EP0471332A1 - Circuit assembly for operating a fluorescent lamp - Google Patents

Abstract

A circuit arrangement for operating a fluorescent lamp (1) on an AC network, for example on an aircraft electrical system, has a switch (8) which is located between the lamp electrodes (3, 4) and can be switched by means of a control circuit (9) closed switch (8) the lamp electrodes (3,4) are heated. In order to increase the service life of the fluorescent lamp (1), it is proposed to evaluate the positive temperature coefficient of the lamp electrodes (3, 4) to determine a preheating time and / or to increase the preheating current and / or to switch the switching time by means of a transformer (13, 14) move and / or switch off the fluorescent lamp (1). <IMAGE>

Description

The invention relates to a circuit arrangement for the operation of a fluorescent lamp on an AC network, for example on an aircraft electrical system, with a switch located between the lamp electrodes and which can be switched by means of a control circuit, the lamp electrodes being heated when the switch is closed.

Such a circuit arrangement is described in DE 33 27 189 A1. The control circuit is used to dim the brightness. It generates phase-shifted ignition voltage pulses by opening the switch in every mains half-wave. When the switch is closed, a heating current flows through the lamp electrodes. The ignition voltage pulses occur immediately in every mains half-wave when a mains switch is opened.
Ignition voltage pulses also occur when the lamp electrodes are not sufficiently preheated.

It has been shown that the service life of a fluorescent lamp and its lifespan are significantly shortened if ignition attempts are carried out when the lamp electrodes are still cold. In the circuit arrangement according to DE 33 27 189 A1, many ignition pulses occur with lamp electrodes which have not yet been sufficiently heated, so that only a short lifespan of the fluorescent lamp can be expected.

In CH-PS 595 036 a circuit arrangement of the type mentioned is also described. It is mentioned that a cold start is avoided in the dark control of the fluorescent lamps by preheating the lamp electrodes after the fluorescent lamps have been switched on. How this dark control is to be carried out is not specified.

The object of the invention is to propose a circuit arrangement of the type mentioned at the outset with which the life of the fluorescent lamp is automatically extended.

According to the invention, the above object is achieved in a circuit arrangement of the type mentioned at the outset in that a measuring element connected on the input side to the control circuit detects the voltage drop across at least one of the lamp electrodes and in that the control circuit only opens the switch for igniting the fluorescent lamp when the detected voltage essentially is constant, or reaches a threshold.

The invention makes use of the fact that the lamp electrodes have a positive temperature coefficient. The ohmic resistance of the lamp electrodes increases from the cold state to the preheated state, for example by a factor of 4.

If a power switch is closed to start up the fluorescent lamp, the switch connecting the lamp electrodes initially remains closed, so that a heating current flows through the lamp electrodes. With increasing heating of the lamp electrodes, the voltage detected by the measuring element increases. If this no longer changes or reaches a predetermined threshold value, the lamp electrodes are sufficiently preheated. First then the control circuit ignites the fluorescent lamp. Ignition attempts are therefore avoided during the preheating time. This increases the life of the fluorescent lamp considerably. When the fluorescent lamp is cold, the preheating time will generally take considerably longer than a half-wave of the AC mains voltage.

It is also favorable that the respective preheating time is proportional to the cooling of the lamp electrodes, which occurs when the fluorescent lamps are switched off. If the fluorescent lamp was only switched off briefly before restarting, then the preheating time is shorter than if the fluorescent lamps were switched off for a longer period and the lamp electrodes were thus cooled further.

It is also advantageous in the evaluation of the temperature-dependent voltage of the lamp electrodes that if the lamp electrodes did not cool very far due to the ambient temperature, the subsequent heating time will be correspondingly short.

It is achieved by the invention that the necessary preheating of the lamp electrodes is achieved before an ignition attempt and that all cooling which has occurred is automatically taken into account, with a correspondingly short preheating time being obtained in each individual case.

In an embodiment of the invention, the control circuit closes the switch for heating the electrodes when the voltage detected by the measuring element changes or falls below the threshold value. It is thereby achieved that the start of a preheating time is initiated automatically when the lamp electrodes are cold.

In one embodiment of the invention, a timing element is provided which increases the preheating time to a maximum value limited. This ensures that overheating of the lamp electrodes is avoided in any case.

In a circuit arrangement of the type mentioned, a second solution to the problem is characterized in that a transformer is provided for at least one of the lamp electrodes, the primary winding of which is connected in series with the switch and the lamp electrode is located on the secondary side, so that the lamp is closed Switch flowing preheating current is increased.

This solution also extends the life of the fluorescent lamps. In practice, the lamp electrodes have manufacturer-specific variations and specimen variations of the ohmic resistances of the lamp electrodes. With the same preheating current and the same preheating time supplied by a ballast, the different fluorescent lamps would then be preheated differently. If the ballast supplies a sufficient preheating current for lamps with high resistance, then this does not lead to the desired preheating for lamps with low resistance lamp electrodes. This can lead to lamp start attempts with insufficiently heated electrodes. This would considerably reduce the lifespan of the fluorescent lamps in question. This is avoided by the transformer. Because the transformer amplifies the current flowing for the lamp electrode on the primary side when the switch is closed.

The transformer preferably has a non-linear core, so that a higher current gain occurs on the secondary side in the case of low-resistance lamp electrodes than in the case of high-resistance lamp electrodes.

It is also favorable that high Magnetic reversal losses of the core are avoided. As a result, the transformer heats up less than with high magnetic loss.

In the case of a circuit arrangement of the type mentioned at the beginning, the control circuit opening the switch periodically in phase-shifted ignition times in dimming mode, a third solution to the stated problem is characterized in that a measuring element connected to the control circuit detects the operating voltage or the choke current in dimming mode and that If the operating voltage or the choke current falls below or exceeds a limit value, the control circuit shifts the ignition times.

This extends the life of the fluorescent lamps. A drop in the operating voltage inevitably leads to a reduction in the lamp current. This causes the lamp electrodes to cool down. There is a risk that the emission layers of the lamp electrodes will be damaged or destroyed. This is avoided by the invention, since the control circuit amplifies the heating current when the operating voltage drops. It counteracts the cooling of the lamp electrodes due to the lower operating voltage. The lifespan of the fluorescent lamp is thus extended. The same applies to impermissibly high operating voltages.

A fourth solution to the above problem in a circuit arrangement of the type mentioned is characterized in that a voltage monitor monitors the operating voltage and that the voltage monitor switches off the ballast and the fluorescent lamp by means of a further switch in the event of voltage deviations which could damage a ballast or the fluorescent lamps.

This further switch is preferably a semiconductor switch. It is preferably switched on in the de-energized state and switched off at zero current.

The circuit arrangements described solve the task individually. An improved task solution is achieved in that the circuit arrangements are used in combination.

• Figur 1 eine Schaltung einer Leuchtstofflampe zur Verhütung eines Kaltstartens,
• Figur 2 eine Schaltung einer Leuchtstofflampe mit Transformatoren an dem Lampenelektroden,
• Figur 3 einen Kern eines Transformators nach Figur 2,
• Figur 4 eine Schaltung einer Leuchtstofflampe mit einer Überwachung der Betriebsspannung
• Figur 5 ein Stromdiagramm
• Figur 6 eine Schaltung einer Leuchtstofflampe mit einem zusätzlichen Schalter und
• Figur 7 und 8 Schaltungen mit mehr als einer Leuchtstofflampe.

Advantageous configurations result from the following description of exemplary embodiments. The drawing shows:

• FIG. 1 shows a circuit of a fluorescent lamp to prevent cold starting,
• FIG. 2 shows a circuit of a fluorescent lamp with transformers on the lamp electrodes,
• FIG. 3 shows a core of a transformer according to FIG. 2,
• Figure 4 shows a circuit of a fluorescent lamp with a monitoring of the operating voltage
• Figure 5 is a current diagram
• Figure 6 shows a circuit of a fluorescent lamp with an additional switch and
• Figure 7 and 8 circuits with more than one fluorescent lamp.

A fluorescent lamp (1) has a fluorescent tube (2) with two lamp electrodes (3, 4). The lamp electrodes (3, 4) are connected to a passive or active ballast (5) and a power switch (6) AC voltage source (7), for example an aircraft electrical system, connected. In the exemplary embodiments according to FIGS. 4 and 6, the passive ballast is formed by a choke (5).

The poles of the electrodes (3, 4) facing away from the AC voltage source (7) are connected to one another via a switch (8). This is controlled by a control circuit (9).

The control circuit (9) enables brightness control of the fluorescent lamp (1) (dimming operation) in a manner known per se. For this purpose, the control circuit (9) has a phase shifter (10) and opens the switch (8) more or less out of phase with respect to the zero crossing in each half-wave of the AC mains voltage. This switching frequency does not have to be twice the mains frequency. It can also have a network frequency or a multiple thereof or a fraction thereof. Opening the switch (8) generates an ignition pulse which ignites the fluorescent tube (2) (cf. ignition times t1 in FIG. 5). The combustion current or lamp current I2 then flows.

As long as the mains switch (6) and the switch (8) are closed, a heating current I3 flows through the lamp electrodes (3, 4).

A measuring element (11) is connected to the control circuit (9) (see Fig. 1). This detects the voltage U4 dropping across the lamp electrode (4). The measuring element (11) could also be connected in such a way that it detects the voltage U3 falling across the lamp electrode (3). It could also be switched so that it detects the voltage U2, which is the sum of the voltages U3 and U4 and the voltage drop across the switch (8).

The ohmic resistance of the lamp electrodes (3, 4) has a positive temperature coefficient. As long as the lamp electrodes (3, 4) are comparatively cold because, for example, the lighting was switched off by opening the mains switch (6), or the operating voltage U1 had briefly failed, or the fluorescent lamp (1) was replaced, the ohmic resistance of the lamp electrodes ( 3.4) much smaller than in operation.

The circuit works as follows:
If the mains switch (6) is closed to switch on the lamp electrodes (3, 4), a heating current flows via the lamp electrodes (3, 4) and the switch (8). The control circuit (9) detects the voltage drop across the measuring element (11) and thereby keeps the switch (8) closed. It is therefore not clocked according to the network frequency.

Due to the flowing heating current, the lamp electrodes (3, 4) gradually heat up and the voltage U4 changes accordingly. During this time, no ignition attempts are made by opening the switch (8).

Once the lamp electrodes (3, 4) have reached their operating temperature, the voltage U4 does not change further, but remains essentially constant. This evaluates the control circuit (9) and only now periodically opens the switch (8).

Instead of evaluating the changes in voltage U4, an evaluation of reaching a threshold value of voltage U4 can also be provided. In this case, the threshold value is set on the control circuit (9). Only when the threshold value has been reached, the lamp electrodes (3, 4) so that they are sufficient Temperature are heated, the control circuit (9) cycles the switch (8) periodically.

In both cases, a timer (12) can also be provided. This limits the preheating time to a maximum. This ensures that the lamp electrodes (3, 4) are not overheated.

Preheating the lamp electrodes (3, 4) before the mains frequency switching operation of the switch (8) has the advantage that the service life of the lamp electrodes (3, 4) is considerably increased, since cold start attempts are not made.

In the exemplary embodiment according to FIG. 2, parts corresponding to FIG. 1 are provided with the same reference symbols.

A transformer (13) is connected on the secondary side to the two poles (3 ', 3' ') of the lamp electrode (3). A transformer (14) is connected on the secondary side to the two poles (4 ', 4' ') of the lamp electrode (4). The transformers (13, 14) are designed as "autotransformers", with one winding part (13 'or 14') of the primary winding (13 '' or 14 '') forming the secondary winding. The primary windings (13 '' or 14 '') are in series with the switch (8).

The current flowing through the primary windings (13 '', 14 '') when the mains switch (6) and switch (8) are closed is transformed higher, so that a correspondingly higher current flows through the lamp electrodes (3, 4). It is thereby achieved that the preheating time described with reference to FIG. 1 is shortened.

In particular, it is also achieved that when the ohmic resistances of the lamp electrodes (3, 4) are lower than is intended per se according to the design of the ballast (5), the low resistance Lamp electrodes (3,4) are preheated with a highly transformed current. This prevents cold start attempts from being carried out on the low-resistance, not yet sufficiently preheated lamp electrodes (3, 4). This extends their lifespan.

The transformers (13, 14) each have a magnetic core (13 '' 'or 14' '') (see FIG. 3). The core is preferably non-linear in that it has a greatly reduced magnetic cross section in one leg (15). This is shown in Figure 3 as a partial gap (16).

It then turns out that with low-resistance lamp electrodes (3, 4) only a small magnetizing current flows through the primary inductance of the transformer, which does not overdrive the core. In the case of high-resistance lamp electrodes (3, 4), on the other hand, a higher voltage is generated on the winding due to the impressed current. This reduces the effective primary inductance. The secondary current then only increases slightly.

The nonlinear core thus advantageously achieves a strong increase in the heating current in low-resistance lamp electrodes, whereas the heating current increases only slightly in high-resistance electrodes. At the same time it is also achieved that high magnetization losses, which could lead to a considerable increase in the temperature of the transformers, are avoided.

The circuit according to FIG. 2 also makes it possible to use fluorescent lamps (rapid start lamps) which require a higher heating current than other fluorescent lamps (starter lamps).

The circuit measures according to FIGS. 1 and 2 can be used together in a circuit arrangement. The The voltage U4 can then be measured between the poles (4 'and 4''). It can also be done on the primary winding (14 ''). The same applies to the voltage U3. In this case, too, a measurement of the voltage U2 can be provided.

If the control circuit (9) is designed so that it does not ignite the switch (8) in each half-wave of the mains AC voltage, but only in positive or negative half-waves, it may be sufficient to provide only one of the transformers (13, 14).

In the exemplary embodiments according to FIGS. 4 and 6, too, the parts corresponding to the exemplary embodiment according to FIG. 1 are identified by the same reference numerals.

In the embodiment of Figure 4, a measuring element (17) is provided which is connected to the control circuit (9). If the operating voltage U1 drops, then the choke current I1 and thus also the heating current I3, which flows in dimming mode in each mains half-wave and which is limited by the switching times t1, t2 controlled by the control circuit (9) (see FIG. 5). This could result in undesired cooling of the lamp electrodes (3, 4), which would shorten the life of the fluorescent lamp (1).

The measuring element (17) measures the choke current I1. This is compared in the control circuit (9) with a target value. Depending on this, the switching times t1 and t2 can be shifted. If the heating current I3 has decreased so that it only insufficiently heats the lamp electrodes (3, 4), ie cool the lamp electrodes, then the ignition times t1 are shifted towards the current maximum, ie to the right in FIG. This increases the peak value of the heating current I3. This is initially also connected with an extension of the heating current duration. This could result in a reduction in the brightness of the fluorescent lamp (1), since the duration of the combustion current I2 is shortened. In order to avoid this, the points in time t2 at which the switch (8) is closed, ie the combustion current I2 ends and the heating current I3 starts, can be shifted to the right in the direction of the following current zero crossing, ie in FIG. 5, within control limits.

Conversely, an increase in the choke current I1, which could also damage the lamp electrodes (3, 4), can also be avoided if the operating voltage U1 rises. For this purpose, the ignition times t1 and t2 in FIG. 5 are shifted accordingly to the left.

The additional measuring element (17) according to FIG. 4 can also be used in a circuit arrangement which realizes the features of the circuit arrangements according to FIGS. 1 and / or 2.

In the exemplary embodiment according to FIG. 6, a voltage monitor (18) is provided for the operating voltage U1 of the AC voltage source (7). This controls a further switch (19) which is in series with the power switch (6).

The voltage monitor (18) is designed such that it opens the switch (19) in the event of overvoltages or undervoltage in the operating voltage U1 and closes it again when the overvoltage or undervoltage is no longer present. It is thereby achieved that such overvoltages or undervoltages are deactivated which could damage the ballast (5) or the fluorescent lamp (1).

The further switch (19) is a semiconductor switch that withstands at least the maximum permissible operating voltage U1. The voltage monitor (16) is preferably designed such that it switches on the switch (19) in the de-energized state and switches it off in the de-energized state.

The circuit of the exemplary embodiment according to FIG. 6 can also be integrated into one or more of the circuits according to FIG. 1 and / or FIG. 2 and / or FIG. 4.

The circuit arrangements described can also be used when two or more fluorescent lamps are connected together. According to FIG. 7, two fluorescent lamps (1, 1 ') are connected in series on a single ballast (5). Only a single control circuit (9) for the two switches (8,8 ') is required here. It is sufficient to provide the measuring element (11) on one of the lamp electrodes (3, 4, 3 ', 4'). If necessary, only one measuring element (17) is required.

According to FIG. 8, the ballasts (5,5 ') of the fluorescent lamps (1,1') are connected in parallel on the AC voltage source (7). Here too, only one control circuit (9) to which the measuring elements (11 and / or 17) are connected is required for the two switches (8, 8 ').

Claims (9)

Circuit arrangement for the operation of a fluorescent lamp on an AC network, for example on an aircraft electrical system, with a switch located between the lamp electrodes and which can be switched by means of a control circuit, the lamp electrodes being heated when the switch is closed,
characterized,
that a measuring element (11) connected on the input side to the control circuit (9) detects the voltage drop at at least one lamp electrode (3, 4) and that the control circuit (9) only opens the switch (8) for igniting the fluorescent lamp (1) when the sensed voltage is substantially constant or reaches a threshold. Circuit arrangement according to claim 1,
characterized,
that the control circuit (9) closes the switch (8) for heating the lamp electrodes (3, 4) when the voltage detected by the measuring element (11) changes or falls below the threshold value. Circuit arrangement according to claim 1 or 2,
characterized,
that the preheating time during which the measuring element (11) keeps the switch (8) closed via the control circuit (9) extends over several half-waves of the AC network. Circuit arrangement according to one of the preceding claims,
characterized,
that a timer (12) is provided which limits the preheating time to a maximum value. Circuit arrangement according to the preamble of claim 1,
characterized,
that a transformer (13 or 14) is provided for at least one of the lamp electrodes (3, 4), the primary winding (13 '' or 14 '') of which is connected in series with the switch (8) and the lamp electrode on the secondary side (3 or 4), so that the preheating current flowing when the switch (8) is closed is increased. Circuit arrangement according to claim 5,
characterized,
that the transformer (13 or 14) has a single winding which forms the secondary winding (13 'or 14') and the primary winding (13 '' or 14 ''). Circuit arrangement according to claim 5 or 6,
characterized,
that the transformer (13 or 14) has a non-linear core (13 '''or14'''), so that with low-resistance lamp electrodes (3, 4) a higher current gain occurs on the secondary side than with high-resistance lamp electrodes (3 or 4 ). Circuit arrangement according to the preamble of claim 1, wherein the control circuit in dimming mode opens the switch periodically at phase-shifted switching times,
characterized,
that a measuring element (17) connected to the control circuit (9) detects the operating voltage (U1) or the choke current (I1) in dimming mode, and that if the operating voltage (U1) or the choke current (I1) falls below or exceeds a limit value, the Control circuit (9) the switching times (t1, t2) shifts. Circuit arrangement according to the preamble of claim 1,
characterized,
that a voltage monitor (16) monitors the operating voltage U1 and that the voltage monitor (16) in the event of voltage deviations which could damage a ballast (5) or the fluorescent lamp (1), by means of a further switch (17), the ballast (5) and the fluorescent lamp (1) switches off.

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