Abstract:
An emergency ballast for a low profile fluorescent lamp fixture including an AC ballast including an end of lamp life shut down circuit. The emergency ballast includes a timing circuit which operates when AC power is restored to the lamp fixture after the fixture has operated for some period of time due to AC power failure. The timing circuit delays the application of AC power to the AC ballast for a given period of time (conveniently about 5 seconds) during the cessation of operation of the emergency ballast because of the restoration of AC power.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application relates to Provisional Patent Application Ser. No. 60/133,439, filed May 11, 1999. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION: 
     1. Field of the Invention 
     The invention relates to emergency lighting, and particularly to fluorescent lighting wherein a ballast for a fluorescent lamp is connected to a source of electrical energy other than normal AC line current in the event that the normal AC current fails. 
     Emergency lighting is required in commercial, industrial, and institutional buildings just as fire extinguishers, smoke alarms and other safety equipment. Three types of emergency lighting are common in such installations: unit equipment, engine generators and central battery systems. Unit equipment falls into two principle types: fluorescent and incandescent. 
     The fluorescent units are customarily combined with and within a conventional fluorescent lighting unit by merely adding the emergency ballast consisting of a battery, a battery charger, inverter and sensing circuitry adjacent the standard fluorescent AC ballast. The sensing circuit of the emergency ballast observes the interruption of normal AC power to the lamp unit and immediately switches on the emergency ballast to power individual lamp(s) or the light fixture for the required period which, under most state safety codes, is a period of at least ninety (90) minutes, a standard called out in the National Electrical Code, NFPA Article 70, and NFPA Article 101 Light Safety Code. 
     2. General Background of the Invention 
     U.S. Pat. No. 5,004,953 entitled Emergency Lighting Ballast for Compact Fluorescent Lamps with Integral Starters, assigned to the assignee of the present invention is illustrative of the general fluorescent type of emergency lighting with a ballast. It is common in the installation of emergency fluorescent lighting that an emergency ballast is added to a conventional fluorescent fixture or provided integrally in a fixture having internal regular and emergency ballast installed. When main AC power fails, voltage sensing circuitry instantly connects DC current from a battery (in the emergency ballast) to an inverter which produces high frequency, high voltage power to illuminate the emergency fluorescent lamp(s) for the required period. 
     The present invention is directed to fluorescent lighting fixtures which incorporate small fluorescent lamps, such as those which have a smaller diameter than conventional fluorescent bulbs (e.g. about ⅝″). These lamps are coming into more common usage and are employed in single or multiple lamp, low profile fixtures. In such small diameter lamps, the cathodes at the lamp ends are very close to the glass envelope. When this type of fluorescent lamp approaches end of its normal life, high power is generated in the cathodes, which may get very hot and can crack the glass open adjacent the cathode heaters. Standard ballasts would continue to supply high voltage to the cracked lamp, which would create potentially dangerous exposure to laceration if someone would try to unknowingly replace cracked glass lamps, as by causing further cracking or open breakage of the glass envelope and impingement of the sharp edges into the skin. The continued operation of the AC ballast with the damaged (unilluminated) lamp may also create an electrical shock hazard were the glass to disintegrate and allow an individual to touch the “hot” cathode (i.e., one carrying high voltage). 
     To prevent this electrical shock hazard, the electronic AC ballasts for small fluorescent lamps now include an end-of-lamp-life shut down circuit. These A.C. ballasts now incorporate a circuit to sense the increased cathode voltage and shut the high voltage down that normally would be supplied to the cracked lamp. Manufacturers that sell ballasts incorporating such a feature are Energy Savings Inc. (Lamp Guard, or Super Lamp Guard), Osram Quicktronic, and Magnetek. 
     These new electronic shutdown circuits conventionally sense any sudden change in power supplied to the lamp, such as a sudden increase in AC voltage or any DC voltage developed across the lamp. If any of these conditions is detected, the AC ballast operation is shut down such that no high voltage appears at the lamp. 
     This shut down capability frequently interferes with the inclusion of an emergency ballast which otherwise will operate the lamp in the event of AC power failure. The problem is actually created with the restoration of normal AC power after the lamp has been powered by the emergency ballast as a result of A.C. power failure. With the shift from battery operation of the lamp by the emergency ballast, there are transient swings of voltage and power to the lamp as the emergency ballast output shuts down and the AC ballast resumes operation. These transient voltages frequently trigger the shut down circuit since the transients exhibit symptoms similar to the voltage swings of the small lamp reaching its end-of-life state. The present invention coordinates the restarting of the fluorescent lamps with normal AC power with the cessation of the supply of emergency power from the emergency ballast. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a low profile emergency ballast for operation in conjunction with a low profile AC ballast having an end of lamp life shut down circuit. More particularly, the present invention is directed to an emergency ballast for a fluorescent lamp having means to avoid erroneous action of an end of lamp life shut down circuit in a low profile AC ballast. 
     One of the objectives of the present invention is the momentary delay of the powering of the lamp by the resumed A.C. power in order to allow the transients exhibited by the shut down of the emergency ballast to subside. A further objective of the present invention is the inclusion of a delay circuit which operates only on A.C. power application directly following operation of the emergency ballast. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a preferred embodiment of an emergency ballast for a low profile fluorescent fixture utilizing small fluorescent lamps including a circuit to avoid erroneous action of an end of lamp life shut down circuit in an AC ballast. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention is illustrated in the context of a conventional low profile or small (e.g. ⅝″ in diameter) fluorescent lamp including an emergency ballast for standby lighting during a period when the main AC power fails. FIG. 1 illustrates the circuit diagram of an emergency ballast according to the present invention which is connected in parallel with a conventional small lamp fluorescent ballast (not shown) for providing emergency lighting in the event of main AC power failure. 
     The circuitry for a low profile emergency ballast B is illustrated in FIG.  1 . This system includes an input/charging circuit I n  which provides charging current to the battery BT 1  and disables the emergency operation mode, i.e., places it in standby during the period that AC power is being supplied. The input/charging circuit has first and second input terminals J 1 - 1  and J 1 - 2 , respectively, connectable to standard AC voltage sources such as 120 AC and 277 volts AC. Inclusion of alternative voltage connections enables the system to be selectively connected to either standard commercial voltage AC (277 volts AC) or normal residential voltage (120 volts AC). Common, or ground potential, connector J 1 - 4  completes the A.C. power connections to the system input. 
     The two A.C. supply voltage terminals J 1 - 1  and J 1 - 2  and the common terminal J 1 - 4  are connected to the AC inputs of a rectifier D 1  (which in the preferred embodiment is a full wave rectifier), the high voltage (277 v. AC) input terminal J 1 - 2  being connected by means of a series arrangement of a first circuit composed of a capacitor, C 1 , and a resistor, R 1 , and a second circuit composed of a capacitor, C 2 , and a resistor, R 2 . The lower voltage (e.g. 120 volts AC) terminal is connected to rectifier D 1 , only via the second circuit including C 2  and R 2 . The capacitors in the circuits serve to limit the charging current supplied to rectifier D 1  to a level consistent with the requirements for a charging current to battery BT 1 . The resistors are included as a safety measure to limit the discharge of power from the capacitors after the A.C. power is removed from the circuits. 
     The DC output from rectifier D 1  is supplied to battery BT 1  by means of the coils of two relays, K 1  and K 2 , and a capacitor C 3  which filters the current supplied to relay coils K 1  and K 2 . A resistor R 3  is connected in series with an LED indicator to show the charging status of the emergency ballast B. 
     Battery BT 1  may be composed of, for example, a high temperature 6 volt (sub-C) nickel cadmium battery. Alternate battery configurations are possible, dictated by the power requirements of load LAMP and size of the battery space available in the emergency ballast. 
     The output circuit I o  includes a secondary winding S of transformer T having a primary winding P and a feedback winding F on the inverter circuit I v  side of transformer T. Output circuit I o  provides current limiting to the fluorescent lamp load LAMP only to the degree that is necessary to keep the lamp within its normal operating limits. The output I o  circuit also provides switching by switches K 1 A and K 1 B and K 2 A between normal lamp operation (K 1  and K 2  energized) and the emergency ballast mode (K 1  and K 2  de-energized) during which the AC power is not available. The output circuit I o  is composed of a capacitor, C 9 , connected across the output of the secondary winding, S, of transformer T 1 . Capacitors C 7  and C 8  are selectively connected as discussed later, in series with the fluorescent lamp LAMP which the output circuit I o  powers. As may be observed by those skilled in the art, the output circuit is remarkably simple in that the output circuit of the emergency ballast B provides only that current limiting necessary to keep the fluorescent lamp within its normal operating limits and allows the lamp to be connected to the otherwise unregulated full-wave AC output created from the energy supplied by battery BT 1  through switching performed by the inverter circuit I v . 
     Emergency power is supplied to load LAMP by battery BT 1  through the operation of inverter circuit Iv. The operation of the emergency ballast B is through switch K 2 A which serves to place the inverter circuit in operation enabling the oscillation of switching transistors Q 3  and Q 4 , including a higher current operation enabled by the timing circuit Ti 1  for a short interval (which may be in the order of a few seconds) after AC power failure to permit the starting of the “cold” fluorescent lamp. Those familiar with fluorescent lighting will recognize that an application of an initial voltage of as much as approximately 600 volts may be required to initiate the ignition of the gasses in the standard fluorescent lamp. Irmediately after ignition, as switch Q 5  (in addition to battery BT 1  through resistor R 12 ) supplies base current to Q 3  and Q 4  as later discussed, the current regulating capacitors C 7  or C 8  in the output circuit I o  regulate the current level to that required to operate the fluorescent lamp at its normal rated illumination. 
     The inverter I v  constitutes a current-fed, self-resonant, switch-mode converter supply, also known as a push-pull converter which includes primary P 1  of transformer T 1 , the transformer having an inductance setting gap in its core. Transformer T 1  is composed of a center tapped primary winding P, a feedback winding F and a high-voltage secondary winding S, composed of a large number of turns of fine magnet wire. Two transistors, Q 3  and Q 4 , are connected so that the emitter/collector pad of each is connected between a respective end of the primary winding P 1  and the negative terminal battery BT 1  as shown. A low-voltage feedback winding, F, of transformer T 1  is connected between the bases of transistors Q 3  and Q 4  to provide positive feedback from winding F to cause Q 3  and Q 4  to alternately switch the battery current through primary winding P 1  creating the alternating current in secondary winding S. 
     Timing circuit T i   1  controls the high voltage and current necessary to initiate the lighting of lamp LAMP. A mosfet transistor Q 2  is connected through its gate to capacitor C 5  and its source/drain through relay coil K 4  so that on loss of AC power and the operation of switch K 2 A the firing of transistor Q 2  causes current to flow in relay coil K 4  and activate relay switches K 4 A and K 4 B to direct transformer secondary S output to load LAMP through capacitor C 7  for the period of time Q 2  conducts (about 10 seconds in the preferred embodiment). Thereafter, the current through relay coil K 4  ceases and switch K 4 B returns to its normally closed condition (as shown) such that capacitor C 8  regulates the current to load LAMP. 
     Timing Circuit T i   2  provides the delay of the return of operation of the load LAMP when AC power is returned to the emergency ballast B. Capacitor C 4  is charged when the emergency ballast B operates to power load LAMP. When AC power returns, the current to relay coils K 1  and K 2  cause load LAMP to be switched from output circuit I o  to the AC ballast (not shown) except that as mosfet transistor Q 1  fires, being powered by the charge on capacitor C 4 , current flows in relay coil K 3  causing switch K 3  to open. Switch K 3  is connected in series with the main AC power line to the AC ballast and upon opening it interrupts AC ballast being supplied with AC line power, delaying the turning on of lamp LAMP for a time sufficient (approximately 5 seconds in the preferred embodiment) for the transient currents from the cessation of emergency supply to dissipate. Once transistor Q 2  ceases conduction, switch K 3  closes and normal supply to load LAMP from the AC ballast resumes. 
     During normal operation when main AC power supply is provided to the AC ballast and the emergency ballast B, charging current is supplied from the rectifier, D 1 , to battery BT 1 , causing the energizing of relay coils K 1  and K 2  so that the timing circuits T i   1  and T i   2  and the oscillating switches Q 3  and Q 4  and the output circuit I o  are inactive. At the time the main AC power supply fails, and for that continuing period of time prior to the AC power returns to normal operation such that input voltage through J 1 - 1  or J 1 - 2  again powers rectifier D 1 , relays K 1  and K 2  are de-energized (and associated switches K 1 A and K 1 B and K 2 A assume the normally closed position indicated in FIG. 1) whereby the fluorescent lamp load LAMP is connected to the output circuit I o  and the inverter I v  is triggered into operation. 
     Upon initial loss of AC power, charging current from the output of diode bridge D 1  ceases, causing switch K 2 A to close (NC position). This allows the battery BT 1  to supply current through coil K 4  and diode D 4  to the drain of transistor Q 2 . Current is also provided to transistors Q 3  and Q 4  which are driven into saturation resulting in a current flow through the primary P 1  of the inverter Iv. Resistor R 12  is given a sufficiently low resistance to supply a base current which will alternately drive transistors Q 3  and Q 4  to oscillate the battery BT 1  current through the primary winding P. Concurrently with the initial conduction of oscillators Q 3  and Q 4 , Q 5  conducts, being driven into conduction by current supplied from the battery through relay coil K 4  and the residual charge on capacitor C 5  holding Q 2  on. With Q 2  turning Q 5  on, additional bias current is provided to oscillators Q 3  and Q 4  to provide the increased current necessary to strike or initially illuminate the load LAMP. Concurrently with the additional bias current to Q 3  and Q 4 , the current flow in relay coil K 4  causes relay switch K 4 B to supply load LAMP through capacitor C 7 . Once the charge on capacitor C 5  has dissipated to the point that transistor Q 2  no longer conducts, Q 5  also ceases conducting and oscillators Q 3  and Q 4  are biased only through R 12  and their output decreases to the steady-state switching current of inverter I v . Likewise, current through coil K 4  ceases and relay switches K 4 A and K 4 B assume the normally closed position shown in FIG. 1 whereby load LAMP is supplied current through capacitor C 8  from transformer T. 
     Once transistors Q 3  and Q 4  are alternately biased to the on condition, they act effectively as switches drawing current from battery BT 1  through their respective emitter/collectors to the center of primary P of transformer T 1 . Current flow through feedback coil F of transformer T 1  effectively diverts the base current to transistors Q 3  and Q 4  alternatively in a positive feedback mode whereby Q 3  and Q 4  oscillate in an on and off condition creating an AC current from battery BT 1  to the center tap of primary P of transformer T 1  which is stepped up to a suitably high AC voltage to run the selected small fluorescent lamps making up the load LAMP by selection of the turns ratio between P and the secondary coil S of transformer T. Resistor R 12  functions to limit the current from battery BT 1  through the feedback winding F such that transistors Q 3  and Q 4  are biased appropriately. Likewise, capacitor C 6  across the collector circuits of Q 3  and Q 4  in parallel with the primary winding P serves to smooth the AC current generated by virtue of the alternative switching action of transistors Q 3  and Q 4  creating the battery supplied AC through primary P. 
     The output circuit I o  which includes the fluorescent lamp load LAMP to be illuminated attached to terminals J 2 - 1  and J 2 - 2  includes also in the secondary winding S, one of current limiting capacitor C 7  or C 8  and capacitor C 9  across secondary winding S. In operation, when the inverter I v  produced high AC voltage is initially generated at secondary S as switches Q 3  and Q 4  fire off, assisted by switch Q 5 , high voltage in the order of 600 hundred volts AC is applied to the fluorescent lamp making up load LAMP. This causes the circuit containing load LAMP, which is essentially capacitive, to receive a voltage spike which ensures that the lamp is started by there being sufficient voltage and current applied to the gases within the lamp to ensure initial conduction. As the lamp initiates its illumination and transistor Q 5  shuts down, current will flow through capacitor C 8  which is sized to limit the current to fluorescent lamp LAMP at its operational level so that the lamp will provide the requisite illumination in emergency operation. Capacitor C 9  across secondary S is also a current limiting impedance in the circuit to ensure that a load is always connected against secondary S. 
     As AC power is restored to the emergency ballast B, power again flows through charging circuit D 1  providing current again through relay coils K 1  and K 2 . Accordingly, relay switch K 2 A opens (position NO) and terminates the function of the inverter circuit I v . Concurrently as the charge on capacitor C 4 , which was built up from battery BT 1  through D 3  during the functioning of the inverter circuit I v , provides a bias to the gate of mosfet transistor Q 1 . With the bias applied to its gate, transistor Q 1  goes into conduction and the charging voltage of diode D 1  is applied to the drain of Q 1 , whereby current is drawn through relay coil K 3  causing relay switch K 3  to open, interrupting the supply of AC line power to the AC ballast, thereby delaying the start-up of the AC ballast. Once the charge on capacitor C 4  has dissipated such that Q 1  no longer conducts (about 4 to 5 seconds), current flow in coil K 3  ceases and switch K 3  resumes its normally closed position (NC) and normal AC ballast operation begins providing power to operate the fluorescent fixtitre (not shown). The delay time is selected to allow the transient voltage and current spikes introduced into the output circuit I o  by the shutting down of emergency ballast to subside such that they are not detected by the end of life cycle circuit in the AC ballast. It should be recognized by those skilled in the art that the sensitivity of the end of cycle circuits of different AC ballasts may require more or less time for the settling of the transients, depending upon those circuits&#39; sensitivity. Such timing adjustments are made by changing the values of capacitor C 5  and resistors R 7  and R 8 . 
     In the embodiment described above and illustrated in FIG. 2, the following components were utilized: 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                 Designator 
                 Description 
                 Component value/description 
               
               
                   
               
             
             
               
                 C1 
                 capacitor 
                 1.5 uF/250 VDC 
               
               
                 C2 
                 capacitor 
                 2.0 uF250 VDC 
               
               
                 C3 
                 capacitor 
                 220 uF/25 VDC 
               
               
                 C4, C5 
                 capacitor 
                 0.68 uF/63 VDC 
               
               
                 C6 
                 capacitor 
                 0.047 uF/100 VDC 
               
               
                 C7 
                 capacitor 
                 1200 pF/2 kV 
               
               
                 C8 
                 capacitor 
                 330 pF/2 kV 
               
               
                 C9 
                 capacitor 
                 470 pF/2 kV 
               
               
                 R1, R2, R8 
                 resistor 
                 10 MΩ /0.25 W 
               
               
                 R3 
                 resistor 
                 270 Ω /0.25 W 
               
               
                 R4 
                 resistor 
                 47 Ω /0.25 W 
               
               
                 R5, R7 
                 resistor 
                 10 kΩ /0.25 W 
               
               
                 R6 
                 resistor 
                 4.7 MΩ /0.25 W 
               
               
                 R9 
                 resistor 
                 1 kΩ /0.25 W 
               
               
                 R10 
                 resistor 
                 not used 
               
               
                 R11 
                 resistor 
                 300 Ω /0.25 W 
               
               
                 R12 
                 resistor 
                 1 kΩ /0.25 W 
               
               
                 D1 
                 diode bridge 
                 1 A, 600 V 
               
               
                 D2, D3 
                 diode 
                 1 A, 600 V 
               
               
                 D4 
                 diode 
                 1N4148 
               
               
                 K1, K2 
                 DPDT relay 
                 3 V, 45Ω  coil 
               
               
                 K3 
                 SPDT relay 
                 5 V, 55Ω  coil 
               
               
                 K4 
                 DPDT relay 
                 5 V, 42Ω coil 
               
               
                 Q1, Q2 
                 mosfet transistor 
                 60 V, 0.15A, 0.4 W 
               
               
                 Q3, Q4 
                 transistor 
                 80 V, 5A, 1.2 W 
               
               
                 L1 
                 inductor 
                 100 turns, 25 GA wire 
               
               
                 BT 
                 nickel-cadmium battery 
                 7.2 V, 1.5 Ah, subC cell 
               
               
                 T1 
                 E187 inverter transformer: 
               
             
          
           
               
                   
                  Winding 
                 Wire 
                 Number 
               
               
                   
                 Description 
                 GA 
                 of Turns 
               
               
                   
                 Secondary 
                 35 
                 800 
               
               
                   
                 Primary 
                 30 
                  9 
               
               
                   
                 Primary 
                 30 
                  9 
               
               
                   
                 Feedback 
                 30 
                  2 
               
               
                   
                   
               
             
          
         
       
     
     The disclosed embodiment is to be considered in all respects as illustrative and not restrictive. The scope of the invention is to be defined by the appended claims rather than the foregoing descriptions and other embodiments which come into the meaning and range of equivalency of the claims are therefore intended to be included within the scope thereof.