Patent Application
20080185968
The present invention relates to a method for operating a gas discharge lamp, a gas discharge lamp, and a lamp driver circuit. In particular, the present invention relates to a method for controlling a lamp life of a gas discharge lamp, and to a gas discharge lamp, a lamp driver circuit, and an assembly thereof configured for performing said method.
It is known that the lamp life of a low-pressure gas discharge fluorescent lamp, such as a TL-lamp or a CFL-lamp, depends inter alia on the deterioration of the electrodes of the lamp, since the condition of the electrodes deteriorates during operation. However, the amount of damage to the electrodes depends mainly on an operating temperature of the electrodes. Two types of damage govern the deterioration of the electrodes: sputter damage and evaporation damage.
A sputter damage rate typically is large when the electrode is relatively cold, but the damage rate decreases with increasing temperature. When the temperature is high enough for thermionic emission, the sputter damage rate becomes small.
An evaporation damage rate increases with increasing temperature. At a certain temperature, the sputter damage rate becomes negligible with respect to the evaporation damage rate.
The electrode temperature is at an optimum with respect to the lifetime of the lamp when the sum of the sputter damage rate and the evaporation damage rate, i.e. a total damage rate, is at a minimum. In practice, with electrodes made of a coiled tungsten wire covered with a mixture of Ca, Ba and Sr oxides, the total damage rate is relatively small when the temperature lies in an operating temperature range, i.e. between certain boundaries T1 and T2. For ignition the operating temperature may be in the range of about 900 K-1000 K, and the total damage is at a minimum at a temperature of about 950 K.
For the above-mentioned kind of electrodes it is known that thermionic emission occurs at temperatures which correspond to a resistance ratio of tungsten
in which Rcold ld represents the electrical resistance at room temperature and Rhot the electrical resistance at the operating temperature.
Conventionally, using the above technical considerations, lamp driver circuits and gas discharge lamps are standardized and designed such that the electrodes may be preheated before ignition such that the resistance ratio is about 4.75.
Similar considerations lead to similar rules for steady-state operation. The temperature range, however, may be different. An appropriate spot temperature for the above-indicated coated tungsten wire may be in the range 1400-1600 K, while the rest of the electrode may be at lower temperature. According to empirical findings lamp life is high for lamps without additional heating current when the lamp current Ilamp is in the range 1-1.5 times the current IR4, IR4 being the current at which the above-mentioned resistance ratio is 4 (measured for the electrode without discharge).
Although the average lifetime of the lamps may be acceptable with such a standardized lamp and driver circuit, the lifetime of individual lamps may be shorter than expected e.g. due to manufacturing tolerances and manufacturer differences.
It is desirable to have a method for optimizing a lifetime of a gas discharge lamp. Further, it is desirable to have a gas discharge lamp and lamp driver circuit for performing said method.
In an aspect of the present invention there is provided a method for controlling a life time of a gas discharge lamp, the method comprising: providing a temperature signal representing a temperature of an electrode of said gas discharge lamp to a lamp driver circuit operating said lamp; and controlling at least one operating signal supplied by said lamp driver circuit to said lamp in response to said temperature signal for controlling the electrode temperature to lie within a predetermined temperature range.
In particular in applications where replacement of defect lamps may be difficult and/or expensive, it is advantageous to control the temperature of the electrodes, since controlled electrode temperature may result in an increased lamp life.
In an embodiment of the method, the temperature signal corresponds to a cathode fall voltage. In an embodiment the cathode fall voltage may be determined by a conducting band positioned around the lamp, said cathode fall voltage being determined by measuring a potential of said conducting band. In a further embodiment, the potential of a lamp cap near the electrode is determined in order to determine the cathode fall voltage. In another embodiment, the cathode fall voltage is determined by measuring a potential of an electrode shield. Such an electrode shield is provided in some known lamps.
In another embodiment the temperature signal corresponds to a electrode coil voltage. The electrode coil referred to is a coiled tungsten electrode, for example. The voltage drop over the electrode coil is, for a given discharge current and a given heating current, a measure for the effective coil resistance, and thereby for the effective coil temperature. Thus, in this embodiment, the voltage over the electrode coil is used as the temperature signal.
In a further embodiment, the cathode fall voltage and the electrode coil voltage are used in combination as a temperature signal. The cathode fall voltage may be more accurate for determining a temperature of a cold electrode, whereas the electrode coil voltage may be more suited to determine a temperature of a hot electrode. Using both signals enables an accurate measurement for both a cold and a hot electrode.
In response to a cold or a hot electrode, the control circuit of the lamp driver circuit controls at least one operating signal. In an embodiment, the at least one operating signal is a heating current supplied to the lamp. The heating current is an operating signal known in the art for heating the electrode and keeping the electrode at a suitable temperature. If the temperature indicated by the temperature signal is not at a desired level, the control circuit may adjust the heating current to adjust the temperature. In particular, if the temperature is below a desired temperature, the heating current is increased; if the temperature is above a desired temperature, the heating current is decreased, if possible.
In another embodiment an electrical connection between an electrode shield and a current carrying lead wire is controlled in response to the temperature signal. For a given discharge current the electrode temperature is lower when the electrode shield is connected to a lead wire, in particular the current carrying lead wire, compared to a situation without a connection. The lead wires provide a current (discharge and heating current) to the electrode. The current carrying lead wire is the one of the two lead wires that carries the highest current (discharge current and heating current; the other lead wire carries the lowest current, possibly only the heating current, if present). As mentioned above, establishing a connection between the current carrying lead wire and the electrode shield is in particular suitable for lowering the electrode temperature.
In a further embodiment the lamp driver circuit controls a variable impedance element connected between the electrode shield and the current carrying lead wire. By controlling a variation of the impedance, the temperature may be controlled. In particular, if the temperature is above a predefined temperature, the impedance is decreased, if possible. In a further aspect of the present invention, there is provided an assembly of a low-pressure gas-discharge fluorescent lamp and a lamp driver circuit for performing the method according to the present invention. The lamp driver circuit and the gas discharge lamp are electrically connected for supplying at least one operating signal from the lamp driver circuit to the lamp and for supplying at least one temperature signal representing an electrode temperature from the lamp to the lamp driver circuit. The lamp driver circuit comprises a control circuit for controlling the at least one operating signal in response to said temperature signal for controlling the electrode temperature to lie within a predetermined temperature range.
In another aspect, the present invention provides a gas discharge lamp for use in said assembly. In an embodiment, such a lamp may comprise a conducting band positioned around the lamp for determining a cathode fall voltage by measuring a voltage of said conducting band.
In another embodiment of the lamp an electrode shield is provided around the electrode of the lamp and a feed through conductive wire provides an electrical connection between a terminal on the outside of the lamp and the electrode shield for electrically connecting the lamp driver circuit and the electrode shield.
In yet another embodiment of the lamp a switching element is connected between an electrode shield and a lead wire and is electrically connectable to the lamp driver circuit for making an electrical connection between the electrode shield and the lead wire in response to an operating signal provided by the lamp driver circuit.
In a further embodiment of the lamp according to the present invention, a controllable variable impedance element is connected between an electrode shield and a lead wire and is electrically connectable to the lamp driver circuit for making an electrical connection between the electrode shield and the lead wire by controlling the impedance of the variable impedance element by the lamp driver circuit.
In another embodiment of the lamp, the gas discharge lamp is provided with said feed through wire for connecting the electrode shield and the lamp driver circuit and is provided with the variable impedance element connected between the electrode shield and the current carrying lead wire. The impedance may be controlled by the lamp driver circuit. As mentioned above, a combination of the above-described measures provides more accurate control and thus a longer lamp life.
In another aspect, the present invention provides a lamp driver circuit for use in said assembly. The lamp driver circuit comprises a control circuit for generating an operating signal in response to a temperature signal.
In an embodiment of the lamp driver circuit, the operating signal is a heating current. In another embodiment of the lamp driver circuit, the operating signal is a switch signal for controlling a switch of the lamp connected between an electrode shield and a lead wire. In yet another embodiment, the operating signal is an impedance signal for controlling a variable impedance element of the lamp connected between an electrode shield and a lead wire.
These and other aspects of the present invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
The annexed drawing shows a non-limiting exemplary embodiment, wherein:
In the drawings, identical reference numerals indicate similar components or components with a similar function.
The temperature signal 12 is supplied to the control circuit 42 comprised in the lamp driver circuit 4. In response to the temperature signal 12, the control circuit 42 may adjust at least one of the operating signals 10.
The electrode temperature may be determined using different practical embodiments. A number of such exemplary embodiments are illustrated in
In operation, a potential is generated on the conducting band 70. The lamp driver circuit may detect said potential as a voltage compared to ground or, more accurately, compared to the floating, i.e. not-grounded contact terminal 62. The detected voltage is a measure for the cathode fall voltage. The cathode fall voltage is a measure for the temperature of the electrode 63. Thus, in this embodiment, the conducting band 70 may generate a suitable temperature signal to be supplied to the lamp driver circuit.
The embodiment of
Referring to
The cathode fall voltage is a more accurate measure of the temperature for determining whether the electrode is cold, i.e. has a temperature that in operation results in more or less severe sputter damage. The electrode coil resistance is a more accurate measure of the temperature for determining whether the electrode is hot, i.e. has a temperature that in operation results in more or less severe evaporation damage. Therefore, in a practical embodiment, the cathode fall voltage and the electrode resistance may be determined using one of the embodiments according to
A control circuit comprised in a lamp driver circuit receiving one or more of the above-indicated temperature signals (a cathode fall voltage signal and an electrode resistance signal) may need to heat or cool the electrode of the gas discharge lamp in order to bring the temperature of the electrode at a desired temperature. The desired temperature may be a temperature within a predefined temperature range T1-T2, or it may be a predefined optimum or near-optimum temperature Topt, for example.
To heat the electrode, it is known to provide a heating current to the electrode. Thus, the control circuit may control the heating current. Increase of the heating current results in an increase of the temperature and a decrease of the heating current results in a decrease of the temperature.
In practice, it has been shown that making a connection between the electrode shield 75 and the current carrying lead wire 61 results in a decrease in temperature of the electrode 63. Thereto, the element 65 may be a switch for providing a connection, or a disconnection, or the element 65 may be a variable impedance element. The variable impedance (resistance) of the connection between the electrode shield 75 and the current carrying lead wire 61 provides a control range for adjusting the temperature of the electrode 63.
Above, in relation to
In an embodiment, the connecting element for connecting the electrode shield and the current carrying lead wire may be comprised in the lamp driver circuit. With an electrical connection between the electrode shield and the lamp driver circuit for determining a cathode fall voltage, it is possible to connect one of the contact terminals configured to receive a supply voltage or current to the electrode shield in the lamp driver circuit. Thus, in such an embodiment, the lamp is provided with a terminal connected to a feed through wire for enabling an electrical connection between the lamp driver circuit and the electrode shield; the lamp driver circuit or the control circuit thereof being provided with the connecting element for making a connection between the current carrying lead wire and the electrode shield.
The method may be performed by defining an upper limit and a lower limit for the temperature, and only adjusting the operating signals when the temperature does not lie within the range defined by said lower and upper limit. As well, the method may be performed by continuously controlling the operating signals in order to control the temperature of the electrode such that it is at or near a predefined optimum temperature at any time. A person skilled in the art will readily recognize these and other methods as suitable control methods for performing the method according to the present invention.
In the above description as well as in the appended claims, ‘comprising’ is to be understood as not excluding other elements or steps and ‘a’ or ‘an’ does not exclude a plurality. Further, any reference signs in the claims shall not be construed as limiting the scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 05102618.5 | Apr 2005 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2006/050918 | 3/27/2006 | WO | 00 | 9/28/2007 |
