time, the VT device is also heated.
[0014] The resistance of the temperature sensitive device in the VT device increases with
the rise of temperature, that is, the voltage of the two terminals of the light tube
increases continuously ( V=IR), finally, when this voltage reaches the heat conducting
voltage of the tube, ie 300 to 400V, the light tube becomes conductive and the gas
inside the tube discharges, subsequently the voltage at the two terminals of the light
tube or the two terminals of the VT device lowers to the normal working voltage of
the light tube, ie approximately 70-90V, and at this stage the VT device converts
to an "off" state.
[0015] Thus, in spite of the fact that there is a capacitance of about 350 PF in the VT
device together with an internal resistance of about 1kW, there will still be a weak
current under the high frequency power supply. During the passing through of these
currents in the VT device, there will be a power consumption of about 0.015 Watt,
but as compared with the self-power-consumption of ordinary PTC (approximately 0.4
Watt), it has been greatly reduced.
[0016] When the fluorescent lamp preheating starter of the embodiments is utilised, in addition
to the fact that the temperature controlled impedance feature of ordinary PCT is maintained,
there is also the function of a voltage switch, thereby, the power consumption of
the preheating starter of the present invention during the normal working period of
the fluorescent lamp is only 0.4 to 5% of that of the PTC.
[0017] The consequent advantages are that the rise of temperature of said device is low,
it is easy to realise parameter matching with the light tube and the starting time
is also appropriate. The reliability of the operation of the light tube can be improved,
and the life of the light tube can be prolonged.
[0018] Embodiments of the invention will now be described by way of example only with reference
to the accompanying illustrative drawings in which :
Fig 1 shows a circuit diagram showing a fluorescent lamp utilizing the prior art PTC
preheating starting device;
Fig 2 shows a circuit diagram showing a fluorescent lamp utilising the VT preheating
starting device of the present invention;
Fig 3 is a schematic drawing showing the structure of the VT preheating starting device
of the present invention; and
Fig 4 is a schematic drawing showing a modification of the VT preheating starting
device of the present invention.
[0019] As shown in Fig 2, the device indicated by the symbol VT is a fluorescent lamp preheating
starting device or automatic glow starter/switch device of the present invention,
which constitutes a serial circuit with the electronic ballast and the filaments and
is connected in parallel with the resonant capacitor C.
[0020] Fig 3 shows a schematic diagram illustrating the structure of a fluorescent lamp
preheating starting device, wherein the temperature sensitive resistance layer or
"T" layer or sheet 1 comprises a mixture of oxides comprising more than 70% by weight
of a barium and strontium and titanium complex oxide. In this embodiment the complex
oxide has the molecular formula (0.878 Ba + 0.122 Sr + Ti) 0
3. The mixture contains oxides in the proportions given by one mole of this latter
oxide mixed with the mole percentages now indicated of the following additional oxides
: 0.05-0.3mol% NB
2O
5 and 0.8-3mol% SiO2 and 0.2-0.8mol% Al203 and 0.4-1.5mol% TiO2 and 0.03-0.08mol%CaO
and 0.005 (or 0.05) - 0.01mol% MnO2 and 0.04-0.8mol%MgO.
[0021] The resistance layer has the properties of a heat sensitive resistor and is coated
with a conductive layer, the Curie Point of which is 60°C to 100°C, and its resistance
at normal temperature is 700 to 1200 . It corresponds to an MZ11PTC heat sensitive
resistor.
[0022] The voltage sensitive switching resistance V layer or sheet 2 comprises a mixture
comprising more than 80% zinc oxide with minor additions of oxides of bismuth, antimony,
cobalt and manganese. In this embodiment the molecular percentages are as follows
: 95-98mol% ZnO plus 0.1-5mol% of each of Bi2O, Sb2O3, CoO and MnO.
[0023] The V layer or sheet is a voltage sensitive structure, the voltage value of the voltage
sensitivity is 120-160 volts, and the mean features of which are similar to that of
an MYD-07K95 voltage sensitive resistor.
[0024] One surface of this voltage sensitive switching resistance layer (sheet) 2 is hot
bonded with the above-mentioned temperature sensitive resistance layer (sheet) 1 by
solder 4. Said solder is a tin plated brass wire solder attached with D4D-87 silver
epoxy conductive glue. The thickness of the silver layer is 0.06 mm and being able
to withstand temperatures of 300°C, it is in fact a conductive layer. Leads 3 are
connected to the temperature sensitive resistance layer (sheet) 1 and the voltage
sensitive switching resistance layer (sheet) 2 respectively. The two layers 1 and
2 are hot bonded together to form an integrated body.
[0025] The above apparatus is sealed by a coating film 5 comprising epoxy powder to form
an automatic starter/switch VT device having both temperature controlled impedance
characteristics and voltage switching characteristics.
[0026] The main parameters of the automatic starter/switch VT device are as follows :
Switching voltage : 120 to 160 Volt
Curie point : 65°C to 100°C
Internal resistance (normal temperature) : 700 Ω to 1200 Ω.
Temperature resistance coefficient : >(R-700)10%/1°C (positive characteristic)
Power consumption : <0.03 Watt (during normal operation of the light tube)
Starting time : 0.6 to 1.5 sec (under normal temperature)
Life of switch : 100,000 times (under normal temperature)
Withstand voltage : >1000 Volt
Static capacitance : <350 PF
[0027] The above-mentioned materials constitute a fluorescent lamp starting device having
both the function of a voltage switch and the characteristic of self-overload-current-protection,
which is capable of automatic preheating, automatic switching, and self-protection.
The lamp starting time of this automatic preheating/switching device (VT) is appropriate,
ie 0.6 to 1.5 second, the life of the switch can be controlled in 20,000 to 100,00
times.
[0028] Fig 4 illustrates an embodiment of another structure of the fluorescent lamp preheating
starter of the present invention as a modification of the above-mentioned structure.
Therein, a current limiting resistance film 6 in parallel with the temperature sensitive
device is coated on the peripheral (one or more peripheral surfaces) thereof. On the
basis of the basic structure of the above-mentioned VT device, the resistance of this
film is 12K Ohms to 20K Ohms. A current limiting resistor having the same resistance
range may also be connected in parallel with the temperature sensitive device.
[0029] A further embodiment of the VT device of the present invention is one in which the
voltage sensitive resistance layer (sheet) 2 and the temperature sensitive resistance
layer (sheet) 1 in the above-mentioned structure are made separated, and the same
effect is achieved merely by connecting them in series with a conductor and then mounting
them in the location indicated by VT as shown in Fig 2.
1. A discharge lamp preheating start-up device comprising a voltage-sensitive switching
element and a temperature-sensitive resistance having a positive temperature coefficient,
characterised in that
hot bonded into an integrated body with the voltage-sensitive switching element comprising
a switching resistance layer (2) comprising more than 80% zinc oxide together with
the temperature-sensitive resistance comprising a layer (1) comprising a mixture of
oxides comprising more than 70% by weight of a barium and strontium and titanium complex
oxide, there is provided a silver-coated conductive layer between said two resistance
layers (2,1), said device having leads (3) and said layers (2,1) being sealed into
a coating film (5) to form an automatic preheating/switching device.
2. A start-up device according to claim 1 characterised in that the diameter of said
two resistance layer (2,1) is approximately in the range of 4mm to 7 mm.
3. A start-up device according to claim 1 or claim 2 characterised in that said voltage-sensitive
resistance layer (2) and said temperature-sensitive resistance layer (1) are separate
devices connected in series by a conductor.
4. A start-up device according to any one of claims 1 to 3 characterised in that the
periphery of said temperature-sensitive layer (1) is coated with a current-limiting
resistance film (6) coupled in parallel therewith.
5. A start-up device according to any one of claims 1 to 3 characterised in that a current-limiting
resistor is connected in parallel with said temperature-sensitive layer (1).
6. A start-up device according to any one of claims 1 to 5 characterised in that the
voltage-sensitive resistance value of said voltage-sensitive switching device lies
in the range of 120 volts to 160 volts.
7. A start-up device according to any preceding claim characterised in that said temperature-sensitive
resistance layer comprises a mixture comprising titanium oxide, the Curie point thereof
lying in the range from 65°C to 100°C, and the resistance under normal temperatures
being from 700 Ohms to 1200 Ohms.
8. A start-up device according to any one of the preceding claims characterised in that
said oxide comprising barium is a complex oxide of barium strontium and titanium.
9. A start-up device according to any one of the preceding claims characterised in that
the oxide comprising barium is a barium carbonate.
10. A method of producing a discharge lamp preheating start-up device comprising a voltage-sensitive
switching element and a temperature resistance having a positive temperature coefficient,
characterised in that
the voltage-sensitive switching element is a layer (2) comprising more than 80% zinc
oxide compressed to form a circular sheet of 4mm to 7mm diameter and shaped by sintering,
that the temperature-sensitive resistance is a layer (1) comprising a mixture of oxides
comprising more than 70% by weight of a barium and strontium and titanium complex
oxide compressed to form a circular sheet of 4mm to 7 mm diameter and shaped by sintering,
that leads (3) are connected to the temperature-sensitive resistance layer (1) and
the voltage-sensitive switching resistance layer (2) respectively and that the two
layers (1, 2) are hot bonded by a silver conductive layer to form an integrated body.
1. Leuchtstofflampen-Vorheizstartvorrichtung, umfassend ein spannungsempfindliches Schaltelement
und einen temperaturempfindlichen Widerstand mit einem positiven Temperaturkoeffizienten,
dadurch gekennzeichnet, daß heißkontaktiert bzw. verbunden in einem integrierten Körper
mit dem spannungsempfindlichen Schaltelement, welches eine Schalt-Widerstandsschicht
(2), mit mehr als 80 % Zinkoxid, umfaßt, gemeinsam mit dem temperaturempfindlichen
Widerstand, welcher eine Schicht (1) aus einer Mischung von Oxyden mit mehr als 70
Gew.% eines Barium- und Strontium- und Titan- Komplexoxydes bzw. Mischoxydes umfaßt,
eine silberbeschichtete, leitfähige Schicht zwischen den zwei Widerstandsschichten
(2, 1) vorgesehen ist, wobei die Vorrichtung Anschlüsse (3) aufweist und die Schichten
(2, 1) in einem Beschichtungsfilm (5) versiegelt sind, um eine automatische Vorheiz/
Schalt-Vorrichtung zu bilden.
2. Startvorrichtung nach Anspruch 1, dadurch gekennzeichnet, daß der Durchmesser der
zwei Widerstandsschichten (2, 1) ungefähr im Bereich von 4 mm bis 7 mm liegt.
3. Startvorrichtung nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß die spannungsempfindliche
Widerstandsschicht (2) und die temperaturempfindliche Widerstandsschicht (1) getrennte
Vorrichtungen sind, welche in Serie miteinander durch einen Leiter verbunden sind.
4. Startvorrichtung nach einem der Ansprüche 1 bis 3, dadurch gekennzeichnet, daß der
Umfang der temperaturempfindlichen Schicht (1) mit einem strombegrenzenden Widerstandsfilm
(6), welcher parallel damit gekoppelt ist, beschichtet ist.
5. Startvorrichtung nach einem der Ansprüche 1 bis 3, dadurch gekennzeichnet, daß ein
strombegrenzender Widerstand parallel mit der temperaturempfindlichen Schicht (1)
verbunden ist.
6. Startvorrichtung nach einem der Ansprüche 1 bis 5, dadurch gekennzeichnet, daß der
spannungsempfindliche Widerstandswert der spannungsempfindlichen Schaltvorrichtung
in dem Bereich von 120 V bis 160 V liegt.
7. Startvorrichtung nach einem der vorangehenden Ansprüche, dadurch gekennzeichnet, daß
die temperaturempfindliche Widerstandsschicht eine Mischung, umfassend Titanoxid,
umfaßt, deren Curiepunkt im Bereich vom 65 °C bis 100 °C liegt, und daß der Widerstand
bei normalen Temperaturen von 700 Ohm bis 1200 Ohm beträgt.
8. Startvorrichtung nach einem der vorangehenden Ansprüche, dadurch gekennzeichnet, daß
das Barium-enthaltende Oxid ein komplexes Oxid von Barium, Strontium und Titan ist.
9. Startvorrichtung nach einem der vorangehenden Ansprüche, dadurch gekennzeichnet, daß
das Barium-enthaltende Oxid ein Bariumcarbonat ist.
10. Verfahren zur Herstellung einer Leuchtstofflampen-Vorheizstartvorrichtung, umfassend
ein spannungsempfindliches Schaltelement und einen Temperaturwiderstand mit einem
positiven Temperaturkoeffizienten, dadurch gekennzeichnet, daß das spannungsempfindliche
Schaltelement eine Schicht (2), mit mehr als 80 % Zinkoxid, ist, welche komprimiert
wird, um ein kreisförmiges Blatt mit einem Durchmesser von 4 mm bis 7 mm auszubilden,
und durch Sintern geformt wird, daß der temperaturempfindliche Widerstand eine Schicht
(1), mit einer Mischung von Oxyden mit mehr als 70 Gew.% eines Barium- und Strontium-
und Titan- Komplexoxydes bzw. Mischoxydes umfaßt, welche komprimiert wird, um ein
kreisförmiges Blatt mit einem Durchmesser von 4 mm bis 7 mm auszubilden, und durch
Sintern geformt wird, daß Anschlüsse (3) mit der temperaturempfindlichen Widerstandsschicht
(1) bzw. der spannungsempfindlichen Schalt-Widerstandsschicht (2) verbunden werden
und daß die zwei Schichten (1, 2) durch eine leitfähige Silberschicht heißkontaktiert
bzw. verbunden werden, um einen integrierten Körper zu bilden.
1. Dispositif d'amorçage avec préchauffage pour lampe à décharge, comprenant un élément
de commutation sensible à la tension et une résistance sensible à la température ayant
un coefficient de température positif, caractérisé en ce que, liée à chaud en un corps
intégré avec l'élément de commutation sensible à la tension, comprenant une couche
de résistance de commutation (2) qui contient plus de 80% d'oxyde de zinc, et avec
la résistance sensible à la température comprenant une couche (1) d'un mélange d'oxydes
qui contient plus de 70% en poids d'oxyde complexe de baryum, strontium et titane,
il est prévu une couche conductrice à revêtement d'argent entre lesdites deux couches
de résistance (2, 1), ledit dispositif comportant des conducteurs (3) et les dites
couches (2,1) étant scellées dans un film de revêtement (5) pour constituer un dispositif
de préchauffage/ commutation automatique.
2. Dispositif d'amorçage selon la revendication 1, caractérisé en ce que le diamètre
desdites deux couches de résistance (2,1) est approximativement dans la plage de 4
mm à 7 mm.
3. Dispositif d'amorçage selon la revendication 1 ou la revendication 2, caractérisé
en ce que la dite couche de résistance sensible à la tension (2) et ladite couche
de résistance sensible à la température (1) sont des dispositifs séparés connectés
en série par un conducteur.
4. Dispositif d'amorçage selon une quelconque des revendications 1 à 3, caractérisé en
ce que la périphérie de ladite couche sensible à la température (1) est revêtue avec
un film de résistance de limitation de courant (6) couplé en parallèle avec elle.
5. Dispositif d'amorçage selon une quelconque des revendications 1 à 3, caractérisé en
ce qu'une résistance de limitation de courant est connectée en parallèle avec ladite
couche sensible à la température (1).
6. Dispositif d'amorçage selon une quelconque des revendications 1 à 5, caractérisé en
ce que la valeur de résistance sensible à la tension dudit dispositif de commutation
sensible à la tension est dans la plage de 120 volts à 160 volts;
7. Dispositif d'amorçage selon une quelconque des revendications précédentes, caractérisé
en ce que la dite couche de résistance sensible à la température comprend un mélange
contenant de l'oxyde de titane, dont le point de Curie est situé dans la plage de
65°C à 100°C, et la résistance dans des conditions de température normale est de 700
Ω à 1200 Ω.
8. Dispositif d'amorçage selon une quelconque des revendications précédentes, caractérisé
en ce que le dit oxyde contenant du baryum est un oxyde complexe de baryum, strontium
et titane.
9. Dispositif d'amorçage selon une quelconque des revendications précédentes, caractérisé
en ce que l'oxyde contenant du baryum est un carbonate de baryum.
10. Procédé de fabrication d'un dispositif d'amorçage avec préchauffage pour lampe à décharge,
comprenant un élément de commutation sensible à la tension et une résistance sensible
à la température ayant un coefficient de température positif, caractérisé en ce que
l'élément de commutation sensible à la tension est une couche (2) contenant plus de
80% d'oxyde de zinc comprimée pour former une feuille circulaire de 4 mm à 7 mm de
diamètre etmise en forme par frittage, en ce que la résistance sensible à la température
est une couche (1) comprenant un mélange d'oxydes qui contient plus de 70% en poids
d'oxyde complexe de baryum, strontium et titane, comprimée pour former une feuille
circulaire de 4 mm à 7 mm de diamètre et mise en forme par frittage, en ce que des
conducteurs (3) sont connectés à la couche de résistance sensible à la température
(1) et à la couche de résistance de commutation sensible à la tension (2) respectivement,
et en ce que les deux couches (1,2) sont liées à chaud par une couche conductrice
en argent pour former un corps intégré.