US 3909668 A
The disclosed variable arc lamp modulator comprises a bistable switch connected in series with an arc lamp and a power supply. The switch is triggered ON (closed) by a series of pulses of constant frequency from a clock pulse generator, and the switch is triggered OFF (open) by a series of pulses from another pulse generator. The time interval between the ON and OFF pulses is varied in accordance with the impedance of the arc lamp as detected by a current integrator and a level sensor.
Description (OCR text may contain errors)
United States Patent Laakmann et al.
[4 1 Sept. 30, 1975  Inventors: Peter Laakmann, Los Angeles,
C alif.; Charles U. Boutin, Tucson, Ariz  Assignce: Hughes Aircraft Company, Culver City, Calif.
 Filed: Dec. 22, 1966 [211 App]. No; 605,139
 U.S. Cl 315/209; 315/246 ] Int. Cl. HOlj 29/00  Field of Search,..-... 315/209, 362, 246; 307/96, 307/97, 98, 99
 References Cited UNITED STATES PATENTS 3,265,930 8/1966 Powell 3l5/209 VARIABLE DUTY CYCLE ARC LAMP MODULATOR 3,278,800 l0/1966 Snell... ,3l5/209 Primary ExaminerMaynard R. Wilbur Assistanr E.\'aminerJ. M. Potenza Attornqv, Agenflor Firm-James K. Haskell; Paul M. Coble 5 7 ABSTRACT The disclosed variable are lamp modulator comprises a bistable switch connected in series with an arc lamp and a power supply, The switch is triggered ON (closed) by a series of pulses of constant frequency from a clock pulse generator, and the switch is triggered OFF (open) by a series of pulses from another pulsegenerator. The time interval between the ON and OFF pulses is varied in accordance with the impedance of the arc lamp as detected by a current integrator and a level sensor.
4 Claims, 2 Drawing Figures The invention herein described was made in the course of or under a contract or subcontract thereunder, (or grant) with the Department of the Army.
This invention relates generally to are lamps and more particularly to a method and apparatus for electronically modulating arc lamps.
Arc lamps, such as mercury vapor or xenon arc lamps used as optical signal sources, require that the current supplied thereto be controlled (this control is often referred to as modulating or keying the arc lamp). This modulation serves the purpose of increasing the signal to noise ratio at the optical sensor by rejecting all signals that are not modulated (keyed) at the desired rate. The signal in these systems is usually recovered through a narrow band filter at the keying rate.
This keying, however, can cause severe instabilities in the arc discharge mechanism that can cause the arc to extinguish. This occurs particularly if the keying frequency is above about 1 Kl-l The instabilities are caused by the exitation of acoustical plasma resonances that are exited by the keying frequency. Additional instabilitics can be caused by high frequency mechanical vibration or mechanical shock.
These instabilities are in addition to the instabilities found even in unmodulated arcs such as temporary shifts in the attachment point of the are on the cathode due to, for example, thermal convection. Instabilities due to all causes are characterized by an increase in arc impedance, and all of them occur at random times and may stretch over time intervals from seconds to fractions of milliseconds. When these instabilities occur, it is imperative to maintain the total power supplied to the arc lamp, or better to increase the power level, as this has the tendency to increase the electrode and plasma temperatures temporarily and will thereafter better maintain the emission process.
One of the present methods of powering arc lamps is to use as much of a constant current source as is possible. A constant current source has the capability to adjust the power flow to the arc lamp immediately. Since, in this case the ON current is constant, the power is proportional to the voltage drop. The voltage drop is always higher during periods of instability. A lamp operated in this mode will never extinguish during periods of increased arc impedance.
Another method is to modulate or key the arc lamp (intermittently apply power thereto). However, if the duty cycle is controlled in order to maintain and increase power to the arc lamp over some range of arc impedance, such controlling action has to be very fast. In keyed applications instability has been observed to occur on a cycle to cycle basis, in extreme cases. It becomes apparent that any such control should have as little delay as possible. The above complications have in the past required a DC supply of relatively high voltage and a large series resistance, all in series with a modulating electronic switch, for example, a transistor. An alternate approach would be the use of a constant current regulator.
Whether a high supply voltage is used or an electronic regulator, reliable arc lamp operation was in the past only possible by dissipating considerable power in the regulator or series resistance. Usually the power loss has to exceed the power delivered to the arc by a factor of two to five in order to achieve stable and reliable operation. In many instances such power loss is intolerable and it is always undesirable.
It is therefore a primary object, of the present invention to provide a method and apparatus for electronically modulating arc lamps.
It is another object of the present invention to control the power supplied to an arc lamp by varying the ON time of a keying circuit as a function of the arc impedance.
It is a further object of the present invention to provide a method and apparatus for controlling the duty cycle and power flow as a function of arc impedance with zero delay and with increased efficiency These objects are accomplished according to the present invention as follows. An arc lamp is provided with a power supply and a bistable switch in series with the arc lamp. The electronic modulating system of this invention includes a clock pulse generator operating at the modulating frequency to deliver a series of narrow pulses at equal time intervals to trigger the bistable switch into the ON (closed) state. A separate pulse generator is employed to turn the bistable switch OFF after a preset value of the time integral of the current which has passed through the arc lamp has been reached. Consequently, when the arc impedance is low, a high current is flowing and the bistable switch will be ON (closed) only for a short time. On the other hand, if the arc impedance is high only a small current will flow but for a longer portion of the cycle. If the are impedance changes during the ON cycle itself, the change will be immediately reflected in the integration and the bistable switch will only turn OFF after the preset value of the time integral of current has been reached.
The present invention thus provides a method and apparatus for controlling the duty cycle by varying the ON time of the keying circuit as a function of arc impedance with zero delay. Compared to the constant current method it is also much more efficient in delivering power from a supply to the arc lamp.
These and other features of the present invention will be more fully understood by reference to the following detailed description when read in conjunction with the accompanying drawings in which like reference characters refer to like elements, and in which:
FIG. 1 is a block diagram which illustrates a preferred embodiment of the present invention; and
FIG. 2 is a schematic circuit diagram of the embodiment shown in FIG. 1.
Referring now to the drawings, FIG. 1 shows an arc lamp 2, the operation of which is to be controlled by the present invention. The are lamp 2 may consist of, for example, a mercury vapor arc lamp. The circuit to be described was originally designed for an arc lamp to be used as an optical signal source on a one-shot mission. This invention, however, is applicable to any restartable arc modulator. A power supply 4 provides power for the circuit. The keying system consists of a clock pulse generator 6, operating at the modulating frequency, which delivers narrow pulses 8 to trigger a bistable semiconductor switch 10 into the ON (closed) state and a separate pulse generator 12 to turn the bistable switch 10 OFF (open) after the preset value of the time integral of current has been reached.
In order to measure the time integral of current a current integrator 14 is provided in series with the arc lamp 2. The current integrator 14 feeds a signal, corresponding to the time integral of current, to a level sensor 16. When the preset value of the time integral is reached at the level sensor 16, the level sensor 16 feeds a signal to the turn-off pulse generator 12. This signal triggers the pulse generator 12 into delivering a pulse to the switch to turn it OFF. Consequently, when the arc impedance is low, a high current is flowing and the switch 10 will be closed for only a short time. On the other hand, if the arc impedance is high only a small current will flow but for a longer portion of the cycle.
If the arc impedance changes during the ON cycle it-' power dissipated in the arc would be constant for a constant voltage power source. If the power source has internal resistance, the power dissipated in the lamp would increase with arc impedance. This is a more de sirable situation as was pointed out earlier. The presence of power source resistance (or additional series resistance), of course, does cause losses. These losses, however, are much smaller than the losses caused by operation with a high source voltage and series resistance for the same power regulation. In a practical case where the power source might be a battery, much resistance is usually present anyway.
. If efficiencies approaching 100 percent from a constant voltage power source are desired, the rising power versus rising arc resistance characteristics can be obtained easily by a nonlinear integration. Suchnonlinear integration can be obtained by weighting? high currents differently from low currents. For example, in a capacitive integrator a shunt resistor paralleled to the capacitor will do this effectively.
If a detector tuned to the fundamental of the modulating frequency is used, it is good design practice to limit the expected range of duty cycle to below 50 percent by the proper choice of supply voltage, average current and are impedance. The fundamental component falls off rapidly if the duty cycle exceeds 50 percent. Duty cycles less than 50 percent cause a slight increase in fundamental component for such detectors as silicon or lead sulfide, depending of course on the power versus peak arc current characteristic of the circurt.
Referring now to the circuit diagram of FIG. 2, the bistable semiconductor switch 10 shown in FIG. 1 comprises a power transistor V a silicon gate controlled switch V anda silicon transistor V in connection with the latching loop which comprises a resistor R and diodes D and D The saturated transistor V removes the drive from the transistor V and charges the capacitor C through the resistor R to turn the gate controlled switch V ON. This in turn will turn ON the transistor V, by keying the base to collector path to be shorted. The latch ing loop which comprises a resistor R and the diodes D and D will keep the transistor V cut off.
The transistor V is saturated periodically at the clock pulse rate by a standard unijunction relaxation oscillator which comprises a unijunction transistor V coupled in a relaxation oscillator circuit including a resistor R and a capacitor C The transistor V is turned ONby the integrating unijunction transistor V This will cause a negative turn of pulse to appear at the gate of the switch V to turn off the arc lamp after the preset integral of current is passed therethrough.
The integration is performed in the capacitor C The charging current, which is proportional to the are current, is derived from the inverting transistor V The collector current in the transistor V is proportional to the emitter current, which in turn is proportional to the voltage drop across the resistor R Saturation at the clock pulse rate of the transistor V resets the integrator at the start of each ON cycle. The arc lamp 2 used in this embodiment is of the type which is started by means of a fusible wire link between the electrodes. During the fusing period the integrator is inhibited by the capacitor C and the diode D The above described embodiment of the present invention was successfully operated using the following values for the components of the circuit.
V, 2N2359 R ohms V 2N3262 R 75 ohms V 2N2222 R, 330 ohms V 2N492A R 18K ohms V 2N492A R 5 l0 ohms V 2N398B R ohms V 2N2222 R I50 ohms V TlCll R 75 ohms D IN36OU' C In F o, IN36OO C .22 F' D IN360O C .022 F 1),, IN3600 C, 6.8 1. F R ,1 ohm C .(llu F R 25 ohms C lSu F R 560 ohms L, H
Since the present invention relates to are lamps, the
following definition thereof will be helpful. Arc lamps are two or three electrodedevices operating atgas pressures above one atmosphere, whose emission proof flash lamps is dueto higher-currents. Stability in flash lamps is no problem.
What is claimed is: 1. Apparatus. for controlling the supply of power from a power source to an arc lamp comprising:
an arc lamp; a power supply connected to said are lamp; a'bistable switch connectedin series with the lamp and the power supply having an ON state in which the power supply is electrically connected to the arc lamp and an OFF state in which the power supply is not electrically connected to the arc lamp; first means to supply first electrical pulsesto the switch at regular intervalsto trigger the switch into the ONstate; and second means to supply second electrical pulses to the switch to trigger the switch into the OFF state, said second means comprising a current integrator for sensing current flow through the arc lamp and pulse generating means for generating the second 2. The apparatus according to claim 2 in which the closing said switch at predetermined time intervals; pulse generating means comprises: integrating, with respect to time, the amount of curlevel sensor meansconnected to the current integrarent which flows through said arc lamp each time tor for receiving from the current integrator a sigsaid switch closes; and nal corresponding to the time integral of current 5 opening said switch when the time integral of current which has passed through the arc lamp, and for which passes through said are lamp reaches a pregenerating an electrical pulse when the signal from determined value. the current integrator reaches a predetermined 4. The method of controlling the supply of power value; and from a power supply to an arc lamp comprising: a turn-off pulse generator connected between the 10 initiating a flow of current from said power supply to level sensor and the switch for delivering a pulse to said are lamp at intermittent time intervals; the switch when the generator receives a stimulus integrating the amount of current which passes from the level sensor to trigger the switch into the through said arc lamp each time said flow of cur- OFF state. rent is initiated; and 3. The method of controlling the supply of power interrupting said flow of current when the time intefrom a power supply to an arc lamp comprising: gral of current reaches a predetermined value.
providing a switch in series with said arc lamp;