Electricity: electrical systems and devices – Safety and protection of systems and devices – Voltage regulator protective circuits
Reexamination Certificate
2002-02-11
2003-11-18
Sircus, Brian (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Voltage regulator protective circuits
C361S018000, C315S291000
Reexamination Certificate
active
06650514
ABSTRACT:
DESCRIPTION
The present invention relates to a protective circuit for a fluorescent lamp with a first and a second lamp filament, comprising a DC voltage source with a positive and a negative pole, a half-bridge arrangement with a first and a second switch, the half-bridge arrangement being fed by the DC voltage source, and the first and the second switch being interconnected to form a first reference point, the first reference point being connected to the negative pole via a first resistor, a decoupling capacitor that is arranged in a serial connection between the half-bridge arrangement and the first or the second lamp filament, the connection of the decoupling capacitor on the filament side forming a second reference point that is connected to the negative pole via a second resistor, a comparator that has a first and a second input and an output, the first input being connected to the first reference point, and the second input being connected to the second reference point, the output being connected to the negative pole via a detection capacitor and an evaluation circuit with the aid of which the voltage dropping across the detection capacitor can be evaluated in order to deactivate the half-bridge arrangement upon overshooting of a predetermined voltage level.
PRIOR ART
Such a protective circuit is known and is, for example, installed by the Applicant of the present invention in ballasts for fluorescent lamps. The protective circuit consists in evaluating at the end of the service life of the fluorescent lamp, that is to say when the lamp is not yet defective, a criterion that leads in good time before overheating in the filament region (risk of fusing of the base) to a shutdown of the half-bridge arrangement (also known as end-of-life shutdown). Use is made in this case of the fact that the filaments of a fluorescent lamp are covered with emitter in order to reduce the work function of the electrons. In the closure phase, the absence of the emitter on one of the two lamp filaments of the fluorescent lamp becomes noticeable by virtue of the fact that the work function slowly increases again, and the voltage dropping across the decoupling capacitor changes thereby. In normal operation, that is to say when both filaments still have emitter, the two reference points lie on average at a potential that corresponds to half the voltage which is made available by the DC voltage source. At the end of service life, the second of the two reference points lies at a different potential, and the reference points are therefore at different potentials. The potential difference is used in order to charge a detection capacitor, the evaluation circuit advantageously being realized such that it is possible to set a voltage level upon the overshooting of which a deactivation of the half-bridge arrangement is effected.
The term “relamping” is known in conjunction with the replacement of a defective lamp. In the case of a lighting system comprising a plurality of lamps, this is understood as making possible the use of a new lamp without the need to switch off the supply voltage and therefore switch off the other lamps. However, the aim is to ensure that the line voltage connected during the entire replacement operation causes the newly inserted lamp to come on again immediately. Circuit structures are also known for this purpose. The disadvantage in the mode of procedure of the prior art resides in that the additional realization of the relamping function makes mass produced ballasts substantially more expensive, for which reason they are frequently omitted. The result is therefore expensive ballasts for which an end-of-life detection and relamping are realized, and there is a second category of ballasts, for which only the end-of-life detection is realized. In the case of the last mentioned ballasts, it is necessary, for example, for all the lamps to be switched off when, for example, replacing a fluorescent lamp in a factory hall, in order thereby to reset the end-of-life detection. Only after all the lamps have been switched off can a new lamp be inserted instead of the aging lamp. Subsequently, all the lamps can be switched on again. Such interruptions are undesirable, especially in large factory halls.
SUMMARY OF THE INVENTION
The object of the present invention therefore consists in making available a cost-effective realization of the end-of-life detection, and of the relamping function.
This object is achieved according to the invention by virtue of the fact that the generic protective circuit also has a third resistor that bridges the decoupling capacitor, and a fourth resistor that connects the first reference point to the positive pole of the DC voltage source, the first, the second, the third and the fourth resistor being selected such that the first and the second reference point are at the same potential without the fluorescent lamp inserted.
The invention is based on the idea of designing the end-of-life detection circuit or realizing the relamping function such that as many components as possible are used jointly. It is thereby possible in the case of a mass produced product such as the present protective circuit to realize the relamping additional function cost-effectively virtually without additional outlay, the result being a very desirable price reduction.
Here, the idea consists in that, with the fluorescent lamp removed, two inputs of the comparator, which detects asymmetry, are supplied with identical potentials which reset the switching-off of the half-bridge arrangement.
As already mentioned above, the two reference points lie on average at half the potential of the DC voltage made available by the DC voltage source. This is usually what is termed the DC link voltage, and is usually provided at a DC link capacitor. In a particularly preferred realization of the invention, the ratio of a first resistor to the fourth resistor is of the same magnitude as the ratio of the second resistor to the third resistor. Particularly in the case when the ratio is selected as 1, even with the fluorescent lamp removed, the two reference points lie at a potential that corresponds to half the DC voltage made available by the DC voltage source.
It is also preferred to use suitably dimensioned voltage dividers to apply only a lower voltage to the comparator. This results in a further cost reduction. For this purpose, the first resistor comprises a first and a second component resistor connected together in series, and the second resistor comprises a third and a fourth component resistor connected together in series, the first reference point being connected to the tie point of the first component resistor and the fourth resistor, and the second reference point being connected to the tie point of the third resistor and the third component resistor, and the first input of the comparator being connected to the tie point between the first and second component resistor, and the second input of the comparator being connected to the tie point between the third and the fourth component resistor. It is not necessary in this embodiment for all the resistors of the voltage dividers to be designed as high-voltage resistors. The comparator and evaluation circuits need likewise only be suitable for low voltage. However, it is sufficient to provide one high-voltage resistor per voltage divider, which results in a further cost reduction.
It is preferred for the ratio of the sum of the first and second component resistor to the fourth resistor to be equal to the ratio of the sum of the third and fourth component resistor to the third resistor. In the case in which the ratios are again selected as one, with the fluorescent lamp removed, the two reference points lie in turn at a potential that corresponds to half the DC voltage made available by the DC voltage source.
A particularly expedient realization of the comparator provides that the comparator comprises a first and a second switching element, which in each case comprise a working, a control and a reference electrode, the fourth component resistor compr
Nguyen Danny
Patent-Treuhand-Gesellschaft für Elektrische Gluehlampen mbH
Sircus Brian
LandOfFree
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