DE1061438B - Operating arrangement for preheating and current limitation of fluorescent lamps - Google Patents

Description

Fluorescent lamps are also increasingly being used to illuminate passenger coaches and vehicles of all types in which only a DC voltage is available. The DC voltage must therefore first be converted into an alternating voltage of the appropriate level and frequency to normal To be able to feed fluorescent lamps with it. Machine converters or inverters are used for this. The latter, with predominantly inverters with rotating mercury beams or, more recently, also transistor inverters are used, supply an alternating voltage with a rectangular waveform.

The advantages of the operating arrangement according to the invention now come in particular in connection with a transistor inverter to its full advantage.

The fluorescent lamp has a falling characteristic curve and therefore requires a series resistor for stable operation. In order to avoid the losses of an ohmic series resistor, a choke is usually used for AC voltage. In the case of a sinusoidal curve shape of the voltage, the current lags ^{the voltage by about 6Q C} , ie the power factor is about 0.5. This can be raised again to about 1 by means of a compensation capacitor.

When using a transistor inverter, it is important to avoid reactive power, because otherwise harmful reverse currents flow and voltage peaks and increased losses occur .. As a result of these effects the utilization of the transistors would be significantly reduced. Even the well-known one at Inverter with rotating mercury beam applied circuit, in which the harmonics, the square wave voltage are screened out and the reactive current is compensated, is for the transistor inverter unfavorable, since harmonic currents flow through the inverter, especially with partial load, which increase Generate losses.

In the operating arrangement according to the invention, the fluorescent lamp current is intended to be limited by a transistor ballast instead of the previously common choke. This results in an ohmic load for the inverter with all the advantages set out above without increased losses. For this purpose, at least one transistor in the current limiting circuit and at least one transistor in the preheating circuit are connected upstream in an operating arrangement for preheating and current limiting of fluorescent lamps in connection with a generator of the fluorescent lamp which emits a voltage with a rectangular waveform Potentiometer, a control transistor and a further operating arrangement connected between the collector and emitter
for preheating and current limitation of fluorescent lamps

Applicant;

ίο LICENTIA Patent-Verwaltungs-GmbH _r Hamburg 36, Hohe Bleichen 22

Dipl.-Ing. Claus §chtmema.nn, BerlinaWilmersdorf,
has been named as ErjEirider

Potentiometer is regulated to keep the collector-emitter voltage constant.

With the help of FIGS. 1 to 5 of the drawing, the arrangement according to the invention will be explained in more detail.

1 b shows the basic characteristic curve a of a fluorescent lamp (also denoted by the same letter in FIGS. 3 b, 3 a and 3 d) with a drawn resistance line b. The lamp 16 works here with an ohmic series resistor; Fig. La shows the associated circuit. From the two intersection points A and B of the two curves in Fig. Ib, stable operation of the circuit is only possible at point B , since if the current were to increase, the voltage at the series resistor R would be greater than the difference between the nominal voltage and the operating voltage available for this purpose , so * - Ug _r (see Fig. lb). A substantial part of the mains voltage is therefore applied to the series resistor R _1, ie the efficiency of the arrangement is very poor.

Any non-linear resistor whose characteristic curve only has to intersect the lamp characteristic curve once from above (point B in FIG. 1 b) could also be used as the series resistor. Due to its kinked characteristic curve, the transistor fulfills this condition, so that, on the one hand, an efficiency of the system of practically 100% is achieved and, on the other hand, the voltage source is subjected to a purely ohmic load.

Fig. 2a shows the characteristics of a flat transistor suitable for the present purpose in an emitter circuit, namely the collector current as a function of the collector-emitter voltage at different base currents as a parameter, ie Ic -f {U ce ) iß- Pig-2b is the characteristic curve base current I _b as a function of the basds-emitter-span

909 577/188

Claims (1)

1 voltage UBE, that is Ib = f (Ube), and in Fig. 2c the known emitter circuit is shown. In FIG. 3 b, the characteristic curves of FIG. 2 a are plotted together with the characteristic curve of the fluorescent lamp a, with the same nominal voltage Un as in FIG. 1b. Fig. 3a shows the associated basic circuit with a DC voltage as the mains voltage. For reasons of good efficiency and a low load on the transistor, the nominal voltage Un must now be reduced to a value somewhat above the operating voltage UBr (FIG. 3 c). The transistor characteristic intersects the lamp characteristic almost vertically from above, and thus complete stability is guaranteed. Although only about the operating voltage is now available as the mains voltage and not more than twice as high a voltage as before in normal operation with a choke, the lamp, once in operation, will ignite again after each zero crossing of the voltage, because of the rectangular shape of the curve the time of no voltage is small compared to the deionization time of the lamp. In practical operation, the mains voltage is by no means constant, nor are the transistor and lamp characteristics each uniform. Thus, with a fixed transistor characteristic, i.e. with fixed modulation JB, the operating state of FIG. 3c will not be established from the outset, but there may be no intersection of the curves at all, or an impermissibly high voltage drop UCE is present at the transistor. As FIG. 3 b shows, it is now possible through various modulation of the transistor to shift the point of intersection of the transistor characteristic with the lamp characteristic and thus to change the voltage drop Uce across the transistor. If, for example, the mains voltage increases, then, as shown in FIG. 3 d, if the limiting transistor (FIG. 3 a) is rigidly driven, the operating point would remain at Ci1 and the transistor would be considerably loaded by the increased collector-emitter voltage Uce. By changing the base current from JB1 to Tb2, the most favorable operating point bt can be set again. Here, the collector current changes from Ial to J6r. This setting should take place automatically and is explained in connection with the exemplary embodiment according to FIG. The conversion of the direct voltage of a vehicle into alternating voltage by means of transistor inverters can now be done either in such a way that each lamp is assigned its own inverter as a ballast or that a common inverter feeds all lamps. Fig. 4 shows a practical embodiment of a circuit of the transistor current limiter and preheater in connection with a single inverter. Part 1 is a well-known transistor inverter that will not be described further. Depending on the direction of the half-wave, the current is fed back from the output transformer 1 a via the two valves 2 a and 2 b to the intermediate transformer 3 and via both center taps and the actual current limiting transistor 4. The arrangement with Zwischenentransformato.r and center tap is chosen to get by with only one limiter transistor and also to keep the current in the intermediate circuit and thus the transistor small by means of suitable transformation ratios. Via the voltage divider 5, 6, which is connected to the DC mains supply voltage Un, the base of the transistor 4 is controlled so far that a desired voltage Uce of about 1 V is applied to the transistor at the low mains voltage UNmin. If the mains voltage now increases, the voltage Uce at the transistor 4 will also increase (see FIG. 3 d). The base of the control transistor 9 is now controlled via the voltage divider 7, 8, namely due to the curvature of the [7ß £ - / ß characteristic of the transistor (see Fig. 2 b) in such a way that only when the voltage Uce is exceeded 1 V a base current flows. The resistance at the path g-h of the transistor 9 is thus smaller. Thus, the voltage at part g-h of voltage divider 5, 6 is also smaller, and the modulation of limiter transistor 4 decreases. The point of intersection of the two characteristics in FIG. 3d moves to the left, and the voltage Uce at transistor 4 drops again. To ignite the lamp, which has to be started without a starter in the absence of a suitably high ignition voltage, the electrodes of the lamp are preheated in a known manner; this is done by heating transformer 10, which, like intermediate transformer 3, is connected to inverter output transformer la via valves 11a, 11b and heating current switching transistor 12. The transistor 12 blocks the heating current in that the voltage on the path g-h drops below the value required for the modulation, as soon as the lamp has ignited and the main current flows. The sudden complete blocking is achieved by interposing the non-linear resistor or valve with a kinked characteristic curve 18 in the basic circuit. The ignition itself can take place in a known manner in that a high voltage is applied between the supply network and an ignition electrode attached to the outer surface of the lamp. The use of small and inexpensive transistors is possible through a suitable design of the transformation ratios of the transformers and through the regulation described. When all lamps are fed by a single, common inverter, each via two-pole supply lines, the center point c of the transformer windings required for the mode of operation can be produced by connecting an autotransformer 13 upstream, as shown in FIG. The DC voltage required for modulating the transistors can then, for. B. can be taken via an auxiliary winding 15 of the autotransformer 13 and rectifier 14. If three-pole feed lines are possible when the lamps are fed by a common inverter, each complete set of lamps can be connected in parallel to the inverter output transformer 1a, (FIG. 4) directly at points d, c and e. In this case, the DC voltage required for the transistor modulation must be derived, for example, by an auxiliary winding from the intermediate transformer 3 via a rectifier, similar to that shown in FIG. The autotransformer is then unnecessary. Patent claims: 1. Operating arrangement for preheating and current limitation of fluorescent lamps in connection with a generator outputting a voltage with a rectangular waveform, characterized in that that the fluorescent lamp has at least one transistor in the current limiting circuit and