Abstract:
A variable output strobe unit includes a variable current limiting circuit to limit peak current draw after each flash. Parameters of strobe output and current limiting circuit are related and can be set manually or electronically.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention pertains to alarm indicating output devices. More particularly, the invention pertains to such devices which emit visual outputs on a periodic basis while limiting peak current requirements.  
       BACKGROUND  
       [0002]     Strobe units are often used as visual alarm indicating output devices in fire alarm systems. As is known, such units emit a high intensity light periodically, for example once a second, to provide an ongoing indication that an alarm condition has been detected somewhere in the region being monitored. One such unit has been disclosed in U.S. patent application Ser. No. 10/040,968 filed Jan. 2, 2002 for Processor Based Strobe with Feedback assigned to the Assignee hereof and incorporated by reference herein.  
         [0003]     Known units include an energy storage device, for example one or more capacitors, coupled to a gas discharge tube. When the tube is triggered with an appropriate control signal it emits high intensity light while discharging the storage device.  
         [0004]     Known strobe units exhibit maximum peak current values subsequent to discharge of the storage element when the tube is triggered. The peak or surge current is primarily due to the fact that electrolytic capacitors in the device need to be recharged for the next flash.  
         [0005]      FIG. 1  illustrates a representative timing diagram of peak capacitor recharge current values I REP . These peak current values are of a type exhibited by known strobe units each time the gas filled tube is triggered. At startup, a substantially larger initial current surge I 0 , which might be as large as 10 amps is exhibited by known units. In contrast, the peak repeating current values I REP  fall in a range of 5 to 7 amps. In contradistinction, the steady state I RMS  current typically falls in range of 50 milliamps to 800 milliamps.  
         [0006]     It is also known that the magnitudes of the peak initial current surge I 0  as well as the repetitive peak current values I REP  vary continuously, from one second to the next, in response to discharge characteristics of the capacitors, the form of electrical energy being supplied to the unit as well as the phase thereof.  
         [0007]     In view of the fact that the initial peak current draw I 0  as well as the repetitive peak current draw I REP  are exhibited by each of the strobe devices in the system it would be desirable to be able to limit not only the initial peak current surge but also the repetitive ongoing current surges as the unit flashes. Preferably, limiting the amplitudes of the peak current surges can be done without affecting the ability of the units to recharge adequately during the available one-second period to provide the next flash of light. Additionally, it would be desirable if peak current limiting could be achieved without substantial increase in heat generated by the respective strobe units or without substantially increasing the size, cost or manufacturing complexity of such units. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a graph illustrating current draw characteristics of an exemplary prior art strobe unit;  
         [0009]      FIG. 2  is a block diagram of a strobe unit in accordance with the invention;  
         [0010]      FIG. 3  is a graph illustrating reduced peak current draw of the unit of  FIG. 2 ;  
         [0011]      FIG. 4  is a partial schematic diagram of one embodiment of the strobe unit of  FIG. 2 ;  
         [0012]      FIG. 5  is a partial schematic diagram of another embodiment of the strobe unit of  FIG. 2 ;  
         [0013]      FIG. 6  is a partial schematic diagram of another embodiment of the strobe unit of  FIG. 2 ;  
         [0014]      FIG. 7  is a partial schematic diagram of yet another embodiment of the strobe unit of  FIG. 2 ; and  
         [0015]      FIG. 8  is a system of strobe units as in  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     While this invention is susceptible of an embodiment in many different forms, there are shown in the drawing and will be described herein in detail specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principals of the invention. It is not intended to limit the invention to the specific illustrated embodiments.  
         [0017]      FIG. 2  illustrates a device  10  in accordance with the present invention. Device  10 , which in exemplary form is illustrated as a warning or emergency indicating strobe unit as an exemplary application only, incorporates a housing  12  and at least a pair of input terminals  14   a,b.    
         [0018]     The input terminals  14   a, b  receive electrical energy and/or control signals from a remote switchable source  16 . For example and without limitation, source  16  could provide a reversible 5 to 24 volt input to terminals  14   a, b  to energize and control the device  10 . In a first mode, the power supply  16  could apply a negative 5 volts between the terminals  14   a, b  which would be a nonoperational condition but could be used for supervision purposes.  
         [0019]     To activate the device  10 , the source  16  could reverse polarity and couple a plus 24 volts across the terminals  14   a, b  along with embedded control signals as desired. Those of skill in the art will understand how such systems work in general in connection with warning or alarm indicating output devices wherein a device such as the device  10  could be used.  
         [0020]     Device  10  further includes a current sensor  20  coupled to a current regulator  22 . An output from the current sensor  20  is also coupled to a comparator  24 . A second input to the comparator  24  is received from a set peak current and illumination level element  28 . Both the current regulator  22  and the illumination parameter setting element  28  are coupled to charging circuitry  30 .  
         [0021]     The charging circuitry  30  is in turn coupled to one or more energy storage devices, such as capacitors and/or inductors or the like,  34  as would be understood by those skilled in the art. The energy storage devices  34  are in turn coupled to a gas filled member or tube  36 .  
         [0022]     As is conventional in the art, the tube or member  36  can be energized with energy stored in devices  34  and triggered by charging circuit  30 , trigger line  30   a . When triggered, a device  36 , due to ionized gases therein, emits an intense radiant energy output R while discharging the energy storage devices  34 .  
         [0023]     The process of recharging the energy storage devices  34  causes a greater than normal current draw via terminals  14   a, b . A peak value of this current draw can be limited in device  10  as a result of an output  20   a  of current sensor  20  moving away from a set point established by the set peak current element  28 , line  28   a . This difference, via comparator  24  is coupled to regulator  22  which in turn increases an input impedance of the device  10  thereby limiting the peak value of recharge or surge current of the device  10 .  
         [0024]      FIG. 3  illustrates a reduced value of peak recharge current achievable with device  10 .  FIG. 3  is plotted on the same scale and time base as is  FIG. 1 . As is apparent from a comparison of  FIGS. 1 and 3 , device  10  with the current sensor  20  and comparator  24  providing control inputs to current regulator  22  exhibits a substantial reduction in peak surge current.  
         [0025]     The peak repeating surge current I REP  of  FIG. 1  can be reduced from a range of 5 to 7 amps for example to a selectable range based on the type of application and imposed maximum, surge current values as illustrated in  FIG. 3 .  
         [0026]      FIG. 4  illustrates in more detail an exemplary embodiment of current sensor  20 , regulator  22  and comparator  24  configured to limit peak surge current in a device such as warning or alarm device  10 . As illustrated in  FIG. 4 , current sensor  20  can be implemented with resistor R 1 . Comparator  24  can be implemented using transistor Q 2 . Regulator  22  can be implemented using field effect transistor Q 1 , a Zener diode Z 1  and resistor R 2 . An output from the drain D of regulator transistor Q 1  is in turn coupled to charging circuit  30 .  
         [0027]     In a normal operating condition, between flashes, where source  16  is applying a positive 24 volt potential to terminals  14   a, b  as illustrated in  FIG. 4 , Zener diode Z 1  in combination with resistor R 2  establish a bias for regulator transistor Q 1  resulting in an input current I IN  corresponding to the steady state current I RMS  illustrated in  FIGS. 1 and 3 . In this condition transistor Q 2  is biased off.  
         [0028]     The drop across resistor R 1  in combination with current I RMS  is insufficient to turn on transistor Q 2 . Current limiting becomes effective when transistor Q 2  turns on. This will occur when the drop across resistor, R 1  substantially equals or slightly exceeds the voltage necessary to forward bias base-emitter junction of transistor Q 2  which will be on the order of about 0.6 volts. This will take place when the current I IN  increases toward I PEAK  in response to needing to recharge the energy storage devices  34 .  
         [0029]     As I IN  increases, transistor Q 2  conducts which in turn raises the gate voltage at node  22   a . Increasing the gate voltage at node  22   a  reduces the magnitude of the gate-to-source voltage of transistor Q 1  which in turn reduces current flow through Q 1 .  
         [0030]     A circuit as in  FIG. 4  can be incorporated into device  10  to limit peak surge currents, as in  FIG. 3 , where only a single candela output is desired from device  10 .  
         [0031]      FIG. 5  illustrates variable input circuitry  50  usable in device  10 . Circuit  50  would in turn be coupled to charging circuit  30 . Structural elements common to the circuit of  FIG. 4  and the circuit of  FIG. 5  have been assigned the same identifiers.  
         [0032]     Circuitry  50  includes potentiometer R 5  which provides a manually or electrically adjustable analog input, voltage V B  which can be varied to adjust the peak value of the surge current I PEAK  which occurs as the energy storage elements  34  are recharged each time the tube  36  is flashed. Voltage V B  is used to adjust and vary current I 0  via a transistor Q 3 .  
         [0033]     In the configuration  50  of  FIG. 5 , the turn-on point for transistor Q 2  corresponds to the voltage drop across resistor R 3  plus the base emitter voltage of transistor Q 2 . In this regard, the transistor Q 3 , resistor R 4  and resistor R 5  in combination form a variable current source for the current I 0 . Hence, by adjusting resistor R 5  the current I 0  can be adjusted which in turn alters the voltage across resistor R 3  and the turn on point for transistor Q 2 . Table 1 illustrates exemplary peak values of input current I IN  for configuration  50  for various values of V B . Those of skill in the art will understand that I PEAK  can be varied based on values chosen for R 1 , R 3  and R 4 .  
                                         TABLE 1                                   V B  (Volts)   I PEAK  (mA)                                        0   200           1   300           2   500           3   700           4   900           5   1100                      
 
         [0034]     It will be understood that a variety of circuit configurations could be used to implement a system having a block diagram of the system  10  all without departing from the spirit and scope of the present invention. Similarly, neither specific semiconductor types nor specific component values represent a limitation of the present invention. Those with skill will understand that where the device  10  is intended to provide a multi candela output, the circuit  50  would be adjusted to an appropriate peak current value in accordance with a desired candela output.  
         [0035]      FIG. 6  illustrates a circuit configuration  50 ′ with a plurality of different peak surge currents selectable via a mechanical or electrical switch indicated generally at S 1 . Switch S 1  sets the voltage V B  to provide the selected maximum surge current. Via a line  28   b  indicated in phantom, that setting is also coupled to charging circuit  30  to set the selected respective candela output from member  36 . Hence, switch S 1  enables an installer to simultaneously set the desired output candela as well as limit the surge current to a predetermined maximum associated with the selected candela output.  
         [0036]     Where appropriate, the circuitry  50 ′ can be used to limit initial surge current I 0  to be less than or equal to 10 times the average current I RMS  the unit  10  draws. Additionally, the peak surge current I P  can be limited so that it is not greater than 5 times the average current draw by the unit  10 , I RMS , between output pulses.  
         [0037]      FIG. 7  illustrates yet another embodiment of a circuit  50 ″ which incorporates a programmed processor  54  in combination with the plurality of resistors R 5 A . . . R 5 D to set the voltage V B . Those with skill will understand that data provided to the processor  54  can in turn cause that processor to select one or more of the resistors R 5 B . . . R 5 D alone or in combination so as, via transistor Q 3 , to set the peak surge current for the unit  10  as discussed above.  
         [0038]     It will be understood that neither the exact form of current sensor  20  nor comparator  24  are limitations of the present invention.  
         [0039]     As illustrated in  FIG. 8 a  plurality  60  of units  10  such as  10 - 1 ,- 2  . . . - w  could be driven via switchable source  16  through a wire medium, such as medium  16   a  as might be used in a monitoring or alarm system  62 . Such systems have been disclosed and claimed in U.S. Pat. No. 5,598,139 for fire detecting system synchronized strobe lights and U.S. Pat. No. 5,850,178 for alarm system having synchronizing pulse generator and synchronizing pulse motion detector both of which were assigned to the Assignee hereof and incorporated by reference herein.  
         [0040]     Those of skill will understand that alarm control circuits  64 , in response to alarm indicating signals from detectors  66  could cause supply  16  to switch from −5 volts applied to medium  16   a , strobe units inactive, to plus 24 volts to activate strobes  60 . In such systems, current limiters, as described above are especially advantageous in that they minimize peak surge currents produced by numerous strobe units  10  triggering and recharging at the same time.  
         [0041]     From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.