Electric lamp and discharge devices: systems – Current and/or voltage regulation
Reexamination Certificate
2000-07-05
2001-05-22
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
C315S224000, C315S289000, C315S287000, C315SDIG007
Reexamination Certificate
active
06236169
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a supply circuit for a fluorescent tube installation with a high voltage transformer.
STATE OF THE ART
Special transformers, so called neon transformers, are used for the operation of high voltage fluorescent tubes, particularly those for neon signs or display systems. They are constructed as stray field transformers i.e. with an air gap, so that they can both produce the high voltage necessary for ignition of the fluorescent tube, and also deliver the tube voltage necessary for long service life.
When referring to operational frequency, a distinction is made between low frequency installations, which run directly with the normal 50 or 60 Hz network frequency of the mains alternating voltage, and high frequency installations for frequencies from around 12,000 Hz upwards.
As the size of inductive parts decreases with frequency, it is possible to build high frequency installations that are considerably more compact and lighter than low frequency installations because of the transformer size. However, EMC problems (EMC stands for electromagnetic compatibility) occur with high frequency installations, since fluorescent tubes inevitably function as antennae. According to the regulations in force, this limits the installable efficacy with lasting effect. There are also restrictions as far as the connecting wires are concerned. Due to the EMC problem they have to be shielded. The resulting impedance limits wire length to about 10 m. Additional problems arise with certain wire lengths due to the formation of standing waves.
There are various reasons why no solutions exist for frequencies covering the range from low frequency areas above mains frequency up to the high frequencies mentioned. Among other reasons is the fact that losses at the air gap in the transformer and frequency connected physical consequences sharply increase. The physical problems become especially critical around 800 Hz. With increasing frequency the buzzing arising from the stray field transformer increasingly becomes a whistling. This whistling becomes particularly disturbing in the 1000 Hz range, since not only is a 1000 Hz tone found to be unpleasant, but the sensitivity of the human ear is at its greatest in this frequency range.
Neon installations are divided into different classes according to practical demands (above all regarding lighting efficacy and length of fluorescent tubes), and are classified with regard to voltage level and tube current. At present there are standards defined for 72 efficacy classes covering twelve different voltages between 750 V. and 15,000 V., each with eight values of current. A special transformer is used for each voltage and each current, optimised according to the construction size, weight and materials used.
Older models of transformer were sometimes equipped with an air gap regulator on the air gap so the transformer could be adjusted for use at different current levels. However, this type of transformer was considerably larger and heavier than types of transformer used at present and could not gain acceptance over non-adjustable models because of the extra cost.
DESCRIPTION OF THE INVENTION
The invention has the object of describing a supply circuit for a fluorescent tube with a high voltage transformer in the lower frequency range which can be made more simply, more economically and smaller than mains frequency installations commonly used at present, and where the mentioned wide variety of different types of transformers is reduced.
This object is achieved according to the invention by a supply circuit having the characteristics given in patent claim
1
.
The circuit according to the invention is characterised in that, the transformer is connected on the primary side to an input voltage with a frequency of several hundred Hz, but is constructed without an air gap. An electronic control which emulates the current limiting function of an air gap is provided in the input circuit of the transformer to replace the air gap in the transformer.
As the input voltage has a frequency that is several hundred Hz greater than mains frequency, and which according to claim
2
is particularly between 300 and 800 Hz, preferably 400 Hz, the size and weight of the transformer can be substantially reduced.
The increase in transformer losses to be expected due to the frequency increase is avoided by the use of a transformer without an air gap, together with electronic emulation of the current limiting function of an air gap.
Additionally, the inevitable physical problems mentioned which occur from 800 Hz are avoided by the choice of a frequency in the given range, particularly by using a frequency of 400 Hz.
It is also particularly advantageous that the present multiplicity of neon transformer types can be considerably reduced through electronic current limitation or current regulation.
A factor in support of a frequency of 400 Hz is that suitable components are known in aircraft construction and can be more or less adopted for use from that source.
A further final advantage of using a transformer without an air gap is that transformers without air gaps do not produce a buzzing sound.
Preferred forms are described in the additional dependent claims.
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Mu-Shen Lin et al., Primary-side dimming control driver for cold-cathode fluorescent lamps, Electronic Letters Jul. 18, 1996, vol. 32, No. 15, pp. 1334-1335.
Burtscher Heinz
Hug Alfred
Meier Patrick
Moser Roman
Amteca AG
Morgan & Lewis & Bockius, LLP
Philogene Haissa
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