Apparatus and methods for making capacitive measurements of cathode fall in fluorescent lamps
View Patent ↗Apparatus and methods for measuring cathode fall in fluorescent lamps are disclosed. Together with measurements of cathode temperature, such measurements of cathode fall may inform a determination of cathode heater voltage as a function of discharge current (i.e., a cathode-heating-profile) that avoids both sputtering and excess-evaporation.
1. A method for making capacitive measurements of cathode fall in a fluorescent lamp, the fluorescent lamp containing an electrode, the method comprising:
electrically coupling to the electrode an alternating-current power supply that supplies a discharge current to the electrode;
electrically coupling to the electrode a direct-current power supply that supplies a heating current to the electrode;
surrounding a portion of the lamp that contains the electrode with an electrically conductive sleeve;
electrically coupling a first input terminal of an operational amplifier to the conductive sleeve;
electrically coupling an output terminal of the operational amplifier to a second input terminal of the operational amplifier; and
determining a cathode fall magnitude from an electrical signal provided by the output terminal of the operational amplifier.
2. The method of claim 1 , wherein the direct-current power supply has a negative terminal that shares a common ground with a terminal of the alternating-current power supply.
3. The method of claim 1 , wherein the electrical signal represents a potential of the sleeve.
4. The method of claim 1 , further comprising:
determining from the electrical signal a lower limit for at least one of cathode heater voltage and cathode heater current at a reduced lamp discharge current.
5. The method of claim 1 , further comprising:
electrically connecting the output terminal of the operational amplifier to an input terminal of an oscilloscope.
6. The method of claim 1 , further comprising:
electrically connecting a first terminal of an operational amplifier to the conductive sleeve via a shielded cable, the shielded cable comprising an electrical shield; and
electrically coupling an output terminal of the operational amplifier to a second input terminal of the operational amplifier and to the electrical shield.
7. A method for controlling heater current supplied to an electrode contained within a fluorescent lamp, the method comprising:
receiving an electrical signal from an output terminal of an operational amplifier, the operational amplifier having a first input terminal that is electrically coupled to a conductive sleeve and a second input terminal that is electrically coupled to the output terminal, the conductive sleeve surrounding a portion of the lamp that contains the electrode;
determining a magnitude of cathode fall from the electrical signal; and
causing a direct-current power supply to apply a heater voltage to the electrode, wherein the heater voltage is based on the magnitude of cathode fall.