Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Periodic switch cut-out
Reexamination Certificate
1998-04-22
2001-02-27
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Periodic switch cut-out
C315S119000, C315S127000
Reexamination Certificate
active
06194842
ABSTRACT:
Be it known that Antonio Canova, residing at Via Po No.79/A, Montevarchi, Arezzo, Italy, 52025, a citizen of Italy, has invented a new and useful “Supply Circuit For Discharge Lamps With Overvoltage Protection.”
BACKGROUND OF THE INVENTION
The present invention relates to a supply circuit with inverter for discharge lamps. More particularly the present invention relates to a supply circuit for discharge lamps with heated electrodes, in which an inverter comprising a set of electronic switches, turned on and off alternately, supply a load circuit having at least one lamp and an LC resonant circuit in series with the lamp.
Systems for controlling the voltage on the electrodes of a discharge lamp and which have the objective of modifying the behavior of the load circuit or of turning off the supply thereto in case of defective operation of the lamp, are often used in supply circuits of the type mentioned above, with the objective of preventing excessively high voltages from arising between the electrodes.
From European Patent Application EP-A-0 610 642, there is known a supply circuit with an inverter for discharge lamps, in which associated with the load circuit is a control circuit comprising a voltage-dependent resistor (VDR) in series with a dissipative element. When the voltage at one terminal of the VDR exceeds a threshold value (which occurs in the case of failure of the lamp to light following a defect therein, for example), the VDR becomes conducting with the consequence that the resonant circuit in series with the lamp receives an additional dissipative element. This modifies the quality factor of the circuit and hence reduces the voltage at the terminals of the lamp. Provision is further made for a timer circuit which turns off the supply to the load circuit should the overvoltage condition last for a time greater than a pre-set threshold value.
From European Patent Application EP-A-0 113 451, there is known a different overvoltage control system, in which a voltage-dependent resistor in series with a capacitor are inserted in parallel with a branch of the load circuit. In this case, when a voltage difference greater than a specified threshold value is generated between the terminals of the VDR, it becomes conducting and inserts an auxiliary capacitor into the load circuit, modifying the frequency of resonance of the resonant circuit in series with the lamp.
Traditional circuits for protection from overvoltages come into operation when the voltage between the electrodes of the lamp exceeds a threshold value. In the case of a defective lamp, and hence of the failure of this lamp to light, the voltage between the electrodes of the lamp reaches values of the order of 1000 V. Conversely, when the lamp is removed from the load circuit, the potential difference between the electrodes is of the order of 700 V. The circuits currently available are unable to discriminate between these two voltage values, which may moreover vary from one instance to another of the circuit. Consequently, the overvoltage circuits cut in anyway, turning off the supply with a pre-specified time (of the order of 300 ms) having elapsed from the onset of the establishment of a situation of overvoltage between the electrodes of the lamp. They are further unable to distinguish between the two conditions of a defective lamp and a removed lamp.
On the other hand, it would be appropriate to make provisions for a circuit which is able to discriminate between a situation of actual defective operation and a situation where a lamp is removed from the load circuit in such a way that the substitution of the lamp does not entail the disabling of the supply inverter of the load circuit. Thus, in currently known circuits, the disabling of the supply inverter is permanent and hence requires the intervention of the operator in order to reactivate the supply for the lamp, even when the disabling has occurred through simple substitution of the lamp rather than through a defect in operation thereof. On the other hand, a high current flows in the case of overvoltage due to a defect in the operation of the lamp in the load circuit which passes through the capacitor in parallel with the lamp. This anomalous condition may lead to the over-stressing of the inverter and hence to damage thereto. Protection from overvoltage has the objective of preventing this consequence. Conversely, when the voltage in the load circuit increases on account of the lamp being removed, the current which flows in the circuit is practically zero, and hence the inverter does not experience the dangerous stresses which occur under conditions of a faulty lamp.
SUMMARY OF THE INVENTION
The objective of the present invention is the construction of a supply circuit for discharge lamps, with an overvoltage protection system, not exhibiting the drawbacks of the traditional circuits briefly described above.
More particularly the objective of the present invention is the construction of a supply circuit for discharge lamps with an overvoltage protection circuit which is able to discriminate between the conditions of failure to light on account of a defective lamp and the conditions of a removed lamp, and which will cut off the supply only when necessary, i.e., in the case of a defective lamp.
Essentially, starting from a supply circuit of the type defined above, the objectives indicated above, and other objectives and advantages which will become clear to those skilled in the art by reading the text which follows, are achieved by making provision for the overvoltage control circuit to comprise a band-pass filter centered on the switching frequency of the inverter, the input signal of which is dependent on the voltage at a specified point of the load circuit and the output signal from which is sent to control means associated with the inverter so as to turn off the supply for the load circuit in the case of defective operation.
The invention is based on the observation that, although the amplitude of the voltage between the electrodes of the lamp is of the same order of magnitude both in the case of defective operation and in the case of a removed lamp, the waveform of the voltage signal is, conversely, qualitatively different in the two cases. In case of failure to light through a defect in the lamp, the voltage between its electrodes has a substantially sinusoidal profile with a frequency corresponding to the switching frequency of the inverter. In case of a removed lamp, conversely, the waveform of the voltage signal at the terminals of the lamp exhibits, as well as a relatively limited component at the switching frequency, a strong signal content at the higher harmonics.
By detecting this voltage signal and filtering it through a band-pass filter centered on the switching frequency, a signal is therefore obtained at the output of the band-pass filter. This signal will be a high signal when the overvoltage established between the electrodes is due to defective operation of the lamp inserted into the load circuit. This signal will, conversely, be low when the overvoltage established between the electrodes is due to a removed lamp. In this last case, in fact, the signal component at the switching frequency is of modest strength with respect to the components at the higher harmonics, which are blocked by the band-pass filter.
The control circuit thereby becomes capable of discriminating between the conditions of defective operation and the conditions of a removed lamp and will be able, with suitable logic, to intervene on the supply inverter, selectively disabling the operation thereof.
Further and advantageous characteristics and embodiments of the circuit according to the invention are indicated in the appended claims and will be described in greater detail below.
In particular, a voltage divider, to which the band-pass filter is linked, can be arranged in parallel with a branch containing at least one component of the load circuit (for example the inductive component). More particularly the voltage divider can be placed in parallel with a branch comprisin
MagneTek, S.P.A.
Patterson Mark J.
Vu David
Waddey & Patterson
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