Abstract:
Optical element driving circuits flexibly configure energy sources to cause a flash tube to produce illumination at one of multiple output intensities. The driving circuits allow a single strobe alarm to take the place of multiple strobe alarms individually dedicated to specific output intensities. The driving circuits may also mitigate or eliminate high voltage arcing within the driving circuit.

Description:
PRIORITY CLAIM 
   This application is a Continuation of application Ser. No. 11/432,120, filed 11 May 2006, titled “Optical Element Driving Circuit.” 

   BACKGROUND OF THE INVENTION 
   1. Technical Field 
   This disclosure relates to optical element driving circuits. In particular, this disclosure is directed to flexible driving circuits which may produce any of multiple different output intensities from a flash tube. 
   2. Related Art 
   Visual emergency warning systems, including strobe alarms, have recently incorporated output intensity adjustments. The intensity adjustments allow the warning systems to output light at different intensities, thereby eliminating the need for a manufacturer to produce multiple separate devices each with a fixed output intensity. The ability to adjust the intensity of the light output provides an installer the flexibility to adapt one model of a strobe alarm for many different environments, each of which may call for a different output intensity. To adapt the warning system for any particular environment, an installer configures the strobe alarm (e.g., using a switch or a jumper) at the time of installation to select one of the output intensities that the strobe alarm supports. 
   Many strobe alarms include basic driving circuits which rely on a step-up transformer to prime a flash tube for illumination and a voltage doubler to start the flash tube. At high candela settings, the high voltages in the driving circuits can cause damaging arcing at and around the flashtube, step-up transformer and the voltage doubler. Therefore, a need exists for an optical element driving circuit that provides the flexibility of different light output intensities and reliable flash tube operation and which also mitigates or eliminates high voltage arcing. 
   SUMMARY 
   The present disclosure describes optical element driving circuits. An installer may configure the driving circuits to select a specific output intensity. The driving circuits also exercise intelligent control over the voltages developed to mitigate or eliminate arcing. 
   In one implementation, an optical element driving circuit includes a first energy source, a second energy source, and trigger input. The trigger input is coupled to an optical element triggering circuit. The optical element driving circuit additionally includes a boost control input and a boost circuit. The boost control input is responsive to a selected output intensity. The boost circuit is selectively configurable in response to the boost control input. In a first circuit configuration, the first energy source, but not the second energy source, drives an optical output element. In a second circuit configuration, the first and second energy sources both drive the optical output element. 
   In another implementation, an optical element driving circuit includes a first energy source and a second energy source that drive an optical output element. The optical element driving circuit additionally includes a trigger input that is coupled to an optical element trigger circuit. The optical element driving circuit further includes a bypass circuit input and a bypass circuit. The bypass circuit input is responsive to a selected output intensity. The bypass circuit is selectively configurable in response to the bypass circuit input to bypass a voltage control circuit. In a first configuration, the first and second energy sources are charged to substantially the same voltage. In a second configuration, the bypass circuit and the voltage control circuit cause the second energy source to charge to a voltage that is different than the voltage of the first energy source. 
   Other systems, methods, features and advantages of the invention will be, or will become, apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the following claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The optical element driving circuits can be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like referenced numerals designate corresponding parts or elements throughout the different views. 
       FIG. 1  is a circuit diagram of an optical element driving circuit. 
       FIG. 2  is a circuit diagram of an optical element driving circuit. 
       FIG. 3  is a circuit diagram of an optical element driving circuit. 
       FIG. 4  is a circuit diagram of an optical element driving circuit. 
       FIG. 5  shows a microcontroller which may control an optical element driving circuit. 
       FIG. 6  is a flow diagram of the acts which an illumination control program may take to control the optical element driving circuit. 
       FIG. 7  shows another implementation of the optical element driving circuit shown in  FIG. 1 . 
       FIG. 8  is a circuit for generating a voltage doubling signal and a trigger signal for the optical element driving circuit shown in  FIG. 7 . 
       FIG. 9  is a circuit diagram of an optical element driving circuit. 
       FIG. 10  is a circuit diagram of an optical element driving circuit. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows an optical element driving circuit  100  for an optical element  101 . In one implementation, the optical element driving circuit  100  is a flash tube driving circuit and the optical element  101  is a flash tube. The optical element driving circuit  100  includes energy sources such as a trigger capacitor  102 , an illumination capacitor  106 , and a doubling capacitor  108 . The optical element driving circuit  100  also includes a step-up transformer  104 , a doubling silicon-controlled rectifier (“SCR”)  110 , a diode  112 , a trigger SCR  114 , and a trigger zener diode  116 . Control logic  121 , such as a microcontroller, intelligently controls a switch  118  as will be explained in more detail below. A charge pump  120 , or other power supply, charges the trigger capacitor  102 , illumination capacitor  106  and doubling capacitor  108 . The trigger capacitor  102 , step-up transformer  104 , and trigger SCR  114  form an optical element triggering circuit  103 . The doubling capacitor  108 , doubling SCR  110 , and switch  118  form a configurable boost circuit  109 . 
   When a trigger signal is applied to a trigger input  122  coupled to the optical element triggering circuit, the flash tube is primed for illumination and one or more energy sources are placed across the flash tube for illumination. Specifically, when a trigger signal is applied to the trigger SCR  114 , the energy stored in the trigger capacitor  102  energizes the primary winding of the step-up transformer  104 . The secondary of the step-up transformer  104  provides a high voltage output which causes initial ionization of the gas in the flash tube  101  to prime the flash tube for illumination. The trigger signal additionally causes the boost circuit to selectively place the voltage of either the illumination capacitor  106  or both the illumination capacitor  106  and the doubling capacitor  108  across the flash tube  101  for illumination depending on the setting of the switch  118 . The switch  118  may be set to place only the illumination capacitor  106  across the flash tube  101  for selected output intensity settings (e.g., high candela settings), while the switch  118  may be set to place both the illumination capacitor  106  and the doubling capacitor  108  across the flash tube  101  for other selected output intensities (e.g., low candela settings). 
   The charge pump  120  (or other power supply) charges the illumination capacitor  106  and the doubling capacitor  108  to the full voltage selected according to the desired output intensity. For example, for relatively low candela output, the illumination capacitor  106  and the doubling capacitor  108  may be charged to 140 volts for 15 candela output and 185 volts for 30 candela output. Similarly, for relatively high candela output, the illumination capacitor  106  and the doubling capacitor  108  may be charged to 250 volts for 75 candela output and 286 volts for 110 candela output. Any of the voltages, capacitances, or types of energy sources may be modified, adjusted, or substituted to provide any desired set of output intensities. 
   The charge pump  120  charges the trigger capacitor  102  through a resistor. However, the voltage on the trigger capacitor  102  is controlled by the trigger zener diode  116  so that it does not rise above, for example, 180 volts. As a result, arcing that might ordinarily occur due to the large step-up voltage ratio (e.g., 1 to 36-38) of the transformer  104  may be avoided. 
   In one implementation, the circuit may additionally include a high frequency filter capacitor  119  connected in parallel with the illumination capacitor  106 . The filter capacitor  119  helps to reduce noise in the optical element driving circuit  100 . More specifically, the filter capacitor  119  absorbs high frequency transients in the charging pulses that charge the trigger capacitor  102 , illumination capacitor  106 , and doubling capacitor  108 . 
   The control logic  121  applies a trigger signal at a trigger input  122 . In response to the trigger signal, the trigger SCR  114  conducts. When the trigger SCR  114  conducts, a circuit is completed for the trigger capacitor  102  to energize a primary coil of the step-up transformer  104 . A secondary coil of the step-up transformer  104  includes one lead connected to ground a second lead connected to the flash tube  101 . When the primary coil is energized, the secondary coil generates a damped multi-KV oscillation which is applied to the outside of the flash tube  701 . In one implementation, the voltage developed across the pair of leads of the secondary coil has a maximum value of about 5,500 V at 15 candela output to about 6,900 V at 110 candela output. The high voltage output of the transformer secondary coil causes an initial ionization of the gases inside the flash tube  101 . The flash tube  101  is then primed for current flow through the tube  101  to generate illumination. 
   The step-up transformer  104  has a large step-up ratio (e.g., 1 to 36-38) so that the magnitude of a voltage input to the step-up transformer is significantly increased. However, the trigger zener diode  116  controls the voltage on the trigger capacitor  102  so that the step-up transformer  104  does not generate such an excessive voltage that arcing results. 
   For relatively low candela settings such as 15 and 30 candela, the control logic  121  closes the switch  118 . The trigger signal at the trigger input  122  is thereby provided to the doubling SCR  110 . When the doubling SCR  110  conducts, the doubling capacitor  108  is placed in series with the illumination capacitor  106  across the flash tube  101 . Therefore, even though the doubling capacitor  108  and illumination capacitor  106  are individually charged to a relatively low voltage, that voltage is doubled across the tube to reliably start the tube. The charge on the doubling capacitor  108  dissipates through the doubling SCR  110  and the illumination capacitor  106  discharges through the flash tube  101  and diode  124 , causing the flash tube  101  to start and emit light at the selected output intensity. 
   The diode  112  provides a voltage clamp to prevent voltage oscillations at the doubling capacitor  108  from going too negative. The diode  112  additionally protects the doubling SCR  110  from voltage ringing at the doubling SCR  110 . Ringing at the SCR  110  can decrease the normal lifespan of the SCR  110 . The diode  112  provides better clamping response than a zener diode and therefore provides increased protection for the SCR  110 . 
   For relatively high candela settings such as 75 or 110 candela, the control logic  121  opens the switch  118 . The trigger signal initiates initial ionization in the flash tube  101 . The illumination capacitor  106 , which has been charged to a voltage high enough to reliably start the tube, dissipates through the flash tube  101  and diode  124  causing the flash tube  101  to start and emit light at the selected output intensity. While the doubling capacitor  108  has been charged to the same voltage as the illumination capacitor  106 , the doubling capacitor does not assist with starting the flash tube  101  or delivering illumination energy through the flash tube  101 . 
   As noted above, the control logic  121  selectively opens or closes the switch  118  to intelligently control the voltages applied to the flash tube  101  and the transformer  104  in the circuit  100 . When the switch  118  is closed, the trigger signal causes the doubling SCR  110  to conduct and complete a circuit to bring the doubling capacitor  108  in series with the illumination capacitor  106  across the flash tube  101 . When the switch  118  is open, there is no path for the trigger signal to reach the doubling SCR  110 . Accordingly, the boost circuit is configured to drive the flash tube  101  with the illumination capacitor  106  and not the doubling capacitor  108 . The switch  118  may be a manually adjustable circuit, such as a switch, jumper, or other circuit. The switch  118  may also be a transistor, logic gate, or other switch circuit opened or closed under control of the control logic  121 . 
   The optical element driving circuit of  FIG. 1  operates in two modes. For relatively low output intensities (e.g., intensities for which the voltage on the illumination capacitor  106  alone may not be sufficient to reliably start the flash tube  101 ), the boost circuit  109  implements a first circuit configuration in which the optical element driving circuit uses a voltage doubler to drive the flash tube  101  with both the illumination capacitor  106  and the doubling capacitor  108  in series. For relatively high output intensities, both the illumination capacitor  106  and the doubling capacitor  108  are fully charged, but the boost circuit  109  implements a second circuit configuration which drives the flash tube  101  only with the illumination capacitor  106 . 
   Table 1 shows examples of component values for the optical element driving circuit  100 . Table 2 shows examples of output intensities and capacitor voltages. 
   
     
       
             
             
             
           
             
             
             
             
           
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Component 
               Component Value 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Trigger Capacitor 102 
               0.047 
               uF 
             
             
                 
               Illumination Capacitor 106 
               68 
               uF 
             
             
                 
               Doubling Capacitor 108 
               0.047 
               μF 
             
             
                 
               High Frequency Filter Capacitor 119 
               0.0005 
               μF 
             
             
                 
               Zener diode 116 
               182 
               V 
             
           
        
         
             
                 
               Transformer step-up ratio 
               36-38 to 1 
             
             
                 
                 
             
           
        
       
     
   
   
     
       
             
             
             
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
               Output 
               Illumination 
               Doubling 
                 
               Trigger 
             
             
               Intensity 
               Capacitor 106 
               Capacitor 108 
               Tube 101 
               Capacitor 102 
             
             
                 
             
           
           
             
               15 cd 
               144 V 
               144 V 
               288 V 
               144 V 
             
             
               30 cd 
               185 V 
               185 V 
               370 V 
               182 V 
             
             
               75 cd 
               250 V 
               250 V 
               250 V 
               182 V 
             
             
               110 cd  
               286 V 
               286 V 
               286 V 
               182 V 
             
             
                 
             
           
        
       
     
   
     FIG. 2  shows an alternative implementation of an optical element driving circuit  200  for an optical element  201 . The optical element driving circuit  200  includes energy sources such as a trigger capacitor  202 , an illumination capacitor  206 , and a boost capacitor  208 , as well as a step-up transformer  204 . The circuit  200  also includes a voltage control circuit  211  such as a voltage control zener diode  210  and a bypass circuit  212 . The circuit  200  also includes a trigger diode  214 , and a trigger SCR  216 . The trigger capacitor  202 , step-up transformer  204 , trigger diode  214 , and trigger SCR  216  form an optical element triggering circuit  203 . 
   A charge pump  218  (or other power supply) charges the trigger capacitor  202 , illumination capacitor  206 , and boost capacitor  208 . The charge pump charges the illumination capacitor  206  to the full voltage selected according to the desired output intensity. When the bypass circuit  212  is active, the charge pump charges the boost capacitor  208  and trigger capacitor  202  to the full voltage by providing a current path around the voltage control zener diode  210 . When the bypass circuit  212  is inactive, the charge pump charges the boost capacitor  208  and the trigger capacitor  202  to the full voltage minus the voltage across the voltage control zener diode  216 . 
   The bypass circuit  212  may be implemented in many different ways.  FIG. 2  shows an example in which the bypass circuit  212  includes a pnp transistor controlled by the applied base voltage. In other implementations, the bypass circuit  212  may employ a Field Effect Transistor (FET), switch, jumper, or other switch circuit to selectively bypass the voltage control zener diode  210 . 
   A trigger signal applied to the trigger SCR  216  causes the energy stored in the trigger capacitor  202  to energize the primary winding of the step-up transformer  204 . The secondary winding generates a damped oscillating high voltage signal to perform first stage ionization in the flash tube  201  to prepare for illumination. The SCR  216  additionally places the illumination capacitor  206  and the boost capacitor  208  in series across the flash tube  201 . The relatively small amount of energy in the boost capacitor  208  discharges first through the SCR  216 . The trigger diode  214  protects the SCR from voltage ringing. 
   The total voltage provided by the illumination capacitor  206  and the boost capacitor  208  allows the flash tube  201  to start. Accordingly, the illumination capacitor  206  discharges through the flash tube  201  and the diode  209 . This discharge produces the selected output intensity. Control logic  219 , such as a microcontroller, selectively activates or deactivates the bypass circuit  212  to intelligently control the voltages applied to the flash tube  201  and the transformer  204  in the circuit  200 . 
   For relatively low output intensities such as 15 or 30 candela, doubling the voltage on the illumination capacitor  206  across the flash tube  201  may still result in a total voltage across the flash tube  201  that avoids arcing. Accordingly, the control logic  219  may assert a bypass control signal on the bypass control line  240  to activate the bypass circuit  212 . Therefore, the charge pump  218  fully charges the illumination capacitor  206  and the boost capacitor  208  (e.g., to 144 volts for 15 candela, or 185 volts for 30 candela). When the control logic  219  asserts a trigger signal on the trigger control line  242 , the illumination capacitor  206  and the boost capacitor  208  are placed in series across the tube. Because both capacitors are charged to the same voltage, the driving circuitry acts as a voltage doubler for the low output intensity modes, and reliably starts the flash tube  201 . 
   For relatively high output intensities, such as 75 or 110 candela, the control logic de-asserts the bypass control signal on the bypass control line  240  to deactivate the bypass circuit  212 . The charging path for the boost capacitor  208  and the trigger capacitor  202  therefore includes the voltage control zener diode  210 . The charge pump  218  fully charges the illumination capacitor  206  (e.g., to 250 volts for 75 candela or 286 volts for 110 candela). However, the voltage control zener diode  210  controls the voltage on the boost capacitor  208  and the trigger capacitor  202 . In particular, the voltage on the boost capacitor  208  and the trigger capacitor  202  charge to the full charging voltage minus the drop (e.g., 90 volts) across the voltage control zener diode. 
   When the control logic  219  asserts a trigger signal on the trigger control line  242 , the illumination capacitor  206  and the boost capacitor  208  are placed in series across the tube. Because the boost capacitor is charged to a lower voltage than the illumination capacitor, less than double the voltage on the illumination capacitor is placed across the tube. Nevertheless, the tube starts reliably because the total voltage is still sufficient to start the tube. Furthermore, because the trigger capacitor voltage is also controlled, the high voltage oscillation applied from the transformer secondary has a lower maximum value than it otherwise would. The voltage control zener diode thereby helps to prevent two types of high voltage arcing in the circuit  200 : arcing from the total voltage applied to the flash tube  201 , and arcing from the high voltage secondary winding of the transformer  204 . 
   The optical element driving circuit  200  operates in two modes. In a low output intensity mode, the charge pump fully charges the illumination capacitor  206  and the doubling capacitor  208 . In the low intensity mode, the optical element driving circuit  200  uses a voltage doubler to place both the illumination capacitor  206  and the doubling capacitor  208  in series across the flash tube  201  when the optical element driving circuit  200  is triggered. In a high output intensity mode, the charge pump fully charges the illumination capacitor  206 , but the voltage control zener diode  210  controls the voltage on the boost capacitor  208 . When the optical element driving circuit  200  is triggered, the optical element driving circuit  200  places both the illumination capacitor  206  and the boost capacitor  208  in series across the flash tube, but less than double the voltage on the illumination capacitor is applied to the flash tube  201   
   Table 3 shows examples of component values for the optical element driving circuit  200 . Table 4 shows examples of output intensities and capacitor voltages 
   
     
       
             
             
             
           
             
             
             
             
           
             
             
             
           
         
             
                 
               TABLE 3 
             
             
                 
                 
             
             
                 
               Component 
               Component Value 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Trigger Capacitor 202 
               0.047 
               μF 
             
             
                 
               Illumination Capacitor 206 
               68 
               μF 
             
             
                 
               Boost Capacitor 208 
               0.047 
               μF 
             
             
                 
               Voltage Control Zener Diode 210 
               90 
               V 
             
           
        
         
             
                 
               Transformer step-up ratio 
               36-38 to 1 
             
             
                 
                 
             
           
        
       
     
   
   
     
       
             
             
             
             
             
           
         
             
               TABLE 4 
             
             
                 
             
             
               Output 
               Illumination 
               Doubling 
                 
               Trigger 
             
             
               Intensity 
               Capacitor 206 
               Capacitor 208 
               Tube 201 
               Capacitor 202 
             
             
                 
             
           
           
             
               15 cd 
               144 V 
               144 V 
               288 V 
               144 V 
             
             
               30 cd 
               185 V 
               185 V 
               370 V 
               185 V 
             
             
               75 cd 
               250 V 
               160 V 
               410 V 
               160 V 
             
             
               110 cd  
               286 V 
               196 V 
               482 V 
               196 V 
             
             
                 
             
           
        
       
     
   
     FIG. 3  shows an alternative implementation of an optical element driving circuit  300  for an optical element  301 . The optical element driving circuit  300  includes energy sources such as a trigger capacitor  302 , an illumination capacitor  306 , and a boost capacitor  308 . The circuit  300  also includes a voltage control circuit  311 . In the example shown in  FIG. 3 , the voltage control circuit  311  includes a voltage control zener diode  310  and a bypass circuit  312 . The trigger capacitor  302 , step-up transformer  304 , and trigger SCR  314  form an optical element triggering circuit  303 . 
   A charge pump  318  (or other power supply) charges the trigger capacitor  302 , illumination capacitor  306 , and boost capacitor  308 . The charge pump charges the trigger capacitor  302 , illumination capacitor  306 , and boost capacitor  308  to the full voltage determined by the control logic  321  according to the desired output intensity. When the control logic  321  activates the bypass circuit  312  and when the trigger event occurs, the full voltage is applied from the boost capacitor  308  and the trigger capacitor  302  by providing a current path around the voltage control zener diode  310 . When the control logic  321  deactivates the bypass circuit  312  and when the trigger event occurs, the full voltage to which the boost capacitor  308  and the trigger capacitor  302  were originally charged is effectively reduced by an amount equal to the voltage drop across the control zener diode  310 . Accordingly, the total voltage across the flash tube is less than double the voltage on the illumination capacitor  306 , and the high voltage on the transformer secondary is controlled to help prevent arcing. 
   The bypass circuit  312  may be implemented in many different ways.  FIG. 3  shows an example in which the bypass circuit  312  includes a pnp transistor controlled by the applied base voltage. In other implementations, the bypass circuit  312  may employ a Field Effect Transistor (FET), switch, jumper, or other switch circuit to selectively bypass the voltage control zener diode  310 . 
   Control logic  321 , such as a microcontroller, selectively activates or deactivates the bypass circuit  312  to intelligently control the voltages applied to the flash tube  301  and the transformer  304  in the circuit  300 . For relatively low output intensities (e.g., 15 cd or 30 cd), the charge pump charges the trigger capacitor  302 , illumination capacitor  306 , and boost capacitor  308  to the full voltage provided by the charge pump  318  (e.g., 144 V for 15 cd or 185 V for 30 cd). The control logic  321  activates the bypass circuit  312  to provide a current path around the voltage control zener diode  310 . A trigger signal then causes the energy stored in the trigger capacitor  302  to energize the primary winding of the step-up transformer  304 . The secondary winding generates a damped oscillating high voltage signal to perform first stage ionization in the flash tube  301  to prepare for illumination. The SCR  316  additionally places the illumination capacitor  306  and the boost capacitor  308  in series across the flash tube  301 . In this configuration, the circuit  300  implements a voltage doubler to reliably start the flash tube  101 . 
   The total voltage provided by the illumination capacitor  306  and the boost capacitor  308  allows the flash tube  301  to start. The relatively small amount of energy in the boost capacitor  308  discharges first through the SCR  316 . The trigger diode  314  protects the SCR  316  from voltage ringing. The illumination capacitor  306  discharges through the flash tube  301  and the diode  309 . This discharge produces the selected output intensity. 
   For relatively high output intensities, such as 75 or 110 candela, the control logic  321  de-asserts the bypass control signal on the bypass control line  340  to deactivate the bypass circuit  312 . With the bypass circuit  312  deactivated, the charge pump also charges the trigger capacitor  302 , illumination capacitor  306 , and boost capacitor  308  to the full voltage (e.g., to 250 volts for 75 candela or 286 volts for 110 candela). However, the voltage control zener diode  310  controls the voltage to which the boost capacitor  308  and the trigger capacitor  302  discharge. Specifically, the voltage on the boost capacitor  308  and the trigger capacitor  302  discharge to a voltage no less than the voltage control zener diode voltage. 
   The trigger signal initiates ionization in the flash tube using the trigger circuit, and places the illumination capacitor  306  and the boost capacitor  308  in series across the flash tube  301 . The voltage control zener diode  310  prevents the application of double the voltage of the illumination capacitor  306  across the flash tube  301 . Furthermore, because the trigger capacitor voltage is controlled, the high voltage oscillation applied from the transformer secondary has a lower maximum value than it otherwise would. The voltage control zener diode  310  thereby helps to prevent two types of high voltage arcing in the circuit  300 : arcing from the total voltage applied to the flash tube  301 , and arcing from the high voltage secondary winding of the transformer  304 . 
   For relatively low output intensities, the control logic  321  activates the bypass control line  340 . The illumination capacitor  306 , boost capacitor  308 , and the trigger capacitor  302  charge to the full voltage for the selected output intensity under control of the charge pump  318 . When the control logic  321  asserts a trigger signal on the trigger control line  342 , the SCR  316  provides a discharge path for the trigger capacitor  302  and causes the illumination capacitor  306  and the boost capacitor  308  to be placed in series across the flash tube  301 . Because the boost capacitor  308  is charged to the same voltage as the illumination capacitor  306 , the circuit  300  acts as a voltage doubler for the low output intensities to reliably start the flash tube  301 . 
   In summary, the optical element driving circuit  300  operates in two modes. In a low output intensity mode, the charge pump  318  fully charges the illumination capacitor  306 , the doubling capacitor  308 , and the trigger capacitor  302 . In the low intensity mode, the optical element driving circuit  300  implements a voltage doubler to place both the illumination capacitor  306  and the doubling capacitor  308  in series across the flash tube  301  when the optical element driving circuit  300  is triggered. In a high output intensity mode, the charge pump fully charges the illumination capacitor  306 , the doubling capacitor  308 , and the trigger capacitor  302 , but the voltage control zener diode  310  controls the voltage on the boost capacitor  308  and the trigger capacitor  302 . When the optical element driving circuit  300  is triggered, the optical element driving circuit  300  places both the illumination capacitor  306  and the boost capacitor  308  in series across the flash tube  301 , but less than double the voltage on the illumination capacitor is applied to the flash tube  301   
   Table 5 shows examples of component values for the optical element driving circuit  300 . Table 6 shows examples of output intensities and capacitor voltages. 
   
     
       
             
             
             
           
             
             
             
             
           
             
             
             
           
         
             
                 
               TABLE 5 
             
             
                 
                 
             
             
                 
               Component 
               Component Value 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Trigger Capacitor 302 
               0.047 
               μF 
             
             
                 
               Illumination Capacitor 306 
               68 
               μF 
             
             
                 
               Boost Capacitor 308 
               0.047 
               μF 
             
             
                 
               Voltage Control Zener Diode 310 
               90 
               V 
             
           
        
         
             
                 
               Transformer step-up ratio 
               36-38 to 1 
             
             
                 
                 
             
           
        
       
     
   
   
     
       
             
             
             
             
             
           
         
             
               TABLE 6 
             
             
                 
             
             
                 
                 
               Doubling 
                 
                 
             
             
                 
                 
               Capacitor 
             
             
                 
                 
               308 
               Tube 301 
               Trigger 
             
             
               Output 
               Illumination 
               Pretrigger/ 
               (During 
               Capacitor 302 
             
             
               Intensity 
               Capacitor 306 
               trigger 
               trigger) 
               Pretrigger/trigger 
             
             
                 
             
           
           
             
               15 cd 
               144 V 
               144 V/144 V 
               288 V 
               144 V/144 V 
             
             
               30 cd 
               185 V 
               185 V/185 V 
               370 V 
               185 V/185 V 
             
             
               75 cd 
               250 V 
               250 V/160 V 
               410 V 
               250 V/160 V 
             
             
               110 cd  
               286 V 
               286 V/196 V 
               482 V 
               286 V/196 V 
             
             
                 
             
           
        
       
     
   
     FIG. 4  shows an alternative implementation of an optical element driving circuit  400  for an optical element  401 . The optical element driving circuit  400  includes energy sources such as a trigger capacitor  402 , an illumination capacitor  406 , and a doubling capacitor  408 . The circuit  400  also includes a first switch  410 , a second switch  412 , a trigger diode  414 , and a trigger SCR  416 . The trigger capacitor  402 , step-up transformer  404 , and trigger SCR  416  form an optical element triggering circuit  403 . Further, the first and second switches  410 ,  412  form a boost circuit. 
   A charge pump  418  (or other power supply) charges the trigger capacitor  402 , illumination capacitor  406 , and boost capacitor  408 . The charge pump  418  charges the illumination capacitor  406  to the full voltage selected according to the desired output intensity. When the first and second switches  410 ,  412  are closed, the charge pump charges the doubling capacitor  408  to the full voltage selected according to the desired output intensity. When at least one of the first and second switches  410 ,  412  are open, the charge pump  418  does not charge the doubling capacitor  408 . 
   The first and second switches  410 ,  412  may be implemented in many ways. In other implementations, the first and second switches  410 ,  412  may be a pnp transistor, a Field Effect Transistor (FET), jumper, relay, or other switch circuit to selectively remove the doubling capacitor  408  from the optical element driving circuit  400 . Furthermore, both switches  410  and  412  need not be provided. Instead, a single switch (e.g., switch  410  or switch  412  alone) may connect or disconnect the doubling capacitor  408  in the driving circuit  400 . 
   Control logic  419 , such as a microcontroller, selectively activates or deactivates the first and second switches  410 ,  412  to intelligently control the voltages applied to the flash tube  401 . A trigger signal applied to the trigger SCR  416  causes the energy stored in the trigger capacitor  402  to energize the primary winding of the step-up transformer  404 . The secondary winding generates a damped oscillating high voltage signal to perform first stage ionization in the flash tube  401  to prepare for illumination. 
   The trigger SCR  416  additionally places the illumination capacitor  406 , or the illumination capacitor  406  and the doubling capacitor  408 , across the flash tube  401 . The total voltage provided by the illumination capacitor  406  and the boost capacitor  408  allows the flash tube  401  to start. Accordingly, the illumination capacitor  406  discharges through the flash tube  401  and the diode  409 . This discharge produces the selected output intensity. 
   For relatively low output intensities such as 15 or 30 candela, doubling the voltage on the illumination capacitor  406  may still result in a total voltage across the flash tube  401  that avoids arcing. Accordingly, the control logic  419  may assert a control signal to close the first and second switches  410 ,  412 . Therefore, the charge pump  418  fully charges the illumination capacitor  406  and the doubling capacitor  408  (e.g., to 144 volts for 15 candela, or 185 volts for 30 candela). 
   When the control logic  419  asserts a trigger signal on the trigger control line  442 , the illumination capacitor  406  and the boost capacitor  408  are placed in series across the tube. Because both capacitors are charged to the same voltage, the driving circuitry acts as a voltage doubler for the low output intensity modes, and reliably starts the flash tube  401 . The relatively small amount of energy in the boost capacitor  408  discharges first through the SCR  416 . The trigger diode  414  is protects the SCR  416  from ringing. 
   For relatively high output intensities, such as 75 or 110 candela, the control logic  419  asserts a control signal to open at least one of the first and second switches  410 ,  412 , thereby removing the doubling capacitor  408  from the circuit  400 . The charge pump  418  fully charges the illumination capacitor  406  (e.g., to 250 volts for 75 candela or 286 volts for 110 candela). However, the charge pump  418  does not charge the doubling capacitor  408 . The voltage on the illumination capacitor  406  is sufficient to start the flash tube  401 . The energy in the illumination capacitor  406  provides the selected output intensity. 
   The optical element driving circuit of  FIG. 4  operates in two modes. In a low light mode, the optical element driving circuit uses a voltage doubler to place both the illumination capacitor  406  and the doubling capacitor  408  in series across the flash tube  401  at the same time. In a high light mode, only the illumination capacitor  106  is fully charged and placed across the flash tube  401 . 
   Table 7 shows examples of component values for the optical element driving circuit  400 . Table 8 shows examples of output intensities and capacitor voltages. 
   
     
       
             
             
             
           
         
             
                 
               TABLE 7 
             
             
                 
                 
             
             
                 
               Component 
               Component Value 
             
             
                 
                 
             
           
           
             
                 
               Trigger Capacitor 402 
               0.047 μF 
             
             
                 
               Illumination Capacitor 406 
                 68 μF 
             
             
                 
               Boost Capacitor 408 
               0.047 μF 
             
             
                 
               Transformer step-up ratio 
               36-38 to 1 
             
             
                 
                 
             
           
        
       
     
   
   
     
       
             
             
             
             
             
           
         
             
               TABLE 8 
             
             
                 
             
             
               Output 
               Illumination 
               Doubling 
                 
               Trigger 
             
             
               Intensity 
               Capacitor 406 
               Capacitor 408 
               Tube 401 
               Capacitor 402 
             
             
                 
             
           
           
             
               15 cd 
               144 V 
               144 V 
               288 V 
               144 V 
             
             
               30 cd 
               185 V 
               185 V 
               370 V 
               185 V 
             
             
               75 cd 
               250 V 
               — 
               250 V 
               250 V 
             
             
               110 cd  
               286 V 
               — 
               286 V 
               286 V 
             
             
                 
             
           
        
       
     
   
     FIG. 5  shows control logic  500  in the form of a microcontroller  502  for controlling the optical element driving circuits described above. The microcontroller  502  includes one or more input lines  504  and one or more output lines  506 . The microcontroller  502  connects to a memory  508  that stores an illumination control program  510  and configuration data  512 . The configuration data  512  may provide a mapping between selected output intensity and whether to assert or de-assert a boost control input, switch control input, bypass control input, or any other output. For example, assuming the control circuit shown in  FIG. 1 , for 110 cd output intensity, the configuration data  512  may specify that the switch control input  123  should be de-asserted so that only the illumination capacitor  106  drives the flash tube  101 . 
   The microcontroller  502  executes the illumination control program  510  stored in the memory  508 . The illumination control program  510  directs the microcontroller  502  to generate control signals on the output lines  506  dependant on signals received on the input lines  504  and the configuration settings in the lookup table  512 . For example, the input lines  504  may include a candela selection input line connected to a jumper, switch, or other selector. The candela selection input line provides a selection signal representative of the desired output intensity. The output lines  506  may drive the boost control input, switch control input, bypass control input, trigger input, or any other input to the control circuits in accordance with the selected output intensity. 
     FIG. 6  is a flow diagram of the acts which the illumination control program  510  may take to control an optical element driving circuit. The illumination control program  510  determines the desired output intensity (Act  602 ). For example, the illumination control program  510  may read a digital input or an analog voltage (e.g., tapped with a jumper on a resistor ladder) to determine the selected output intensity. With the selected output intensity, the illumination control program  510  accesses the configuration data  512  to determine whether to assert or de-assert voltage configuration signals, such as the bypass control input (Act  604 ). Alternatively, the illumination control program  510  may incorporate logical tests to determine whether to assert or de-assert any particular voltage configuration signal. Thus, the illumination control program  510  outputs the control signals which configure elements such as the switch  118  of  FIG. 1 , the bypass circuit  212  of  FIG. 2 , the bypass circuit  312  of  FIG. 3 , or the first and second switches  410 ,  412  of  FIG. 4  for the selected output intensity (Act  606 ). 
   The illumination control program  510  then allows the illumination, boost, and trigger capacitors to charge (Act  608 ). The illumination control program  510  may then determine when to issue a trigger signal to the driving circuit (Act  610 ). The trigger signal initiates the ionization of the gas in the flash tube, and the optical output from the flash tube at the selected output intensity. 
     FIG. 7  shows a specific implementation of the driving circuit presented in  FIG. 1 . The driving circuit  700  produces illumination from the flash tube  701  at one of four different output intensities. A 2-pin jumper may be used to select the intensity: either 15 candela, 30 candela, 75 candela, or 110 candela. The output intensity may be set in many different ways, however. For example, the output intensity may be set under software control by local or remote entities in communication with the control circuitry. 
   The driving circuit  700  includes a trigger capacitor C 6  connected to a step-up transformer T 1 . Two terminals of a flash tube  701  connect to the sockets SKT 1  and SKT 2 . An illumination capacitor C 8  and a doubling capacitor C 5  are present to drive the flash tube. A high frequency filter capacitor C 9  is connected in parallel across C 8  to help reduce noise. The high frequency filter capacitor C 9  smoothes high frequency transients in the charging pulses which charge the capacitors C 5 , C 6 , C 8 , and C 9 . 
   Charging circuitry fully charges the capacitors C 5 , C 8 , and C 9  to a specific voltage which depends on the selected candela output. In addition, the two series connected 91 V zener diodes D 12  and D 13  control the voltage on the trigger capacitor C 6  so that it does not charge above 182 V. The capacitors C 8 , C 9 , and C 5  always charge to the full voltage determined by the charging circuit, without limitation. In other words, the capacitors C 8 , C 9 , and C 5  are never charged to different voltages; they are always charged to the full voltage determined by the charging circuitry. Depending on the selected candela output, the driving circuit  700  either operates in a first mode that applies C 8  and C 9  to the tube, or in a second mode that doubles the voltage across the tube. The voltage doubler uses C 5  in series with C 8  and C 9 . The driving circuit  700  uses the voltages shown below in Table 5. 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 5 
             
             
                 
             
             
               Candela Output 
               C8/C9 
               C5 
               Tube 
               C6 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               15 
               144 V 
               144 V 
               288 V (C8/C9 + C5) 
               144 V 
             
             
               30 
               185 V 
               185 V 
               370 V (C8/C9 + C5) 
               182 V 
             
             
               75 
               250 V 
               250 V 
               250 V (C8/C9) 
               182 V 
             
             
               110 
               286 V 
               286 V 
               286 V (C8/C9) 
               182 V 
             
             
                 
             
           
        
       
     
   
   To prime the tube  701  to provide a light output, the driving circuit  700  provides a trigger signal on the trigger input labeled SCR to trigger the SCR Q 3 . The trigger signal causes the SCR Q 3  to conduct, thereby completing a circuit for the trigger capacitor C 6  to energize the primary coil of the step-up transformer T 1 . The transformer secondary winding includes one lead connected to ground and a second lead connected to the flash tube  701 . The transformer secondary winding generates a damped multi-KV oscillation applied to the outside of the tube  701 . The voltage developed across the pair of leads in the secondary of the transformer has a maximum value of about 5,500 V at 15 candela output to about 6,900 V at 110 candela output. The high voltage output of the transformer secondary winding causes an initial ionization of the gases inside the tube  701 . The tube  701  is then primed for current flow through the tube  701  to generate illumination. 
   At the 15 candela and 30 candela output intensities, the driving circuit  700  uses a voltage doubler to reliably start the tube and generate the desired light output. At the 15 candela and 30 candela output levels, the driving circuit  700  asserts the doubling input labeled DSCR (at the same time as the input labeled SCR) to trigger the SCR Q 4 . When Q 4  conducts, it brings the previously positive node of C 5  to ground, placing C 5  across the tube with C 8 /C 9  to double the voltage applied to the tube  701 . The diode D 4  is temporarily reverse biased. The doubled voltage reliably starts the tube  701 , and capacitor C 5  quickly discharges through the SCR Q 4 . The energy in the illumination capacitor C 8  then provides the selected light output level as current flows from C 8 , through the tube  701 , and through D 4  to ground. 
   At the 75 candela and 110 candela output intensities, the voltage on the illumination capacitor C 8  is sufficient to reliably flash the tube  701 . Therefore, in the 75 candela and 110 candela output modes, the driving circuit  700  uses C 8 /C 9  to drive the flash tube  701  without doubling. Though there may be insignificant leakage of C 5  through the 1M Ohm resistor R 72  through the flash tube  701 , it is the voltage on C 8 /C 9  that fires the tube  701  and the energy in C 8  that produces the selected output light level. More particularly, at the 75 candela and 110 candela output levels, the driving circuit  700  does not assert the DSCR signal. As a result, the driving circuit  700  applies the voltage of C 8 /C 9  across the flash tube  701  without doubling. The energy in the illumination capacitor C 8  provides the selected light output level as current flows from C 8 , through the tube  701 , and through D 4  to ground. 
   In other words, the driving circuit  700  operates in one of two modes. In the low light mode, the driving circuit  700  uses a voltage doubler to simultaneously place C 8 /C 9  and C 5  in series across the tube  701 . In the high light mode, the driving circuit  700  drives the flash tube  701  using C 8 /C 9  connected across the tube  701 . In the high light mode, C 5  is charged to the same voltage as C 8 /C 9 , but is not used in conjunction with C 8 /C 9  to start the tube  701  or provide illumination. 
     FIG. 8  is a control circuit  800  for controlling the trigger input and the doubling input connected to the driving circuit  700 . The control circuit  800  includes a first NOR gate  802  and a second NOR gate  804 . The NOR gates are connected to two inputs. The microcontroller  502  or other control logic may assert or de-assert the inputs to control the voltages developed in the driving circuit  700 . The control circuit  800  may be implemented with any other circuitry, and is not limited to an implementation in NOR gates, or hardware. 
   The first input is a strobe trigger input  814  coupled to a first input  806  and a second input  808  of the first NOR gate  802 . The strobe trigger input  814  is additionally coupled to a first input  810  of the second NOR gate  804 . The second input is a voltage doubling control input  816  connected to a second input  812  of the second NOR gate  806 . 
   When the strobe trigger input  814  is asserted, the first NOR gate  802  generates a trigger pulse on the trigger output labeled SCR. In response to the trigger signal, SCR Q 3  conducts to complete a circuit for the trigger capacitor C 6  to energize the primary coil of the step-up transformer T 1 . When the voltage doubling control input  816  is also asserted, the control circuit  800  generates a trigger pulse on the doubling input DSCR. Otherwise, no trigger pulse is generated on the doubling input DSCR. 
   At low output intensities (e.g., 15 candela and 30 candela), the voltage doubling control input  816  is asserted. Accordingly, the doubling input DSCR causes Q 4  to conduct, thereby placing C 5  across the flash tube with C 8 /C 9  to double the voltage applied to the flash tube. At high output intensities (e.g., 75 candela and 110 candela), the voltage doubling control input  816  is not asserted. Accordingly, Q 4  does not conduct and the driving circuit uses C 8 /C 9  to drive the flash tube without doubling. While the control circuit  800  has been explained with respect to the optical element driving circuit of  FIG. 7 , the same control circuit  800  could also be adapted to control, as examples, the bypass control input, boost control inputs, and switch control inputs discussed above with respect to  FIGS. 1 ,  2 ,  3 , and  4 . 
     FIG. 9  shows an alternative implementation of an optical element driving circuit  900  for an optical element  901 . In  FIG. 9 , a power source  926  (e.g., an AC or DC voltage source, charge pump, or other power source) charges the trigger capacitor  902 . The power source  926  operates independently from the charge pump  920  that charges the illumination capacitor  906  and boost capacitor  908 . Accordingly, the control logic  921  may set the voltage on the trigger capacitor  902  independently of the voltage on the illumination capacitor  906  and boost capacitor  908 . Additionally or alternatively, a third power source may be provided to independently charge the boost capacitor  908 . In other words, the control logic  321  may exercise direct and independent control over the voltage on any of the illumination capacitor  906 , boost capacitor  908 , and trigger capacitor  902 . Accordingly, the control logic  321  may specifically control the voltages to provide a wide range of desired output intensities, while avoid arcing. 
   As noted above, the power source  926  charges the trigger capacitor  902  to a selected trigger voltage independently of the voltage to which the charge pump  920  charges the illumination capacitor  906  and the doubling capacitor  908 . For example, for relatively high candela settings, such as 75 or 110 candela, the power source  926  may charge the trigger capacitor  902  to a relatively low voltage, such as 182 V, while the charge pump  920  independently charges the illumination capacitor  906  and the doubling capacitor  908  to a relatively high voltage, such as 250 V or 286 V. The power source  926  thereby operates as an independent control on the voltage produced by the secondary winding of the transformer  904 , helping to prevent arcing at and around the flashtube  901  and the step-up transformer  904 . 
   In the driving circuit  100  of  FIG. 1 , the trigger zener diode  116  controls the voltage at the trigger capacitor  102 . In the implementation shown in  FIG. 9 , however, the voltage source  926  directly controls the voltage on the trigger capacitor  902 . As a result, the trigger zener diode  916  may be omitted (or may be retained as a safeguard against overcharging the trigger capacitor  902 ). One or more independent power sources for the illumination capacitors, boost capacitors, or trigger capacitors may also be employed in any of the driving circuits explained above. 
     FIG. 10  shows an alternative implementation of an optical element driving circuit  1000  that provides an independent power source for the trigger capacitor  1002 . In particular, the driving circuit  1000  includes a PWM charge pump  1020  under control of the control logic  1021 . While the driving circuit  100  in  FIG. 1  (for example) charged both the trigger capacitor and the illumination capacitor with the same charging output from the charge pump  120 , the implementation shown in  FIG. 10  splits the charging output into a separate illumination charging output  1030  and a trigger charging output  1028 . 
   As a result, the circuit  1000  may include circuitry connected to the trigger charging output  1028  for independent control over charging the trigger capacitor  1002 . As shown in  FIG. 10 , the charge pump  1020  charges the trigger capacitor  1002  through a diode  1032 , a supply capacitor  1034 , and a resistor  1038 . The diode  1032  allows current pulses to flow from the charge pump  1020  to the supply capacitor  1034  to thereby charge the supply capacitor  1034 . The supply capacitor  1034  provides a stable voltage source that charges the trigger capacitor  1002  through the relatively large 1M Ohm resistor  1038 . 
   The driving circuit  1000  optionally includes voltage control circuitry  1036  connected to the trigger charging output  1028 . The voltage control circuitry  1036  helps to set the voltage to which the trigger capacitor  1002  charges. For example, the voltage control circuitry  1036  may include a zener diode, or any other circuitry that boosts or reduces the voltage to which the trigger capacitor  1002  charges. The voltage control circuitry  1036  may be used in addition to or as an alternative to the trigger zener diode  1016 . While  FIG. 10  shows a modified version of the driving circuit  100 , an independent illumination charging output and trigger charging output may be provided in any of the driving circuits explained above. 
   The disclosed driving circuits may be modified and still fall within the spirit of the disclosure. For example, the bypass circuits may be implemented with other types of transistors, such as field effect transistors, with switches, jumpers, relays, or other circuits. The flash tube may be any source of illumination (or energy output in the visible or non-visible spectrum), including a Xenon flash tube or other light source. The zener diodes voltages may vary to accommodate any particular design or application. The driving circuit may produce output intensities other than 15, 30, 75, and 110 candela. Batteries, or other energy sources, may be used in addition to or as alternative to the capacitors, while other types of switches may be used instead of SCRs. The charge pump may be implemented with another type of power supply. The control circuitry may be analog or digital control circuitry, including discrete circuits, processors operating under programmed control, or other circuitry. It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to define the spirit and scope of this disclosure.