EP0201624A2 - Ballast circuit for a fluorescent lamp - Google Patents

Abstract

The fluorescent lamp (14) is connected in a bridge circuit of electronic switches T1 to T4. This bridge circuit is supplied via a series circuit consisting of an electronic switching element (TS), a current sensor (I) and an inductor (L). The signals from the current sensor (I) are compared with threshold values in a comparator (16). The switching element (TS) is alternately opened and closed so that the lamp current (iL) remains constant within a predetermined band width. The reversing of the polarity of the lamp (14) is carried out by the control mechanism (15) at relatively large time intervals of e.g. one hour. The time integral of this current is formed during one half period of the lamp current. The current of opposite polarity is maintained until its integral has reached the same value. In this way, electrophoresis on the lamp 14 is avoided. The balanced circuit can be operated equally well at different supply voltages and with lamps of different power, without having to carry out any changeovers. <IMAGE>

Description

The invention relates to a fluorescent lamp ballast with a power supply circuit that feeds two parallel circuit branches, each of which leads via an electrode of the fluorescent lamp and contains two electronic switches in series, a control unit for controlling the electronic switch, an inductor for generating the ignition voltage for the fluorescent lamp and a current sensor.

The usual electronic fluorescent lamp ballasts contain a series resonant circuit that is triggered by an inverter and generates an oscillation whose frequency is above the hearing range of the human ear. The fluorescent lamp is connected in parallel to the capacitor of the series resonant circuit. Such ballasts have the Disadvantage that the fluorescent lamp is operated at a relatively high frequency and causes high-frequency electromagnetic interference. At the high frequency, parasitic line capacitances become effective, so that in order to achieve reproducible properties, a defined line routing from the device to the fluorescent lamps is required. Difficulties arise particularly when the fluorescent lamp is removed from the socket or when its resistance has become too great due to aging. In such cases, the series resonant circuit is damped insufficiently or not at all, which can lead to the destruction of components. For this reason, the ballasts mentioned require additional safety circuits to ensure that operation only takes place when a fluorescent lamp used is functioning properly. Finally, the known ballasts are each designed only for a narrow power range of the fluorescent lamp and for a very limited range of supply voltages. Different ballasts must be used for different lamp powers, lamp voltages and supply voltages.

In a known ballast of the type mentioned (DE-OS 29 42 468) four electronic switches, which are controlled by a logic control unit, are connected in the manner of a bridge circuit, in the transverse branch of which the fluorescent lamp is located. In the preheating phase, certain switches are conductive so that direct current flows through the electrodes of the fluorescent lamp. To ignite the gas discharge, two diagonally opposite switches are blocked, so that a current which is supported by the discharge of an inductor now flows through the fluorescent lamp. The switches are then with egg ner high-frequency AC voltage of over 10 kHz clocked, whereby the fluorescent lamp is periodically reversed with the frequency mentioned. A current sensor is provided in one of the circuit branches, but is only used to determine the ignition of the fluorescent lamp and to switch off the control unit after a certain number of unsuccessful ignition attempts.

In the known ballasts, the fluorescent lamp is supplied with a high-frequency voltage, the lamp current depending on various parameters, for example the lamp resistance. The consequence of this is that lamps of different wattages each require a different ballast and that the lamp wattage can change over the course of the life of the lamp due to the increasing resistance.

The invention has for its object to provide a fluorescent lamp ballast of the type mentioned, which can be used equally for lamps with different lamp powers and different lamp voltages.

The solution to this problem is, according to the invention, that the current sensor and the inductor are connected in series with the two circuit branches and that the current sensor controls a switching element which is also connected in series in such a way that this switching element opens the circuit when the current has a specific value exceeds the first threshold and closes the circuit when the current falls below a certain second threshold that is lower than the first threshold.

Due to the interaction of the current sensor with the switching element and the inductor is achieved that the current drawn by the circuit branches current, ie the _L ampenstrom constantly remains in the range between the first and the second threshold value and is thus kept almost constant. A defined current is therefore supplied to the lamp, which is independent of the level of the externally applied voltage and also independent of the lamp resistance. The lamp current is kept almost lossless because the switching element is either in the on state or in the off state in each phase. When the switching element is in the on state, the current through the inductance in the lamp circuit increases slowly until the first threshold value is reached. The switching element is then blocked so that the inductance discharges and the current slowly drops to the second threshold value. The inductance, which ensures a slow increase and slow decrease in the lamp current within the bandwidth specified by the two threshold values, can simultaneously be used to generate the high voltage required to ignite the fluorescent lamp.

According to a preferred development of the invention, it is provided that the threshold values can be changed individually or together depending on at least one control signal. In this way, the ballast can, for example, perform a dimmer function if the threshold values are changed by a control signal which can be influenced manually. If the thresholds are high, the lamp current is high and the lamp is bright; If the threshold values are lowered, the lamp current becomes smaller and the lamp shines less brightly. There is also the possibility that To regulate threshold values automatically, for example depending on the illuminance prevailing in the room in which the fluorescent lamp is located or depending on the lamp temperature.

A particular advantage is that the ballast can be supplied with different supply voltages, for example with a mains voltage of 110 V and a frequency of 60 Hz or with a mains voltage of 220 V and 50 Hz. Since the lamp current is kept constant regardless of the supply voltage , the ballast works with different supply voltages without switching. The two threshold voltages can preferably be changed together, so that the bandwidth of the current fluctuations does not change when the current amplitude changes.

According to a preferred development of the invention, the electronic switches are controlled in such a way that the fluorescent lamp is reversed with a frequency below 100 Hz. The polarity reversal frequency should be below 1 Hz and preferably below 1 mHz. The fluorescent lamp is operated to a certain extent with direct current, with a polarity reversal taking place at relatively large time intervals. The polarity reversal is necessary to prevent cataphoresis in the fluorescent lamp. Cataphoresis occurs when the lamp is operated with the same polarity for a long time. It is a mercury displacement for a _K-Thode out, which can be dismantled by reversing the polarity of the lamp. The polarity reversal can take place at relatively large intervals of, for example, one hour. This would correspond to a pole reversal frequency of approx. 0.3 mHz. While the usual ballasts with one above the According to the invention, the polarity reversal frequency of the lamp is made low. The lamp current is kept constant in every half period, the switchover is only from time to time to avoid cataphoresis.

An integration device is preferably provided which forms the time integral over this current during each period of the current flowing through the fluorescent lamp and initiates the next period when this integral has become zero. The polarity reversal of the fluorescent lamp does not necessarily take place with a constant half-period. In the relatively long half-periods, the lamp current in the second half-period may differ from that in the first half-period, e.g. if the lamp current has been changed by dimming. In order for the lamp to be equally loaded in both half-periods, the time integral is formed over the current in each of the half-periods, and when the integrals of the positive and negative half-periods are equal to one another, the period is ended. The integral formation takes place, particularly in the case of long half-periods, preferably by converting the current values into digital form and adding them up cumulatively.

Since the current is regulated in the ballast according to the invention, there are numerous possibilities for influencing this current in order to achieve different operating states of the fluorescent lamp. A particular advantage is that the ballast can be used without structural changes for fluorescent lamps with different powers by the size of the lamp current according to the lamp power is set. This setting can be made either on a manually adjustable signal transmitter or depending on the parameters of the fluorescent lamp used in the ballast. In the latter case, the lamp current required by the fluorescent lamp in question is set automatically.

The ballast according to the invention also enables the use of a dimmer circuit with which the lamp current is changed as a function of an external control signal. This control signal can be generated either by manually adjusting the dimmer circuit or by an external sensor.

A particular problem with fluorescent lamp ballasts is starting the fluorescent lamp. Before starting, the electrodes of the fluorescent lamp are preheated for a certain time with direct currents. Thereafter, several ignition attempts are made in succession in the known ballasts by interrupting both circuit branches which lead over the electrodes of the fluorescent lamp, ie by briefly blocking one of the electronic switches for the passage of current. The inductance then briefly supplies a high ignition voltage. According to a preferred embodiment of the invention, numerous short firing pulses are with short time _A b stands supplied to one pair of diagonally arranged electronic switch to open and close periodically. The repetition frequency of the ignition pulses is so high that the charge carriers in the interior of the fluorescent lamp cannot essentially recombine between two ignition pulses. In this way, the ignition takes place after several ignition pulses. The ignitability of the Fluorescent lamp builds up like a staircase curve. A particular advantage is that the inductance required for ignition can be kept relatively small. This inductance is reduced by a factor of n compared to the usual ignition with individual pulses, where n represents the number of briefly successive ignition pulses. The smaller inductance not only reduces the required coil dimensions and iron weight, but also the stored energy. The electronic switches are not destroyed even in the event of voltage breakdowns, since the low energy stored in the coil is not sufficient for this. The electronic switch used for ignition is only held in the blocking state for as long as a reversible voltage breakdown occurs at this switch, but the switch is not destroyed. In the event of a voltage breakdown, the blocked electronic switch behaves like a zener diode. The second breakdown leading to destruction, which is referred to as thermal breakdown, is avoided by the current limitation.

Exemplary embodiments of the invention are explained in more detail below with reference to the drawings.

• Fig. 1 ein schematisches Schaltbild eines ersten Ausführungsbeispiels des Vorschaltgeräts,
• Fig. 2 ein Diagramm zur Verdeutlichung der Erzeugung des Lampenstroms,
• Fig. 3 ein Diagramm zur Erläuterung der Umpolung des Lampenstroms,
• Fig. 4 ein Blockschaltbild eines zweiten Ausführungsbeispiels des Vorschaltgerätes,
• Fig. 5 verschiedene Spannungs- und Stromverläufe bei dem Vorschaltgerät nach Fig. 4 während des Zündens und danach,
• Fig. 6 eine Darstellung der Schaltung zur Erzeugung des Referenzsignals bei dem Vorschaltgerät nach Fig.4,
• Fig. 7 eine Modifikation des Signalgebers aus Fig.6, und
• Fig. 8 eine weitere Modifikation des Signalgebers.

Show it:

• 1 is a schematic circuit diagram of a first embodiment of the ballast,
• 2 shows a diagram to illustrate the generation of the lamp current,
• 3 is a diagram for explaining the polarity reversal of the lamp current,
• 4 shows a block diagram of a second exemplary embodiment of the ballast,
• 5 different voltages and currents in the ballast of FIG. 4 during ignition and afterwards,
• 6 shows the circuit for generating the reference signal in the ballast according to FIG. 4,
• Fig. 7 shows a modification of the signal generator from Fig. 6, and
• Fig. 8 shows a further modification of the signal generator.

The ballast shown in FIG. 1 has a rectifier circuit 10, which is fed with an alternating voltage via a low-pass filter 11 for eliminating interference voltages from or into the supply network. The rectifier circuit 10 generates a DC voltage on the capacitor C, which does not have to be regulated, and the level of which may depend on the applied mains voltage.

The switching element T _S , which is a switching transistor, is connected to the positive pole of the supply voltage. For reasons of easier understanding, the switching element T and the electronic switches to be described below are shown in the drawing as mechanical switches. The switching element T _S is connected in series with the current sensor I and the inductance L. The parallel connection of the two circuit branches 12 and 13 is connected to this series connection. The other end of this parallel connection is connected to the negative pole of the rectifier 10.

The circuit branch 12 contains the electronic switches T ₁ and T ₃ , between which the one electrode 14 ₁ the fluorescent lamp 14 is switched. The other circuit branch 13 contains the series connection of the transistors T ₂ and T ₄ , between which the second electrode 14 _{2 of} the fluorescent lamp 14 is connected.

The switches T ₁ to T ₄ are controlled by the logic control unit 15, which can be a microprocessor, for example.

It is assumed that the switches T ₁ to T _{4 are} in the conductive state or that a current can flow in another way via the bridge circuit formed by the circuit branches 12, 13 and the lamp 14. Then, when the switching element _S T becomes conductive, a current flows through the current sensor I, the inductance L and the bridge circuit. This current i _L , which is shown in FIG. 2, builds up slowly as a result of the inductance L, and the increase can be assumed to be linear. The output of the current sensor I is connected to the B input of a comparator 16 with hysteresis, the output of which controls the switching element T _S. The A input of the comparator receives a reference voltage U _ref from the control unit 15. The comparator 16 generates two threshold values from the reference voltage, one of which corresponds to a maximum lamp current i _L max and the other to a minimum lamp current i _L min. When the lamp current reaches the maximum threshold value i _Z max, the comparator 16 blocks the switching element T _S , ie the series circuit is interrupted. The inductance L now tries to maintain the current that has previously flowed, so that the coil current i _L slowly drops. When this current reaches the lower limit value i _Z min, the comparator 16 switches the switching element T _S back into the conductive state, as a result of which the lamp current i _L in the series circuit rises again. In Fig. 2 the time course of the voltage drop U _L at the inductance L and the time course of the current i _{L is} shown. It can be seen that the current changes over time within the limit values i _L max and i _L min, which are relatively close to one another, and remains almost constant. The height of the band between i _L max and i _L min can be changed by the control voltage supplied from the control unit 15 to the comparator 16. The time course of the current i, which is taken from the rectifier 10, is shown in FIG. 1. This stream is sawtooth-shaped, with the individual saw teeth being separated from one another by gaps. So that the lamp current can flow when the switching element T _{S is} blocked, the output side of this switching element is connected via a free-wheeling diode 17 to the negative pole of the rectifier circuit 10. The anode of the freewheeling diode 17 is connected to this negative pole.

Before starting the lamp 14, the electrodes 14 ₁ and 14 _{2 are} first heated. For this purpose, all four switches T ₁ to T _{4 are} controlled in the conductive state. After a defined preheating time, two diagonally opposite switches, for example switches T ₁ and T _{4, are} blocked. At the same time, the current i _L can be changed by appropriately controlling the comparator 16. As a result of the change in current, the inductance L generates the voltage rise required for the ignition of the lamp 14. The lamp is now operated for a longer period of time, for example over an hour, using switches T ₂ and T ₃ , electrode 14 ₂ having a positive potential and electrode 14 ₁ having a negative potential.

3 shows the lamp current i _L over time t, but the time scale is much larger than that of FIG. 2. During the first half period 1 HP, in which this polarity is maintained, the output signal of the current sensor I is integrated by the integrator 18. From Fig. 3 it can be seen that the integral f iL dt increases linearly with time. This integral is fed from the integrator 18 to the control unit 15. When the integral has reached a predetermined maximum value, the control unit 15 causes the switches T ₁ and T _{4 to} be kept conductive and the switches T ₂ and T ₃ are subsequently blocked. The current then flows through T ₁ , the lamps 14 and T ₄ , so that the lamp 14 is reversed. Simultaneously with the polarity reversal, the integrator 18 is reversed so that it is now integrated downwards. The upward and downward integration is controlled by the logic control unit 15 via the lines 19. When the integral has reached the value 0, the second half period 2HP is ended and the switches T ₂ and T ₃ become conductive again, and subsequently the switches T ₁ and T _{4 are} blocked. It is thus achieved by the integrator 18 that the current integral in the first half period is equal to the current integral in the second half period. In this way, different current intensities, which may in the meantime be caused by the comparator 16, are taken into account, and it is achieved that the current applied to the lamp 14 is the same in both directions.

The embodiment of FIGS. 4 to 6 largely corresponds to the first embodiment, so that only the differences are explained below. Components that have the same function in both exemplary embodiments are provided with the same reference symbols, so that only the differences or additional features are explained below. 4, the electronic switching element T is connected to the inductor L and the other end of the inductor L is connected to one end of the two parallel circuit branches 12, 13. The other end of these circuit branches 12, 13 is connected to ground potential via the current sensor I designed as a low-resistance resistor 20.

The electronic switches T ₁ and T ₄ are controlled by the control unit 15 via the control circuit 21 and the electronic switches T ₂ and T ₃ are controlled by the control unit 15 via the control circuit 22. A further transistor drive circuit 23 is connected between the output of the comparator 16 and the switching element T.

The voltage generated at the current sensor I is present at the negative input of the comparator 16 and the reference signal U _{ref is} fed to the positive input. This reference signal determines the value that the lamp current assumes.

The ballast contains a signal generator 24, which in the present exemplary embodiment consists of four manually adjustable switches. A switch combination can be set at these switches in accordance with the respective power level to which the fluorescent lamp 14 belongs. A line leads from each of the switches of the signal generator 24 to the encoder 25. A further input of the encoder 25 is supplied with a signal VH, which is supplied by the control unit 15 and which indicates whether the ballast is in the preheating phase or not. The encoder 25 also has a sixth input, to which a signal M is supplied, which indicates whether a lamp circuit is on _M indeststrom flows or not. The minimum current is a quiescent current that is maintained as long as the lamp 14 is inserted in the socket. It is a quiescent current through which the circuit is monitored and kept ready for operation. As long as the minimum current is not exceeded, the signal M is "ONE" and if the minimum current is exceeded, this signal is "ZERO".

The encoder 25 generates from the supplied thereto six _B inärsignalen a digital signal representing a measure of the instantaneous lamp current. This digital signal is fed to a digital / analog converter 26, which applies a corresponding analog signal to the A input of the multiplier 45.

The minimum current signal M is generated by a differential amplifier 46, which receives a threshold signal 47 at one input and whose other input is connected to the output of the current sensor I.

The encoder 25 consists, for example, of an encoding matrix. It is designed such that it forms a digital value from the output signal of the signal generator 24 and the signal VH, which indicates the size of the lamp current for the respective lamp type, both for the preheating phase and for the operating phase. However, this only applies as long as the signal M is "ZERO". If the signal M is "ONE", then, regardless of the signal from the signal generator 24 and the signal VH, a very specific signal is output which corresponds to the size of the minimum current to be generated.

In order to be able to dim the fluorescent lamp, a dimmer 27 is connected to the ballast. The digita The output signal of the dimmer 27 is fed via an optocoupler 28 in serial form to a series / parallel converter 29 which contains a shift register. The output signal of the series / parallel converter 29 is fed to the B input of the multiplier 45 via an encoder 30. Encoder 30 is blocked from passing signals when at least one of VH or M signals occurs. In this case, the signals from the dimmer 27 become ineffective. If the B input of multiplier 45 receives no signal, the signal applied to the A input is multiplied by a factor of 1. If, on the other hand, a signal occurs at the B input, the analog signal A is multiplied by an analog value corresponding to the digital signal B. The signal B indicates the percentage of the lamp current from the nominal current. This percentage can be less than 100% or more than 100%. Fluorescent lamps can also be operated temporarily with a lamp current above the nominal current without being damaged.

The dimmer 27 does not necessarily have to be manually adjustable. In the present exemplary embodiment, it is connected to a sensor MF, which, for example, determines the illuminance and supplies a corresponding control signal to the dimmer 27 in order to regulate this illuminance to a constant value. Instead of the sensor MF, a timer or the like can also be used. be provided.

The transmission behavior of the multiplier 45 is influenced by the signal of a temperature sensor TF, which is attached to the ballast. In this way, the lamp output can be reduced, for example, when the ballast temperature rises above a predetermined value.

5 shows various pulse profiles when the fluorescent lamp is ignited. The voltages U _S1 , U _S2 , U _S3 and US ₄ are the control voltages for the electronic switches T ₁ to T ₄ . A high voltage level means that the switch in question is controlled in the conductive state and a low voltage level means that the switch is blocked.

In the preheating phase, which lasts a few hundred milliseconds, all four transistors T ₁ to T _{4 are} conductive, so that the electrodes 14 ₁ and 14 _{2 are} heated. The preheating phase ends at time t ₁ . The control unit 15 then first controls the switch T ₄ into the blocking state and a few microseconds later also switches T ₁ . When the switch T ₄ was blocked, the circuit branch 13 was interrupted. When the switch T ₁ is blocked, the circuit branch 12 is additionally interrupted, so that no more current can flow through any of the electrodes. However, the inductance L strives to maintain the current flow and generates a high voltage which reaches the electrode 14 ₂ via T ₂ , while the electrode _{141 is} connected to ground potential via _T3 . The high voltage is above the first breakdown voltage of the switches T ₁ and T ₄ , so that the transistor with the lower breakdown voltage becomes conductive, although it is controlled in the blocking state. The breakdown voltage of T ₁ or T4 thus corresponds to the ignition voltage for the fluorescent lamp. The inductance L can therefore discharge when the lamp is not ignited via T ₁ and T ₃ .

As FIG. 5 shows, numerous ignition pulses are generated in the ignition phase in order to periodically make T ₁ and T ₄ conductive and block them. The duration of each ignition pulse 31, in which T1 and T _{4 are} blocked, is 7 microseconds and the duration of the subsequent pulse pause, in which T ₁ and T _{4 are} conductive, is approximately twice as long, 15 microseconds in the present exemplary embodiment. The period of the ignition pulses 31 is thus 22 microseconds. The second curve in FIG. 5 shows the course of the coil current i _L through the inductance L. In the preheating phase, this current is at the value determined for preheating. If the switches T ₁ and T ₄ are blocked by an ignition pulse 31, then the coil current flowing through the switches T ₁ and T ₃ drops to zero. In the subsequent leading phase of T ₁ and T ₄ , the coil current rises again to the value specified by U _ref . This value is maintained until the next firing pulse 31 occurs.

During each ignition process, the control unit 15 generates a certain number of control pulses 31 for, for example, 100 ignition pulses. The pulse pauses, in which T ₁ and T _{4 are} conductive, are on the one hand so long that T ₁ or T ₄ can recover from the voltage breakdown in the conductive state, and on the other hand so short that there is no substantial recombination of the charge carriers in the fluorescent lamp 14 can. Due to the brief successive ignition pulses, the gas in the fluorescent lamp is increasingly ionized until the ignition takes place. In this case, as is customary, a single ignition pulse is not generated, which must apply all of the ignition energy, but the ignition energy is distributed over a plurality of ignition pulses. This has the advantage that the value of the inductance L to a fraction of the otherwise usual value can be reduced.

The control unit 15 delivers the full number of control pulses for e.g. 100 ignition pulses, regardless of when the ignition actually takes place. As a rule, only about 5 to 10 ignition pulses are required. The remaining ignition pulses are then generated when the fluorescent lamp is already lit, which is not detrimental to the operation.

The right part of FIG. 5 shows the voltage and current profiles after the ignition. It is assumed that the switches T ₁ and T _{4 first} become conductive immediately after the ignition, while the switches T ₂ and T ₃ block. In order to avoid cataphoresis on the lamp 14, the control unit 15 switches over the aforementioned switch pairs at predetermined time intervals. At the time t ₃ , those switches T ₂ and T ₃ which were previously blocked are first switched to the conductive state. For a short period of time until time t ₄ , all four switches are in the conductive state. Then the switches T ₁ and T ₄ , which were conductive in the previous phase, are controlled in the blocking state.

In order to prevent the same pair of switches T ₁ , T ₄ or T ₂ , T _{3 from} becoming conductive each time after switching on, the random generator 32 is provided in connection with the control unit 15 according to FIG. The random generator 32 randomly generates "ZERO" signals and "ONE" signals. It is controlled by the control pulses 31. The state of the output signal of the random generator 32 at the time of ignition of the fluorescent lamp determines which of the two switch pairs is initially switched on and which is off is switched. In this way it is avoided that the fluorescent lamp is always operated with the same polarity after being switched on. The random generator 32 replaces the integrator 18 according to FIG. 1.

6 shows the circuit diagram of the circuit for generating the reference voltage U _ref in a detailed form. The signal generator 24 contains four switches S ₁ , S ₂ , ^S ₃ , ^S ₄ , which are each connected to one another at one end and connected to ground and the other ends of which are connected to the supply voltages via resistors 32 with different values. Each switch is connected to an input line of the encoder 25. The signals M and VH described above are at two further inputs of the encoder 25.

The encoder 25 encodes the six-digit binary signal supplied to it into a four-digit binary signal which is fed to the digital / analog converter 26. Each of these signals controls an electronic switch ^A ₁ , A ₂ , A _{3 '} A ₄ . One end of these switches is connected to the supply voltage + U and the other ends are each connected to a weighted resistor 33. The ends of all resistors 33 are connected to one another and to the input of an inversion amplifier 34. This input also forms the A input of the multiplier 45.

The amplifier 34 has several parallel feedback loops. The one feedback loop contains a series connection of several resistors 35, each of which can be bridged by a switch 36. The switches 36 are controlled by the various outputs of the encoder 30. When all switches 36 are closed, all resistors 35 are in first feedback branch ineffective. The amplifier 34 then has a gain factor of ONE. By opening one or more switches 36, the gain factor can be increased to over 100%.

The first feedback branch consisting of the resistors 35 is connected in parallel to a plurality of further feedback branches, each of which consists of a weighted resistor 37 and a switch 38 connected in series therewith. The switches 38 are also controlled by output signals from the encoder 30. By closing one or more switches 38 (with switches 36 closed) the amplification factor of the amplifier 34 is reduced to below 100%.

A third feedback branch of the amplifier 34 contains the series connection of a resistor 39 and a switch 40. The switch 40 is controlled by the temperature sensor TF, which reacts with an NTC resistor 41 to the temperature of the ballast. When the temperature rises, the amplifier 42 provides a signal to close the switch 40. This reduces the amplification factor of the amplifier 34, so that the reference voltage U _ref at the output of this amplifier also decreases. The result is that the comparator 16 (Fig.4) is set to a lower lamp current.

The signals for controlling each switch 36 and 38 are supplied from encoder 30 to the B input of multiplier 45. However, encoder 30 is disabled when either M or VH is "ONE". In this case the switches 36 are closed and the switches 38 are open, ie the amplifier 34 has a gain factor of ONE, so that the reference chip _Uref is set to the minimum current or the preheating current depending on which of the signals M or VH is "ONE". In this case, the lamp current cannot be influenced by the signals of the dimmer 27.

Fig. 7 shows a modified embodiment with signal generator 24a. The signal transmitter is not set manually here, but rather generates the signal dependent on the lamp type or the lamp power automatically. For this purpose, the two electrodes 14 _. and 14 _{2 of} the fluorescent lamp 14 connected to the input of an analog / digital converter which generates a digital output signal which (with a defined lamp current) corresponds to the lamp resistance. This output signal is fed to the encoder 25, which forms a signal for generating the reference signal. Thanks to its lamp resistance, the fluorescent lamp ensures that the current corresponding to the lamp power is automatically set.

In the embodiment of FIG. 8, the voltage drop at each of the two electrodes 14 ₁ or 14 _{2 is} fed to an analog / digital converter 43 or 44. Provided that the current flowing through the electrode has a certain value, the voltage drop across the electrode corresponds to the electrode resistance. The output signals of the analog / digital converters 43 and 44 are converted in the signal generator 24b in the encoder 25 into a control signal which is further processed by the digital / analog converter 26 in the manner described above.

Claims (21)

1. Fluorescent lamp ballast with a power supply circuit that feeds two parallel circuit branches, each of which leads over an electrode of the fluorescent lamp and contains two electronic switches in series, a control unit for controlling the electronic switches, an inductor for generating the ignition voltage for the fluorescent lamp and a current sensor,
characterized in that the current sensor (I) and the inductance (L) are connected in series with the two circuit branches (12, 13) and in that the current sensor (I) controls a switching element (T _S ) which is also connected in series in the manner that this switch opens the circuit when the current exceeds a certain first threshold (i _L max) and closes the circuit when the current falls below a certain second threshold (i _L min), which is lower than the first threshold. 2. fluorescent lamp ballast according to claim l, characterized in that the threshold values are variable as a function of at least one control signal (U _ref ). 3. fluorescent lamp ballast according to claim 1 or 2,
characterized in that the electronic switches (T ₁ , T) are controlled in such a way that the fluorescent lamp (14) is reversed with a frequency below 100 Hz. 4. fluorescent lamp ballast according to claim 3, characterized in that the polarity reversal frequency is below 1 Hz and preferably below 1 mHz. 5. fluorescent lamp ballast according to one of claims 1 to 4,
characterized in that an integration device (18) is provided which forms the time integral of this current during each period of the current flowing through the fluorescent lamp (14) and initiates the next period when this integral has become zero. 6. fluorescent lamp ballast according to claim 5, characterized in that the first half period 1HP of the period is ended after a predetermined time. 7. fluorescent lamp ballast according to claim 5, characterized in that the first half period 1HP of the period is ended when the integral reaches a predetermined maximum value. 8. fluorescent lamp ballast according to one of claims 1 to 7,
characterized in that a random generator (32) is provided which switches on the electronic switches (T ₁ ) to (T4) in pairs before, during or after the lighting of the fluorescent lamp (14), so that the probabilities for each of the two polarities of the fluorescent lamp (14) are equal to each other. 9. fluorescent lamp ballast according to one of claims 1 to 8,
characterized in that the current supplied to the fluorescent lamp (14) as a function of a reference signal (Uref); It can be changed that a signal transmitter (24) is provided which provides an indication of the nominal power of the fluorescent lamp (14) and that the reference signal (U _ref ) is generated by a modulation circuit (45) which delivers the one supplied by the signal transmitter (27) Combined signal with an external control signal. 10. fluorescent lamp ballast according to claim 9, characterized in that between the signal generator (24) and the modulation circuit (45) an encoder (25) is connected, which in addition to the signals of the signal generator (24) at least one other of the control unit (15th ) received signal (VH, M) about the operating state of the fluorescent lamp and generates the signal to be fed to the modulation circuit (45) as a function of the signal from the signal generator (24) and the additional signal (VH, M). 11. fluorescent lamp ballast according to claim 9 or 10,
characterized in that the signal transmitter (24a, 24b) is controlled by an electrical or mechanical characteristic value of the fluorescent lamp (14). 12. fluorescent lamp ballast according to claim 11, characterized in that the signal transmitter (24a) is controlled by the voltage drop occurring on the fluorescent lamp (14). 13. Fluorescent lamp ballast according to claim 11, characterized in that the signal transmitter (24b) is controlled by the voltage drop occurring on at least one electrode (14 ₁ , 14 ₂ ) of the fluorescent lamp (14). 14. fluorescent lamp ballast according to claim 10, characterized in that the signal transmitter (24) is a manually adjustable memory. 15. fluorescent lamp ballast according to one of claims 9 to 14,
characterized in that the modulation circuit (45) is a multiplier. 16. fluorescent lamp ballast according to one of claims 9 to 15,
characterized in that the modulation circuit (45) is designed such that, in the event that the control signal is derived from the ON state of an ON / OFF switch, the content of the signal generator (24) is passed on unchanged as a reference signal (U _ref ). 17. fluorescent lamps _- ballast according to one of claims 9 to 16,
characterized in that several ballasts equipped with fluorescent lamps (14) of different nominal loads are controlled by the same control signal. 18. fluorescent lamp ballast according to one of claims 9 to 17,
characterized in that the external control signal is supplied by a dimmer device (27) as a function of the signal from a sensor (MF). 19. fluorescent lamp ballast according to one of claims 1 to 18,
characterized in that the control unit (15) periodically switches on and off at least one of the electronic switches (T ₁ to T ₄ ) to start the fluorescent lamp (14), the switch-off phases being so short that a reversible voltage breakdown occurs at the set current this switch (T ₁ ) occurs and the switch-on phases are dimensioned such that there is no substantial recombination of the charge carriers in the fluorescent lamp (14). 20. Fluorescent lamp ballast according to claim 19, characterized in that the switch-off phases are shorter than the switch-on phases. 21. Fluorescent lamp ballast according to claim 20, characterized in that the duration of the switch-off phases is approximately 5 to 20 µs and that of the switch-on phases is approximately 10 to 40 µs.

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