Patent Publication Number: US-7218205-B2

Title: Variable candela strobe with constant trigger voltage

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This is a continuation-in-part application of U.S. patent application Ser. No. 10/429,900 filed May 5, 2003 now U.S. Pat. No. 6,856,241, entitled “Variable Candela Strobe”. 

   FIELD OF THE INVENTION 
   The invention pertains to strobe devices of a type used in alarm systems. More particularly, the invention pertains to strobe devices with selectable candela outputs. 
   BACKGROUND OF THE INVENTION 
   Strobe devices require the application of a relatively high voltage across a flash tube in order to produce a gaseous discharge in the tube. In known devices, this high voltage is achieved by using a charge pump to transfer energy to an internal capacitor from an external energy source. The external source typically can be nominally 12 volts or 24 volts. 
   The capacitor is coupled in parallel with the flash tube and provides the energy for the flash. The amount of light from the flash tube is directly proportional to the energy stored in the capacitor that is discharged into the flash tube. 
   A single capacitor can be charged to various voltages in order to provide a multi-candela (multi-intensity) unit. However, there are limitations on the range of candela (intensity) that can be reliably achieved. One problem is that to flash the tube requires that the voltage across it be greater than a predetermined threshold amount (e.g. 180 volts) for reliable operation. 
   Present designs for multi-candela strobes include a range of 15 candela to 100 candela. To achieve such outputs, the capacitor needs to be charged to 240 volts for the 100 candela, but will only need to be charged to 120 volts for the 15 candela output. The 120 volts is, however, below the exemplary 180 volts needed for reliable operation. 
   In order to overcome this low voltage problem, known designs incorporate a voltage booster circuit to increase the voltage across the flash tube. One type of a voltage booster circuit is a voltage doubler circuit. One known voltage doubler design is disclosed in a “flashtubes” EG&amp;G Heimann Optoelectronics Catalog, pg. 7, 1991. This document discloses a voltage doubler circuit to be used with a flash tube. 
   A prior art strobe unit with a known doubler is illustrated in  FIG. 1 . A capacitor C 3  stores the energy that is going to determine the candela of the flash. It is coupled across a series combination of a flash tube LP 1  and a diode D 13 . 
   Capacitor C 13  is the doubler capacitor. It is charged through resistor R 15  to the same voltage Vc as capacitor C 3  is charged. The polarities of the voltages on the capacitors C 3  and C 13  are the same. Capacitor C 4  is used for the trigger function and is charged to the same voltage and polarity as is capacitor C 13 . Hence, the trigger voltage, across capacitor C 4  tracks the value of Vc. As Vc varies, based on desired candela output, so does the trigger voltage. Thus, the trigger voltage may be excessive at high candela outputs. Additionally, at low candela settings, the trigger voltage may not be high enough to produce reliable ignition of the flash tube LP 1 . 
   When the unit is triggered, by a signal from the trigger circuit, SCR Q 8  will conduct and pull node A low. This causes C 4  to discharge through Q 8  and the primary winding of TR 2 , the trigger transformer. 
   Until the flash tube is triggered by the voltage out of the secondary winding of TR 2 , C 13  and C 3  cannot discharge. However, the voltage across the flash tube at this time is double the voltage VC of C 3  (far exceeding the minimum required voltage). In  FIG. 1 , the voltage across capacitor C 4  is the same as the voltage stored in the capacitor C 3 . The turns ratio of transformer TR 2  is designed to provide 6 KV output when the capacitor C 3  has been charged up to about 220 volts. As noted above, at low candela settings, the trigger voltage may be too low to be sure that the tube will be triggered. At high candela settings, the voltage can become so high that the transformer may start to arc and to fail. 
   When the tube flashes, it first discharges capacitor C 13 , then capacitor C 3 . The energy stored in capacitor C 3  provides the preselected candela output from tube LP 1 . 
   While known devices provide a selectable candela output, the use of a voltage doubler does have some disadvantages. At high output intensities, the voltage across the tube LP 1  is substantially equal to 2 VC which can be quite high. This high voltage requires the use of components rated therefore. In addition, in compact units with the circuitry implemented on a printed circuit board, arcing is a potential problem. Further, it would be preferable if the trigger voltage was not dependent on candela setting since the voltage Vc can vary widely as a function thereof. 
   There thus continues to be a need for multi-candela strobe units which provide reliable, triggerable light of a selected intensity. Preferably such reliability could be achieved in compact, high density packaging, without the necessity of high voltage components. It would also be preferable if operational reliability could be achieved while simultaneously eliminating arcing during normal operation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a prior art strobe alarm; 
       FIG. 2A  is a schematic diagram of a first embodiment of the invention; and 
       FIG. 2B  is a schematic diagram of a second embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   While embodiments of this invention can take many different forms, specific embodiments thereof are shown in the drawings and will be described herein in detail with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiment illustrated. 
   The disadvantages of the prior art above can be overcome with a power supply in accordance with the invention. A more controlled voltage is achieved across the flash tube using a voltage booster circuit that is not a “doubler” but rather an “adder” type circuit. The present circuit operates significantly differently than the prior art circuits. The parent application hereto Ser. No. 10/429,900 filed May 5, 2003 and assigned to the assignee hereof is hereby incorporated by reference. 
   It would be advantageous to achieve a broad candela range, for example 15–185 candela, by adjusting the charge pump rate and controller circuit thresholds, and without physically changing component values in the circuit. This means that the voltage across the main energy storage capacitor, C 3  must vary across a wide range. This present problem is solved by the present invention, by making sure that the gas discharge tube is driven within its specified limits even near the ends of the candela output range. 
     FIGS. 2A ,  2 B illustrate two different embodiments  10 ,  10 - 1  of strobe alarms in accordance with the invention. In both  FIGS. 2A ,  2 B the maximum voltage capable of being applied across tube LP 1  is the sum of Vc, the voltage across capacitor C 3  plus the voltage across an off-set circuit  12 ,  FIG. 2A  or  12 - 1 ,  FIG. 2B . This total voltage, as discussed below, will be substantially less than 2 Vc but great enough to produce reliable, repeatable flashing of tube LP 1 . 
   In  FIG. 2A , a power supply PS provides energy for circuit  10 , from a remote source. Supply PS is coupled to controller CC, and charge pump circuit CP as well as other circuitry as would be known to those of skill in the art. The output of charge pump CP couples energy to capacitor C 3  and other components of circuit  10  as discussed subsequently. Trigger circuit TC provides trigger gate signals to SCR Q 8 . 
   Candela selecting switches S or S′ can be coupled to either controller CC or charge pump CP without limitation. Switch settings can be established manually, electronically or both without limitation. 
   In  FIG. 2A , an off-set circuit  12  includes a Zener diode D 11  which is coupled, in parallel, with the capacitor C 13 . Diode D 11  causes the voltage of node A′, Q 8 , C 4 ′, and C 13 ′ to be clamped to a known, predetermined voltage (e.g. 100 volts). This voltage is limited by the off-set circuit  12 , for example by the voltage across D 11 . It will not vary with the different charging voltages of C 3  used to obtain the different candela rating. Alternate sources, such as a power supply could be used instead of capacitor C 13  without limitation. 
   When the capacitors have been charged, the voltage across the flash tube is VFT=Vc+VD 11  at the time that the tube LP 1  is flashed. The addition of the diode D 11  limits the voltage variations across tube LP 1 . It is possible to control the total voltage across the flash tube LP 1  to tighter limits than is achieved by a prior art voltage doubler. The tighter voltage range is demonstrated in Table 1 which compares a voltage limiter as in  FIG. 2A  to a doubler, as in  FIG. 1 . 
   In the embodiment of  FIG. 2A , the flash tube LP 1  requires 180 volts minimum to flash reliably. Table 1 illustrates that both the circuits of  FIG. 1  and  FIG. 2A  create voltages greater than 180 volts to satisfy this requirement. However, at the highest candela rating, the circuit of  FIG. 2A  applies 140 volts less to tube LP 1  than does the circuit of  FIG. 1 . This is a significant difference. 
   The limiter circuit  12  couples a smaller range of voltages across the flash tube LP 1  while operating from 15 candela to 100 candela than do the prior art doubler-circuits. The lower over-all voltage translates to reduced breakdown voltage specifications for the various components (less expensive) and possibly to a more compact spacing, higher density of circuit board points, without arcing, than is the case with the voltage doubler configuration of  FIG. 1 . 
   
     
       
         
             
             
             
           
             
               TABLE 1 
             
             
                 
             
             
               Candela 
               Voltage With 
               Voltage With 
             
             
               Rating 
               Limiter 12 
               Doubler 
             
             
                 
             
           
          
             
               15 
               220 
               240 
             
             
               30 
               240 
               280 
             
             
               110  
               340 
               480 
             
             
                 
             
          
         
       
     
   
   It will be understood that the specific minimum threshold voltage to flash the tube LP 1  reliably may vary as a result of tube geometry, gas and the like without limitation. Such variations come within the spirit and scope of the invention. As discussed below, for a given tube and geometry, it is preferable to provide a constant trigger voltage irrespective of candela setting. Both circuits  10 ,  10 - 1  produce a stable trigger voltage. Table 1 makes it clear that the voltage across the flash tube varies less across a range of different candela outputs with the limiter circuit  12  than in the prior art. Both capacitor values and the value of the constant voltage used in the limiter circuit of  FIG. 2A  can be changed to different values without departing from the scope of this invention. Even with such variations in circuit components, a constant trigger voltage will still be applied to tube LP 1 . 
     FIG. 2B  illustrates an alternate embodiment  10 - 1 . Components common to those in  FIG. 2A  have been designated with the same identification numerals. In  FIG. 2B , a limiting circuit  12 - 1  includes capacitor C 13  and Zener diode D 11 , between node A″ and ground. Circuit  12 - 1  limits the total voltage across LP 1  prior to a trigger event. In circuit  12 - 1 , current through Zener D 11  does not flow through diode D 13 . 
   As illustrated in  FIG. 2B , Zener diode D 11  sets the trigger voltage across capacitor C 4  to about 100 volts. This voltage is independent of candela setting. This provides a stable trigger circuit input voltage. 
   The trigger voltage is generated by step-up transformer TR 2 . The output of transformer TR 2  is intended to provide about 6 KV at the outside surface of the tube, or bulb LP 1 . Based on a 100 volt input, the 6 KV output initiates a flash sequence. This corresponds to a turns ratio on the order of 60:1. Other turns ratios come within the spirit and scope of the invention. This high voltage is produced when the switching action of the circuit causes the voltage across capacitor C 4  to be coupled to the primary side of transformer TR 2 . 
   The voltage across the flash tube LP 1  can also be established by the use of a non-booster type circuit design. In this embodiment, two capacitors are still used. They are coupled in parallel, not in series with the flash tube as in the prior art. 
   In summary, the embodiments  10 , and  10 - 1  illustrate driving circuits which provide alternates to voltage doubler circuitry for purposes of generating selectable candela output levels in a strobe alarm. In all instances, a voltage substantially less than twice the voltage on the major illumination providing storage element is added to that voltage to initiate ignition of an ionization gas discharge tube. Subsequently, the energy stored in the primary storage capacitor is used to provide the selected candela output level for the circuit. A constant trigger voltage is provided to drive the trigger electrode of the discharge tube via the step-up transformer. 
   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.