Abstract:
A ballast for a low pressure gas discharge lamp, preferably of the heated-filament type, includes an on-off indicator for a lamp. The ballast includes a load circuit with a lamp, and a driver for supplying AC load current to the lamp. Such driver includes circuitry for shutting off the load current in the presence of a lamp fault condition. The ballast also includes a pair of nodes having voltage across them when the lamp operates normally, and having substantially no voltage across them when the lamp is off. An on-off lamp indicator circuit includes a light-emitting device and is coupled to the pair of nodes for causing the light-emitting device to emit light when the driver supplies load current to the lamp and for causing the light-emitting device to cease to emit light when the driver no longer supplies load current to the lamp.

Description:
FIELD OF THE INVENTION 
     This invention relates to ballasts for ultraviolet (UV) or other gas discharge lamps that include an indicator of the on or off condition of the lamp. More particularly, the invention relates to ballasts including a pair of nodes that have voltage across them when a lamp operates normally, and that have substantially no voltage across them when the lamp is off. This condition typically occurs in ballasts that power lamps having heated filaments. 
     BACKGROUND OF THE INVENTION 
     Ultraviolet (UV) lamps are widely used for sterilization of water and air in water supplies, air ventilation systems and the like. UV lamps are quite similar to conventional low pressure discharge lamps, the principal difference being that there is no fluorescent coating on the inside of the lamp. Without a fluorescent coating, UV radiation from the plasma inside the lamp flows directly out of the lamp, but is almost imperceptible to the human eye. In particular, the human eye is very insensitive to UV radiation, and may perceive only a faint, dim glow even when the intensity is such that immediate damage to the eye and other parts of the body may occur. Consequently this kind of UV light is dangerous to people, and UV light sources are always kept completely enclosed, for instance, inside a water tank or air duct where they sterilize the water or air. 
     Faced with the foregoing safety concern, the otherwise routine issue of verifying that the lamp is still running becomes a somewhat complex problem. Verification is important because failure of a UV lamp to sterilize the air or water may have serious health consequences. It is not acceptable for a person to view the light source to verify its operation. Instead, it is customary to provide some kind of electrical sensing circuitry to indicate satisfactory operation of the lamp. 
     One prior art approach involves modifying a lamp ballast to include a resistor in series with a UV lamp. The voltage across the resistor is used to drive a bidirectional light-emitting diode (LED). When lamp current is flowing, a voltage generated across the resistor is used to drive the bidirectional LED. A problem with this technique is that it only works for instant-start ballasts, which have only one wire going to the lamp ends. Also, it has the property that the UV ballast has to be constructed quite differently from a conventional ballast for gas discharge lamps such as fluorescent lamps. For instance, the mentioned resistor needs to be interconnected within the ballast circuitry, and wires for the LED need to be connected from the ballast circuitry to a point outside the ballast. 
     Another technique to provide an indication of on-off UV lamp operation is to place a light pipe close to the UV lamp to receive UV energy, and then to place a fluorescent element at an external end of the light pipe that lights up from the UV energy impinging upon it. The use of light pipes with fluorescent elements is relatively expensive and undesirable for that reason. 
     It would be desirable to provide an on-off indicator signal for a UV lamp. 
     If would be further desirable to provide an on-off indicator for a UV lamp that may incorporate a mass-produced ballast used to power fluorescent lamps. 
     If would also be desirable to provide an on-off indicator signal for non-UV lamps that may be economically implemented. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the invention, a ballast for a low pressure gas discharge lamp includes an on-off indicator for a lamp that preferably has heated filaments. Three examples of ballasts for lamps with heated filaments are: (1) a program start ballast, (2) a rapid start ballast, and (3) a controlled preheat ballast. 
     The ballast includes a load circuit with a lamp. The ballast further includes a driver for supplying AC load current to the lamp. Such driver includes circuitry for shutting off the load current in the presence of a lamp fault condition. The ballast also includes a pair of nodes that have voltage across them when the lamp operates normally, and that have substantially no voltage across them when the lamp is off. This condition typically occurs in ballasts for powering lamps that have heated filaments. 
     An on-off lamp indicator circuit includes a light-emitting device and is coupled to the pair of nodes for causing the light-emitting device to emit light when the driver supplies load current to the lamp and for causing the light-emitting device to cease to emit light when the driver no longer supplies load current to the lamp. 
     Beneficially, the ballast can be of the mass-produced type for powering conventional fluorescent lamps, such as that of U.S. Pat. No. 6,366,032, by Allison and Moore and which is assigned to the instant assignee. The ballast described in the foregoing patent automatically shuts off power to the lamp when a lamp fault is detected. In a preferred form, the on-off indicator circuit can be powered from available nodes across which voltage of a lamp filament is present, so that the ballast does not need to be changed mechanically. 
     The invention may be useful in connection with a conventional fluorescent lamp, to economically provide a remote confirmation of proper operation, for instance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic diagram, partially in block form, of a prior art circuit for providing an on-off indication of UV lamp operation. 
     FIG. 2 is a schematic diagram, partially in block form, of a ballast circuit for powering two lamps and including an on-off indicator circuit in accordance with one embodiment of the invention. 
     FIG. 3 is a schematic diagram of an alternative on-off indicator circuit that can be used in the circuit of FIG.  2 . 
     FIG. 4 is a schematic diagram, partially in block form, of another ballast circuit for powering a single lamp and including an on-off indicator circuit in accordance with the invention. 
     FIG. 5 is a schematic diagram, partially in block form, of another ballast circuit for powering a pair of lamps and including an on-off indicator circuit in accordance with the invention. 
     FIG. 6 is a schematic diagram, partially in block form, of ballast circuit similar to that of FIG. 5 for powering a single lamp and including an on-off indicator circuit in accordance with the invention. 
     FIG. 7 is a schematic diagram, partially in block form, of another ballast circuit for powering a single lamp and including an on-off indicator circuit in accordance with the invention. 
     FIG. 8 is a schematic diagram of another form of an on-off indicator circuit in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will become clearer after considering the prior art approach of FIG.  1 . In FIG. 1, an ultraviolet (UV) lamp  10  has electrodes  10   a  and  10   b  that are not provided with a filament for heating the electrodes. A conventional inverter circuit  12  supplies alternating current (AC) load current  13  for a load circuit  14  via a transformer  16 . A capacitor  18  limits current through the lamp. 
     Bidirectional (or anti-parallel connected) light-emitting diodes (LEDs)  20  and  22  emit light when load current  13  flows through lamp  10 . Thus, AC load current  13  creates an AC voltage drop across a resistor  24  when the lamp conducts current, and that voltage drop causes LEDs  20  and  22  to emit light, indicating that the lamp is operating. As mentioned above, the UV lamp itself emits UV light that is almost imperceptible to the human eye, so LEDs  20  and  22  give a visual indication to the human eye that the lamp is operating. 
     When load current  13  ceases to flow, the AC voltage drop across resistor  24  ceases, and the LEDs stop emitting light. The cessation of light from the LEDs visually indicates that the UV lamp has stopped operating, and so needs to be inspected or replaced. 
     FIG. 2 shows a typical arrangement that can be used in accordance with the invention for ballasting a pair of UV lamps  28  and  30 , while providing an on-off lamp indicator circuit  32  that emits light when the lamps are operating and that stops emitting light when the lamps stop operating. More broadly, lamps  28  and  30  could comprise other low pressure discharge lamps, such as fluorescent lamps. Lamps  28  and  30  may each have electrodes in the form of filaments  28   a ,  28   b  and  30   a ,  30   b , respectively. Current flow through the filaments, discussed below, heats the filaments to maintain a desirably elevated temperature for operation. 
     For powering lamps  28  and  30 , a DC source  36 , such as rectified AC current from power mains, supplies current to a load circuit  38   a  through a switching arrangement including field-effect transistors (FETs)  40  and  42 , which are controlled by a control circuit  44 . Control circuit  44  may be of the type described in U.S. Pat. No. 6,366,032, entitled “Fluorescent Lamp Ballast Using Integrated Circuit,” by Allison and Moore, which is incorporated herein in its entirety. 
     Control circuit  44  causes FETs  40  and  42  to alternately conduct; that is, in a manner in which upper FET  40  first conducts while the lower FET  42  is off, providing current flow from DC source  36 , left to right through an inductor  46 ; and then lower FET  42  conducts while upper FET  40  is off, causing current flow from right to left through inductor  46  to a reference or ground  48 . Accordingly, current flow in inductor  46  alternates direction in synchrony with the alternate periods of conduction of FETs  40  and  42 . The current flow in load circuit  38   a  is thus AC current. Meanwhile, a DC blocking capacitor  45  prevents DC current flow through the lamps. 
     Other ballasting circuits with two FETs comparable to FETs  40  and  42  or with other switching means will be apparent to those of ordinary skill in the art from the present specification. 
     To set a resonant frequency of operation of current in load circuit  38   a , a resonant capacitor  50  cooperates with a resonant inductor  46 . A current-supply circuit  52  supplies current for filament  28   a  of lamp  28 . Circuit  52  includes an inductor winding  54  coupled to inductor  46 , for instance, to receive energy from that inductor. Thus, current in inductor  46  induces current in inductor winding  54 , which flows through filament  28   a . Since filament  28   a  is resistive, current flowing through it heats the filament. A capacitor  56  limits the current in filament  28   a , and assures that there is no net DC current flow in current-supply circuit  52 . 
     A similar current-supply circuit  60  supplies current to filament  28   b  of lamp  28  and also to filament  30   a  of lamp  30 . Circuit  60  includes an inductor winding  60  coupled to receive energy from inductor  46 , and a capacitor  64  for limiting current in filaments  28   b  and  30   a  while assuring that there is no net DC current flow in current-supply circuit  60 . 
     Another current-supply circuit  68  supplies current to filament  30   b  of lamp  30 . Circuit  68  includes an inductor winding  70  coupled to receive energy from inductor  46 , and further includes a capacitor  72  for limiting current in filament  30   b  while assuring that there is no net DC current flow in current-supply circuit  68 . 
     In accordance with the invention, on-off lamp indicator circuit  32  is coupled to receive energy from current-supply circuit  60 . For instance, circuit  32  may be coupled to a pair of nodes  74  and  76 , across which the voltage of filament  28   b  (and of parallel-connected filament  30   a ) is present. Circuit  32  in the version shown includes diodes  80  and  82 , poled in the opposite direction, and coupled together in anti-parallel fashion. One or both of the diodes comprise an LED. A resistor  84  limits the current through diode  80  if such diode  80  comprises an LED, and another resistor  86  limits current in diode  82  if such diode  82  comprises an LED. If diode  80  is not an LED, associated resistor  84  may be omitted; and if diode  82  is not an LED, associated resistor  86  may be omitted. 
     As used herein, “coupling” of diodes  80  and  82  in anti-parallel fashion allows other devices (e.g., resistors  84  and  86 ) to be included so long as they do not significantly detract from the indication of whether a lamp is on or off by whichever of diodes  80  or  82  is an LED. 
     The use of two diodes  80  and  82  allows current to flow first through one diode and then through another. In this way, whichever diode(s) is an LED will provide light when the ballast or driver circuitry (e.g., DC source  36 , FETs  40  and  42 , and control circuit  44 ) supplies current to lamps  28  and  30 , and will stop providing light when the ballast or driver circuitry stops supplying current to the lamps. The light from one or both of diodes  80  and  82  indicates that the lamps are operating, and the cessation of such light indicates that the lamps are off. 
     As is conventional, control circuit  44  additionally includes circuitry for sensing a fault condition of the lamps. The fault condition preferably comprises the condition that the load current has ceased. As disclosed in U.S. Pat. No. 6,366,032 by Allison and Moore, the fault conditions that a control circuit may sense include, by way of example, lamp current not reaching a level, for instance, of 50% of normal current within, for instance, 6 seconds. 
     On-off lamp indicator circuit  32  may be realized in other forms. For, instance, as shown in FIG. 3, an alternative on-off lamp indicator circuit  90  may include anti-parallel coupled diodes  92  and  94 , at least one of which is an LED. Circuit  90  can replace circuit  32  of FIG. 2 at the pair of nodes  74  and  76 . An impedance  96  limits current in diodes  92  and  94 , and may be resistive, inductive, or capacitive. A resistive impedance  96  can be used with the circuit of FIG. 1. A possible circuit that could use capacitive or inductive impedance is described below. 
     Rather than using the resistor  84  or  86  shown in the on-off indicator circuit  32  of FIG. 1, or the impedance  96  of the indicator circuit  90  of FIG. 3, other ways of limiting the current in an LED will be apparent to those of ordinary skill in the art in view of the present specification. This also applies to circuits (not shown) using only a single diode, which is possible when DC current is provided for heating lamp filaments. 
     From the perspective of an LED (e.g.,  80  or  82 , FIG.  2 ), circuits for supplying current to filaments of a lamp appear as a constant voltage source. Without some means to limit the current in the diodes, the current in an LED would tend to increase indefinitely and destroy the diodes. So, current-limiting means are used to limit the current in an LED so that the LED can be driven from a constant voltage source. 
     Thus, in addition to the specific on-off indicator circuits  32  (FIG. 2) and  90  (FIG. 3) shown, three other approaches (not shown) to allow the diodes to be driven by a constant voltage source are as follows. First, more than one diode can be placed in series. Second, a two-transistor (e.g., PNP and NPN bipolar transistors) circuit can be configured to provide two external terminals while supplying essentially constant current regardless of how much driving voltage is impressed. Third, a single bipolar transistor can have its base terminal biased with an essentially constant voltage so as supply essentially constant current from its collector terminal for driving the diodes. 
     An alternative to using an LED to provide light to indicate the operational state of a lamp is described below. 
     FIG. 4 shows a ballast arrangement similar to that of FIG. 2, but for powering only a single lamp  28  in a load circuit  38   b . The same reference numerals as between FIGS. 4 and 2 refer to like parts, and so reference is made to the prior description in connection with FIG.  2 . 
     FIG. 4 includes an on-off indicator circuit  100  that may be embodied as shown at  32  in FIG. 2 or at  90  in FIG. 3, for instance. Circuit  100  is coupled to nodes  102  and  104 , which are not connected to either filament of lamp  28 . Rather, nodes  102  and  104  are coupled to a circuit  60 , including inductor winding  62  and capacitor  64 , which may normally be designed to supply current to filament  28   b  of lamp  28  and a filament of another lamp (not shown). 
     On-off indicator circuit  100  may comprise the circuit of FIG.  3 . In this case, impedance  96  in FIG. 3 may be inductive or capacitive. Beneficially, impedance  96  may comprise the leakage inductance of inductor winding  62 , which would be acceptable since that winding would not be supplying current to a filament of a lamp. Alternatively, since current supply circuit  60  would not be supplying current to a filament of a lamp, a capacitive impedance  96  (FIG. 3) could be used. In this case, moreover, such capacitance can be combined together with the capacitance of capacitor  64  (FIG. 4) so that only a single capacitor is used. 
     On-off indicator circuit  100  operates in the same manner as described above for the previous on-off indicator circuits. That is, circuit  100  provides light when current flows through the lamp, and ceases to provide light when current stops flowing through the lamp. Current in the lamp, in turn, is controlled by control circuit  44 , which shuts off current to the lamp when it senses a lamp fault condition. 
     FIG. 5 shows a ballast arrangement for powering a pair of lamps  28  and  30  that are included in a load circuit  38   c . The same reference numerals as between FIGS. 5 and 2 refer to like parts, and so reference is made to the prior description in connection with FIG.  2 . 
     FIG. 5 shows how the principles of the invention may be used with a so-called current-controlled preheat circuit. A single current path passes through the lamp filaments  28   a ,  28   b ,  30   a  and  30   b , as shown. Capacitors  106  and  108  cooperate with inductor  46  to set a resonant frequency of operation of load circuit  38   c . Capacitor  110  blocks DC current flow through the lamps. 
     An on-off indicator circuit  112  may be embodied as shown at  32  in FIG. 2 or at  90  in FIG. 3, for instance. Circuit  112  is connected across nodes  114  and  116 , which are connected across filament  30   b  of lamp  30 . 
     On-off indicator circuit  112  operates in the same manner as described above for the previous on-off indicator circuits. That is, circuit  112  provides light when current flows through the lamps, and ceases to provide light when current stops flowing through the lamps. Current in the lamps, in turn, is controlled by control circuit  44 , which shuts off current to the lamps when it senses a lamp fault condition. 
     FIG. 6 shows a ballast arrangement, similar to that of FIG. 5, but for powering a single lamp  28  in a load circuit  38   d . The same reference numerals as between FIG.  6  and FIG. 5 or  2  refer to like parts, and so reference is made to the prior description in connection with FIG. 5 or  2 . 
     In FIG. 6, an on-off indicator circuit  112  is connected across nodes  118  and  120 , which are, in turn, connected across filament  28   b  of lamp  28 . Circuit  112  operates in the same manner as the like-numbered circuit in FIG.  5 . 
     FIG. 7 shows another ballasting arrangement, with similarities to aspects of both FIGS. 5 and 4. The same reference numerals as between FIG.  7  and FIG. 5 or  2  refer to like parts, and so reference is made to the prior description in connection with FIG. 5 or  2 . 
     In FIG. 7, a single current path in load circuit  38   e  passes through only two lamp filaments, that is, filaments  28   a  and  30   b . A capacitor  122  serves the same function as capacitors  106  and  108  in FIG.  5 . As in FIG. 2, inductor winding  62  of circuit  60  is coupled to receive energy from inductor  46 , and, in turn, provides current to heat lamp filaments  28   b  and  30   a . As shown, filaments  28   b  and  30   a  are connected in series. However, an alternative arrangement (not shown) for other types of lamps (e.g. so-called T8 lamps) would be to connect filaments  28   b  and  30   a  in parallel. In either case, on-off indicator circuit  112  would be connected across the pair of nodes (e.g.,  124 ,  126 ) on which power to drive the filaments is supplied. 
     On-off indicator circuit  112  is coupled across nodes  124  and  126 , which, in turn, are coupled across lamp filaments  28   b  and  30   a . On-off indicator circuit  112  operates in the same manner as described for the earlier on-off indicator circuits. 
     FIG. 8 shows an alternative to using an LED to provide light to indicate the operational state of a lamp. In particular, FIG. 8 shows an on-off indicator circuit  128  comprised of an incandescent lamp  130 . Nodes  74  and  76  could be those shown in FIG. 2, for instance, or other similar nodes described herein, such as nodes  102  and  104  in FIG.  4 . The typical filament voltage of about 3 volts would be adequate to drive an appropriately chosen incandescent lamp  130 . Such a lamp would not require, as in the case of an LED, means to limit the current to the incandescent lamp so that the lamp can be driven by a constant voltage source, since the impedance of the incandescent lamp limits the current. 
     Exemplary component values for the circuit of FIG. 2 are as follows for UV lamps  28  and  30  each rated at 26-watts, with a voltage from DC source  36  of 470 volts; and with an operating frequency of 48 kHz: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 Switches 40 &amp; 42 
                 Type 3NB50, n-channel, enhancement mode 
               
               
                   
                 MOSFETs, sold by ST Microelectronics, an 
               
               
                   
                 international company 
               
               
                 Lamp filaments 28a, 28b, 
                 2 ohms each 
               
               
                 30a &amp;  30b 
               
               
                 DC blocking capacitor 45 
                 0.1 microfarads 
               
               
                 Resonant inductor 46 
                 2.6 millihenries 
               
               
                 Resonant capacitor 50 
                 3.3 nanofarads 
               
               
                 Capacitors 56, 64 &amp; 72 
                 0.1 microfarads each 
               
               
                 Inductors 54 and 70 
                 Turns ratio with inductor 46 of 7:230 
               
               
                 Inductor 62 
                 Turns ratio with inductor 46 of 9:230 
               
               
                 LEDs 80 and 82 
                 Part No. 160-1052-ND sold by DigiKey of 
               
               
                   
                 Thief River Falls, Minnesota 
               
               
                 Resistors 84 and 86 
                 400 ohms each 
               
               
                   
               
             
          
         
       
     
     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.