EP1343360A2 - Circuit for operating a discharge lamp with early EOL recognition - Google Patents

Abstract

A discharge lamp (1) end of life warning circuit has microcontroller (7) measurement of the DC voltage across the electrodes (2, 3) with an offset voltage (10) connected using the voltage divider (4, 5).

Description

Technical field

The invention relates to an operating circuit for a low-pressure discharge lamp.

State of the art

Low pressure discharge lamps have lamp electrodes, usually two electrodes per lamp, which have a limited lifespan. The end of the lamp life is usually given by the end of the life of an electrode.

It is known that low-pressure discharge lamps should, if possible, be replaced when an electrode fails. This is mainly due to the fact that an electrode has an unusually high electrode drop shortly before the end of its life, which leads to high temperatures of the electrode and the adjacent region of the discharge lamp. This can result in safety problems, particularly in the case of small, low-pressure discharge lamps and heat-sensitive installation situations.

For this purpose, detection circuits are used for the detection of the end of the life of the electrodes ( _" end-of-life" detection: hereinafter referred to as EOL detection). A well known option For early detection of EOL, the voltage is measured on a so-called coupling capacitor, which connects an electrode to the positive or negative connection of the supply and decouples the lamp in terms of direct current, and couples it to the supply in terms of alternating current. In normal operation, this coupling capacitor charges up to half of the supply voltage on average over time. Deviations from this value can be recorded by a comparator and used to identify an impending end of life.

This solution has proven to be disadvantageous in terms of accuracy and technical complexity.

Presentation of the invention

Based on this, the invention is based on the technical problem of specifying an operating circuit for a low-pressure discharge lamp with an EOL detection circuit which is simple and enables reliable and safe lamp operation.

According to the invention, an operating circuit is provided for this purpose, in which the EOL detection circuit can measure the DC voltage between the electrodes in order to carry out early detection on the basis of the measured DC voltage, and the DC voltage between the electrodes can be changed by an offset voltage such that during the measurement only one polarity occurs in the changed DC voltage between the electrodes by the EOL detection circuit.

The special feature of the operating circuit according to the invention is that the EOL detection circuit now measures the direct voltage between the electrodes of the low-pressure discharge lamp. Ideally, when the electrodes are completely intact, no DC voltage occurs during operation. there It should be recalled that the low-pressure discharge lamp is operated with pure alternating current and is DC-decoupled from the operating circuit.

It has been found, however, that with increasing electrode degeneration, a DC voltage occurs, specifically because a somewhat stronger electrode drop area is formed in front of the electrode, which is likely to have a shorter service life. The low-pressure discharge lamp thus has an overall rectifying effect. This asymmetry increases with the progressive aging of the electrode with the shorter life until its failure. A voltage threshold value can be determined empirically, at which the early detection of an expected electrode failure takes place.

The advantage is that comparatively small voltages are measured, which can be processed with semiconductor components, without the need for excessively large voltage divider ratios. Precision problems are linked to voltage divider circuits with large division ratios, which can only be remedied by costly component selection. Otherwise, the procedure according to the invention for the direct measurement of the direct voltage between the electrodes is particularly simple and hardly depends on further details of the operating circuit.

The invention also provides that this DC voltage measurement is carried out in such a way that the DC voltage caused by the lamp-internal processes is shifted between the electrodes with an offset voltage, so that ultimately only voltage values of one in the range of the DC voltages permitted before the EOL early detection Polarity occur. This allows the structure of the DC measuring device used can be significantly simplified.

Furthermore, a voltage divider circuit can also be used in the invention for dividing the DC voltage to be measured between the electrodes, a tap point being provided for the EOL detection circuit. In contrast to the prior art described, however, the invention only has to detect comparatively small voltages between the electrodes, which require a much weaker voltage division than the half supply voltage conventionally used on the so-called coupling capacitor. The sensitivity to errors in the components used is therefore also lower.

According to the invention, these advantages can be linked to the fact that the EOL detection circuit has an electrode interrogation function. With the electrode interrogation function, the safety advantage of the operating circuit that has already been achieved by the EOL early detection can be further increased. The electrode query determines whether the connection or connections of a socket connected to the operating circuit for the low-pressure discharge lamp is / are connected to the associated electrode. If an electrode is not present, the low-pressure discharge lamp is not inserted correctly or is defective. If there is no electrode, then probably no discharge lamp is used at all, which results in the need to prevent the socket from being exposed to high voltage in order to prevent any danger to persons.

The electrode interrogation function takes place in that the EOL detection circuit can detect a reference potential via the respective electrode. If the connection to the reference potential is missing, it will detected by the EOL detection circuit, which provides information about the presence of the electrode.

There are already advantages if only one electrode can be queried in the manner described. The safety aspect of preventing the application of voltage when the discharge lamp is missing already arises. In particular, a _" nearer mass" electrode can be queried because touching the _" far away" electrode would be less dangerous (querying _" the cold end").

Advantageously, however, an interrogation of all existing electrodes is provided, that is, usually two electrodes. This has the advantage, for example, of being able to detect a defect in a discharge lamp that has just been inserted in any situation. In this embodiment, the EOL detection circuit must therefore be connected to a first connection of all electrodes, the other connection of which is connected to the respective reference potential.

The use of the potential of the operating circuit serving as ground for the or at least one of the reference potentials is a particularly advantageous, because simple, variant of the invention.

Furthermore, one embodiment provides that the electrode query uses the same measurement input and the same electrode taps as the DC voltage measurement for the purpose of early EOL detection.

The measurement of the - possibly voltage-divided - DC voltage between the electrodes and the electrode interrogation function are preferably carried out via a microcontroller. This microcontroller can also be used to generate the offset voltage Deliver output voltage. The output of the microcontroller used for the offset voltage is preferably connected via a resistor to the tap point of the voltage divider circuit already mentioned. Reference is made to the exemplary embodiment.

Furthermore, the operating circuit according to the invention can be designed such that it only responds in the early detection of EOL if the DC voltage triggering the detection has already occurred between the electrodes for a certain minimum time. Experience has shown that short-term phenomena can occur in the discharge lamp at the start of operation and also in continuous operation, which could trigger early detection of EOL, i.e. cause correspondingly high DC voltages between the electrodes. Such error detections can be prevented by defining a minimum recording time. In the case of the microcontroller already mentioned, loop queries or averaging over a certain number of measured values are possible. Because of the thermal inertia of the discharge lamp itself, this time delay can be safely tolerated.

Otherwise, the operating circuit can also be designed for a plurality of discharge lamps, for example for two discharge lamps. A series connection of the electrodes of one of the discharge lamps and one electrode of the other discharge lamp is then preferably provided. The remaining electrode can then be connected to ground. Reference is made to the exemplary embodiment.

Brief description of the drawings

Figur 1

zeigt ein Prinzipschema des Schaltungsaufbaus einer erfindungsgemäßen Betriebsschaltung für eine Niederdruckentladungslampe;

Figur 2

zeigt einen entsprechenden Aufbau einer Betriebsschaltung für zwei Niederdruckentladungslampen; und

Figur 3

zeigt einen entsprechenden Aufbau in der Betriebsschaltung für zwei Niederdruckentladungslampen nach einer alternativen Ausführungsform.

Two exemplary embodiments are described below to illustrate the invention in more detail, it being possible for the individual features disclosed to be essential to the invention in other combinations.

Figure 1

shows a schematic diagram of the circuit structure of an operating circuit according to the invention for a low-pressure discharge lamp;

Figure 2

shows a corresponding structure of an operating circuit for two low-pressure discharge lamps; and

Figure 3

shows a corresponding structure in the operating circuit for two low-pressure discharge lamps according to an alternative embodiment.

Preferred embodiments of the invention

In FIG. 1, 1 denotes a low-pressure discharge lamp which contains two electrodes 2 and 3. As is common with low-pressure discharge lamps, these are preheatable filament electrodes. The electrodes 2 and 3 are supplied with a high-frequency supply power by a conventional half-bridge oscillator circuit, which is not shown here and is otherwise conventional, so that a discharge can be ignited and maintained in the discharge lamp 1. Corresponding preheating circuits are provided for preheating the electrodes 2 and 3, which could also be conventional and are not shown in detail.

The connections on the left of FIG. 1 of the electrodes 2 and 3 are connected to a voltage divider circuit consisting of two resistors 4 and 5, with which a DC voltage present between the electrodes 2 and 3 is divided. The reference potential (ground) is due to the other Connection of the electrode 3. An input 6 of a microcontroller 7 is connected to the tap between the resistors 4 and 5. This voltage input 6 is connected to ground via a capacitor 8, so that the microcontroller 7 only evaluates DC voltage signals.

The tapping point between the resistors 4 and 5 and thus the voltage input 6 of the microcontroller 7 are connected via a further resistor 9 to an auxiliary voltage source 10, which in this example is actually also provided by the microcontroller 7. Furthermore, the connection of the upper electrode 2 shown in FIG. 1, which is not connected to the voltage divider circuit 4, 5, is connected via a resistor 11 to a further auxiliary voltage source 12. Accordingly, all voltages are defined against ground. In this exemplary embodiment, the auxiliary voltage source 12 corresponds to an already existing supply voltage for the analog electronics (for example of MOSFET drivers) in the range from 12 to 18 V. In this example, its potential is therefore somewhat higher than that of the auxiliary voltage source 10 of the microcontroller 7.

If a direct voltage occurs between the electrodes 2 and 3 during the continuous operation of the discharge lamp 1, this is divided down in accordance with the resistors 4, 5 and 9 at the voltage input 6 of the microcontroller 7. The resistors 4, 5 and 9 can therefore be used to adapt the level to the technical requirements of the microcontroller 7 with regard to the voltage input 6. Since the high-frequency supply voltage components between the electrodes 2 and 3 are short-circuited to ground via the capacitor 8 with a relatively low impedance, on the other hand the resistors 4 and 5 have relatively large values, the voltage input 6 is practically free of such high-frequency components.

With the help of the auxiliary voltage source 10, the voltage level between the electrodes 2 and 3 can be effectively shifted via the resistor 9. For this purpose, the auxiliary voltage source 10 specifies an offset voltage, so that, taking into account the numerical ratios between the resistors 4, 5 and 9, the same polarity always results for all permissible direct voltages between the electrodes 2 and 3 at the voltage input 6 of the microcontroller 7. This inevitably leads to a certain change in the potential relationships in the discharge lamp 1 itself. However, this effect is rather theoretical if the resistors 4 and 5 are sufficiently large. This does not have any practical effects. If faults occur here, the auxiliary voltage sources 10 and 12 could also be operated intermittently, that is, they could only be activated at certain time intervals in order to carry out a query. Then the influence on discharge physics would be limited to these comparatively short periods of time.

The second auxiliary voltage 12 offers the possibility of interrogating the electrode with respect to the electrode 2. If this electrode 2 is present and conductive, the potential at the voltage input 6 is influenced by the auxiliary voltage source 12. If the electrode 2 is not present or no longer conducts, the potential at the voltage input 6 is only influenced by the voltage divider circuit 9, 4. The resistor 11 is used to feed an auxiliary current into the measuring branch.

The electrode interrogation with respect to the electrode 3 works in a similar way, the ground connection serving as reference potential. If the electrode 3 fails, the potential at the voltage input 6 is caused by the voltage divider circuits 5, 9 and 11 and the auxiliary voltage sources 10 and 12. If no discharge lamp 1 is used or both electrodes 2, 3 have failed, the auxiliary voltage source 10 alone determines the level of the voltage input 6.

Using two auxiliary voltage sources 10 and 12 (theoretically also with only one auxiliary voltage source), both a very simple EOL early detection and a double electrode query can be carried out with a single voltage measurement input 6 of the microcontroller 7.

The microcontroller 7 can take care of disregarding the early detection of EOL by simple digital processes such as averaging over a certain number of measuring processes (e.g. of 0.5 s or a little more) or loop queries if the effect occurs only briefly. In addition to the microcontroller, only four additional resistors are required (at least if the offset voltage and the double electrode query are available at the same time). Because of the relatively moderate division ratio of the voltage divider circuit, there are no practical problems with the accuracy of the resistors. With a skilful choice of auxiliary voltages and resistance values, the conceivable voltage values at voltage measurement input 6 are in a direct 1: 1 relationship to the various operating states to be determined. Typical quantitative values are 0 - 5V as the measuring range for the voltage measurement input 6, 1V - 5V as the voltage value of the auxiliary voltage source 10 and 5V - 500V as the voltage value for the auxiliary voltage source 12. The values of the resistors can be, for example, from 3.9 kΩ to 1 MΩ for 4, at 47 kΩ to 2.2 MΩ for 5, at 3.9 kΩ to 330 kΩ for 9 at 47 kΩ to 10 MΩ for 11, and at 100 pF to 1 µF for capacitor 8.

As an example, the resistance should be 4 56 kΩ, the resistance 5 330 kΩ and the resistance 9 47 kΩ, the resistance 11 470 kΩ and the capacitor 8 100 nF. The values of the auxiliary voltage sources 10 and 12 are 5V and 15V. The following example assignments then result between different operating states and voltage values at voltage measurement input 6: when lamp 1 has not yet started but is intact, the voltage at point 6 is 3.10V.

If lamp 1 has not yet started and the upper filament is defective, the measured value is 2.72 V, if the lower filament is defective, it is over 5 V and can be limited by measuring input 6. If lamp 1 is started and OK, the measured value is 2.52V. If the lamp is started and a DC voltage between the electrodes has developed in the positive direction, for example 20 V, the measured value is 3.96 V, for the same DC voltage in the negative direction it is 1.09 V. This shows that at suitable dimensioning, the voltage value at the measurement input 6 can be brought into clear connection with the different operating states.

The above statements apply accordingly to the second exemplary embodiment from FIG. 2, which is distinguished from FIG. 1 by the fact that two discharge lamps 1 and 1 'are provided. The electrodes are accordingly designated 2, 3, 2 ', 3'. FIG. 2 shows that the electrodes 2, 3 and 2 'are connected to the auxiliary voltage source 12 with the aid of a further resistor 13 (to prevent a short circuit between the electrodes 2 and 3), while the electrode 3' is in turn connected to ground. The rest of the structure (apart from the dimensioning of the actual supply circuit) is identical to FIG. 1. It can be seen that both a DC voltage between electrodes 2 and 3 and a DC voltage between electrodes 2 'and 3' can be detected because they are add in the voltage divider circuit 4, 5. The theoretically conceivable case that the direct voltages between the electrodes 2 and 3 on the one hand and 2 'and 3' on the other hand temporally Developing in opposite directions in parallel in exactly the right ratio so that they fully compensate one another is so unlikely, especially with regard to the time course of the development of the DC voltages between electrodes, that it is not significant for practical use.

Furthermore, the electrodes 2, 3 and 2 'can be queried via the auxiliary voltage source 12. In this embodiment, therefore, the failure or absence of each electrode can be detected.

Failure of the electrodes 2, 3 and 2 ', however, cannot be distinguished from the electrode query.

Figure 3 shows a third embodiment with an operating circuit, which is also designed via two discharge lamps 1 and 1 '. In this exemplary embodiment, the helix interrogation described only takes place for the lower electrode 3 or 3 ', because this forms the _" cold end" of the lamp 1 or 1' when used. For this reason, two lamps 1 and 1 'working in parallel can be monitored here in a particularly simple manner with a uniform circuit. The EOL early detection takes place via the already explained resistors 4 and 5 or 4 'and 5'. If the DC voltage between the electrodes 2 and 3 or between the electrodes 2 'and 3' becomes too great, this is detected in the same way as in the first exemplary embodiment from FIG. 1. The only difference is that there are 6 DC voltages between the voltage measurement input 6 make the electrodes of both lamps 1 and 1 'noticeable. The theoretically conceivable situation of an exactly opposite development of DC voltages in the same lamps, which compensate each other at the voltage measurement input 6, is irrelevant for practice because it is extremely unlikely. However, it can happen that voltages have already developed in both lamps 1 and 1 'and thus triggering when exceeded a place value takes place if neither of the two DC voltages exactly corresponds to this threshold value. On the other hand, in practice the exact size of the threshold value is not essential, so that it is possible to work practically well in the manner outlined in FIG. 3.

Claims (11)

Operating circuit for a low-pressure discharge lamp (1, 1 ') with lamp electrodes (2, 3, 2', 3 ') and an EOL detection circuit (4-13) for early detection of an expected electrode failure,
characterized in that the EOL detection circuit (4-13) can measure the DC voltage between the electrodes (2, 3, 2 ', 3') in order to carry out early detection on the basis of the measured DC voltage,
and the DC voltage between the electrodes (2, 3, 2 ', 3') can be changed by an offset voltage (10) such that when the changed DC voltage between the electrodes is measured by the EOL detection circuit (4-13) only one polarity occurs. Operating circuit according to Claim 1, in which a voltage divider circuit (4, 5) with a tap point for the EOL detection circuit (4-13) is provided between the electrodes (2, 3, 2 ', 3'). Operating circuit according to Claim 1 or 2, in which the EOL detection circuit (4-13) is combined with a helix interrogation function,
the EOL detection circuit (4-13) being connected to a first connection of at least one electrode (2, 3, 2 ', 3'), the other second connection of which is connected to a reference potential (12), so that by checking the electrical connection can be carried out via the electrode to the reference potential (12). Operating circuit according to Claim 3, in which the EOL detection circuit (4-13) is connected to a first connection of both electrodes (2, 3, 2 ', 3'), the other second connection of which is connected to a respective reference potential (12) is connected, so that a spiral interrogation can be carried out by checking the electrical connection via the respective electrode (2, 3, 2 ', 3') to the respective reference potential (12). Operating circuit according to claim 3 or 4, wherein the / one of the two reference potential (s) is ground. Operating circuit according to one of the preceding claims, in which the EOL detection circuit (4-13) carries out the coil interrogation via the same measuring input (6) and the same electrode taps as the measurement of the direct voltage between the electrodes (2, 3, 2 ', 3' ). Operating circuit according to one of the preceding claims, in which the EOL detection circuit (4-13) has a microcontroller (7) for measuring the DC voltage between the electrodes (2, 3, 2 ', 3') and optionally for the helix interrogation function. Operating circuit according to Claim 7, in which the microcontroller (7) can supply an output voltage (10) which is used to generate the offset voltage. Operating circuit according to Claim 2 and Claim 8, in which the output (10) of the microcontroller for the offset voltage is connected via a resistor (9) to the tap point of the voltage divider circuit (4, 5). Operating circuit according to one of the preceding claims, in which the EOL detection circuit (4-13) is designed to only provide an early detection signal when the DC voltage between the electrodes (2, 3, 2 ', 3') is above a certain value Generate signal when the DC voltage has already occurred for a certain minimum time. Operating circuit according to one of the preceding claims, which is designed for two discharge lamps (1 1 '), the electrodes (2, 3) of one of the discharge lamps (1) and an electrode (2') of the other discharge lamp (1 ') via a resistor (13) are connected in series and connected to an electrode tap and the other electrode (3 ') of the other discharge lamp is connected to ground.

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