EP0165893B1 - Brightness control device for a fluorescent lamp - Google Patents

Abstract

1. Device for controlling the brightness of fluorescent lamps by means of an adjustable phase control of each voltage half wave of a feeding alternating-current system, the current switched from the time of the phase control to the next zero transition of the voltage halfwave being supplied via a current-limiting ballast to the fluorescent lamp, the cathodes of the fluorescent lamp furthermore being continuously and uncontrolledly heated by the alternating current system, and means being provided to supply higher-frequency energy to the fluorescent lamp for its ignition, characterized by a trigger circuit (18) which can be fed by the voltage of the phasecontrolled halfwave of the system alternating voltage and a damped resonant circuit (31) which can be charged, which is connected to the trigger circuit and to which the fluorescent lamps can be connected with respect to alternating current, the damped oscillation of the resonant circuit (31) triggered by the trigger circuit having a higher frequency than the frequency of the system alternating voltage and exhibiting at the beginning an electric power which is sufficient for igniting the fluorescent lamp.

Description

The invention relates to a device for controlling the brightness of fluorescent lamps according to the preamble of patent claim 1.

Numerous control devices of this type are known, but they are disadvantageous because they do not or only with complex auxiliary circuits for controlling the brightness of fluorescent lamps with high ignition voltage and poor control properties, such as the so-called slimline tubes with a diameter of 26 mm can be used, and because they only allow a limited control range with insufficiently low minimum brightness.

A control of the brightness is not possible by means of a phase cut of the supplying alternating current if the glow cathodes of the fluorescent lamps are preheated by a heating transformer since the lamps do not ignite. However, if a heating transformer is dispensed with and a conventional glow starter device is provided for it, the lamps can be ignited at full control. With additional circuit measures in the phase gating device, a control range down to about 20% can be achieved. However, since the glow cathodes of the lamps are not heated enough when the lamp current falls below a certain value, so that their emissivity drops sharply and the lamps become unusable after only a few hours of operation following destruction, the permissible control range is limited to around 50%.

It is also known to feed fluorescent lamps with an alternating current of higher frequency, that is to say with a frequency above 20 kHz. By means of an electronic ballast, the higher-frequency supply voltage or the higher-frequency supply current is changed to control the brightness. An uncontrolled power supply for heating the hot cathode is not necessary. Since the output power of the electronic ballast is greatly reduced by the upstream voltage or current control carried out by means of phase angle control or a regulating transformer and the hot cathode heating supplied from the same source is thereby insufficient, the achievable control range is limited to approximately 10%.

A device has also become known in which, on the one hand, an uncontrolled current is continuously fed to the hot cathodes of the fluorescent lamps for their heating, so that there is no fear of the lamps being destroyed. On the other hand, the supply current of the lamps is controlled by means of phase gating, with a low, higher-frequency energy being continuously supplied in parallel with the lamps for igniting and re-igniting the lamps in the current which is incomplete due to the phase gating control. As a result of this higher-frequency energy, there is minimal brightness when the phase gating device is fully controlled. When the phase gating device is fully controlled, full light is achieved. Since a certain minimum higher-frequency energy must now be supplied to the lamps for a reliable ignition of the lamps on the one hand and a stable burning of the lamps in the controlled state on the other hand, the achievable control range is limited to approximately 4% (1:25).

However, it has been shown that it is desirable in numerous cases to achieve a significantly smaller controllable minimum brightness, which is not possible with the known devices.

It is therefore an object of the present invention to provide a device of the type mentioned at the outset with which the brightness of the fluorescent lamps can be continuously controlled in a range from full light to a minimum brightness which is substantially below 4% without impairing the emissivity of their hot cathodes.

To achieve this object, the device according to the invention has the features stated in the characterizing part of patent claim 1.

The fact that in the inventive device of the fluorescent lamp a higher-frequency ignition energy of high power, but only at the cut-in time of each half-wave of the AC mains voltage, the duration of the feeding of the higher-frequency energy can be shortened considerably. This means that a minimum brightness of well under 1% can be achieved. If the power of the damped ignition pulse oscillation triggered by the trigger circuit is reduced in a suitable manner when the phase gating is controlled back, the control range can be expanded to a minimum brightness of less than 0.01%. Although the ignition of the fluorescent lamp is guaranteed regardless of the time of the phase control, its emissivity remains unchanged in any state of the brightness control, because the hot cathodes of the lamp are continuously fed via an uncontrolled power supply, in particular a heating transformer.

• Fig. 1 ein Schaltungsschema einer erfindungsgemässen Einrichtung zusammen mit einem Phasenanschnittgerät, einem Vorschaltgerät und einer Leuchtstofflampe,
• Fig. 2a und 2b Diagramme des zeitlichen Verlaufs verschiedener Spannungen im Schaltungsschema der Fig. 1, und
• Fig. 3 ein Teil des Schaltungsschemas der Fig. 1 in einer Ausführungsvariante.

Embodiments of the subject matter of the invention are explained below with reference to the drawing. Show it :

• 1 shows a circuit diagram of a device according to the invention together with a phase gating device, a ballast and a fluorescent lamp,
• 2a and 2b diagrams of the time course of different voltages in the circuit diagram of Fig. 1, and
• Fig. 3 shows a part of the circuit diagram of Fig. 1 in an embodiment.

In Fig. 1 is by dash-dotted lines limits the circuit diagram of an exemplary embodiment of the control device 1 according to the invention. The control device 1 has various connecting terminals, namely a connecting terminal 2 for connecting the phase conductor L1 of an AC network, a connecting terminal 3 for connecting the neutral conductor N of the AC network, a connecting terminal 4 for connecting the earth conductor E of the AC network and a connecting terminal 5 for connecting the controlled output of a phase gating device 6 shown only schematically, which in turn is connected to the phase conductor L1 and the neutral conductor N of the alternating current network and is of a conventional type which is not explained in detail here. The control device 1 has further connection terminals on the load side, namely two connection terminals 7 for connecting a current-limiting ballast 8, usually a choke, and two pairs of connection terminals 9 and 10, which are provided for this purpose, each with a glow cathode filament 11 or 12 of a fluorescent lamp 13 to become.

For permanent and uncontrolled heating of the hot cathode filament 11, 12 there is a secondary winding 14 and 15, respectively, of a heating transformer 16 at the paired connection terminals 9 and 10, the primary winding 17 of which, accordingly, with the connection terminals 2 and 3 for the phase conductor L1 and the neutral conductor N of the AC network connected is. It can be seen that the heating transformer 16 could also be arranged outside the control device 1 with dash-dotted lines, as is shown for the phase gating device 6 and the ballast 8. One of each pair of connecting terminals 9, 10 also serves to supply the connected fluorescent lamp 13 with the fuel supply current of the AC network during the phase gating periods. Accordingly, one of the connection terminals 9 is essentially connected to the connection terminal 5 via the connection terminals 7 for the ballast 8, and one of the connection terminals 10 is essentially connected to the connection terminal 3.

Between the controlled output of the connected phase gating device 6 and the connected neutral conductor N of the AC network, a first trigger circuit 18 is connected, which consists of the series connection of a resistor 19, a first capacitor 20 and a second capacitor 21 and one at the connection point of the two capacitors 20 , 21 connected trigger diode 22. A second trigger circuit 23, which is connected between the output of the connected ballast 8 and the connected neutral conductor N of the AC network, has the series connection of a resistor 24 and a capacitor 25 and a resistor 26, which also connects the connection point of the resistor 24 and the capacitor 25 the already mentioned trigger diode 22 connects.

The control device 1 shown also contains a further capacitor 27, which can be charged with respect to the connected neutral conductor N by a resistor 28 connected to the connected phase conductor L1 of the AC network via the connection terminal 2, and which via a primary winding 29 of a pulse transformer by means of the switching path of a electronic switch 30, preferably a triac, can be discharged. The trigger diode 22 belonging to the trigger circuits 18 and 23 is connected to the control electrode of the triac 30. When the triac 30 is in the conductive state, the capacitor 27 and the primary winding 29 of the pulse transformer form a damped oscillating circuit 31. The damped oscillation which is excited when the triac 30 is closed by the discharge current of the capacitor 27 which flows through the primary winding 29 has here a frequency that is significantly greater than the frequency of the AC network and that is, for example, above 20 kHz.

Two separate secondary windings 32 and 33 of the pulse transformer mentioned with the primary winding 29 are connected into the feed lines of the fluorescent lamp 13. They are connected to one another at their ends facing away from the fluorescent lamp 13 by a bypass capacitor 34, so that the fluorescent lamp 13, the two secondary windings 32 and 33 of the pulse transformer and the bypass capacitor 34 form a closed ignition circuit, as will be explained below.

The operation of the device 1 shown in Fig. 1 in connection with the outer components 6, 8 and 13 also shown and the AC network is the following.

The phase gating device 6 supplies a voltage _UPA to the connecting terminal 5 of the control device 1, the course of which is shown in FIG. 2a in the period of a half-wave between the specified times t _o and t ₆ for three different settings of the phase gating device. In one case, which is shown with a solid line of the voltage profile, the phase gating takes place at a time t ₃ (relatively large current flow angle). In a further case, which is shown with a dash-dotted line of the voltage curve, the phase gating takes place at a later point in time t ₄ (smaller current flow angle). In a last case, which is shown with a dashed line of the voltage curve, the phase gating takes place at a time t ₅ shortly before the zero crossing of the voltage at time t ₆ (very small current flow angle). The voltage still present in the periods t _o to t ₂ or t _o to t comes from the current still flowing when the fluorescent lamp is on and disappears at the zero crossing of the current (time t or t ₂ ) with a steep flank this edge with a decreasing current flow angle to the zero crossing t _o shifts.

The voltage _UPA then has a vertical flank 35, 36 or 37 at the times t ₃ , t ₄ or t ₅ set in the phase gating device 6. In the control device 1 (FIG. 1), the occurrence of the edge 35 or 36, for example, generates a control pulse on the control electrode of the triac 30 via the trigger circuit 18. Since at the beginning of the relevant half-wave of the AC line voltage Uu, the capacitor 27 is charged via the resistor connected to the connection terminal 2, the voltage U _{c of} the capacitor 27 is also above the triac 30, so that the latter is turned on by the control pulse of the trigger diode 22 is switched. As a result, the capacitor 27 can discharge via the primary winding 29 of the pulse transformer, which is now connected in parallel with it. In the parallel resonant circuit formed by the inductance of the primary winding 29 and the capacitance of the capacitor 27, a damped oscillation is generated, the first voltage and current amplitudes of which are relatively high. Correspondingly powerful, temporally weakened vibrations occur with additive polarity on the secondary windings 32 and 33 of the pulse transformer, so that the supply voltage applied to the fluorescent lamp 13 via the ballast 8 is superimposed on the transformed voltage of the resonant circuit 31 and the fluorescent lamp 13 is reliably ignited. After the ignition has taken place, the regular lamp current begins to flow, always within the same half-wave of the AC mains voltage and fed from the phase gating device 6 and limited by the ballast 8, until the natural next zero crossing of the AC mains current is reached. After a pause determined by the setting of the phase gating device 6 (FIG. 2a), the same process takes place with the opposite polarity in the next half-wave of the AC mains voltage.

As already mentioned, with the series feed shown in FIG. 1, the higher-frequency, damped voltage generated by the oscillating circuit 31 into the ignition circuit is closed via the secondary windings 32 and 33, the fluorescent lamp 13 and the capacitor 34. The ignition voltage applied to the fluorescent lamp 13 is thus kept away from the ballast. In a typical example, the frequency of the damped oscillation excited in the oscillating circuit 31 is approximately 30 kHz and the duration of the oscillation is approximately 0.5 ms. Since the voltage u _c across the capacitor 27 of the resonant circuit 31 is approximately 150 V at the time the triac 30 is triggered and a current of greater than 0.5 A flows through the connection of the primary winding 29 in parallel with the capacitor 27, the oscillation train of the damped ones produced Vibration very powerful, including the ignition pulse applied to the fluorescent lamp 13 and transformed to a voltage of approximately 1000 V.

Instead of two separate secondary windings 32, 33 of the pulse transformer, the latter can also have only a single secondary winding. The division shown in FIG. 1 into two identical secondary windings 32, 33 with an additive winding sense has the advantage that the high transformed ignition voltage of the resonant circuit 31 is halved against the potential of the neutral conductor N or against the earth potential. The voltage stress on the heating transformer 16 is also reduced by half due to the above-mentioned division relative to the earth potential.

In addition to the previously described operating state, in which, according to the diagram in FIG. 2a, the flank 35 or 36 of the cut half-wave corresponding to the respective setting of the phase gating device 6 (FIG. 1) at any middle point t ₃ or t ₄ between the two successive ones Zero crossings of the mains voltage _UL1 , but not in the immediate vicinity of one of these zero crossings, still have to be taken into account the operating _states that exist when the current flow angle is very small (control of the phase _{gating device} 3 back to the time t ₅ in FIG. 2a) or maximum is (full control of the phase control device).

With regard to the first-mentioned case of a very small set current flow angle, it must be taken into account that the voltage u _{c applied} to the capacitor 27 and supplied via the resistor 28 compared to the line voltage U _{L1 'carried} by the phase conductor L1 of the AC network, from which the capacitor voltage u _{c is} derived, lagging out of phase. The diagram of FIG. 2b shows the time profiles of the two voltages u _L1 and u _c in a half wave, which corresponds to that of the diagram of FIG. 2a between the times t _o and t ₆ . 2b shows the values of the voltages u _c at the capacitor 27 in the phase gating times t ₃ , t ₄ and t ₅ of FIG. 2a before the triggering or discharge of the capacitor 27 by the primary winding 29. It can be seen from this that the voltage u _c at the time t ₅ shortly before the zero crossing of the mains voltage U _L1 at the time t _{5 is} still comparatively large, so that even if the phase gating device 6 is almost completely controlled back (time t ₅ with edge 37 in Fig. 2a), a sufficient ignition pulse power is generated for the ignition of the fluorescent lamp 13. In other words, a reliable ignition of the fluorescent lamp 13 can also be achieved with the present control device 1 if a very small current flow angle, ie a very low brightness of significantly less than 1%, is set with the phase gating device 6.

If, under such conditions of a very small set current flow angle, the mains AC voltage present at the connecting terminal 5 is no longer sufficient to maintain a combustion current in the ignited fluorescent lamp 13, the decaying effect causes over the secondary winding 32, 33 of the pulse transformer fed in higher-frequency ignition oscillation a short further burning of the fluorescent lamp 13, so that a low light brightness is maintained. Since, according to the diagram in FIG. 2b, with a further reduction in the phase _{gating angle} , that is to say a further approximation of the phase _gating to the zero crossing of the mains voltage _UL1 , the voltage at the capacitor 27 decreases and thus the power of the higher-frequency oscillation supplied to the fluorescent lamp decreases, a further reduction in brightness. This makes it possible to continuously achieve a minimum brightness of less than 0.01%.

If the fluorescent lamp is controlled from the low brightness state to a higher brightness state by setting a larger current flow angle according to a shift from the time t _s to the time t ₄ with the phase gating device 6, the ignition torque shifts forward. However, according to the diagram in FIG. 2b, the voltage u _c across the capacitor 27 is greater. This has the advantageous consequence that the fluorescent lamp 13 burns stably when the higher light level is set. By a suitable choice of the values of the resistor 28 and the capacitor 27, an optimal phase shift of the voltage u _c at the capacitor 27 with respect to the mains alternating voltage _UL1 can be obtained, whereby a stable burning of the fluorescent lamp and a large control range for its brightness is achieved.

With regard to the second case of a maximum set current flow angle, which is also present when the phase gating device 6 is bridged, a pure sinusoidal voltage is present at the connection terminal 5 of the control device 1, since the times t ₂ and t ₃ in the diagram in FIG approximate to cover the two flanks. In this case, the steep gating flank 35 for triggering the ignition oscillation by the trigger circuit 18 and the triac 30 is no longer present, so that the fluorescent lamp 13 itself is not ignited.

In order to achieve a reliable ignition of the fluorescent lamp 13 even with such a full modulation set on the phase gating device 6, the second trigger circuit 23 is provided, which takes advantage of the voltage across the two feed lines of the fluorescent lamp 13 or the voltage across the bypass capacitor 34. In normal operation, the _operating voltage of the fluorescent lamp is essentially at the capacitor 34, which is considerably _lower than the mains voltage _UL1 . The voltage divider with the resistor 24 and the capacitor 25 of the trigger circuit 23 is dimensioned such that in this case the divided operating voltage of the fluorescent lamp 13 applied to the trigger diode 22 via the resistor 26 does not reach the trigger voltage of the diode 22.

If, however, the fluorescent lamp 13 does not burn, i.e. no current flows through the ballast 8, the voltage across the bypass capacitor 34 is identical to the mains voltage U _{L1 '} when the phase gating device 6 is fully activated. Since this voltage is significantly higher than the burning voltage of the fluorescent lamp, it is sufficient voltage on capacitor 25 to trigger periodic switch-on of triac 30 via trigger diode 22 until fluorescent lamp 13 has ignited due to the damped vibrations generated thereby. As soon as the fluorescent lamp burns, the second trigger circuit 23 becomes inactive due to a lack of sufficient trigger voltage. It should also be noted here that when the triac 30 is switched on, which, according to the diagram in FIG. 2b, is slightly earlier than the time t ₃ or should be by appropriate selection of the values of the components of the trigger circuit 23, the voltage _Uc at the capacitor 27 is sufficiently large in spite of lag to generate a powerful ignition pulse.

In the trigger circuit 18, the voltage divider composed of the capacitors 20 and 21 essentially determines the minimum voltage reaching the trigger diode 22 for its control. At times when the voltage _UPA applied to the trigger circuit 18 is so high that the trigger diode 22 could be damaged, the resistor 19 limits the current flowing through the capacitors 20 and 21, so that the control voltage at the trigger diode 22 is safe Value is limited.

The feeding of the higher-frequency ignition energy shown in FIG. 1 via the secondary windings 32 and 33 in series with the AC network feeding the fluorescent lamp 13 has the disadvantage that the secondary windings 32, 33 have to be dimensioned for the full lamp current. Instead of this series feed, a parallel coupling of the output of the resonant circuit 31 to the circuit of the fluorescent lamp 13 is also possible. FIG. 3 schematically shows the part of the control device 1 of FIG. 1 that has been changed for this case.

The part shown again shows the fluorescent lamp 13 with hot cathode filaments 11 and 12, which are fed with heating current via connecting terminals 9 and 10, respectively, from the secondary windings 14 and 15 of the heating transformer 16 of FIG. 1. Furthermore, the ballast 8 connected via the connection terminals 7 to the phase-controlled mains supply line is shown in accordance with FIG. 1.

The primary transformer 29, which belongs to the resonant circuit 31 of FIG. 1, has a single secondary winding 38, which is connected on the one hand to the neutral line connected to the connecting terminal 3 for the neutral conductor N (FIG. 1) and on the other hand via a capacitor 39 to the neutral line this neutral line lying hot cathode coil 11 or is connected to one of the terminals 9. The Ueber shown in Fig. 1 bridge capacitor 34 is of course omitted. The remaining circuit parts of FIG. 1 are essentially unchanged (not shown in FIG. 3).

It can be seen that in the device according to FIG. 3 the ignition oscillation occurring at the secondary winding 38 is fed in parallel into the mains feed circuit of the fluorescent lamp 13. As a result of the blocking capacitor 39, which has a relatively small capacitance value, since it is dimensioned for the passage of the significantly higher-frequency ignition oscillation, the current of the fluorescent lamp 13 does not flow via the secondary winding 38. On the other hand, there are the disadvantages that, in relation to the capacitance of the coupling capacitor 39, it does not negligible parasitic winding capacitances of the heating secondary windings 14, 15 and of the ballast 8 against the earth or neutral potential reduce the voltage of the ignition oscillations which is effective at the fluorescent lamp 13, so that the ignition properties are less good. In addition, the tension of the ignition vibrations is applied to the ballast 8, so that its winding must be dimensioned for a considerably higher than the actual operating voltage. However, much better control properties for the fluorescent lamp 13, in particular with regard to minimum brightness, can also be achieved with the circuit according to FIG. 3 than with the known control devices mentioned at the outset.

Claims (10)

1. Device for controlling the brightness of fluorescent lamps by means of an adjustable phase control of each voltage halfwave of a feeding alternating-current system, the current switched from the time of the phase control to the next zero transition of the voltage halfwave being supplied via a current-limiting ballast to the fluorescent lamp, the cathodes of the fluorescent lamp furthermore being continuously and uncon- trolledly heated by the alternating current system, and means being provided to supply higher-frequency energy to the fluorescent lamp for its ignition, characterized by a trigger circuit (18) which can be fed by the voltage of the phasecon- trolled halfwave of the system alternating voltage and a damped resonant circuit (31) which can be charged, which is connected to the trigger circuit and to which the fluorescent lamp can be connected with respect to alternating current, the damped oscillation of the resonant circuit (31) triggered by the trigger circuit having a higher frequency than the frequency of the system alternating voltage and exhibiting at the beginning an electric power which is sufficient for igniting the fluorescent lamp.

2. Device according to Claim 1, characterized in that the resonant circuit contains a capacitor and an inductance which can be connected to the capacitor via an electronic switch, the electronic switch being connected to the trigger circuit so that it can be controlled, and in that the capacitor can be connected to the system alternating voltage via a charging resistor before being periodically charged.

3. Device according to Claim 2, characterized in that the inductance is the primary winding of a pulse transformer the at least one secondary winding of which is connected to a feed circuit for the fluorescent lamp.

4. Device according to Claim 3, characterized in that the secondary winding of the pulse transformer is series-connected into the feed circuit for the fluorescent lamp.

5. Device according to Claim 4, characterized in that the pulse transformer has two separate secondary windings which are each connected on one side of connections for the fluorescent lamp into the feed circuit for the fluorescent lamp.

6. Device according to Claim 3, characterized in that the secondary winding of the pulse transformer is connected metallically isolated, for example via a capacitor, in parallel to the feed circuit for the fluorescent lamp.

7. Device according to one of Claims 2 to 6, characterized in that the capacitor of the resonant circuit and its charging resistor are dimensioned in such a manner that the charging voltage present across the capacitor is phase-shifted with lag compared with the system alternating voltage.

8. Device according to one of Claims 1 to 7, characterized in that the trigger circuit, which can be fed by the voltage of the phase-controlled halfwave of the system alternating voltage, contains a voltagedividing resistor-capacitor section and a switching diode.

9. Device according to one of Claims 1 to 8, characterized in that a further trigger circuit for triggering the damped oscillation of the resonant circuit exists which, for its feed, is connected in parallel to the feed circuit for the fluorescent lamp in order to trigger the damped oscillation for igniting the fluorescent lamp when the voltage halfwave of the alternating current system does not have a phase-controlled edge.

10. Device according to Claim 2, characterized in that the further trigger circuit contains a voltagedividing resistor-capacitor section and a switching diode.

Images