EP1343359B1 - EOL-detection with integrated electrode interrogation - Google Patents

Abstract

A discharge lamp (1) end of life monitoring 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) and a reference voltage (12) applied to the electrodes.

Description

Technical area

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 life. The end of the life of the lamp is usually given by the end of the life of an electrode. A recognition circuit is from the document US 5,808,422 known.

It is known that low-pressure discharge lamps should be replaced if possible, if the failure of an electrode is emerging. This is mainly due to the fact that with an electrode shortly before the end of its life an unusually high electrode case occurs, which leads to high temperatures of the electrode and of the adjacent region of the discharge lamp. Especially with small low-pressure discharge lamps and heat-sensitive mounting situations, this can result in safety problems.

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 known possibility for the EOL-early detection consists in the measurement of the voltage at a so-called coupling capacitor, which connects an electrode to the positive or negative terminal of the supply and the lamp DC-decoupled and AC-coupled to the supply. This coupling capacitor charges in normal operation in the time average to half of the supply voltage. Deviations from this value can be detected by a comparator and used to detect an imminent end of life.

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

Presentation of the invention

Based on this, the technical problem underlying the invention is to provide an operating circuit for a low-pressure discharge lamp with an EOL detection circuit, which is simple and enables reliable and reliable 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 to perform the early detection based on the measured DC voltage, and the DC voltage between the electrodes can be changed by an offset voltage so that during the measurement the changed DC voltage between the electrodes by the EOL detection circuit only one polarity occurs.

The special feature of the operating circuit according to the invention is that the EOL detection circuit now measures the DC voltage between the electrodes of the low-pressure discharge lamp. In the case of completely intact electrodes, ideally no DC voltage occurs during operation. there It is to 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 is created by the formation of a slightly larger electrode area in front of the electrode, which is expected to have a shorter lifetime. The low-pressure discharge lamp thus has a total rectifying effect. This asymmetry increases with progressive aging of the electrode with the shorter life until its failure. It can empirically set a voltage threshold, in which the early detection of an expected electrode failure occurs.

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

These advantages are linked to the invention in that the EOL detection circuit has an electrode interrogation function. By the electrode interrogation function, the already achieved by the EOL-early detection security advantage of the operating circuit can be further increased. Namely, it is determined by the electrode interrogation whether the one or more terminals of a socket connected to the operating circuit for the low-pressure discharge lamp connected to the associated electrode is / are. If an electrode is not present, the low-pressure discharge lamp is not properly inserted or defective. If there is no electrode, then probably no discharge lamp is used, which results in the need to prevent high voltage loading of the socket, in order to preclude any risk to persons.

The electrode interrogation function according to the invention 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, this is detected by the EOL detection circuit, resulting in a statement about the presence of the electrode.

The invention should already be realized if only one electrode can be interrogated in the manner described. The safety aspect of preventing a voltage application in the absence of a discharge lamp results namely already then. In particular, a _" near-ground" electrode can be interrogated, because touching the _" off-center" electrode would be less dangerous (query the _" cold end").

Advantageously, however, a query of all existing electrodes is provided, that is usually two electrodes. This results, for example, in the advantage of being able to detect a defect of a discharge lamp being used in every situation. In this embodiment, therefore, the EOL detection circuit must be connected in each case to a first terminal of all electrodes, their respective other terminal being connected to the respective reference potential.

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

Furthermore, an 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.

A further preferred embodiment is characterized in that the DC voltage used for EOL early detection is shifted between the electrodes by an offset voltage such that only one polarity of this DC voltage occurs during the measurement by the EOL detection circuit. The offset voltage must therefore be at least as large as the already mentioned voltage threshold. The existence of only one voltage sign results in simplification possibilities for the construction of the voltage measuring device of the EOL detection circuit.

It may also be advantageous in the invention to use a voltage divider circuit between the electrodes in order to be able to pick up a part of the DC voltage between the electrodes at a tapping point for the EOL detection circuit. However, this voltage divider circuit is unproblematic in comparison with the prior art in that the DC voltages between the electrodes are far from reaching the level of half the supply voltage. Therefore, the voltage divider ratios are more moderate, so that the sensitivity to errors of the resistor elements used is not as pronounced as in the prior art.

The measurement of the - optionally offset-shifted and voltage-divided - DC voltage between the electrodes and the electrode interrogation function are preferably carried out via a microcontroller. This microcontroller may also provide an output voltage to be used for generating the offset voltage. Preferably, the output used for the offset voltage of the microcontroller is connected via a resistor to the already mentioned tapping point of the voltage divider circuit. Reference is made to the embodiment.

Furthermore, the operating circuit according to the invention can be designed so that it only responds to the EOL-early detection when the detection triggering DC voltage between the electrodes has already occurred a certain minimum time. Experience shows that short-term phenomena in the discharge lamp can occur at the start of operation and also during continuous operation, which could trigger EOL early detection, ie cause correspondingly high DC voltages between the electrodes. By defining a minimum acquisition time, such misrecognitions can be prevented. In question, in the case of the microcontroller already mentioned, for example, loop queries or averaging over a certain number of measured values. Because of the already given thermal inertia of the discharge lamp itself, this time delay can be safely tolerated.

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

In the following, two exemplary embodiments for a more detailed illustration of the invention will be described, wherein the disclosed individual features may also be essential to the invention in other combinations.

FIG. 1

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

FIG. 2

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

FIG. 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 shows a low-pressure discharge lamp which contains two electrodes 2 and 3. As usual with low-pressure discharge lamps, these are preheatable helical electrodes. The electrodes 2 and 3 are supplied by a non-illustrated here and otherwise conventional half-bridge oscillator circuit with a high-frequency power supply, so that in the discharge lamp 1, a discharge can be ignited and maintained. For preheating the electrodes 2 and 3 corresponding preheating circuits are provided, which could also be conventional and are not shown in detail.

In the FIG. 1 Respectively left terminals of the electrodes 2 and 3 are connected to a consisting of two resistors 4 and 5 voltage divider circuit, with which a voltage applied between the electrodes 2 and 3 DC voltage is divided. The reference potential (ground) is on the other Connection of the electrode 3. At the tapping point between the resistors 4 and 5, an input 6 of a microcontroller 7 is connected. This voltage input 6 is connected via a capacitor 8 to ground, so that the microcontroller 7 evaluates only DC 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 is actually also provided by the microcontroller 7 in this example. Further, the terminal not connected to the voltage dividing circuit 4, 5 is the in FIG. 1 upper electrode 2 connected via a resistor 11 to a further auxiliary voltage source 12. All voltages are defined accordingly to ground. In this exemplary embodiment, the auxiliary voltage source 12 corresponds to an already present supply voltage of the analog electronics (for example of MOSFET drivers) in the range of 12-18 V. In this example, its potential is slightly higher than that of the auxiliary voltage source 10 of the microcontroller 7.

If in the continuous operation of the discharge lamp 1 between the electrodes 2 and 3, a DC voltage, it is divided down according to the resistors 4, 5 and 9 at the voltage input 6 of the microcontroller 7. By the resistors 4, 5 and 9 so a level adjustment to the technical requirements of the microcontroller 7 with respect to the voltage input 6 can be made. Since the high frequency supply voltage components between the electrodes 2 and 3 are shorted to ground via the relatively low impedance capacitor 8, 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 aid 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 is an offset voltage, so that taking into account the numerical relationships between the resistors 4, 5 and 9 at all permissible DC voltages between the electrodes 2 and 3 at the voltage input 6 of the microcontroller 7 always the same polarity results. This inevitably leads to a certain change in the potential conditions in the discharge lamp 1 itself. However, this effect is more theoretical if the resistors 4 and 5 are sufficiently large. Practical effects do not result from this. Should interference arise here, the auxiliary voltage sources 10 and 12 could also be operated intermittently, that is, activated only at certain time intervals in order to perform a query. Then the influence on the discharge physics would be limited to these comparatively short periods of time.

The second auxiliary voltage 12 offers a possibility for electrode detection with respect to the electrode 2. If this electrode 2 is present and conducts, the potential at the voltage input 6 is influenced by the auxiliary voltage source 12. If the electrode 2 is absent or no longer conductive, the potential at the voltage input 6 is influenced only by the voltage divider circuit 9, 4. The resistor 11 serves to feed an auxiliary current into the measuring branch.

Similarly, the electrode sensing works with respect to the electrode 3, with the ground terminal serving as a reference potential. If the electrode 3 fails, the potential at the voltage input 6 is conditioned by the voltage divider circuit 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 measuring input 6 of the microcontroller 7.

The microcontroller 7 can provide averaging operations (e.g., of 0.5 sec or more) extended by simple digital operations such as a certain number of measurements, or loop queries, to disregard EOL early detection if the effect occurs only briefly. In addition to the microcontroller only four additional resistors are necessary (at least if the offset voltage and the double electrode query are present at the same time). Because of the relatively moderate division ratio of the voltage divider circuit, there are no practical difficulties with the accuracy of the resistors. With skillful choice of auxiliary voltages and resistance values, the conceivable voltage values at the voltage measuring input 6 are in direct 1: 1 relationship with the various operating states to be determined. Typical quantitative values are at 0-5V as the measuring range for the voltage measuring input 6, at 1V-5V as the voltage value of the auxiliary voltage source 10 and at 5V-500V as the voltage value for the voltage auxiliary source 12. The values of the resistors can be, for example, 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 the capacitor 8.

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

If the lamp 1 is not yet started and the upper coil is defective, the measured value is 2.72V, if the lower coil is defective, it is above 5V and may be limited by the measuring input 6. When the lamp 1 is started and in order, the reading is 2.52V. When the lamp is started and a DC voltage between the electrodes in the positive direction of, for example, 20 V has developed, the measured value is 3.96 V, at the same DC voltage in the negative direction at 1.09 V. Thus one recognizes that at suitable dimensioning of the voltage value at the measuring input 6 can be brought into clear connection with the various operating conditions.

The above statements apply mutatis mutandis to the second embodiment FIG. 2 facing each other FIG. 1 characterized in 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 'with the aid of another resistor 13 (to prevent a short circuit between the electrodes 2 and 3) are connected to the auxiliary voltage source 12, while the electrode 3' is in turn connected to ground. The rest of the structure is identical (apart from the dimensioning of the actual supply circuit) FIG. 1 , It can be seen that both a DC voltage between the electrodes 2 and 3 and a DC voltage between the electrodes 2 'and 3' can be detected because they add in the voltage divider circuit 4, 5. The theoretically conceivable case that the DC 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 exactly the same proportions, so that they completely compensate each other, is so unlikely, above all, with regard to the time course of the development of the DC voltages between electrodes, that it is of no importance for practical application.

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

However, a failure of the electrodes 2, 3 and 2 'is indistinguishable via the electrode query.

FIG. 3 shows a third embodiment with an operating circuit, which is also designed via two discharge lamps 1 and 1 '. In this embodiment, the described Wendelabfrage takes place only for the lower electrode 3 and 3 ', because this forms the "cold end" of the lamp 1 or 1' in the application. For this reason, two lamps 1 and 1 'operating in parallel here can be monitored in a particularly simple manner with a uniform circuit. The EOL-early detection takes place in each case 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 large, this is detected in the same way as in the first embodiment FIG. 1 , The only difference is that DC voltages between the electrodes of both lamps 1 and 1 'become noticeable at the voltage measuring input 6. 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 in practice, because extremely unlikely. However, it may happen that voltages have already formed in both lamps 1 and 1 'in each case and thus tripping on exceeding a value occurs if neither of the two DC voltages corresponds exactly to this threshold value. On the other hand, in practice, the exact magnitude of the threshold does not necessarily matter, so that in the FIG. 3 sketched way can be worked practically well.

Claims (8)

1. Operating circuit for a low-pressure discharge lamp (1, 1') with lamp electrodes (2, 3, 2', 3') and an END-OF-LIFE detection circuit (4-13) for early detection of an expected electrode failure,
   the END-OF-LIFE detection circuit (4-13) measuring the DC voltage between the electrodes (2, 3, 2', 3') in order to carry out the early detection with the aid of this measured DC voltage,
   characterized in that the END-OF-LIFE detection circuit (4-13) is connected to a first terminal of a first electrode (2, 3, 2', 3') whose other second terminal is connected to a first reference potential (12) such that an electrode interrogation can be carried out by checking the electric connection via the first electrode (2, 3, 2', 3') to the first reference potential (12), and the END-OF-LIFE detection circuit (4-13) being connected to a first terminal of a second electrode (2, 3, 2', 3') whose other, second terminal is connected to a second reference potential such that an electrode interrogation can be carried out by checking the electric connection via the second electrode (2, 3, 2', 3') to the second reference potential,
   and a voltage divider circuit (4, 5) with a tapping point being provided between the in each case first connections of each electrode, which tapping point is connected to a measuring input (6) of the END-OF-LIFE detection circuit (4-13), the END-OF-LIFE detection circuit (4-13) carrying out the electrode interrogation via the same measuring input (6) and the same electrode taps as the measurement of the DC voltage between the electrodes (2, 3, 2', 3').
2. Operating circuit according to Claim 1, in which the/one of the two reference potential(s) is ground.
3. Operating circuit according to one of the preceding claims, in which the DC voltage between the electrodes (2, 3, 2', 3') can be modified by an offset voltage (10) such that only one polarity occurs during measurement of the modified DC voltage between the electrodes (2, 3, 2', 3') by the END-OF-LIFE detection circuit (4-13).
4. Operating circuit according to one of the preceding claims, in which the END-OF-LIFE detection circuit (4-13) has a microcontroller (7) for measuring the DC voltage between the electrodes (2, 3, 2', 3') and for the electrode interrogation function.
5. Operating circuit according to Claim 4, in which the microcontroller (7) can supply an output voltage that is used to generate the offset voltage.
6. Operating circuit according to Claim 4 and Claim 5, in which the output (10) of the microcontroller (7) for the offset voltage is connected via a resistor (9) at the tapping point of the voltage divider circuit (4, 5).
7. Operating circuit according to one of the preceding claims, in which the END-OF-LIFE detection circuit (4-13) is designed to the effect that given a DC voltage between the electrodes (2, 3, 2', 3') that lies above a specific value a signal indicating the early detection is generated only when the DC voltage has already occurred for a specific minimum time.
8. 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') being connected in series via a resistor (13) and connected to an electrode tap, the other electrode (3') of the other discharge lamp (1') being connected to ground.

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