Flashtube trigger circuit with anode voltage boost feature
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electrical trigger circuits for gaseous discharge flashtubes, and more particularly, to trigger circuits for flashtubes that must be triggered reliably at low anode power supply voltages. 
     2. Description of the Prior Art 
     As illustrated in FIGS. 2 and 2B, prior art flashtube trigger circuits generally trigger flashtubes by applying a high voltage pulse to the flashtube gas by either a direct series triggering method of injecting a high voltage pulse in series with the flashtube anode or cathode circuit or by a capacitively coupled external trigger method. FIG. 2A depicts the flashtube anode voltage waveform during the trigger event of FIG. 2. 
     The minimum cathode to anode operating voltage of a flashtube is determined by lamp element geometry, gas fill pressure and lamp construction materials. Flashtube discharge is initiated by the application of a high voltage trigger pulse greater than the static breakdown voltage of the tube, generally ranging between 2000 to 20,000 volts. The difference between the trigger voltage and the lamp operating voltage must be sufficient to avoid spontaneous triggering. A ratio of 10:1 minimum is typically used to prevent spontaneous triggering. 
     The direct series triggering method utilizes a large trigger transformer with a secondary winding connected in series with either the lamp cathode or anode to inject a high voltage pulse when a semiconductor or mechanical switch is closed to initiate a trigger event. Closure of the trigger switch discharges a small trigger capacitor through the trigger transformer primary winding which induces a damped high voltage oscillation in the secondary winding. Direct series trigger components are large and costly because they must carry the full flashtube electrode current. The maximum anode voltage applied to the flashtube during the trigger event is the sum of the voltage of the power supply energy storage capacitor and the trigger transformer voltage. 
     The capacitivity coupled external triggering method is used with flashtubes that have an external trigger electrode fastened to the flashtube which extends over the entire arc length of the tube. 
     The external trigger electrode forms a capacitance of approximately 10 pf against the cathode and anode of the lamp. As a result, a small pulse transformer with a transformation ratio of 1:20 to 1:100 is used to generate a high voltage pulse when a semiconductor or mechanical trigger switch is closed to start a trigger event. The resulting discharge of the small trigger capacitor into the trigger transformer primary winding produces a damped high voltage oscillation in the secondary winding. The maximum anode voltage applied to the flashtube during the trigger event by this circuit equals the power supply energy storage capacitor voltage. 
     Other prior art variations of the capacitive external triggering method provide an increase in flashtube cathode to anode voltage during a trigger event by using an auxiliary anode voltage supply having an output voltage higher than the power supply energy storage capacitor voltage to assist lamp triggering. U.S. Pat. No. 4,900,990 teaches capacitive triggering with an external anode boost voltage source. Page 7 of the 1992 Heimann Optoelectronics Flashtube Guide teaches the use of a voltage doubling circuit that requires four electrical connections to the lamp assembly and a diode and small capacitor to increase the apparent anode voltage on the lamp during the trigger event. 
     As illustrated in FIG. 3, the prior art voltage doubler taught by Heimann requires four electrical connections to the remote lamp assembly and therefore will not work with the large number of three wire flashtube assemblies currently in use. The FIG. 3A timing diagram graphically represents the anode voltage change during a trigger event relating to the circuit illustrated in the FIG. 3 electrical schematic diagram. 
     SUMMARY OF THE INVENTION 
     It is therefore a primary object of the present invention to provide an apparatus for assisting the triggering of a remote three wire flashtube and trigger coil assembly operated at a low anode voltage by using a small coupling or boost capacitor and an isolation diode in the flashtube power supply to increase the flashtube anode voltage at the outset of each trigger event to a level higher than the flashtube power supply energy storage capacitor voltage. This feature of the invention enables capacitive external triggering of flashtubes at a power supply energy storage voltage far below the normal flashtube anode operating voltage. This unique operating mode makes it possible to operate a standard flashtube in a non-standard dim output mode by providing a trigger circuit derived, short duration anode boost voltage at the onset of each trigger event to thereby enable a flashtube to operate with a less than normal minimum anode voltage. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention is pointed out with particularity in the appended claims. However, other objects and advantages together with the operation of the invention may be better understood by reference to the following detailed description taken in connection with the following illustrations, wherein: 
     FIG. 1 is an electrical schematic diagram of a preferred embodiment of the present invention. 
     FIG. 1A is a graphical representation of the anode voltage change during a trigger event facilitated by a preferred embodiment of the present invention. 
     FIG. 2 is a schematic diagram of a prior art flashtube capacitively coupled external trigger circuit. 
     FIG. 2A is a graphical representation of the anode voltage change during the trigger event of a prior art flashtube capacitively coupled external trigger circuit. 
     FIG. 2B is a schematic diagram of a prior art flashtube direct series trigger circuit. 
     FIG. 3 is a schematic diagram of a prior art flashtube voltage doubler circuit. 
     FIG. 3A is a graphical representation of the anode voltage change during a trigger event of the prior art voltage doubler circuit illustrated in FIG. 3. 
     FIG. 4A illustrates the FIG. 1 flash tube trigger circuit configured into the charging state with the SCR in the open circuit configuration. 
     FIG. 4B illustrates the FIG. 1 flash tube trigger circuit configured into the discharge state with the SCR closed. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to drawings of the present invention, the advantages of the invention and its contributions to the art, will be reviewed in detail. 
     Referring now to FIG. 1, the flashtube anode voltage boost circuit of the present invention includes trigger boost capacitor C TB  and diode D which act together to temporarily increase the flashtube anode voltage during the onset of the trigger event by adding the trigger coil oscillating voltage V T  stored in boost capacitor C TB  to the power supply output or flashtube operating voltage V O . Diode D should be a fast recovery type such as a Motorola MUR460 which acts to prevent the boosted anode voltage from being fed back into the energy storage capacitor C B . The Motorola MUR460 diode possesses a t rr  maximum reverse recovery time of 75 nanoseconds when I F  =1.0 amp. and the di/dt=50 A/microseconds and a t fr  maximum forward recovery time of 50 ns when I F  =1.0 amp. and the di/dt=100 A/microsecond, with recovery to 1.0 volt. 
     The capacitance rating of capacitor C TB  can be very small relative to the rating of energy storage capacitor C B , and in the best mode will be approximately 0.047 uF with a voltage rating equal to at least V O . The particular details and operating modes of the remainder of the strobe trigger circuit are well known in the art and have not been shown or explained in detail. 
     The FIG. 4A and 4B circuit diagrams illustrate the two state reconfiguration of the FIG. 1 flash tube trigger circuit. 
     FIG. 4A illustrates the trigger switch or SCR in the normally open state which allows capacitors C Z  and C TB  to be charged through resistor R and diode D FAST by power supply output voltage V O . 
     As illustrated in FIG. 4A, when the SCR trigger switch is maintained in the open or high impedance state, trigger capacitor C Z  and trigger boost capacitor C TB  are effectively coupled in parallel. Because the isolating diode D FAST is forward biased, charging current readily flows from the power supply output terminal into C TB . 
     As illustrated in FIG. 4B, when the trigger switch is closed to initiate a trigger event, trigger capacitor C Z  and trigger boost capacitor C TB  are coupled in series. 
     Because during the FIG. 4A charging state each capacitor C Z  and C TB  is charged to a voltage V X  where V X  typically approximates V O , the summed output from series-connected capacitors C TB  and C Z  in the FIG. 4B discharge state will equal 2 V X , or approximately 2 V O , where that essentially doubled power supply output voltage is applied across the flashtube anode and cathode terminals as illustrated in FIG. 4B. 
     In the FIG. 4B series-coupled state, the isolating diode is reverse biased because voltage 2 V X  substantially exceeds power supply voltage V O  to prevent unwanted discharge of the series-coupled capacitors C TB  and C Z  through the power supply. 
     The 1A timing diagram illustrates the SCR-controlled transition between the parallel-coupled capacitor charging state and the series-coupled capacitor discharge state which temporarily generates a flashtube anode to cathode voltage approximately equal to 2 V 0 . 
     It has been found that the anode voltage boost circuit of the present invention consisting of uniquely connected diode D and capacitor C TB  will allow the minimum lamp anode operating voltage of a typical flashtube to be reduced from 194 VDC to 134 VDC, or approximately thirty percent, while maintaining reliable flashtube triggering. 
     The increase in the triggerability of the flashtube provided by the anode voltage boost circuit of the present invention can be applied in several ways: 
     1. The fill pressure of the flashtube can be increased (which increases the :flashtube minimum anode voltage operating parameters) to increase the efficiency of the flashtube thereby increasing its light output while using the same input power. 
     2. The operating voltage V O  of the energy storage capacitor can be reduced to decrease the brightness of the flashtube thereby allowing the flashtube to be operated at brightness level far below the minimum level attainable with prior art trigger circuits. 
     While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced with modifications within the spirit and scope of the appended claims.