Rejecting noise transients while turning off a fluorescent lamp using a starter unit

• US 8,541,960 B2
• Filed: 05/28/2010
• Issued: 09/24/2013
• Est. Priority Date: 05/28/2010
• Status: Expired due to Fees

• Current Assignee: ZiLOG Incorporated (Littelfuse Incorporated)
• Original Assignee: ZiLOG Incorporated (Littelfuse Incorporated)
• Inventors: Khalsa, Kamlapati, Ball, Roger
• Primary Examiner(s): Owens, Douglas W
• Assistant Examiner(s): PHAM, THAI N
• Application Number:
  US12/802,090
• Publication Number:
  US 20110291581A1
• Time in Patent Office: 1,215 Days
• Field of Search: 315/33, 315/46, 315/94, 315/116, 315/246, 315/250, 315/291, 315/294, 315/313, 315/362, 315/DIG.5
• US Class Current: 315/362
• CPC Class Codes: H05B 41/04 Starting switches
• CPC Class Codes: H05B 41/295 with semiconductor devices and specially adapted for lamps with preheating electrodes, e.g. for fluorescent lamps
• CPC Class Codes: H05B 41/2986 against internal abnormal circuit conditions

Abstract

A local minimum of a current monitoring signal is identified by a starter unit that turns off a fluorescent lamp without using a wall switch. Closing a main switch in the starter unit stops an illuminating current from flowing through a gas in the lamp. The local minimum of the current monitoring signal is reached when an increasing valid sample is identified following four valid samples. A sample is valid if it does not differ from the preceding valid sample by more than a threshold difference based on known properties of the signal. By skipping invalid samples, the local minimum is accurately determined to have been reached despite transient noise spikes in the signal that would trip any voltage threshold used to locate the local minimum. When the main switch is opened at a predetermined time after the local minimum, the illuminating current does not again flow through the gas.

Claims

1. A method comprising:

(a) stopping an illuminating current from flowing through a gas of a lamp by closing a switch;

(b) determining a first magnitude of a waveform at a first time;

(c) determining a second magnitude of the waveform at a second time that occurs after the first time;

(d) determining a third magnitude of the waveform at a third time that occurs after the second time;

(e) determining a first difference between the third magnitude and the second magnitude;

(f) determining a threshold difference for the waveform between the second time and the third time;

(g) comparing the third magnitude to the first magnitude if the first difference is smaller than the threshold difference;

(h) after the comparing in (g), determining that a local minimum of the waveform has been reached if the third magnitude exceeds the first magnitude; and

(i) opening the switch at a predetermined time interval after the third time.

2. The method of claim 1, wherein the lamp is coupled to an inductor-type ballast, and wherein the predetermined time interval is less than one quarter of a cycle of the waveform.

3. The method of claim 1, wherein the lamp is coupled to a capacitor-type ballast, and wherein the predetermined time interval is between one quarter and one half of a cycle of the waveform.

4. The method of claim 1, wherein the waveform represents a shunt current that flows through the switch when the switch is closed, wherein the illuminating current flows through the gas prior to the switch being closed, and wherein the illuminating current does not flow through the gas when the switch is closed.

5. The method of claim 4, wherein the second magnitude represents a voltage drop across a current sense resistor generated when the shunt current flows through the switch.

6. The method of claim 1, wherein the stopping the illuminating current in (a) occurs before the first time.

7. The method of claim 1, wherein the illuminating current does not begin to flow through the gas upon the opening of the switch in (i).

8. A method comprising:

(a) stopping an illuminating current from flowing through a gas of a lamp by closing a switch;

(b) taking samples of a shunt current that flows through the switch when the switch is closed, wherein the samples of the shunt current are decreasing, and wherein the taking of the samples in (b) is performed by measuring a voltage drop across a current sense resistor through which the shunt current flows; and

(c) opening the switch at a predetermined time interval after the samples of the shunt current first begin to increase after the samples of the shunt current are decreasing in (b).

9. The method of claim 8, further comprising:

(d) rejecting a sample of the shunt current taken in (b) when the rejected sample is taken during a transient noise spike in the shunt current such that the samples are not determined to begin to increase during the transient noise spike, and such that the switch is not opened in (c) at the predetermined time interval after the rejected sample.

10. The method of claim 8, wherein the lamp is coupled to an inductor-type ballast, and wherein the predetermined time interval is less than one quarter of a cycle of the waveform.

11. The method of claim 8, wherein the lamp is coupled to a capacitor-type ballast, and wherein the predetermined time interval is between one quarter and one half of a cycle of a waveform of the shunt current.

12. An apparatus comprising:

a ballast adapted to receive an alternating current from an AC line voltage supply;

a fluorescent lamp coupled to the ballast, wherein the alternating current has a waveform and flows through a switch when the switch is closed, and wherein the alternating current flows through a gas of the fluorescent lamp when both the switch is open and the fluorescent lamp is on; and

means for opening the switch when a predetermined time interval elapses following a local minimum of the waveform of the alternating current by determining when samples of the alternating current begin to increase, wherein the means is also for stopping the alternating current from flowing through the gas without disconnecting the AC line voltage supply from the fluorescent lamp, wherein the means determines the local minimum of the waveform despite the waveform exhibiting transient noise spikes within a quarter cycle of the waveform before and after the local minimum.

13. An apparatus comprising:

a ballast adapted to receive an alternating current from an AC line voltage supply;

a fluorescent lamp coupled to the ballast, wherein the alternating current has a waveform and flows through a switch when the switch is closed, and wherein the alternating current flows through a gas of the fluorescent lamp when both the switch is open and the fluorescent lamp is on; and

means for opening the switch when a predetermined time interval elapses following a local minimum of the waveform of the alternating current by determining when samples of the alternating current begin to increase, wherein the means is also for stopping the alternating current from flowing through the gas without disconnecting the AC line voltage supply from the fluorescent lamp, wherein the ballast is an inductor-type ballast, and wherein the predetermined time interval is less than one quarter of a cycle of the waveform.

14. An apparatus comprising:

a ballast adapted to receive an alternating current from an AC line voltage supply;

a fluorescent lamp coupled to the ballast, wherein the alternating current has a waveform and flows through a switch when the switch is closed, and wherein the alternating current flows through a gas of the fluorescent lamp when both the switch is open and the fluorescent lamp is on; and

means for opening the switch when a predetermined time interval elapses following a local minimum of the waveform of the alternating current by determining when samples of the alternating current begin to increase, wherein the means is also for stopping the alternating current from flowing through the gas without disconnecting the AC line voltage supply from the fluorescent lamp, wherein the ballast is a capacitor-type ballast, and wherein the predetermined time interval is between one quarter and one half of a cycle of the waveform.

15. An apparatus comprising:

a ballast adapted to receive an alternating current from an AC line voltage supply;

a fluorescent lamp coupled to the ballast, wherein the alternating current has a waveform and flows through a switch when the switch is closed, and wherein the alternating current flows through a gas of the fluorescent lamp when both the switch is open and the fluorescent lamp is on; and

means for opening the switch when a predetermined time interval elapses following a local minimum of the waveform of the alternating current by determining when samples of the alternating current begin to increase, wherein the means is also for stopping the alternating current from flowing through the gas when the switch is open without disconnecting the AC line voltage supply from the fluorescent lamp, and wherein the means measures a voltage drop across a current sense resistor that is generated when the alternating current flows through the switch.

16. A processor-readable medium having processor-readable instructions for performing the steps of:

(a) closing a switch such that an illuminating current stops from flowing through a gas of a lamp;

(b) determining a first magnitude of a waveform at a first time;

(c) determining a second magnitude of the waveform at a second time that occurs after the first time;

(d) determining a third magnitude of the waveform at a third time that occurs after the second time;

(e) determining a first difference between the third magnitude and the second magnitude;

(f) determining a threshold difference for the waveform between the second time and the third time;

(g) comparing the third magnitude to the first magnitude if the first difference is smaller than the threshold difference;

(h) after the comparing in (g), determining that a local minimum of the waveform has been reached if the third magnitude exceeds the first magnitude; and

(i) opening the switch at a predetermined time interval after the third time.

17. The processor-readable medium of claim 16, wherein the lamp is coupled to an inductor-type ballast, and wherein the predetermined time interval is less than one quarter of a cycle of the waveform.

18. The processor-readable medium of claim 16, wherein the lamp is coupled to a capacitor-type ballast, and wherein the predetermined time interval is between one quarter and one half of a cycle of the waveform.

19. A method comprising:

(a) causing a switch to close and then to open such that a fluorescent lamp is ignited, wherein when the switch is closed in (a) an alternating current flows in series through the switch and a filament of the fluorescent lamp;

(b) after (a) causing the switch to be open during an operation of the fluorescent lamp when the fluorescent lamp is on, wherein when the switch is open and the fluorescent lamp is on in (b) the alternating current flows through a gas in the fluorescent lamp but does not flow through the switch;

(c) causing the switch to close;

(d) taking samples indicative of the magnitude of the alternating current, wherein each sample is a multi-bit digital value;

(e) using a sliding window of the samples to determine a local minimum of the alternating current, wherein a sample in the sliding window is not used in the determining of the local minimum if the sample has a predetermined relationship with respect to one or more other samples in the sliding window; and

(f) causing the switch to open a predetermined time interval after the local minimum determined in (e) such that the fluorescent lamp is extinguished.

20. The method of claim 19, wherein samples that have the predetermined relationship in (e) are invalid samples whereas samples that do not have the predetermined relationship in (e) are valid samples, and wherein a sample has the predetermined relationship if the difference between the magnitude of the sample and the magnitude of the closest preceding valid sample exceeds a threshold difference.

21. The method of claim 19, wherein (a) through (e) are performed by a circuit, and wherein the circuit receives a turn-off command in response to which the circuit causes (c) through (f) to be performed thereby turning off the fluorescent lamp.