Electronic Gas Discharge Tube Starter Having A Semiconductor Switch Element Controlled By A Capacitive Voltage Divider

Abstract

An electronic gas discharge tube starter comprises a semiconductor switch element with a separate control signal terminal connected through a semiconductor switch diode to the node between two capacitors included in a circuit connected in parallel with the switch element. According to the invention, the capacitors can exclusively be charged through a diode. It is thus ensured that after a predetermined time the voltage generated in the node has decreased to the extent that the switch diode is no longer capable of igniting the tube.

Description

This invention relates to a device for electronically igniting gas discharge tubes, such as luminescent lamps.

It is generally known to include such a device in one circuit with the gas discharge tube 1 to be ignited or started, as shown diagrammatically in FIG. 1. In this arrangement the igniter or starter 5 is connected between the connectors A and B and hence across the gas discharge tube 1 to be ignited.

A much-used construction of such a starter, the so-called "neon starter," comprises a neon tube adapted to operate a bimetallic switch contact. In addition to the fact that the service life of such a neon starter is relatively short, the ignition process takes some time and is moreover accompanied by annoying flashing.

According to other proposals, such a starter operates fully electronically, which provides an improvement regarding the starter's service life, because of the absence of mechanical switch contacts. This solution has the disadvantage, however, that it has proved to shorten the service life of the gas discharge tube operated by the starter.

It is an object of the present invention to eliminate the above disadvantages and to provide an igniter which makes it possible for the gas discharge tube to be ignited in a relatively short time and without annoying light flashes, whereby to ensure a long service life for both the gas discharge tube and the igniter proper.

A further disadvantage of the existing igniter is that, after one or a limited number of attempts to start a lamp with insufficient residual emission, it does not stop starting the lamp but continues the starting process until the filaments of the lamp or the starter itself break down.

Since it is the very starting procedure which tends to cause radio interference, it is a further object of the invention to overcome this disadvantage by providing an electronic starter which when the lamp's emission proves to be insufficient, does not repeat the starting process so long as the circuit arrangement remains energized.

The present invention further aims at as simple and compact a construction as possible, which, if so desired, is adapted to be accommodated in an exchangeable housing, as commonly used for the above "neon starter," or in the fitting of the gas discharge tube proper.

According to the present invention an apparatus for electronically igniting gas discharge tubes, such as luminescent lamps is provided comprising a semiconductor switch element having a separate control signal terminal which through a semiconductor switch diode is connected to the node between two capacitors included in a circuit connected in parallel with said switch element, and wherein said capacitors can be charged exclusively through a diode, thereby to ensure that after a predetermined time the voltage generated in said node has decreased to such an extent that the switch diode is no longer capable of igniting the tube.

A first embodiment of an igniter according to the invention is diagrammatically shown in FIG. 2. The connectors A and B of FIG. 2 correspond with connectors A and B shown in FIG. 1; in other words, the circuit arrangement according to FIG. 2 is substituted for the block 5 of FIG. 1.

The circuit arrangement shown in FIG. 2 comprises a triode semiconductor DC switch element 10, such as a thyristor, which is connected to the nodes A' and B'. The gate of the triode semiconductor DC circuit element is connected through a diode semiconductor DC switch element 11, such as e.g., a Shockley diode, with the node of a capacitor 12 and a capacitor 14. These capacitors form together with a resistor 13 and a diode 17 a series circuit, which is connected in parallel with the switch element 10. Capacitors 12 and 14 are in addition bridged by leakage resistors 15 and 16. The functions of capacitor 8 and resistor 9 will be described hereinafter; these elements need not influence the actual switching action.

The operation of the igniter is as follows:

When an alternating voltage is generated across A and B, during the half cycle in which B is positive relative to A, the thyristor 10 will be switched early in this half cycle by the switch diode 11 from the charged capacitor 12, by virtue of the fact that capacitor 14, selected of greater magnitude than 12, is still practically uncharged. Owing to the fact that the former capacitor 12 is charged, capacitor 14 also acquires a certain charge, which it is true, partly flows away through the leakage resistor 16, which has a high-resistance value, but is largely maintained until the next cycle, during which capacitor 14 is further charged. As a consequence, capacitor 14 is charged stepwise in each cycle until the total voltage across capacitors 12 and 14 has become so high that diode 17 is no longer conductive or at any rate capacitor 12 is no longer charged to the extent that ignition is effected through switch diode 11. The starter thus ceases to operate.

Leakage resistors 15 and 16 are so dimensioned that, with charged capacitors 12 and 14, they correspond with the voltage ratio in the capacitors. If the breakdown voltage of the switch diode 11 is termed V.sub.d. and the maximum occurring peak voltage between A' and B' is termed V.sub.s., then we have the following equation: R16:R15.gtoreq. (V.sub.s -V.sub.d ):V.sub.d.

The diode then continues to replenish in each cycle the leakage current through resistors R15 and R16 after the ignition has failed, without however charging capacitor 12 to the extent that ignition occurs. The residual current carried by the starter as such is then very small, while no current effects can arise owing to the fact that the thyristor is no longer switching.

During the above-described process, in which capacitor 12 is charged stepwise, the ignition moment of thyristor 10 is shifted from early in the half cycle (B positive relative to A) to an ever later point of time, and after a number of ignitions the ignition moment has shifted so far that it practically coincides with the peak voltage between A' and B'. At that moment the tube will generally be ignited, while in the preceding cycles a high-filament current was obtained for rapidly heating the filaments. During the half cycles in which A is positive relative to B, an induction voltage is generated from the coil, which can introduce the ignition during this part of the cycle.

Owing to the DC component generated through the choke coil 4, the igniter according to the invention provides a higher ignition current for the filaments 2 and 3 than can be obtained in the case of normal short-circuiting of AB.

As a result, the said filaments are in a short time heated to such an extent that rapid ignition of the tube is possible, while maintaining a long service life of the tube. The shift of the ignition moment thereafter ensures that during a given cycle the ignition moment at all times occurs practically simultaneously with, or shortly after, the moment when the peak voltage is attained. This ensures proper ignition. If the lamp does not have the required emission, the starter is inactivated and is not reactivated before the current has been switched off and switched on again.

The resistor 13 can be omitted, so that the number of parts will become extremely small, and the whole starter can be accommodated in the same housing as that of a glow tube starter.

If, as is normally the case, a noise-suppressing capacitor 8 is used in the starter, in parallel with the tube, then a choke coil or a resistor 9 must be included for protecting the thyristor. In the present case it is attractive, according to the invention, to select a resistor so dimensioned that it can conduct the normal starting current but is overloaded, if, for any reason whatsoever (defective switch element), a starting current is unintentionally maintained. The resistor then acts at the same time as a trip device.

If, as shown in FIG. 1, a series capacitor 6 is included in the circuit, the starter shown in FIG. 3, according to the invention, provides a solution for igniting the tube by the same principle:

According to FIG. 3, after the choke coil or resistor 9, there is included a diode bridge 18, 19, 20 and 21, after which the thyristor circuit with igniter of FIG. 2 is repeated. In this case, owing to the use of the diode bridge, instead of switching being effected during one-half of the AC cycle only, switching takes place in each half cycle with the same shift effect; after being switched-on, the starter now gives in both half cycles first an early ignition, which occurs later and later, so that ultimately upon ignition on or just after the moment when the peak voltage is reached, the tube can be ignited.

The oscillation effects occurring during the starting process in the capacitive circuit, resulting from the self-induction of the choke coil, cause much higher peak voltages across the thyristor 10 than in the case of the circuit shown in FIG. 2, for which the thyristor 10 and especially the capacitor 14 must be designed. Here again, for that matter, the desired switch-off effect occurs.

Although the starter according to FIG. 3 is more expensive owing to the provision of the diode bridge, it is also universally suitable for starting tubes without a series capacitor. Since in the last-mentioned case, no substantial DC component is generated through the choke coil, capacitors 12 and 14 should be so dimensioned that starting current is given during a longer period of time before the optimum ignition voltage is applied.

FIG. 4 shows a triode AC switch element equivalent to FIG. 3. In it, the thyristor has been replaced by the double thyristor 10, such as a triac, and the AC switch diode 11, such as a diac, provides for the ignition.

Capacitor 14 and leakage resistor 16 are now disposed in a rectifying bridge for them to be able to buildup direct voltage charge stepwise. Capacitor 12 is now alternatingly charged, so that capacitor 14 must be larger than in FIG. 2 or 3. The leakage resistor 15 can be omitted.

The demands made on the maximum barrier voltage of the double thyristor, as well as on its low dv/dt sensitivity, however, are great, so that, with the present state of the art, the circuit arrangements of FIGS. 2 and 3 will generally be preferred.

Claims

I claim:

1. A circuit having an electronic device for igniting gas discharge tubes, such as luminescent lamps, comprising two input terminals for connecting the device to a gas discharge tube; a semiconductor switch element having two main terminals and a gate terminal for receiving a control signal, said switch element having its two main terminals included in a circuit across said two input terminals; a semiconductor switch diode; a voltage divider means including capacitive means coupled across said switch element; and means for coupling said gate terminal through said semiconductor switch diode to a junction of said voltage divider means, said voltage divider means being arranged so as to produce at its said junction a voltage for firing said switch element through said switch diode, and said circuit across said switch element having included therein a unilateral means for sustaining a voltage at which said switch diode is no longer capable of firing said switch element, once electric charge, supplied during a predetermined number of successive cycles of voltage waveform is developed across said two input terminals, has builtup a voltage sufficient for bringing said unilateral means to a blocking state.

2. A circuit according to claim 1, wherein said voltage divider means including capacitive means comprises a first capacitor bridged by a first leakage resistor and a second capacitor bridged by a second leakage resistor, the ratio between the first leakage resistor and the second leakage resistor being equal to (V.sub.s -V.sub.d)/V.sub.d wherein V.sub.s is maximum peak voltage which may occur across the switch element and V.sub.d is the voltage at which said switch diode becomes conductive.

3. A circuit according to claim 1, including a diode bridge coupled across said two main terminals for enabling said switch element in each half cycle of voltage which may be applied to said input terminals.

4. A circuit according to claim 1 wherein said unilateral means comprises a rectifying bridge circuit arrangement; said semiconductor switch element is of the type which can be triggered into conduction in both directions; said semiconductor switch diode is of the type which can be brought to conduction in both directions; said voltage divider means including capacitive means comprising a first capacitor connected across a first pair of opposite terminals of said rectifying bridge circuit arrangement, and a second capacitor which is connected in series with a second pair of opposite terminals of said rectifying bridge circuit arrangement; said junction being electrically at a plate of said second capacitor.

5. A circuit according to claim 4, including a leakage resistor connected across said first capacitor.

6. A circuit according to claim 1, including a gas discharge tube coupled across said two input terminals.

7. A circuit according to claim 2 including a gas discharge tube coupled across said two input terminals.

8. A circuit according to claim 3, including a gas discharge tube coupled across said two input terminals.

9. A circuit according to claim 4, including a gas discharge tube coupled across said two input terminals.

10. A circuit according to claim 5, including a gas discharge tube coupled across said two input terminals.

11. A circuit according to claim 6, including a noise-killing capacitor connected in parallel with said gas filled tube, and a noncapacitive impedance connected in series with said noise-killing capacitor across said two input terminals.

12. A circuit according to claim 7, including a noise-killing capacitor connected in parallel with said gas filled tube, and a noncapacitive impedance connected in series with said noise-killing capacitor across said two input terminals.

13. A circuit according to claim 8, including a noise-killing capacitor connected in parallel with said gas filled tube, and a noncapacitive impedance connected in series with said noise-killing capacitor across said two input terminals.

14. A circuit according to claim 9, including a noise-killing capacitor connected in parallel with said gas filled tube, and a noncapacitive impedance connected in series with said noise-killing capacitor across said two input terminals.

15. A circuit according to claim 10, including a noise-killing capacitor connected in parallel with said gas filled tube, and a noncapacitive impedance connected in series with said noise-killing capacitor across said two input terminals.