Abstract:
A power supply for high voltage, low current gas discharge tubes such as neon, argon, and mercury vapor. A free running, flyback oscillator, converts D.C. voltage energy into radio frequency energy by means of a compact, ferrite transformer and associated circuitry. The primary winding is tuned by a resonant capacitor and driven by a power transistor. A high voltage, centertapped winding of a ferrite transformer drives the gas tube load directly. A feedback winding arranged across the transistor base and emitter junction sustains oscillation and controls the drive level of the transistor by means of a regulating circuit which controls the amplitude of the current. Oscillator starting is achieved by means of an on-off switch which supplies a single starting pulse to the power transistor or by means of a time delayed starting pulse. A MOSFET transistor connected to the power transistor base and a current sensing transformer arranged in series with the primary winding, disables the power transistor momentarily at the end of a conducting cycle. Charge carries are depleted in the base-cathode region, resulting in resetting the transistor quickly such that it can withstand a forward voltage of 700 volts in the off state.

Description:
BACKGROUND OF INVENTION 
     This invention relates to power supplies and more particularly to a solid state, high efficient supply which converts D.C. energy to high frequency A.C. energy for the purpose of supplying gas discharge tubes with high voltage at relative low currents in a range of 15-55 milliamperes (ma) in a range of 15-115 watts. The high voltage may vary from one kilovolt to 10 kilovolts depending on the glass diameter, length, bends, type of gas, etc. 
     Upon ionization of a gas discharge tube by means of high voltage resulting in current flow, the atoms of neon are stimulated to emit an orange-red light. Other gases which glow when electrically energized are mercury vapor (blue-green), argon (pale blue), and a mixture of the two (deep blue). Pigmented fluorescent coatings are used with mercury vapor gas to produce many visible hues of light quite efficiently. 
     One type of prior art power supply is simply 60 Hz transformers where 120 volts A.C. is applied to the primary of the transformer and the secondary winding output voltage is connected to the tube load. By utilizing a large ratio of primary secondary turns such as 50-100, high voltages are induced up to 10 kilovolts. Such systems are heavy, for example 10-12 pounds, dangerous, and may be as inefficient as 85% resulting in high internal temperatures and low reliability. Several sizes of transformers are available to prevent an underdrive or overdrive of the tube load. 
     More recent solid state power supplies are lighter, more efficient, and operate silently compared with the 120 Hz audible noise from 60 Hz power supplies. However, specific problems are evident with such power supplies, such as: a) the series resonant type of oscillators employed result in a “beading” of the energized neon gas which is displeasing to the eye; (b) the lack of secondary short circuit protection so the system can fail when the secondary is shorted; (c) the lack of open circuit protection resulting in high voltages up to 16 kilovolts which is dangerous and may result in an arc and a fire; (d) the lack of protection from an open secondary lead or a broken tube which can cause a fire; (e) inadequate protection of persons who may come in contact with the high voltage by touching one of the leads; (f) the absence of a method to set and regulate the amplitude of current to a gas discharge tube often results in failing the tube load; and (g) the absence of circuit capability to connect a millampmeter for the purpose of adjusting the load current to a safe value. 
     It has been found that tubes filled with mercury vapor gas tend to degrade when excess current is allowed to flow in the tubes due to excessive voltage. For example, such degradation has been observed in window neon signs with currents which exceed the nominal current by only 20%. The general symptom resulting from current overdrive is a dimming or darkening of specific sections of the tube caused by condensation of the mercury vapor which results in reducing the secondary emission of light from the flourescent coating. 
     Gas discharge tubes have a negative coefficient of resistance with current. That is, the tube&#39;s resistance decreases as the current through it increases which suggests that a runaway condition exists if the current is not regulated. 
     The glass used for window neon signage range from 9-12 mm. High voltage, gas discharge tubes used for lighting are generally 15 or 18 mm&#39;s, are filled with mercury gas, and emit white light. The area of the glass inside diameter determines the amount of high voltage and resultant current which will be tolerated by mercury vapor sections of signs or lighting systems. In commercial practice, the outside diameter of the glass is used as reference rather than the inside diameter. The following table illustrates the nominal and damaging currents for lighting devices of various sizes. 
     
       
         
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Use 
                 Range mm 
                 Optimum ma 
                 Damaging ma 
               
               
                   
                   
               
             
             
               
                   
                 Sign 
                 8-9 
                 20 
                 24 
               
               
                   
                 Sign 
                 9-10 
                 22 
                 26 
               
               
                   
                 Sign 
                 10-11 
                 24 
                 28 
               
               
                   
                 Sign 
                 11-12 
                 26 
                 31 
               
               
                   
                 Lighting 
                 15 
                 34 
                 41 
               
               
                   
                 Lighting 
                 18 
                 41 
                 49 
               
               
                   
                   
               
             
          
         
       
     
     Neon gas tubing is not easily damaged by excessive voltage and resultant current, however neon and mercury vapor sections generally are arranged in series in signage resulting in the need for regulation of the current because of the mercury vapor sections. Also, when more than one section of tubing is used to configure the sign, such as four sections of different colors, the smallest diameter mercury vapor section determines the safe current limit. Often tubes are bent sharply during the manufacturing process resulting in reducing the area of the tube at these points by the equivalent of 1-2 mm&#39;s. 
     SUMMARY OF INVENTION 
     An object of the invention is to provide a power supply for gas discharge tubes whose high voltage and load current may be adjusted to the optimum value by means of an inexpensive digital V.O.M. meter. 
     Another object of the invention is to provide a power supply for gas discharge tubes which regulates load current over a wide range of gas tube load. 
     An object of the invention is to provide a power supply for gas discharge tubes wherein load current regulation is provided over a wide range of the ambient temperatures. 
     Another object of the invention is to provide a power supply for gas discharge tubes wherein load current regulation is provided over a wide range of the input A.C. voltage. 
     An object of the invention is to provide a power supply for gas discharge tubes wherein high voltage, high frequency energy is provided to the tube load only during the time when the power transistor is turned off, preventing the load impedance from having any immediate effect on the transformer primary circuit. 
     A further object of the invention is to provide a power supply for neon gas filled tubes which does not cause beading. 
     Yet another object of the invention is to provide a power supply for a gas filled tube which is highly efficient. 
     An object of the invention is to provide a power supply which is quiet, compact, light weight, and reliable. 
     Another object of the invention is to provide a power supply which may be packaged in a vented, plastic box without exposed metal and which is only warm to the touch during operation. 
     An object of the invention is to provide a power supply for gas tubes applied to signage where a single setting of the load current is adequate to safely drive all signs over a wide range of wattages. 
     Another object of the invention is to provide a power supply which includes failsafe circuitry which prevents injury to persons who may accidentally touch the circuitry by disabling the high voltage. 
     An object of the invention is to provide a power supply with failsafe circuitry which prevents accidental fires in case either high voltage load is opened, the gas discharge tube is broken, or shorted, or an open connection develops between the high voltage source and the tube load. 
     Another object of the invention is to provide a power supply which can be turned on safely without a load and which disables the high voltage if the high voltage is touched during this condition. 
     An object of the invention is to provide a power supply with which minimum circuit alterations converts low voltage D.C. to high voltage A.C. where the D.C. voltage may be a combination of auto type batteries or D.C. derived by rectifying an A.C. source where the frequency is not critical to performance. 
     Another object of the invention is to provide a power supply operating in a power range of 15-115 watts and providing currents up to 50 ma&#39;s for tubes used for lighting such as 15-18 mm&#39;s. 
     In general terms, the invention comprises a power supply circuit for energizing a gas-filled tube, the circuit including oscillating means for energizing the tube and transformer means having primary winding means and secondary winding means. The secondary winding means are defined by first and second winding portions each having a first terminal means for being connected to the tube and second terminal means. Circuit means is connected between the second terminals of the winding portions for placing the same in a series circuit relation. The circuit means includes terminal means constructed and arranged for connecting an ampere meter in series between the second terminals. 
     In the preferred embodiment, the invention includes an oscillator which is free running, operates in a flyback mode, and is self resonant at 20 KHz. A power transistor configured as a common collector drives the primary of the high voltage transformer where the primary inductance is tuned by a resonating capacitor. The frequency of the oscillator is derived from the equation where F is Hz, L is Henrys, and C 
     
       
         F=½  LC  
       
     
     is Farads. 
     A feedback winding operating in the regenerative mode supplies a rectified DC signal to the power transistor base to sustain oscillation. The amplitude of the feedback signal is controlled by an in series current regulator which samples the tube load current and adjusts the drive level of the power transistor to increase or decrease the high voltage and load current as required by the set value. A potentionmeter is used to set the load current to the desired value The illuminance (brightness) of a gas discharge tube is directly proportional to the voltage across it and the current through it (W=EI). 
     A MOSFET transistor is connected between the base-emitter circuitry of the power transistor and is driven on at the instant the emitter current of the power transistor attempts to decrease resulting in negative drive which instantly disables the power transistor. A pulse transformer connected in series with a one turn primary winding senses the current decrease and generates a gate-source positive pulse enabling the MOSFET which disables the power transistor. 
     The circuit described results in maximum efficiency of the power transistor since it is forced to operate either on or off like a switch resulting in minimum power loss in the device. When the transistor is on, it is saturated and the collector-emitter resistance is very low. When switched off, the resistance is infinite. Another benefit of the MOSFET switch is to provide a base-emitter junction circuit path for charge carriers which assists in rapid turn off of the power transistor with a significant improvement in heat loss of the power transistor. 
     The rapid depletion of the charge carriers allows the power transistor to quickly block the forward voltage between the emitter-collector junction resulting from the flyback voltage. 
     The high voltage transformer includes a split ferrite core with an air gap of 0.60″, for example, which provides leakage reactance for the transformer. The primary winding is wound with stranded litz wire to minimize skin effect IR 2  losses resulting from the high frequency current. 
     When the power transistor conducts, the electrical energy of the primary winding is stored in the air gap in the form of a magnetic field. When the transistor is turned off, the magnetic energy is released to the core and secondary windings which drives the tube load. Induced voltage occurs in the feedback winding which results in oscillation and an auxiliary winding which powers two low voltage supplies; one for the failsafe circuit and the other for the current regulator. 
     The power supply oscillator is not self starting. An on-off switch, operated as a push-pull switch alternately turns the oscillator on and off. When off, the power transistor base is grounded to circuit common. On reversing the switch, a +12 volt, short duration pulse, +12 volts, for example, is applied to the power transistor base which enables the transistor and oscillation begins. 
     A second starting circuit is required by the failsafe circuit. When a problem is detected by the failsafe circuit, the oscillator is disabled. After a delay of five seconds, for example, a timer generates a voltage pulse, +30 volt 100 microsecond pulse which is applied to the power transistor gate which restarts the oscillator if the problem has been corrected. This timer also restarts the power supply in case of a power outage or if the load is controlled by a day/night timer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram illustrating a power supply according to the preferred embodiment of the invention. 
     FISG.  2 - 9  illustrates eight waveforms of the power supply and includes a dotted line where they are synchronized. 
     FIG. 2 illustrates the feedback voltage from the high voltage transformer applied between gate and circuit common of the power transistor. 
     FIG. 3 illustrates the power transistor gate voltage, referenced to common. 
     FIG. 4 illustrates the gate current of the power transistor. 
     FIG. 5 illustrates the current sense pulse applied to the switching MOSFET which terminates the conduction period of the power transistor. 
     FIG. 6 illustrates the emitter current of the power transistor. 
     FIG. 7 illustrates the emitter voltage of the power transistor referenced to +160 volts D.C. 
     FIG. 8 illustrates the resonant current in the resonanting capacitor. 
     FIG. 9 illustrates the current in the tube load, measured at the centertap of the two secondary windings. 
     FIG. 10 illustrates a graph of a beverage sign A where load resistance in K ohms, load current in ma, load voltage in kilovolts, and load watts are plotted. 
     FIG. 11,  12 , and  13  illustrate similar graphs of three other signs B, C, &amp; D. 
     FIG. 14 illustrates the secondary circuit plotted in FIG. 10, sign A. 
     FIG. 15 illustrates the electrical equivalent of the FIG. 14 secondary circuit. 
     FIG. 16 illustrates the vector relationships of the inductive reactance X L  in K ohms vs the dynamic tube load resistance of sign A. 
     FIG. 17 illustrates the voltage relationships IX, IR, and the induced voltage E L  of sign load A. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While various specific voltages, currents and wattages are referred to in the following description, it is to be understood that these are merely values obtained in one specific embodiment and are intended only for purposes of illustration and not to limit the scope of the invention. 
     The power supply circuit  30  is shown generally in FIG.  1 . According to the preferred embodiment of the invention, the circuit includes an oscillator  34  which supplies low current, high voltage energy to a load such as a gas discharge tube  38 . A current regulating circuit  42  is arranged in series with the oscillator feedback winding L 1  and adjusts the high voltage across load  38  by sensing and controlling the current through load  38 . The optimum current for beverage sign loads ranges from 20-26 ma. 
     On-off switch SW of starter circuit  43  is a two pole, two position ( 2 P 2 P) switch which shorts the base of a transistor Q 1  to circuit common  50  in the off mode. When turned on, the switch SW applies a 12 volt positive pulse from capacitor C 1  to the transistor Q 1  base through diode D 1  and opto LED Q 2 , which enables oscillator  34 . Timer  54  provides a starting pulse, delayed by 5 seconds for example, to restart oscillator  34  in case of a power outage or in case the failsafe circuit has disabled the oscillator. 
     A failsafe circuit  58  connects to the centertap of high voltage windings L 2  and L 3  at trace  62  and earth ground trace  66  through opto LED Q 3 . Any unusual increase in the centertap current to ground activities opto coupler Q 3 ′ enabling failsafe circuit  58  and its output triac Q 4  which short circuits the feedback signal at the gate of transistor Q 1 , disabling the oscillator  34  and the high voltage. A restart is attempted every 5 seconds by timer  54 . An open circuit of either high voltage lead also activates the failsafe circuit  58 . 
     A full wave rectifier circuit  70  provides a D.C. power supply of 160 volts at 3.0 amperes for the power supply  30 . 120 volts A.C. connect to terminals  74  and  78 . A tuned, passive filter consisting of capacitors C 2  and C 3 , transformer  94 , and capacitor C 4  reject all but 5 millivolts of the 20 KHz oscillator signal measured at A.C. input terminals  74  and  78 . A varistor  82  clamps noise spikes above 130 volts. The peak voltage of the 120 volt RMS A.C. voltage is 160 volts D.C. and is stored in bulk capacitor C 5 . 
     The primary current path of oscillator  34  begins with the negative end of bulk capacitor C 5 , trace  86 , and consists of a series arrangement of pulse winding L 4 , the primary winding L 5  paralleled by resonant capacitor C 6 , emitter bias circuit, resistor R 1  and capacitor C 7  connected in parallel, and the emitter-collector junction of transistor Q 1  where the collector terminates at the positive end of capacitor C 5 , trace  90 . Several amps of pulsating D.C. current flow in this path during oscillation. 
     In addition to the primary winding L 5 , transformer  98  includes mutually coupled windings L 2  and L 3  which provide high voltage to load  38 , feedback winding L 1  which sustains oscillation, and auxillary winding L 6  which provides A.C. voltage for two low voltage D.C. power supplies required of the current regulator circuit  42  and the failsafe circuit  58 . A common ferrite U core  102  including an air gap complete a magnetic circuit which mutually couples the windings. 
     Pulse transformer  106  includes a single turn primary L 4  and a  100  turn secondary winding L 7  mutually coupled by ferrite core  110 . Secondary L 7  connects directly to circuit common  50  and the gate of MOSFET Q 5 . The gate-source junction of MOSFET Q 5 , Zener D 2 , and the secondary winding L 7  are parallel connected. 
     Biasing network resistor R 1  and capacitor C 7  are parallel connected and complete the circuit between the transistor Q 1  emitter and circuit common  50 . The bias voltage established by the current flowing through resistor R 1  and capacitor C 7  is applied directly to the Q 1  emitter-base junction by means of MOSFET Q 5  when it is enabled by the failsafe circuit  58 . 
     Switch SW is a  2 P 2 P on-off switch. In the off mode, the armature  114  shorts the base of the power transistor Q 1  to circuit common  50  disabling the oscillator. Moving the switch to “on” results in opening armature  114 , removing the base short of transistor Q 1 . Simultaneously armature  118  connects to position  122  of switch SW which allows the voltage in capacitor C 1  to discharge through diode D 1 , switch SW, opto LED Q 2 , and the base-emitter junction of transistor Q 1 ; enabling transistor Q 1  and oscillator  34 . Resistor R 2  charges capacitor C 14  from +160 D.C., trace  90 . Zener diode D 3  regulates the charge to 12 volts. 
     When transistor Q 1  is turned on by the starting pulse from the on-off switch SW, 160 volts D.C. is applied across the primary winding L 5  and its resonant capacitor C 6  introducing sufficient energy to cause a damped wave oscillation. All windings mutually coupled to L 5  are energized including feedback winding L 1  which is required to sustain oscillation. Winding L 1  connects directly to the transistor Q 1  base through resistor R 3  and schottky diode D 4 . On the opposite side of winding L 1 , a closed series circuit is arranged through a network consisting of resistor R 4 , resistor R 5  and capacitor C 8  in parallel, and opto silicon controlled rectifier (S.C.R.) Q 2 ′ to circuit common  50 . Resistor R 6  shunts the gate-cathode junction of opto transistor Q 2 ′. 
     When the on-off switch SW is switched on, C 1  discharges through opto LED Q 2 , turning on opto S.C.R. Q 2 ′ which closes the feedback series circuit momentarily. Once turned-on, opto transistor Q 2 ′ remains on until the D.C. current flowing through capacitor C 8  charges C 8  to a voltage equal and opposite to the source voltage from winding L 1  which removes the voltage from opto S.C.R. Q 2  and opens the circuit feedback to circuit common  50 . 
     Voltage is induced into the oscillator feedback winding L 1  and auxiliary winding L 6  synchronous with the starting pulse. One end of winding L 6  is grounded and the other end charges capacitor C 9  with −8.2 volts through the series circuit of resistor R 7 , diode D 5 , and the 8.2 volt zener D 6  which parallels C 9 . The 8.2 volt charge in capacitor C 9  serves as −8.2 a volt D.C. supply for the current regulator  42 . 
     The −8.2 volts is applied across resistor R 8  and opto transistor Q 6 ′ in series which act as a single ended bridge to control the base voltage of transistor Q 7 . Transistor Q 7  is connected as an emitter follower with the collector connected to −8.2 volts and the emitter returned to circuit common  50  through resistor R 9 , which directly drives the gate-source junction of MOSFET Q 8 . Until opto transistor Q 6 ′ conducts, which will subsequently be discussed, MOSFET Q 8  is turned on completing the series oscillator feedback path from circuit common, MOSFET Q 8 , winding L 1 , resistor R 3 , diode D 4 , base-emitter junction of transistor Q 1 , and the parallel configuration of resistor R 1  and capacitor C 7  to circuit common. The resultant positive feedback to transistor Q 1  sustains oscillation. Capacitor C 10  connected across the collector-base junction of transistor Q 7  operates in a degenerative mode which suppresses oscillation of Q 7  in the regulation circuit. 
     The anode-cathode junction of a triac Q 9  shunts the transistor Q 7  emitter load resistor R 9  with its gate biased from the centertap of zener diode D 7  and resistor R 10  which is also parallel resistor R 9 . A subsequent discussion follows. 
     In circuit  34 , a bridge rectifier D 8  is arranged in series with high voltage windings L 2  and L 3  at terminals  130  and  62  and the gas discharge load  38 . The rectified D.C. output of the bridge rectifier D 8 , traces  138  and  142 , is applied to opto LED Q 6  through series resistor R 11 . A current calibrating potentiometer R 12  shunts LED Q 6  providing an adjustment of the current through opto LED Q 6  which varies the resistance of opto transistor Q 6 ′ in the current regulator  42 . A digital V.O.M.  146 , adjusted to read D.C. ma, directly measures the current in load  38  when connected across resistor R 11 . The amount of load current can be set to the desired value by simply adjusting potentiometer R 12  while viewing the meter. The brightness of the tube  38  varies in proportion to the meter current. Increasing the current increases the high voltage across the tube load. 
     The value of resistor R 11  is not critical and may have a range of 100-500 ohms. In one embodiment of the invention, a 200 ohm resistor was used. When the meter  146  is used, practically all of the current flows through the meter due to its low resistance. When the meter is not used, all of the load current flows through resistor R 11  producing a small drop of 5 volts if the load current is 25 ma (E=IR). In an experimental embodiment of the invention, a female jack was provided such that a millimeter may be plugged-in when needed to set a load current. 
     Opto diode Q 6  is an LED whose light output is directly proportional to the current through it. The emitter-collector resistance of opto transistor Q 6 ′ is directly proportional to the light received from LED Q 6 . When the load current through tube  38  tends to decrease, reducing the light to opto transistor Q 6 ′, the emitter-collector resistance increases. Referring to the current regulator  42 , an increase in the transistor Q 6 ′ resistance increases the base drive voltage of transistor Q 7  increasing its emitter voltage and the gate-source voltage of MOSFET Q 8 . The source-drain resistance of Q 8  reduces increasing the feedback current to the power transistor Q 1  resulting in an increase of current through transistor Q 1  and the primary winding L 5 , as well as the high voltage current to load  38 . Therefore any tendency for the load current through tube load  38  to change is countered by an opposite change resulting from the current regulation of circuit  42 . 
     Trace  62  at the centertap end of high voltage winding L 3  is returned to earth ground at the centertap of capacitors C 2  and C 3  through opto LED Q 3 , shunted by diode D 9 . Any unbalance in resistance or capacitance of tube load  38  at end  150  or  154  relative to earth ground results in current flow from centertap  62  through opto LED Q 3  to earth ground  66 . The resistance of opto transistor Q 3 ′ is reduced by the light from LED Q 3 . 
     Auxiliary winding L 6  provides an A.C. voltage for a −12 volt power supply for the failsafe circuit  58 . One end of L 6  connects to circuit common  50  and the other end to resistor R 13 , diode D 10 , and capacitor C 11  in series. Zener diode D 11  regulates the voltage across capacitor  11  to −12 volts. The isolation breakdown voltage between LED Q 3  and opto transistor Q 3 ′ is 7.5 kilovolts which prevents the high voltage circuit of  34  from effecting any other circuit of power supply  30 . 
     The series arrangement of opto transistor Q 3 ′ and resistor R 14  connect in parallel across capacitor C 11  and share the −12 volt supply. Opto transistor Q 3  and resistor R 14  is a single ended bridge whose output appears across capacitor C 12  which shunts resistor R 14 . The voltage charge in capacitor C 12  is applied to the input of unijunction transistor Q 10  which is connected as a two terminal switch. At 7 volts, UJT Q 10  fires discharging capacitor C 12  through the gate-cathode junction of triac Q 4 , shunted by resistor R 15 . The cathode-anode junction of triac Q 4  conducts shunting the transistor Q 1  base to common  50 , thereby disabling oscillator  34 . 
     Any unbalance of resistance or capacitance at either end of tube load  38 , traces  150  or  154 , causes current to flow through LED Q 3  lowering the emitter-collector resistance of transistor Q 3  charging capacitor C 12 . An unbalance results from a human touch of either end of the tube load  38 , an open lead  150  or  154 , or a broken tube. If the unbalance causes a current flow of 2 ma in LED Q 3 , the charge in capacitor C 12  will exceed the 7 volt threshold of UJT Q 10  causes it to conduct, enabling triac Q 4  and disabling power transistor Q 1  and oscillator  34 . 
     Without a timer to restart the oscillator, a single operation of the failsafe circuit renders the oscillator inoperative until the on-off switch SW is turned off, then on. A timer is illustrated in block  54 . Its purpose is to provide a starting pulse to transistor Q 1  to restart the oscillator  34  after a delay of five seconds. After the initial turn off by the failsafe circuit  58 , the five second timer  54  attempts to restart the oscillator  34  each five seconds until the problem is cleared. 
     If the failsafe circuit continues to detect a failure, the oscillator  34  will not restart, therefore transistor Q 11  cannot discharge C 13 . Opto SCR Q 12 ′ is momentarily switched on each time the diac D 12  fires because opto LED Q 12  is in series with diac D 12 . Therefore opto SCR Q 12 ′ discharges capacitor C 13 , resulting in resetting the five second timer for another  5  seconds. 
     Window neon signs are often turned on and off with real time clocks. In this case, the five second timer  54  starts oscillator  34  after the delay. 
     Resistor R 16  and capacitor C 13  are connected in series from +160 volts D.C., trace  90 , to circuit common  50 . Transistor Q 11  shunts C 13  and normally prevents a charge in capacitor C 13  because the base signal of transistor Q 1  is coupled to the base of transistor Q 11  through resistor R 17  causing the emitter-collector junction transistor Q 11  to conduct preventing a charge in capacitor C 13 . When the oscillator  34  is disabled by the failsafe circuit  58 , transistor Q 11  ceases conduction and capacitor C 13  charges through resistor R 16 . In the experimental embodiment of the invention, these values are chosen to allow capacitor C 13  to charge to 30 volts D.C. in 5 seconds. 
     As capacitor C 13  is charging to 30 volts, capacitor C 14  is charged to the same value of voltage through resistor R 18 . Diac D 12  fires at 30 volts discharging capacitor C 14  through the series path of opto LED Q 12 , diac D 12 , opto LED Q 2 , base-emitter junction of transistor Q 1 , and circuit common through resistor R 1  and capacitor C 7  in parallel. The single positive pulse saturates the base-emitter junction of Q 1  enabling oscillator  34 . Transistor Q 11  is turned on by the signal from the base of transistor Q 1  discharging capacitor C 13  and maintaining a low resistance path across it preventing a recharge. 
     As mentioned above transistor Q 1  is switched on by a current pulse from capacitor C 14  which is simultaneously charged by capacitor C 13  through resistor R 18 . Capacitor C 14  is only 1% of the capacitance value of capacitor C 13  reducing the pulse width to transistor Q 1  and the possibility that transistor Q 1  may receive a feedback signal simultaneous with the starting pulse. In the experimental embodiment of the invention, the pulse width of the capacitor C 14  signal is 100 microseconds. Opto LED Q 12  is pulsed on each time that timer  54  operates which automatically causes opto transistor Q 11 ′ to conduct discharging capacitor C 13  and resetting the 5 second timer, otherwise the failsafe circuit would not reset; disallowing the failsafe circuit from interrogating the load  38  and associated circuitry. 
     The current regulator  42  includes one feature not previously discussed. The power supply  30  can be turned on without the load  38  being connected to the high voltage terminals  150  and  154 . Very little current flows through the regulator opto LED Q 6  under this condition, resulting in opto transistor Q 6 ′ being high in resistance causing transistor Q 7  to develop in excess of 6.2 volts at its emitter and at the gate of MOSFET Q 8  resulting in maximum feedback drive to transistor Q 1  and excessive high voltage. 
     Under this condition, zener diode D 7  interrogates the transistor Q 7  emitter voltage and conducts at −6.2 volts D.C. which causes saturation of the gate-cathode and cathode-anode of triac Q 9  resulting in a law voltage at the gate of MOSFET Q 8  causing a high impedance of MOSFET Q 8  and practically an open circuit of the feedback path, thereby reducing the high voltage to only 1 or 2 kilovolts which is relatively safe. Once turned on, triac Q 9  cannot turn off if any voltage remains between its anode and cathode. Under this condition, the failsafe circuit  58  operates normally and disables the oscillator  34  if either high voltage leads  150  or  154  are touched. The circuit automatically resets with the on-off switch or if the A.C. input voltage is disconnected. 
     The current regulator circuit  42  includes thermistor R 19  which provides thermal compensation of optocoupler Q 6 ′ which has a positive temperature coefficient; that is, the collector-emitter resistance of Q 6 ′ increases as the temperature inside the power supply housing rises as a result of a change in load  38  or an ambient temperature change without thermal compensation, an increase in the opto transistor Q 6 ′ resistance boosts the feedback drive to transistor Q 1  increasing the high voltage and current to load  38 . To off-set an increase in the opto transistor Q 6 ′ resistance, thermistor R 19  shunts opto transistor Q 6 ′ and has a negative temperature coefficient. In the experimental embodiment of the invention, R 19  was 10 K ohms and decreased 4%/° C. between 25 ° C. &amp; 100 ° C. The thermal compensation provided by thermistor R 19  allows the current regulator  42  to meet a specification of +1 ma with load changes of 15-115 watts or an ambient temperature change of ±25 ° C. 
     Power transistor Q 1  is an inexpensive bipolar transistor commonly used in various forms of switching power supplies generally designed for specific D.C. voltage loads such as personal computers. When driven off, it must withstand forward voltages up to 800 volts D.C. and 3 amperes peak when driven on. In the experimental embodiment, transistor Q 1  is mounted to an aluminum, extruded heatsink which dissipates about 3 watts with a tube load of 110 watts. The plastic enclosure of the power supply is slotted providing sufficient draft for air to flow across the heatsink cooling the power transistor Q 1  and MOSFET current regulator Q 8  . Transistor Q 1  mounts on one end of the heatsink and MOSFET Q 8  on the opposite end. 
     In the experimental embodiment, the power transformer  98  is mechanically configured in a rectangular shape with two transformer bobbins positioned over air gaps resulting from butting two ferrite U cores together. As mentioned, the gaps are 0.060″ and consist of phenolic spacers with excellent dielectric properties. The primary bobbin has individual slots for the primary winding L 5 , feedback winding L 1 , and auxiliary winding L 6 ; all wound with litz wire which reduces heat loss. The secondary bobbin is divided into 6 slots with 4 termination pins for the high voltage windings L 2  and L 3 . Winding L 2  is wound in 3 slots on one end of the bobbin and winding L 3  is similarly wound on the other end. Windings L 2  and L 3  are wound from the center of the bobbin to either end to insure equal inductance and distributed capacitance to earth ground of both windings. The centertap traces  130  and  62  terminate on the printed circuit board providing for a series connection of rectifier bridge D 8 . GTO-10, 10 kilovolt cable terminate the ends of windings L 2  and L 3  at traces  150  and  154 . 
     Secondary windings L 2  and L 3  are preferably epoxy encapsulated. In the experimental embodiment of the invention, the wound secondary bobbin inserts into a potting cup which provides a hole on either side of the cup to receive extensions of the secondary bobbin which protrude through the cup holes. An inner hole through the tube of the bobbin allows installation of the ferrite cores after the encapsulation process. A suitable epoxy material, which has been desired, is metered into the cup and bobbin while mounted to a fixture in a vacuum chamber where all air is removed from the windings and epoxy. A heat cure is completed after removing the bobbin and cup combination from the vacuum chamber. During encapsulation, all 4 leads are encapsulated by the epoxy to complete the seal of the high voltage windings. 
     Active, electronic regulation of the load current is desirable to achieve reliable, predictable operation of power supply  30 . The circuit  34  is inherently a passive, constant current source which is necessary in driving gas discharge loads where the tube loads are resistive, vary over a wide range, and have a negative resistance coefficient in relationship to their current and power. 
     In the experimental embodiment of the invention, FIGS. 10 through 13 illustrate the dynamic curves of four different sign systems where the current is varied and the current and wattage are metered. The resistance of the load and the voltage across the load are calculated by: 
     
       
         E=W/I and R=E/I  
       
     
     Using 25 ma as the reference current, sign A parameters are: E=3.75 kilovolts, R=150 K ohms, &amp; W=94. It is observed that the load resistance decreases as the current through the load increases. Expressed as E=IR, the high voltage curve should vary only slightly as the current varies from 20 ma to 30 ma and the wattage from 70 to 115. The high voltage varies from 3.4 kilovolts to 3.8 kilovolts which is a change of only 400 volts over a 45 watt range. Signs B, C and D demonstrate similar results. 
     The inductance sum of L 2  +L 3 =1 Henry. The inductance may be calculated as: 
     
       
           X   L =2 ×PI×FL= 120 K ohms at 20 KHz.  
       
     
     FIG. 14 illustrates the circuit of sign A and the equivalent circuit in FIG.  15 . FIG. 16 illustrates the X R, &amp; Z vector relationships. FIG. 17 plots the voltage drop across the tube load as IR=3.75 kilovolts; the voltage drop across the secondary inductance X L  as IX L =3.0 kilovolts; and the induced volt age E Z =4.8 kilovolts. 
     The equivalent circuit FIG. 15 illustrates that an inductive reactance of 120 K ohm appears in series with any load  38  connected across the secondary windings L 2  and L 3  which clearly demonstrates that circuit FIG. 15 is a constant current source in a passive sense. The circuit can tolerate wide variations of loads in terms of wattage without large changes in current A shorted load  38  between terminals  150  and  154  results in all of the induced voltage E Z  being dropped across X L  of L 2  and L 3 . 
     Observing a wattmeter connected to the input of D.C. power supply  70  reveals that very little energy is dissipated with a shorted load circuit and no damage results. All of the induced voltage in L 2  and L 3  is dropped across the sum of their respective inductive reactances with a zero power factor: Watts=EI×P.F.=0. The limitation on the current is 120 K ohms of X and only 300 ohms of resistance, which is the resistance of inductors L 2  &amp; L 3 . Even if the secondary current increased to 50 ma when shorted, practically zero power results because P=I R=0.75 watts. Current regulator  42  prevents the short circuit current from increasing above the set point which limits the short circuit load power to about 0.4 watt. 
     Load currents of ten gas discharge type signs were compared with and without the active, electronic regulator  42 . To disable the regulator, MOSFET Q 8  was replaced with an appropriate resistor. Without the regulator, the current varied from 23.6 ma to 37 ma over a wattage range of 15-115 watts. With electronic regulator  42 , the current range was 24.0 to 26.0 ma. 
     
       
         
               
               
               
             
           
               
                   
               
               
                 Sign 
                 Without Regulator 
                 With Regulator 
               
               
                   
               
             
             
               
                 1 
                 29.1 ma 
                 25.0 ma 
               
               
                 2 
                 26.4 ma 
                 25.5 ma 
               
               
                 3 
                 33.1 ma 
                 25.2 ma 
               
               
                 4 
                 28.0 ma 
                 25.1 ma 
               
               
                 5 
                 31.7 ma 
                 25.4 ma 
               
               
                 6 
                 31.0 ma 
                 26.0 ma 
               
               
                 7 
                 31.4 ma 
                 24.6 ma 
               
               
                 8 
                 23.6 ma 
                 25.3 ma 
               
               
                 9 
                 34.9 ma 
                 24.0 ma 
               
               
                 10  
                 37.0 ma 
                 24.8 ma 
               
               
                   
               
             
          
         
       
     
     In the experimental embodiment of the invention, the upper limit of the wattage and current control range is 115 watts. At 115 watts of output power, MOSFET Q 8 ′ has less than 1 ohm of source to drain resistance representing a full “on” condition and the limit of its control. Under this condition, R 16  represents the only resistance in series with feedback winding L 1  and the transistor Q 1  base and therefore determines the upper load range of the power supply  30 . At loads less than 115 watts, the current regulator  42  assumes control and regulates at the set current (25 ma in this example). 
     If ten tube sections equal to 11.5 watts each at 25 ma are arranged in series and connected to high voltage traces  150  and  154 , each 11.5 watt section represents a voltage=11.5/25 ma=460 volts drop and a resistance=460 volts/25 ma=18.4 K ohms. The total wattage, volts, and resistance of the ten sections are: 115 watts, 4.6 kilovolts, and 184 K ohms of resistance. 
     Adding one additional section of 11.5 watts to the load results in a drop in current to the load because the high voltage limit is 4.6 kilovolts and the load resistance has increased by 18.4 K ohms. Reducing the load from ten sections to three by successively removing one section results in a constant current of 25 ma and a wattage of 11.5 watts per section with normal brightness. This example best describes the importance of current regulator  42  to power supply  30 . 
     The wattage of oscillator  34  and power supply  30  is limited to 115 watts by the amount of current flow through the power transistor Q 1 . Changing circuit parameters can increase the maximum wattage of the power supply. 
     FIGS. 2-9 represent actual waveforms at key circuit points of the oscillator  34  and transformer  98 , synchronously arranged. As discussed earlier, the oscillator is started by a single pulse from the on-off switch or from a timer whose output is delayed 5 seconds. Transistor Q 1  conducts resulting in 160 volts D.C. being applied across the primary winding L 5  paralleled by resonant capacitor C 6  resulting in a damped wave oscillation of L 5  and C 6 . 
     The waveform illustrated in FIG. 2 is applied regeneratively to the base of power transistor Q 1  resulting in sustained oscillation. The amplitude is 30 volts peak. The resultant transistor Q 1  base voltage and current are represented by FIGS. 3 and 4 and the emitter current and voltage by FIGS. 6 and 7. After turn on, the current through transistor Q 1  and inductor L 5  conduct linearly as shown in FIG. 6 until winding L 5  begins to saturate causing the I/E relationship to change slightly. Pulse transformer  106  detects the change instantly with a one turn primary winding L 4  which is mutually coupled to L 7  resulting in the voltage pulse shown in FIG.  5 . In FIG. 3 the amplitude 15 volts peak; in FIG. 4, the average drive current is 250 ma peak; in FIG. 5 the peak voltage is +6.8 volts; and in FIG. 6 peak current is 3 amperes when the tube load is 90 watts. The +6.8 volt pulse turns on MOSFET Q 5  whose source-drain junction shorts the transistor Q 1  gate to circuit common and reverse biases transistor Q 1  opening the emitter-collector junction. The effect of the sense pulse illustrated in FIG. 5 is shown in FIG. 4 where the base current is turned off removing the base voltage illustrated in FIG. 3, and resulting in cutting off the emitter current shown in FIG.  6  and beginning the flyback voltage shown in FIG.  7 . FIG. 8 illustrates the resonant current in capacitor C 6  which conducts during the flyback period and initiates positive feedback from winding L 1  to start conduction in Q 1  for the succeeding cycle. In FIG. 7 the peak voltage is 600 volts with a 90 watt load and in FIG. 8 the peak-peak current is 4 amperes. 
     An oscilloscope was arranged in shunt with a 100 ohm resistor in series with the centertap trace  62  to display the load current. FIG. 9 illustrates the waveform of the load current of 26 ma with a 90 watt load. 
     The secondary current waveform in FIG. 9 also represents the voltage waveform across the tube load. Generally, it is one alternation of a sine wave which is automatically averaged by the high voltage windings L 2  and L 3  and the load such that equal and opposite average currents flow in the load. No D.C. component is present. Any D.C. component causes electroplating and eventual failure of the tube or electrode. 
     The energy supplied by the power transistor Q 1  to the primary resonant circuit comprising winding L 5  and capacitor C 6  equals the energy dissipated in the load  38 , allowing for small losses resulting from the remaining circuit. When load resistance is decreased, the reflected impedance from windings L 2  and L 3  reduce the primary XL increasing the primary current. If the increase does not satisfy the set current of the load, such as 25 ma, the current regulator  42  increases the flyback drive to power transistor Q 1  until the load current condition is satisfied. 
     Typical values of components of the power supply are listed in the following table to enable those of ordinary skill in the art to practice the invention without undue experimentation. Modifications will be obvious to those of ordinary skill in the art. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 TABLE OF COMPONENT VALUES 
               
             
          
           
               
                   
                 Comp. 
                 Value 
                 Comp 
                 Value 
               
               
                   
                   
               
               
                   
                 R16 
                 6.8 meg 
                 D7 
                 6.2 volt zener 
               
               
                   
                 C13 
                 2.2 MF 
                 R10 
                 4.7 K ohm 
               
               
                   
                 Q11 
                 2N3904 
                 Q9 
                 Triac 
               
               
                   
                 R17 
                 12 K 
                 C10 
                 .01 MF 
               
               
                   
                 R18 
                 47 K 
                 R8 
                 470 ohms 
               
               
                   
                 Q12 
                 Opto transistor 
                 Q6 
                 Opto L.E.D. 
               
               
                   
                 R30 
                 4.7 K 
                 R19 
                 10 K thermistor 
               
               
                   
                 C14 
                 .022 MF 
                 C9 
                 47 MF 
               
               
                   
                 Q12 
                 Opto L.E.D. 
                 D6 
                 8.2 volts zener 
               
               
                   
                 D12 
                 Diac 
                 D5 
                 1N4148 
               
               
                   
                 R2 
                 6.8 meg 
                 R7 
                 100 ohms 
               
               
                   
                 D3 
                 12 volt zener 
                 Q4 
                 Triac 
               
               
                   
                 C1 
                 .01 MF 
                 R15 
                 4.7 K 
               
               
                   
                 SW 
                 2P2P 
                 Q10 
                 2N4990 
               
               
                   
                 Q2 
                 Opto L.E.D. 
                 C12 
                 1 MF 
               
               
                   
                 D4 
                 Schottky diode 
                 R14 
                 3.9 K 
               
               
                   
                 R3 
                 10 ohms 
                 Q3′ 
                 Opto transistor 
               
               
                   
                 Q5 
                 P MOSFET 
                 C11 
                 22 MF 
               
               
                   
                 D2 
                 6.8 volt zener 
                 D11 
                 12 volt zener 
               
               
                   
                 R31 
                 10 ohms 
                 D10 
                 1N4148 
               
               
                   
                 R1 
                 1 ohm 
                 R13 
                 200 ohms 
               
               
                   
                 C7 
                 330 MF 
                 Q3 
                 Opto L.E.D. 
               
               
                   
                 Q1 
                 Bipolar transistor 
                 D9 
                 1N4148 
               
               
                   
                 98 
                 Power transformer 
                 170 
                 3 amp 
               
               
                   
                 L5 
                 Primary winding 
                 C2,C3 
                 .022 
               
               
                   
                   
                 102 turns. Litz wire 
                 C4 
                 .022 
               
               
                   
                 L1/L3 
                 Secondary Windings 
                 82 
                 130 V varistor 
               
               
                   
                   
                 5 turns ea, Litz wire 
                 94 
                 R.F.I. XFormer 
               
               
                   
                 L2/L3 
                 Secondary 3 K turns 
                 D20 
                 4 1N5404 bridge 
               
               
                   
                 102 
                 Ferrite cores “U” type 
                 C5 
                 200 MF 
               
               
                   
                 106 
                 Pulse transformer 
               
               
                   
                 L4 
                 1 turn primary 
               
               
                   
                 L7 
                 100 turn secondary 
               
               
                   
                 110 
                 Ferrite core. “E” type 
               
               
                   
                 D8 
                 4 1N4148 diodes 
               
               
                   
                 R11 
                 200 ohms 
               
               
                   
                 146 
                 100 D.C. ma V.O.M. 
               
               
                   
                 Q6 
                 Opto L.E.D. 
               
               
                   
                 R12 
                 100 ohm potentiometer 
               
               
                   
                 C6 
                 .039 MF 
               
               
                   
                 R4 
                 22 ohms 
               
               
                   
                 C8 
                 330 MF 
               
               
                   
                 R5 
                 1.2 K ohm 
               
               
                   
                 R6 
                 12 K ohms 
               
               
                   
                 Q8 
                 N type MOSFET 
               
               
                   
                 Q7 
                 2N3906 
               
               
                   
                 R9 
                 1.8 K ohms