EP0003528B1 - Electronic device for controlling the light intensity of a gaseous discharge lamp without a heated cathode - Google Patents

Description

The present invention relates to an electronic device for regulating the brightness of an electrical gas discharge lamp fed from an AC network without a hot cathode, in particular a high-pressure metal vapor lamp.

Electronic devices which operate on the principle of so-called leading edge control are known and customary for regulating the brightness of filament lamps. Such a device has a triac or thyristor arranged in the supply circuit of the lamp, to which an ignition voltage is periodically supplied, the ignition angle of which, measured from the zero crossing of the current, is controlled within each AC half-wave. Among the known circuit arrangements for phase control, there are those in which the firing angle can be changed as a function of an adjustable DC control voltage.

If one has a circuit arrangement for phase control of the type described for the brightness regulation of a gas discharge lamp without hot cathode, e.g. a mercury vapor lamp or high-pressure sodium vapor lamp, it turns out that when the brightness is adjusted down, there is a risk of the lamp being completely extinguished, especially if voltage fluctuations or brief voltage interruptions occur in the AC network in the down-regulated brightness state. Once the gas discharge lamp goes out, it can only be switched on again after it has cooled to the ambient temperature and can thus be switched on. In order to prevent the unintentional extinction of a gas discharge lamp when its brightness is regulated downwards, electronic devices have already been created in which a control voltage which is dependent on the current of the current flowing through the gas discharge lamp is generated and is used to automatically control the ignition angle of the ignition voltage for a triac or thyristor measures are taken that a predetermined minimum current = strength of the current flowing through the lamp is ensured with reduced energy supply to the lamp.

From US-A-3 991 344 a device for regulating the brightness of gas discharge lamps of high luminance is known, in which device in the feed circuit of the gas discharge lamp in series with a triac is the primary winding of a current transformer, the secondary winding of which is connected to the input of a rectifier arrangement, of which Output the mentioned control voltage is derived. This control voltage is fed to one input of a differential amplifier, at the other input of which there is a DC control voltage which, for example, according to the desired brightness of the lamp is set manually. The ignition angle of the ignition voltage is controlled in accordance with the difference between the control voltage and the control voltage, which results in automatic regulation of the lamp current in accordance with the control voltage, in the entire range from maximum to minimum lamp brightness. To ensure a predetermined minimum amperage of the current flowing through the lamp with reduced energy supply to the lamp, i.e. at the minimum lamp brightness, resistors ensure that even with the external control voltage zero there is still an internal "control voltage" according to which the automatic current regulation is based. Since in this known device the automatic current control has to function over the entire range between the full power of the lamp and the minimum power, the effectiveness of the control remains relatively the same as the lamp current decreases, but in absolute terms it becomes less and less. Accordingly, when the lamp current is reduced to a critical value at the minimum lamp brightness, the current control becomes most unsafe. Therefore, the lamp current must not be reduced below 25-35% of the nominal current, which still results in a brightness of about 5% of that at nominal current (cf. column 3, lines 44-48).

From US-A-3989976 a device for controlling the brightness of gas discharge lamps of high luminance is also _'known, but in which means the strength of the current flowing through the lamp current is controlled by varying a shunt in parallel with one of two Vorschaltdrosseln depending on the voltage above the lamp. The automatic control ensures that the voltage across the lamp does not exceed a certain value at which the lamp does not go out. This device has no means for generating a control voltage which is dependent on the strength of the current flowing through the lamp, in order to ensure a minimum current strength and allows the brightness to be reduced to approximately 5% of the brightness at the nominal current strength (cf. column 4, line 24 -27, 44-46 and 51-54).

It is the object of the present invention to design an electronic device for regulating the brightness of an electrical gas discharge lamp fed from an AC network without a hot cathode in a relatively simple manner in such a way that an unintentional extinction of the gas discharge lamp even with a greater reduction in the current flowing through the lamp and thus the Brightness of the lamp is reliably prevented.

This object is achieved according to the invention in a device of the type required in the preamble of claim 1 and known from US Pat. No. 3,991,344 in that the additional circuit arrangement has a reference voltage source and a voltage divider. has, the two ends of which are connected to oppositely polarized connections of the output of the rectifier arrangement on the one hand and the reference voltage source on the other, while the respective other connections of the output of the rectifier arrangement and the reference voltage source are connected to one another in such a way that the control voltage is tapped at a tap of said voltage divider and that through at least one electric valve the one polarity of the. Control voltage for the ignition angle control is switched off.

This configuration of the device according to the invention ensures that the control voltage, which is dependent on the lamp current, is effective for the ignition angle control only at relatively low values of the lamp current, but to a large extent. The voltage divider in question forms a bridge arrangement which is fed on the one hand by the reference voltage source and on the other hand by the rectifier arrangement and in each case comes out of equilibrium if the influence of the output voltage of the rectifier arrangement which is dependent on the lamp current outweighs the influence of the reference voltage source in a positive or negative sense, the Control voltage for the ignition angle control arises. Since the one polarity of the control voltage is suppressed by the electrical valve mentioned, the ignition angle is only influenced by the control voltage if the strength of the current flowing through the lamp drops below a predetermined value and the danger of an unwanted extinguishing of the gas discharge lamp only occurs in the first place. These measures make it possible to reduce the brightness of gas discharge lamps, such as mercury vapor lamps in particular, to 3% and less without the lamps going out unintentionally as a result of voltage fluctuations or other influences.

In contrast to mercury vapor lamps, the minimum current required for high pressure sodium lamps depends to a large extent on the temperature of the lamp. The minimum current is relatively high at the normal operating temperature of the lamp and decreases with decreasing lamp temperature. When the energy supply to the lamp is reduced, however, the lamp temperature drops only after a considerable time delay because of the thermal inertia of the lamp. In order to achieve the largest possible range of brightness regulation, the specified minimum current strength should therefore be reduced with the same time delay. It is therefore a further object of the present invention to further develop the device according to the invention in such a way that the control voltage used to ensure the minimum current intensity depends not only on the intensity of the current flowing through the lamp but also indirectly on the respective temperature of the lamp.

This is achieved in a surprisingly simple manner with the aid of a second rectifier arrangement, the input of which lies parallel to the triac arranged in the supply circuit of the lamp, a component of the control voltage which is dependent on the current intensity of the current flowing through the lamp and the output of the rectifier arrangement being dependent on the output second rectifier arrangement, a component of the regulating voltage which is dependent on the voltage across the triac is derived for automatically regulating the minimum current strength of the current flowing through the lamp in adaptation to the characteristic of the lamp.

With this further developed embodiment of the device according to the invention, the brightness of a high-pressure sodium lamp can be regulated in a hitherto unreached large range from full brightness at nominal power - down to less than 1% thereof, without the risk of the lamp being switched off unintentionally.

• Fig. 1 zeigt als erstes Beispiel das elektrische Schaltschema einer Einrichtung zur Helligkeitsregulierung einer Quecksilberdampflampe;
• Fig. 2 zeigt als zweites Beispiel das elektrische Schaltschema einer Einrichtung zur Helligkeitsregulierung einer Natriumdampf-Hochdrucklampe.

Further features, details and advantages of expedient embodiments of the subject matter of the invention result from the claims, from the following description and from the associated drawings, in which the electrical circuit diagrams of preferred embodiments of the inventive device for regulating the brightness of a gas discharge lamp are illustrated purely by way of example.

• 1 shows as a first example the electrical circuit diagram of a device for regulating the brightness of a mercury vapor lamp;
• 2 shows, as a second example, the electrical circuit diagram of a device for regulating the brightness of a high-pressure sodium lamp.

In Fig. 1, 20 denotes a mercury lamp, the brightness of which is to be regulated. In series with the mercury vapor lamp 20 there is a throttle 21 in the usual way for limiting the current flowing through the lamp below a maximum value.

An electronic device 22 and a potentiometer R1 with two end connections 23, 24 and an adjustable tap 25 serve to regulate the brightness of the mercury vapor lamp 20. By adjusting the tap 25, the brightness of the lamp 20 can be between a maximum value and a minimum value that is only about 3 % of the maximum value, arbitrarily regulate.

The electronic device 22 has two mains connection terminals 26, 27 for connection to an AC network with a voltage of e.g. 220 V, also two lamp connection terminals 28, 29 for connecting the mercury vapor lamp 20 and the choke 21, and three control line connection terminals 31, 32, 33 for connecting control conductors that lead to the connections 23, 24, 25 of the potentiometer R1. Within the device 22, the one mains connection terminal 26 and the one lamp connection terminal 28 are connected directly to one another. A triac TR1, a high-frequency blocking inductor L1 and the primary winding L2 of a current transformer 30 are connected in series between the other mains connection terminal 27 and the second lamp connection terminal 29. To suppress high-frequency interference voltages, capacitors C1, C2 and the series connection of a capacitor C3 and a resistor R2 are provided.

The triac TR1 has a control electrode 34, which has to be supplied with an ignition pulse in each half-wave of the mains AC voltage in order to bring about the current flow. The following circuit arrangement known per se is provided for generating the ignition pulses: A commercially available integrated circuit IC1, e.g. the type TCA 280 A from Philips, is connected on the one hand with its connection 13 via a resistor R3 and a rectifier diode D1 to the mains connection terminal 26 and on the other hand with its connection 16 directly to the mains connection terminal 27 in order to be supplied with electrical energy from the AC network . The ground conductor 35 of the circuit arrangement is also connected to the connection 16. At a connection 11, the integrated circuit IC1 provides a constant DC voltage which, for example, with respect to the ground conductor 35 Is + 14V. A capacitor C4 located between the connection 11 and the ground conductor 35 smoothes the DC voltage. Between the connection 11 and the directly connected connections 2 and 6 of the integrated circuit IC1 there is a series connection of a resistor R4 and an adjusting resistor R5, while a capacitor C5 is connected between the ground conductor 35 and the mentioned connections 2 and 6. This creates a sawtooth voltage at terminals 2 and ti, the steepness of which can be changed within certain limits by means of the adjusting resistor R5. The sawtooth voltage is synchronized with the half-waves of the AC line voltage in that a current path containing a resistor R6 is connected between the line terminal 26 and a trigger input 1 of the integrated circuit IC1. A reverse voltage connection 3 of the integrated circuit IC1 is connected via a resistor R7 to the electrode 36 of the triac TR1 facing away from the ground conductor 35, as a result of which the start of the increase in the sawtooth voltage does not lie before the zero crossing of the current in the supply circuit of the mercury vapor lamp.

The control line connection 31 is connected by means of a conductor 37 to the connection 11 of the integrated circuit IC1 carrying the constant direct voltage, while the control line connection 32 is connected to the ground conductor 36, so that the constant direct voltage of e.g. 14V lies. The control line connection 33 connected to the potentiometer tap 25 is connected via a resistor R8 and a conductor 38 to an ignition angle control input 5 of the integrated circuit IC1. The level of the DC voltage at input 5 determines the ignition angle or ignition point within each AC voltage half-wave. The ignition pulses appear at an output 10 of the integrated circuit IC1. This output 10 is connected to the control electrode 34 of the triac TR1 via a resistor R9. Each ignition pulse begins when the instantaneous value of the sawtooth voltage at connection 6 matches the DC voltage at the ignition angle control input 5. The duration of each ignition pulse is determined by a resistor-capacitor combination R10, R11, C6, which is connected to further connections 7, 8 and to the Ground conductor 35 is connected.

The previously described circuit arrangement for controlling the triac TR1 is known, which is why it is not necessary to explain its mode of operation in detail here. To facilitate understanding, it is sufficient to mention that, depending on the position of the tap 25 on the potentiometer R1, there is a more or less high DC control voltage U _s between the connections 32 and 33. If this control voltage U _{s is} equal to zero, ie if the tap 25 is located directly at the end connection 24 of the potentiometer R1 connected to the ground conductor 35, the firing angle is zero. The ignition pulses at the output 10 of the integrated circuit IC1 then begin immediately after each zero crossing of the AC mains voltage, which is why the mercury vapor lamp 20 is supplied with full power and reaches its maximum brightness. If you tap the tap 25 of the potentiometer R1 more and more towards the other end connection 23 of the potentiometer, the control DC voltage U _s between the connections 32 and 33 increases, which increases the ignition angle accordingly and increases the ignition pulses after the zero crossings experienced the AC voltage. As a result, current flows through the triac TR1 and TR1 only during part of each half cycle of the AC voltage through the mercury vapor lamp 20 so that its brightness is reduced.

If the potentiometer tap 25 is located directly at the end connection 23 of the potentiometer R1, the ignition angle is greatest and the brightness of the mercury vapor lamp 20 is lowest. In order to prevent the mercury vapor lamp from going out unintentionally in this case, an additional circuit arrangement is provided in the device 22, which automatically ensures that the intensity of the current flowing through the mercury vapor lamp never falls below a certain minimum value. The additional circuit arrangement for minimum current isolation is described below.

The current transformer 30 has a secondary winding L3, which is loaded with a parallel resistor R13 and is connected to the input terminals 40, 41 of a rectifier arrangement 42. The rectifier arrangement 42 has a positive output terminal 43 connected to the ground conductor 35 and a negative output terminal 44. Between the output terminals 43, 44, a Zener diode D2 for deriving overvoltages in the event of a short circuit in the supply circuit of the mercury lamp 20, a load resistor R14 and a capacitor C7 for smoothing the rectified voltage are connected in parallel. The negative output terminal 44 of the rectifier arrangement 42 is connected via a voltage divider comprising a resistor R15, a potentiometer R16 and a further resistor R17 to the conductor 37, on which the constant DC voltage of, for example, + 14V is applied. The potentiometer R16 has an adjustable tap 45, on which a control voltage U _{R which} is dependent on the current strength in the supply circuit of the mercury vapor lamp is tapped. This control voltage U _R is fed to the base of an npn transistor T1, the emitter of which is connected to the ground conductor 35 and the collector of which is connected via a resistor R12 to the conductor 38 leading to the ignition angle control input 5 of the integrated circuit IC1. The collector-emitter path of the transistor T1 serves as a variable resistor, the resistance of which can be controlled electronically by the control voltage U at the base. The resistors R8 and R12 and the collector-emitter path of the transistor T1 serving as a resistor together form a voltage divider, above which the DC control voltage U _s , which is between the connections 32, 33 and is adjustable by means of the potentiometer R1. The resistance value of the resistor R12 is much smaller than that of the resistor R8 and is practically negligible. At the connection point 39 between the resistor R8 and the series circuit of the resistor R12 and the transistor T1, the voltage supplied to the ignition angle control input 5 of the integrated circuit IC1 by means of the conductor 38 is tapped off. The latter is dependent on the one hand on the position of the tap 25 of the potentiometer R1 and on the other hand on the respective resistance value of the collector-emitter path of the transistor T1. A diode D3 is connected between the base and the emitter of the transistor T1 and is poled in such a way that it prevents the occurrence of negative polarity voltages at the base of the transistor. A resistor R 19 is connected between the reference voltage conductor 37 and the control voltage conductor 38. Another resistor R20 and a charging capacitor C8 connected in parallel lie between the ground conductor 35 and the control voltage conductor 38.

• Solange die Regelspannung U, an der Basis des Transistors T.1 gleich Null oder nicht positiv ist in bezug auf den masseleiter 35 (negativ kann sie wegen der Diode D3 nicht werden), ist die Kollektor-Emitter-Strecke des Transistors T1 hochohmig, weshalb dann der Transistor T1 keinen Einfluss auf die Helligkeitssteuerung mittels der Steuergleichspannung U_s ausübt.

The operation of the additional circuit arrangement described is as follows:

• As long as the control voltage U at the base of transistor T.1 is zero or not positive with respect to ground conductor 35 (it cannot become negative because of diode D3), the collector-emitter path of transistor T1 is high-resistance, which is why then the transistor T1 has no influence on the brightness control by means of the control direct voltage U _s .

An AC voltage is induced in the secondary winding L3 of the current transformer 30, which is proportional to the strength of the current flowing in the supply circuit of the mercury vapor lamp 20. The induced AC voltage is rectified in the rectifier arrangement 42 and smoothed by the capacitor C7. A direct voltage which is substantially proportional to the lamp current and is therefore negative across the capacitor C7 is negative with respect to the ground conductor 35. This DC voltage is related to the constant positive DC voltage on the conductor 37 by means of the series connection of the resistor R15, the potentiometer R16 and the resistor R17. The tap 45 of the potentiometer R16 can be adjusted so that for a given, relatively low strength of the in the supply circuit of the mercury vapor lamp 20 current flowing, the influences of the negative voltage on the output terminal 44 of the rectifier arrangement 42 on the one hand and the positive reference voltage on the conductor 37 on the other hand just cancel out on the potential at the tap 45. At all higher currents of the current flowing through the lamp 20, in particular when the lamp is operated at full power, the rectified voltage at the output terminal 44 of the rectifier arrangement 42 is more negative with respect to the ground conductor 35. As a result, the voltage balance at the tap 45 described above becomes thereby of the potentiometer R16 disturbed in such a way that the potential at the tap 45 would become negative with respect to the ground conductor 35 if this were not prevented by the diode D3. Then lies on the base of the transistor T1 no positive control voltage U _R , and the brightness regulation of the lamp 20 is determined solely by the DC control voltage U _s .

If the ignition angle is increased by increasing the DC control voltage U _s to such an extent that the current in the supply circuit of the lamp 20 threatens to drop below the above-mentioned minimum value, the negative potential at the output terminal 44 of the rectifier arrangement 42 with respect to the ground conductor decreases so far that at the tap 45 of the potentiometer R16 and at the base of the transistor T1 there is a positive potential with respect to the ground conductor 35. As a result, the collector-emitter path of the transistor T1 is controlled in a conductive state. The result of this is that the voltage divider R8, R12, T1 takes effect and the DC voltage at point 39 becomes lower than the control DC voltage U _S set at the potentiometer R1 at point 39. Since the voltage prevailing at point 39 is identical to that at the ignition angle control input 5 of the integrated circuit IC1, the ignition angle also decreases accordingly, as a result of which the energy supply to the lamp 20 is increased and thereby a further decrease in the current intensity in the lamp supply circuit is counteracted.

It can be seen that when the strength of the current flowing through the lamp 20 drops below the value at which there is a voltage equilibrium at the tap 45 of the potentiometer R16, a counter regulation takes place automatically and a certain minimum current strength is thus automatically maintained, even if Due to the control direct voltage U _s set on the potentiometer R1, an even lower current would result, at which there would be the danger of the lamp being inadvertently completely extinguished. The automatic regulation as a function of the current intensity in the supply circuit of the lamp 20 thus has priority over the regulation from the outside by means of the potentiometer R1. In practice, the tap 45 of the potentiometer R16 is adjusted to match the individual properties of the lamp 20 to be controlled so that when the DC control voltage U _{s is increased} to its maximum value, the current-dependent control voltage U _R assumes a value which is sufficient to extinguish it to prevent the lamp 20 with certainty. However, in order for the brightness of the mercury vapor lamp 20 to be reduced to a still tolerable value, the minimum current intensity in the supply circuit of the lamp must be chosen as low as possible. Thus, the control range of the automatic current-dependent ignition angle control is subject to certain limits. This is achieved through resistors R12, R19 and R20.

The automatic control described to ensure a given minimum current in the supply circuit of the mercury vapor lamp 20 also comes into effect if the current would drop below the permissible minimum value for reasons other than by adjusting the potentiometer R1, e.g. in the event of fluctuations in the AC mains voltage or in the event of brief voltage drops.

It should also be mentioned that the changes in brightness of the mercury vapor lamp 20 lag behind the change in current due to the thermal inertia of the lamp.

The capacitor C8 ensures that a cold start of the mercury vapor lamp 20 is also readily possible if the tap 25 of the potentiometer R1 is set to minimum lamp brightness, ie is located directly at the end connection 23 of the potentiometer. After connecting the mains connection terminals 26, 27 to the alternating current network, the capacitor C8 is charged only gradually, for example within 20 seconds, via the resistor R19. Since the voltage across capacitor C8 is initially zero and then rises slowly, the firing angle is initially zero, regardless of the setting of tap 25 of potentiometer R1. The mercury vapor lamp 20 is thus supplied with the full current for a few seconds after it is switched on, so that the lamp quickly reaches its operating temperature, after which the current gradually levels itself out to the value determined by the DC control voltage U _s or the control voltage U _μ .

If a plurality of mercury vapor lamps are to be controlled in terms of their brightness, each of these lamps is assigned its own device 22. The DC control voltage 'U _s, on the other hand, can be generated for all these devices 22 by means of a single potentiometer R1, the end connection 24 and tap 25 with the control line connection terminals 32 and 33 of all devices 22 are connected. Instead of a potentiometer R1 common to all devices 22, it is of course also possible to provide another source for the generation and delivery of the DC control voltage U _s .

The second exemplary embodiment of the subject matter of the invention serves to regulate the brightness of a high-pressure sodium lamp, which in turn is designated by 20 in FIG. 2 and has assigned a current limiting choke 21. 2 again shows an electronic device 22 and a potentiometer R1 with an adjustable tap 25 for arbitrarily changing the brightness of the lamp 20. The difference from the exemplary embodiment described with reference to FIG. 1 lies only in the electronic device 22. Although this contains exactly the same and identically labeled circuit arrangements as in the case of the example according to FIG. 1, it also contains more the further circuit means and electronic components explained below.

Parallel to that in the supply circuit is another voltage divider, which is formed from two resistors R21 and R22. The connection point 47 between the resistors R21 and R22 is connected to the base of a second transistor T2, the collector-emitter path of which is connected in parallel to that of the transistor T1. Between the base and emitter of the second transistor T2 there is a diode D4 which is poled in such a way that it prevents the occurrence of voltages of negative polarity at the base of the transistor T2.

The input 50, 51 of a second rectifier arrangement 52 is connected in parallel with the triac TR1 located in the supply circuit of the high-pressure sodium vapor lamp 20. The positive output terminal 53 of the rectifier arrangement 52 is connected to the ground conductor 35 and the negative output terminal 54 to the one end of a voltage divider composed of two resistors R25 and R26. The other end of the voltage divider R25, R26 is connected to the conductor 37, on which the constant DC voltage of e.g. + 14V is. The connection point 55 between the two resistors R25 and R26 is connected to the base of the transistor T1 via a resistor R27 and a diode D5. Furthermore, a smoothing capacitor C9 and a Zener diode D6 for deriving overvoltages are each connected between the connection point 55 and the ground conductor 35. If the rectifier arrangement 52 consists simply of a diode for one-way rectification, the input terminal 50 and the positive output terminal 53 as well as their connections to the ground conductor 35 are omitted in the diagram shown, since the ground conductor 35 itself then forms the relevant connection.

The mode of operation of the device according to FIG. 2 is basically the same as that described with reference to FIG. 1, insofar as the parts of the circuit arrangement are the same. Therefore, for the sake of simplicity, only the mode of operation of the additional circuit arrangements and electrical components compared to FIG. 1 is explained below.

The control voltage U _R1 at the base of the transistor T1 is composed of two components, namely a first component, which is obtained by means of the current converter 30, the rectifier arrangement 42 and the voltage divider R15, R16, R17 in the manner already described and from which Strength of the current flowing through the high-pressure sodium vapor lamp is dependent, and a second component, which is obtained by means of the second rectifier arrangement 52 and the voltage divider R25, R26 and is dependent on the voltage lying above the triac TR1. The control voltage U _R2 at the base of the second transistor T2 is obtained exclusively by means of the current converter 30, the rectifier arrangement 42 and the voltage divider R21, R22 and is therefore solely dependent on the strength of the current flowing through the lamp 20.

Generation and effect of the first-mentioned component of the control voltage U _R1 are the same as described above with reference to the control voltage U. It should only be mentioned here that the tap 45 of the potentiometer R16 is to be set such that when the DC control voltage U _{s is} rapidly increased to its maximum value, the current-dependent component of the control voltage U _R1 assumes a value which is sufficient to extinguish the sodium vapor Prevent high-pressure lamp 20 with certainty even if the lamp 20 has its normal operating temperature in accordance with the full power.

• Am Eingang 50, 51 der zweiten Gleichrichteranordnung 52 herrscht die über dem Triac TR1 liegende Wechselspannung. Letztere ist praktisch gleich Null, wenn die Lampe 20 mit voller Leistung gespeist wird, d.h. wenn der Zündwinkel gleich Null ist. Wird der Zündwinkel durch Erhöhung der mittels des Potentiometers R, einstellbaren Steuergleichspannung U_S vergrössert zwecks Verminderung der Helligkeit der Lampe 20, so nimmt die Wechselspannung über dem Triac TR1 zu, wobei gleichzeitig die Ausgangsklemme 54 der Gleichrichteranordnung 52 in bezug auf den Masseleiter 35 negativ wird. Am einen Ende. des Spannungsteilers R25, R26 liegt dann das negative Potential der Ausgangsklemme 54 und am anderen Ende das positive Potential des Bezugsspannungsleiters 37. Die Spannung am Abgriff 55 des Spannungsteilers R25, R26 liegt irgendwo zwischen den genannten Potentialen und wird durch den Kondensator C9 geglättet. Solange die Spannung am Abgriff 55 gegenüber dem Masseleiter positiv ist. hat dies keinen Einfluss auf die Regelspannung U_R1' weil die Diode D5 positive Spannungen des Abgriffes 55 von der Basis des Transistors T1 fernhält. Die beiden Widerstände R25 und R26 sind nun derart bemessen, dass am Abgriff 55 ein Spannungsgleichgewicht, d.h. die Spannung Null gegenüber dem Masseleiler 35 herrscht, wenn der Strom im Speisestromkreis der Natriumdampf-Hochdrucklampe 20 auf die vorstehend erläuterte Mindeststromstärke reduziert ist, die nötig ist, um die Lampe 20 mit Sicherheit vor dem gänzlichen Erlöschen zu bewahren, wenn die Lampe noch ihre normale Betriebstemperatur entsprechend der Volleistung hat. Dies ist immer dann der Fall, wenn nach einem Betrieb der Lampe mit Volleistung, d.h. beim Zündwinkel Null, der Zündwinkel durch Erhöhung der Steuergleichspannung U_s verhältnismässig rasch vergrössert wird, um die Helligkeit der Lampe auf den Minimalwert herabzuregulieren.

The following can be said about the generation and the effect of the second component of the control voltage U _R1 :

• At the input 50, 51 of the second rectifier arrangement 52 there is the alternating voltage lying above the triac TR1. The latter is practically zero when the lamp 20 is fed at full power, ie when the ignition angle is zero. If the ignition angle is increased by increasing the control direct voltage U _S which can be set by means of the potentiometer R, in order to reduce the brightness of the lamp 20, the alternating voltage across the triac TR1 increases, the output terminal 54 of the rectifier arrangement 52 simultaneously becoming negative with respect to the ground conductor 35 . At one end. of the voltage divider R25, R26 is then the negative potential of the output terminal 54 and at the other end the positive potential of the reference voltage conductor 37. The voltage at the tap 55 of the voltage divider R25, R26 lies somewhere between the potentials mentioned and is smoothed by the capacitor C9. As long as the voltage at tap 55 is positive with respect to the ground conductor. this has no influence on the control voltage U _{R1 '} because the diode D5 keeps positive voltages of the tap 55 away from the base of the transistor T1. The two resistors R25 and R26 are now dimensioned such that a voltage equilibrium prevails at the tap 55, that is to say the voltage is zero with respect to the ground divider 35, when the current in the supply circuit of the high-pressure sodium lamp 20 is reduced to the minimum current intensity explained above, which is necessary in order to reliably prevent the lamp 20 from completely extinguishing when the lamp is still at its normal operating temperature in accordance with the full power. This is always the case if, after the lamp has been operated at full power, ie at zero ignition angle, the ignition angle is proportional by increasing the DC control voltage U _s is rapidly increased in order to regulate the brightness of the lamp down to the minimum value.

After the current flowing through the lamp 20 has been reduced to the minimum current strength described above at normal operating temperature of the lamp, the latter gradually takes on a lower temperature, the voltage above the lamp 20 decreasing in parallel with the temperature reduction, in accordance with the special burning characteristic of a high-pressure sodium lamp . The sum of the voltages across the lamp 20, the choke coils L1 and 21, the primary winding L2 of the current transformer 30 and the triac TR1 is always the same as the mains voltage at the terminals 26, 27. Since, with the help of the component of the control voltage U _RI which is dependent on the current in the supply circuit of the lamp, the minimum current is kept practically constant, as explained above, the voltage across the choke coils L1 and 21 and across the primary winding L2 of the current transformer 30 remains practically constant. Accordingly, when the voltage above the lamp 20 drops when the lamp cools, the voltage across the triac TR1 must rise to the same extent. The potential of the output terminal 54 of the rectifier arrangement 52 becomes increasingly negative with respect to the ground conductor 35, and the voltage balance at the tap 55 of the voltage divider R25, R26 is disturbed in the sense that the tap 55 becomes negative with respect to the ground conductor 35, and all the more so stronger, the further the high-pressure sodium vapor lamp 20 cools. As soon as the tap 55 becomes negative in the manner described, a current flows from the tap 45 of the potentiometer R16 via the diode D5 and the resistor R27 to the tap 55, as a result of which the control voltage U _R1 positive at the ground conductor 35 at the base of the Transistor T1 is reduced. This results in a corresponding increase in the resistance value of the collector-emitter path of the transistor T1, as a result of which the voltage at the ignition angle control input 5 of the integrated circuit IC1 is increased and the ignition angle is increased. The result is a further reduction in the energy supply to lamp 20, so that its brightness continues to decrease.

It can be seen that when the alternating voltage lying above the triac TR1 rises as a result of the cooling of the high-pressure sodium vapor lamp 20 supplied with reduced current, a control system is automatically set up by means of which the automatically ensured minimum current through the lamp is reduced. In this way it is possible to reduce the brightness of the high-pressure sodium vapor lamp 20 to a negligibly small residual brightness of less than 1% of the brightness at full power without the risk of the lamp being completely extinguished undesirably. The decrease in the brightness of the lamp takes place gradually in accordance with the progressive cooling of the lamp to a minimum temperature, which finally occurs with the reduced energy supply. Since the control voltage U _R1 depends both on the lamp current and also indirectly on the voltage across the lamp, the minimum current regulation described takes into account the actual power consumption of the high-pressure sodium vapor lamp 20 operated with reduced brightness.

With the help of the second transistor T2 and the control voltage U _R2 obtained by means of the voltage divider R21, R22 is ensured. that the strength of the current flowing through the lamp still has a certain minimum value if the lamp has been controlled for its minimum brightness in the manner described above. The two resistors R21 and R22 are dimensioned such that a voltage equilibrium then prevails at the tap 47, ie the potential with respect to the ground conductor 35 is equal to zero when the current in the supply circuit of the lamp 20 is just sufficient to reduce the lamp after it has been lowered their brightness to the minimum value with certainty to prevent them from completely extinguishing. The mode of operation of the transistor T2 is otherwise analogous to the mode of operation of the transistor T1 as detailed above, depending on the strength of the current flowing through the lamp 20.

In the second exemplary embodiment according to FIG. 2, the resistors R12, R19 and R20 additionally limit the ignition angle adjustment range.

The automatic control described to ensure a given minimum current level in the supply circuit of the high-pressure sodium vapor lamp 20 also comes into effect when the current level would drop below the permissible minimum value for reasons other than by adjusting the potentiometer R1, e.g. in the event of fluctuations in the AC mains voltage or in the event of brief voltage drops.

As in the first exemplary embodiment, the capacitor C8 ensures that a cold start of the high-pressure sodium lamp 20 is readily possible even if the tap 25 of the potentiometer R1 is set to minimum lamp brightness, i.e. is located directly at the end connection 23 of the potentiometer.

If a plurality of high-pressure sodium vapor lamps are to be controlled in terms of their brightness, each of these lamps is assigned its own device 22. The DC control voltage U _s, however, can be generated for all these devices 22 by means of a single potentiometer R1, the end connection 24 and tap 25 of which are connected to the control line connection terminals 32 and 33 of all devices 22. Instead of one For all devices 22 common potentiometers R1, another source for the generation and delivery of the control direct voltage U _s can of course also be provided.

The described embodiments of the device according to the invention are e.g. Suitable for the brightness regulation of mercury vapor lamps or high pressure sodium vapor lamps in interior, roadway and tunnel lighting systems. In addition to regulating the brightness of mercury vapor lamps or high pressure sodium vapor lamps, one or the other embodiment of the device described can also be used to regulate the brightness of other electric gas discharge lamps, depending on their burning characteristics.

Claims (9)

1. An electronic device for controlling the brightness of an electrical gas-discharge lamp (20), having no incandescent cathode, from an altemating-current network, by means of a circuit arrangement (TR1, IC1, D1, R3―R11, C4-C6) for controlling the electrical energy fed to the lamp, said circuit arrangement comprising a triac (TR1) arranged in the lamp supply circuit, and means (IC1) for delivering ignition pulses to the triac (TR1), the ignition angle of the said pulses being adjustable, within each alternating-voltage half-wave, as a function of a d.c. control voltage (U_S), an additional circuit arrangement (30, 42, R12-R17, C7, D3, T1; . R21, R22, D4, T2) being provided for the production of a regulating voltage (U_R; U_R1; U_R2) dependent upon the intensity of the current flowing through the gas-discharge lamp (20), for the purpose of an automatic control of the ignition angle for ensuring a given mimimum current intensity flowing through the said lamp (20), as the energy fed to the lamp is reduced, the said additional circuit arrangement comprising a rectifier arrangement (42) and a current transformer (30), of which the primary winding (L2) lies in the supply circuit to the gas-discharge lamp (20), while the secondary winding (L3) thereof is connected to the input (40, 41) of the said rectifier arrangement (42), the said regulating voltage (U_R; U_R1; U_R2) being derived from the output (43, 44) of the said rectifier arrangement (42), characterized in that the additional circuit arrangement (30, 42, R12-R17, C7, D3, T1; R21, R22, D4, T2) comprises a reference-voltage source (IC1, 35, 37) and a voltage divider (R15, R16, R17; R21, R22), the ends of which are connected to terminals (44, 37), of opposite polarity, of the output of the rectifier arrangement (42), on the one hand, and of the reference-voltage source (IC1, 35, 37) on the other hand, whereas the other output terminals (43, 35) of the said rectifier arrangement (42) and the reference-voltage source, respectively, are connected together; in that the regulating voltage (U_R; U_R1; U_R2) is taken from a tap (45; 47) of the said voltage divider (R15, R16, R17; R21, R22); and in that, by means of at least one electric valve (D3; D4; D5), one polarity of the regulating voltage (U_R, U_R1' U_R2) for the ignition-angle-control is suppressed.

2. A device according to claim 1, characterized in that the additional circuit arrangement comprises a second rectifier arrangement (52), the input (50, 51) of which is in parallel with the triac (TR1) arranged in the lamp supply circuit; and in that a component of the regulating voltage (U_R1), dependent upon the intensity of the current flowing through the lamp (20), is derived from the output (43, 44) of the said first rectifier arrangement (42), while another component of the regulating voltage (U_R1), dependent upon the voltage appearing across the triac (TR1), is derived from the output (53, 54) of the said second rectifier arrangement (52), for the purpose of an automatic con- trot of the said minimum current-intensity flowing through the lamp (20), in adaption to the characteristics thereof.

3. A device according to claim 2, characterized in that the additional circuit arrangement also comprises a second voltage divider (R25, R26); in that output terminals (43, 53), of similar polarity, of the first and second rectifier arrangements (42, 52) are connected to each other and to the output terminal (35), of opposite polarity, of the said reference-voltage source (IC1, 35, 37); in that the other output terminals (44, 54) of the first and second rectifier arrangement (42, 52) are connected to one end respectively of, the first and second voltage dividers (R15, R16, R17; R25, R26), the other ends of the two voltage dividers being connected to each other and to the second output terminal (37) of the said reference-voltage source; and in that each component of the regulating voltage (U_R,) is taken from a tap (45, 55) on the first and second voltage dividers (R15, R16, R17; R25, R26).

4. A device according to one or more of claims 1 to 3, characterized in that the d.c. control voltage (U_s) is fed to a further voltage divider (R8, T1) consisting of a fixed resistor (R8) and a variable resistor (T1) controlled electronically by the regulating voltage (U_R; U_Rl), and in that a junction (39) between the fixed resistor (R8) and the variable resistor (T1) is connected to an ignition-angle-control input (5) of the means (lC1) for delivering the ignition pulses.

5. A device according to claim 4, characterized in that the variable resistance is constituted by the collector-emitter section of a transistor (T1), to the base of which the regulating voltage (U_R), respectively the two components of the regulating voltage (U_R,) are applied; and in that at least one diode (D3, D5) serving as the said electric valve is connected to the base of this transistor (T1).

6. A device according to claim 4 or 5, characterized in that a second variable resistor (T2) is arranged in parallel with the variable resistor (T1), the said second variable resistor being controlled electronically by a second regulating voltage (U_R2) which is dependent solely upon the intensity of the current flowing through the lamp (20).

7. A device according to claim 6, characterized in that the second variable resistor is constituted by the collector-emitter section of a transistor (T2), to the base of which the second regulating voltage (U_R2) is applied, and in that a diode (D4) serving as the said electric valve is connected to the base of this transistor (T2).

8. A device according to claim 7, characterized in that a further voltage divider (R21, R22) is arranged in parallel with the first voltage divider (R15, R16, R17), the second regulating voltage (U_R2) being taken from a tap (47) on the said further voltage divider.

9. A device according to one or more of claims 1 to 8, characterized in that the means (IC1 ) for delivering the ignition pulses, in a manner known per se, delivers pulses, the ignition angle of which is zero in the absence of a control voltage and increases as the control voltage is increased, and in that a charging capacitor (C8) and a charging resistor (R19), associated therewith, are connected to the ignition-angle-control input (5) of the means (lC1) for delivering the ignition pulses, so that when the gas-discharge lamp (20) is switched on, the ignition angle is initially zero and assumes the value determined by the d.c. control voltage (U_S), and the regulating voltage (U_R; U_R1; U_R2), only after a time-delay.

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