Patent Document

FIELD OF THE INVENTION 
     The present invention relates to electronic ballasts for fluorescent lamps. More particularly, the present invention relates to fluorescent lamp ballasts including integral circuitry for turning off the lamp after a predetermined period of time has elapsed. 
     BACKGROUND OF THE INVENTION 
     Electronically-ballasted fluorescent lamps are often used as task lighting in office environments. Normally, in these applications the task lights are mounted in workstation furniture and are turned on by the user at the beginning of the day. The problem with this arrangement is that often the user forgets to turn the task lamps off at the end of the day, thus eliminating a large percentage of the energy savings from using a fluorescent lamp due to the lamp remaining on all night. 
     Previously, manufacturers have used mechanical or electronic timers in line between the power source and the light fixture in order to interrupt power to the light fixture after a specified time. Drawbacks of this approach include the significant added expense of a separate timing circuit. Additionally, such prior art solution also requires a significant volume to implement the circuitry required to interrupt the primary power to the fixture. 
     It would, therefore, be desirable to provide an electronic fluorescent lamp ballast with an integral timer feature that will allow the lamp to operate for a predetermined length of time and then shut the lamp off. It would be desirable for such timer feature to add only a marginal increase in cost and size of the lamp ballast. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides, in one embodiment, a gas discharge lamp ballast with an integral shutdown timer. The ballast includes a circuit for receiving AC power, a converter circuit for converting the AC power to DC power and a square wave oscillator powered from the DC power. A resonant circuit powered by the square wave oscillator supplies power to at least one gas discharge lamp. A time-delay circuit disables the square wave oscillator, without interrupting the DC power supplied to the oscillator, upon the passage of a predetermined period of time from power-up of the AC power. 
     The foregoing ballast beneficially allows a gas discharge lamp to operate for a predetermined length of time and then shuts off the lamp. The ballast can be realized with only a marginal increase in cost and size of the ballast. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, in which like reference numerals refer to like parts: 
         FIG. 1  is a block diagram of a prior art ballast for a fluorescent lamp. 
         FIG. 2  is block diagram of another prior art ballast for a fluorescent lamp. 
         FIG. 3  is a block diagram of a further prior art circuit using a mechanical or electronic timer positioned between an AC input and a lamp ballast. 
         FIG. 4  is a block diagram of a fluorescent lamp ballast, which includes a timer function in accordance with an aspect of the present invention. 
         FIG. 5  is a schematic circuit diagram, partially in block form, of one implementation of the ballast of  FIG. 4 . 
         FIGS. 6A-6E  are timing diagrams for various voltages or conditions of the ballast of  FIG. 5 . 
         FIGS. 7A-7E  are timing diagrams for various voltages or conditions of the ballast of  FIG. 5  when utilizing an external reset circuit for the time delay network of that ballast. 
         FIG. 8  is a schematic diagram, partially in block form, of an alternative time delay network. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1-3  show various prior art electronic ballasts, so as to provide a better perspective for understanding the present invention.  FIGS. 1-2  show ballasts that can benefit from the timer feature of the present invention, while  FIG. 3  shows a prior art approach which includes a timer function with a ballast and over which the present invention improves. 
     A list of list of reference numerals and associated parts appears near the end of this description for the convenience of the reader. 
       FIG. 1  shows a prior art ballast for a fluorescent lamp  100 , which receives AC input power as noted by arrow  200 . Lamp  100  may have two-terminal heated filaments as shown. A rectifier and filter circuit  300  receives the AC input power and, in turn, provides DC power to a square wave oscillator  400 , which may comprise a half-bridge circuit formed with a pair of serially connected switching transistors in a totem-pole arrangement. Alternatively, square wave oscillator may comprise a push-pull circuit of parallel resonant design. A resonant tank  500 , typically including a resonant capacitance and resonant inductance, supplies AC voltage to lamp  100 . 
     Resonant tank  500  typically provides a feedback signal, such as from a winding (not shown) coupled to the resonant inductance or from a separate feedback transformer, to an oscillator control and drive circuit  450 . Oscillator control and drive circuit  450  then controls and drives square wave oscillator  400  in a manner to start and then run lamp  100 . As an alternative to using the foregoing feedback winding and corresponding implementation of oscillator control and drive circuit  450 , an integrated control circuit  460  can be incorporated in oscillator control and drive circuit  450 . U.S. Pat. No. 6,366,032 to Allison et al. describes one such integrated control circuit that can be used. 
       FIG. 2  shows another prior art ballast for a fluorescent lamp  100 . The ballast of  FIG. 2  has parts with the same reference numerals as in  FIG. 1 , and since the likeness of reference numerals refers to likeness of parts, the reader can consult the earlier description of such parts for part description. 
     The fluorescent lamp ballast of  FIG. 2  adds to the ballast of  FIG. 1  either an oscillator fault detection circuit  600  or a lamp fault detection circuit  625  (shown in phantom). A fault logic circuit  650  responds to fault signals from either of the oscillator fault detection circuit  600  or the lamp fault detection circuit  625 , and responsively provides a control signal to oscillator control and drive circuit  450 . 
     As mentioned above, either of the electronic ballasts of  FIGS. 1-2  can benefit from the timer feature of the present invention. 
       FIG. 3  shows a prior art circuit using a timer function over which the present invention improves. Lamp ballast  700  can be realized by various circuits including those shown in  FIGS. 1-2 . This prior art approach interposes a mechanical or electrical timer  750  in the power line between AC input power  200  and lamp ballast  700 . One key disadvantage of such a solution to providing a timer function is that a separate mechanical or electrical timer  750  must be used. It, would, therefore be desirable to eliminate the need for a separate mechanical or electrical timer  750 . 
       FIG. 4  shows application of various embodiments of the present invention to the prior art circuit of  FIG. 1  or  FIG. 2 . In  FIG. 4 , a time delay network  800  in accordance with the invention receives either a signal representing AC power  200 , or, alternatively, receives a reset signal  855 . In response to whichever of the foregoing signals is received, time delay network  800  provides an output signal to control either oscillator control and drive circuit  450  or fault logic circuit  650 . 
     Reset signal  855  is shown associated with a phantom line, to indicate that it may typically be used instead of using the input from a signal representing AC power  200 , or only the signal representing AC power  200  may be used. Similarly, the output from time delay network  800  leading to fault logic circuit  650  is shown in phantom to indicate that it may typically be used instead of the output of the time delay circuit leading to oscillator control and drive circuit  450 , or only the output of the time delay circuit to the oscillator control and drive circuit may be used. 
       FIG. 5  shows a preferred embodiment of a ballast circuit for a fluorescent lamp in accordance with the invention. The previously described square wave oscillator  400  of  FIGS. 1-2  is realized by circuitry including a pair of NPN switching transistors  402  and  404  configured in a totem-pole arrangement to alternately switch midpoint connection  406  of the transistors between a DC voltage on the upper-shown node of transistor  402  and the illustrated ground or reference node. NPN switching transistors  402  and  404  may be replaced by other power switching device such as power MOSFETs. A resonant tank circuit includes a DC blocking capacitor  502 , a resonant inductor  504 , a resonant capacitor  510 , and lamp  100 . In customary fashion, windings  506  and  508  sample current through the lamp and provide respective feedback signals to control or base nodes  452  and  454  of switching transistors  402  and  404  via resistors  456  and  458 . Base nodes  452  and  454  comprise parts of oscillator control and drive circuit  400  of  FIGS. 1-2 . 
     In  FIG. 5 , rectifier and filter  300  of  FIGS. 1-2  is realized by circuitry including input inductor  210 , a full-bridge rectifier formed of PN diodes  302 ,  304 ,  306 , and  308 , and a bulk filter capacitor  310 . Diodes  302 - 308  could be formed as an integral bridge rectifier assembly. As an alternative to using four diodes  302 ,  304 ,  306  and  308  and a single bulk filter capacitor  310 , two diodes (not shown) and two series capacitive elements (not shown) could be used to realize a voltage-doubler type of input rectifier and filter circuit. AC input power  200  of  FIGS. 1-2  is realized application of AC voltage to input nodes  202 . 
     A lamp fault detection circuit  625  of  FIGS. 1-2  is realized in  FIG. 5  by a sense resistor  626  in the emitter lead of transistor  404 , resistors  630 ,  634 , and  638 , capacitor  632 , diodes  628  and  636  and SCR  410 . This circuit detects excessive current in the oscillator circuit and switches SCR  410  to the ON state, pulling base  452  of transistor  402  to ground and turning off the oscillator. 
     In  FIG. 5 , an oscillator control and drive circuit  450  of  FIGS. 1-2  is realized by circuitry comprising base drive transformer  506 / 508  and resistors  456  and  458  connected across the bases  452  and  454  of NPN transistors  402  and  404 , respectively. Base drive transformer  506 / 508  may be realized as separate transformers or as part of inductor  504 . This network generates drive signals corresponding to the current through the resonant tank inductor and capacitor and uses regenerative feedback to maintain oscillation of the resonant circuit. 
     In accordance with as aspect of the invention, time delay network  800  of  FIG. 4  is realized by circuitry including a MC14521B 24-stage frequency divider  802  by ON Semiconductor of Phoenix, Ariz. Frequency divider  802  includes a 24-stage chain of flip-flops (not shown) which divide the input frequency by a factor of 2 24 . Resistors  804  and  806  and capacitor  808  function as an RC oscillator with a period of approximately 2*(resistance of resistor  804 )*(capacitance of capacitor  808 ) or 4.4 milliseconds in the case of the resistors  804  and  806  having resistances of 210 k-ohms and 105 k-ohms, respectively, and capacitor  808  having a capacitance of 22 nF. This frequency is then divided by frequency divider  802  such that the initiation of the 24 th  stage occurs after 0.0046*2 23  seconds, or approximately 10.2 hours. This 10.2 hours duration approximately represents a standard business day, but the foregoing resistor and capacitor values can be chosen to produce other durations of time for initiation of the 24 th  stage. In any event, such duration would typically be greater than 15 minutes. 
     Upon initiation of the 24 th  stage in frequency divider  802 , output Q 24  of frequency divider  802  transitions from low to high, providing a signal through resistor  810  to trigger a shutdown SCR  410 , contained in the realization of oscillator control and drive circuit  450  of  FIGS. 4-5 , so as bring the voltage of base node  452  of the upper transistor  402  low and turn the lamp off. In addition, output Q 24  of the frequency divider is connected to a PNP transistor  820  via resistor  822 , which pulls the voltage of the common node or tap  824  of the above-mentioned oscillator components low and prevents further operation of the oscillator. This keeps the output node Q 24  high, keeping the shutdown SCR  410  triggered, as well as keeping the frequency divider  802  itself locked in that state. 
     Reset of the time delay network in  FIG. 5  corresponding to network  800  of  FIG. 4  is caused by turning off AC input power to the ballast. This is sensed by voltage at a filtered power lead  212  of filter inductor  210 , which causes base drive to a reset transistor  828  to be removed and allows the “RESET” pin of frequency divider  802  to go high, resetting the logic bits in the frequency divider. 
     In making the foregoing fluorescent lamp ballast of  FIG. 5B , it was decided to move a reset signal for resetting from a node at the ballast bulk voltage labeled “DC VOLTAGE” to the unrectified AC line via filtered power lead  212  of filter inductor  210 . Selection of a reset signal from a signal on winding tap  212  representing AC input power, rather than from a signal representing the ballast bulk voltage guarantees that the reset signal will occur before the bulk filter capacitor  306  completes the process of becoming fully discharged. Bias voltage VDD and VDD 1  for frequency divider  802  is provided by a tap off the ballast bulk voltage labeled “DC VOLTAGE,” via resistor  812 . This means that frequency divider  802  will still be fully energized when the reset signal—i.e., AC power being turned off—occurs. Beneficially, this ensures that frequency divider  802  remains powered by the residual charge on the bulk capacitor  306  until after resetting of the frequency divider has occurred. As a result, the frequency divider becomes reset in a reliable manner. 
     Finally, regarding  FIG. 5 , a printed-circuit board (PCB)  900  is indicated by phantom-line box  900 . All components shown within phantom-line box  900  are preferably mounted directly or indirectly onto PCB  900 . By “mounting indirectly” is meant that a component may be mounted on small PCB, for instance, which is, in turn, mounted onto PCB  900 . 
     With reference to  FIG. 4 , as to which  FIG. 5  is an implementation, an alternative to using feedback windings  506 / 508  of  FIG. 5  and associated circuitry for self-resonant control of square wave oscillator can be realized as follows. Oscillator control and drive circuit  450  can incorporate integrated control circuit  460 , such as that described in U.S. Pat. No. 6,366,032 to Allison et al. 
     For a further understanding of the fluorescent lamp ballast of  FIG. 5 ,  FIGS. 6A-6E  show various timing waveforms for voltages or conditions of that lamp ballast. 
       FIG. 6A  shows an AC input waveform taken at filtered power lead  212  in  FIG. 5 ;  FIG. 6B  shows a rectified DC waveform taken at the node labeled “DC VOLTAGE” in  FIG. 5 ;  FIG. 6B  shows a rectified DC waveform taken at the node labeled “DC VOLTAGE” in  FIG. 5 ;  FIG. 6C  shows the logic output of pin Q 24  of frequency divider  802 ;  FIG. 6D  shows a reset signal taken at the RESET pin of frequency divider  802 ; and  FIG. 6E  shows the output of lamp  100 . 
     Time points T 1 -T 5  in  FIGS. 6A-6E  are explained as follows.
         T 1 : AC input power is applied to the ballast.   T 2 : DC filter bulk capacitor  306  becomes charged up as shown in  FIG. 6B  and the ballast begins to drive the lamp.   T 3 : Frequency divider  802  reaches its predetermined shutoff point when its output Q 24  goes high as shown in  FIG. 6C , causing shutdown SCR  410  to activate and shut down the square wave oscillator  400  ( FIG. 4 ), thus turning off the lamp. Output Q 24  remains high as shown in  FIG. 6C  in order to keep the lamp off and disable frequency divider  802  so as to prevent any further counts in the internal clock of the frequency divider.   T 4 : The AC input power is turned off. This causes the reset signal of  FIG. 6D  to go low. This causes frequency divider output Q 24  to go low as shown in  FIG. 6C , which deactivates shutdown SCR  410  and clears any clock counts in the frequency divider.   T 5 : Any time after time T 5 , the ballast can be re-energized from application of AC input power, so that the lamp restarts. However, a short delay is necessary to ensure that the reset signal has transitioned from high to low and the frequency divider has been cleared.       

     As described above, fluorescent lamp ballast of  FIGS. 4-5  allows a user to power up lamp  100  and keep the lamp on for a predetermined period of time, such as approximately 10 hours, and as shown by time interval  850  in  FIG. 6E . In an alternative embodiment, a reset circuit  860 , shown in phantom, can be connected to realize reset signal  855  of  FIG. 4  and apply that signal to the control or base node of reset transistor  828 . In this case, the control or base node of reset transistor  828  would no longer be connected to filtered input lead  212 , so that PN diode  862  would be eliminated 
     Reset circuit  860  may comprise a toggle switch that switches between high and low logic levels. Power for reset circuit  860  may be supplied from an AC or DC power supply, by way of example. Reset circuit  860  may be embodied as a conventional wall switch or toggle switch such as is standard in the industry. 
       FIGS. 7A-7E  show various timing waveforms for voltages or conditions of the lamp ballast circuit of  FIGS. 4-5  when reset circuit  860  is used to reset frequency divider  802 . In this case, power-down of the AC input power, as sensed from filtered power lead  212 , would typically not be used for resetting frequency divider  802 . 
       FIG. 7A  shows an AC input waveform taken at filtered power lead  212  in  FIG. 5 ;  FIG. 7B  shows a rectified DC waveform taken at the node labeled “DC VOLTAGE” in  FIG. 5 ;  FIG. 7C  shows the logic output of pin Q 24  of frequency divider  802 ;  FIG. 7D  shows a reset signal taken at the RESET pin of frequency divider  802 ; and  FIG. 7E  shows the output of lamp  100 . 
     Time points T 1 -T 5  in  FIGS. 7A-7E  are explained as follows.
         T 1 : AC input power is applied to the ballast.   T 2 : DC filter bulk capacitor  306  becomes charged up as shown in  FIG. 7B  and the ballast begins to drive the lamp.   T 3 : This time point is related to the following time point T 4 , which collectively constitutes a toggling of a switch (not shown) in reset circuit  802  ( FIG. 5 ). At time point T 3 , the reset signal shown in  FIG. 7D  and provided by reset circuit  802  to the reset transistor  828  causes frequency divider  802  to stop counting. Shutoff of square wave oscillator  400  is disabled.   T 4 : At time point T 4 , the reset signal provided by reset circuit  860  ( FIG. 5 ) is toggled back to high. This causes frequency divider  802  to reset to zero and begin its counting sequence anew. Shutoff of square wave oscillator  400  ( FIG. 5 ) is re-enabled to become active when the frequency divider reaches the end of a predetermined duration   T 5 : This is the shutoff point based on initial activation of frequency divider  802 . However, since the frequency divider was reset by reset circuit  860 , at times T 3  and T 4 , nothing happens to the frequency divider at this time point and no change in output is shown in  FIG. 7C .   T 6 : Frequency divider  802  reaches is predetermined shutoff point based on the new reset signal provided at time point T 4 . The frequency divider applies a shutdown signal as shown in  FIG. 7C , via output Q 24 , to the ballast, and the lamp is turned off as shown in  FIG. 7E . The output of frequency divider  802  remains high, as shown in  FIG. 7C , in order to keep the lamp off and disable the frequency divider from preventing any further counter of its internal timer clock. As can be seen from a comparison with  FIG. 6E ,  FIG. 7E  shows a duration  852  of the lamp being on that exceeds duration  850  shown in  FIG. 6E . So, the lamp can be left on longer using the reset circuit  860  of  FIG. 5 .   T 7 : The lamp can be restarted either by cycling of the AC input power—e.g., turning the power switch to the lamp off and on—or by cycling of the reset signal as described in connection with time points T 3  and T 4 .       

       FIG. 8  shows a resistive-capacitive (RC) time delay network  870  differing from time delay network shown in  FIG. 5 . In  FIG. 8 , an input node  872  receives a signal to start timing for a predetermined duration. Such a signal could come from filtered power lead  212  or reset circuit  860  in  FIG. 5 , for instance. The signal on input node  872  causes a current through resistor  874  to charge a capacitor  876 . A level detect circuit  878  detects when capacitor  876  has charged to a threshold level, which corresponds to elapse of a predetermined period of time such as 10 hours. An output  880  from time delay network  870  is then applied to either fault logic circuit  650  or oscillator control circuit  450 , which are shown in block in  FIG. 4 . 
     In the following list of reference numerals and associated parts, exemplary values or descriptions for various parts are placed in parenthesis after the part name:
       100 . Fluorescent lamp     200 . AC input power     202 . Nodes     210 . Filter Inductor     212 . Filtered Power Lead     300 . Rectifier and filter circuit     302 . PN diode (rectifier 1N4007)     304 . PN diode (rectifier 1N4007)     306 . PN diode (rectifier 1N4007)     308 . PN diode (rectifier 1N4007)     310 . Bulk filter capacitor (33 uF, 200V, aluminum electrolytic)     400 . Square wave oscillator     402 . NPN switching transistor (BUL128)     404 . NPN switching transistor (BUL128)     406 . Midpoint connection     410 . Shutdown SCR (XL0840)     450 . Oscillator control and drive circuit     452 . Base node     454 . Base node     456 . Resistor (3.3 ohm, ¼W, metal film)     458 . Resistor (3.3 ohm, ¼W, metal film)     460 . Integrated Circuit     500 . Resonant tank     502 . DC blocking capacitor (0.47 uF, 250V, metallized polyester)     504 . Resonant inductor (1.2 mH)     506 . Base drive winding     508 . Base drive winding     510 . Resonant capacitor (15 nF, 1000V, metallized polypropylene)     600 . Oscillator fault detector circuit     625 . Lamp fault detector circuit     626 . Sense resistor (0.68 ohms, ½W, metal film)     628 . Diode (switching, 1N4148)     630 . Resistor (470 k ohms, ⅛W)     632 . Capacitor (470 uF, 10V, aluminum electrolytic)     634 . Resistor (220 k ohms, ⅛W)     636 . Diode (rectifier, 1N4936)     638 . Resistor (100 ohms, ¼W)     650 . Fault logic circuit     700 . Lamp ballast     750 . Mechanical or electrical timer     800 . Time delay network     802 . Frequency divider (MC14521B)     804 . Resistor (210 k ohms, ⅛W)     806 . Resistor (105 k ohms, ⅛W)     808 . Capacitor (22 nF, NPO ceramic)     810 . Resistor (510 ohms, ⅛W)     812 . Resistor (200 k ohms, ½W)     820 . Transistor (2N2222)     822 . Resistor (5.1 k ohms, ⅛W)     824 . Common node or tap     828 . Reset transistor (N2222)     850 . Time interval     860 . Reset circuit     855 . Reset signal     862 . Node     864 . Diode (rectifier, 1N4007)     870 . Time delay network     872 . Input node     874 . Resistor     876 . Capacitor     878 . Level detect circuit     880 . Output     900 . Printed-circuit board
 
Part numbers mentioned in the foregoing list are standard part number typically used by multiple manufactures in the United States. Practice of the invention will be routine to a person of ordinary skill in the art based on the foregoing component values and remainder of this description.
   

     The foregoing describes an electronic fluorescent lamp ballast that achieves a timer function, as described herein, while typically only marginally increasing ballast cost and ballast size. 
     While the invention has been described with respect to specific embodiments by way of illustration, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true scope and spirit of the invention.

Technology Category: 4