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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.
Laakmann; Peter (Los Angeles, CA), Boutin; Charles U. (Tucson, AZ)
Primary Examiner: Wilbur; Maynard R.
Assistant Examiner: Potenza; J. M.
Attorney, Agent or Firm:Haskell; James K.
Coble; Paul M.
The invention herein described was made in the course of or under a
contract or subcontract thereunder, (or grant) with the Department of the
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 arc lamp;
a bistable switch connected in 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
first means to supply first electrical pulses to the switch at regular intervals to trigger the switch into the ON state; 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 pulses when a predetermined value of the time integral of the sensed current is provided by the integrator.
2. The apparatus according to claim 2 in which the pulse generating means comprises:
level sensor means connected to the current integrator for receiving from the current integrator a signal corresponding to the time integral of current which has passed through the arc lamp, and for generating an electrical pulse when the signal
from the current integrator reaches a predetermined value; and
a turn-off pulse generator connected between the level sensor and the switch for delivering a pulse to the switch when the generator receives a stimulus from the level sensor to trigger the switch into the OFF state.
3. The method of controlling the supply of power from a power supply to an arc lamp comprising:
providing a switch in series with said arc lamp;
closing said switch at predetermined time intervals;
integrating, with respect to time, the amount of current which flows through said arc lamp each time said switch closes; and
opening said switch when the time integral of current which passes through said arc lamp reaches a predetermined value.
4. The method of controlling the supply of power from a power supply to an arc lamp comprising:
initiating a flow of current from said power supply to said arc lamp at intermittent time intervals;
integrating the amount of current which passes through said arc lamp each time said flow of current is initiated; and
interrupting said flow of current when the time integral of current reaches a predetermined value.
This invention relates generally to arc 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 KH.sub.z. The instabilities are caused by the exitation
of acoustical plasma resonances that are exited by the keying frequency. Additional instabilities 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 arc 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 arc 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 arc 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 10 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 itself, the change will be immediately reflected in the integration and the switch 10 will only turn OFF after the preset value of the time integral of current has been reached.
In this method, since the average current over one keying cycle is independent of the arc impedance, the 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 desirable 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. Such nonlinear 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 arc 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 circuit.
Referring now to the circuit diagram of FIG. 2, the bistable semiconductor switch 10 shown in FIG. 1 comprises a power transistor V.sub.1, a silicon gate controlled switch V.sub.2, and a silicon transistor V.sub.4, in connection with the
"latching" loop which comprises a resistor R.sub.4 and diodes D.sub.1 and D.sub.3.
The saturated transistor V.sub.7 removes the drive from the transistor V.sub.4 and charges the capacitor C.sub.2 through the resistor R.sub.5, to turn the gate controlled switch V.sub.2 ON. This in turn will turn ON the transistor V.sub.1 by
keying the base to collector path to be shorted. The latching loop which comprises a resistor R.sub.4 and the diodes D.sub.1 and D.sub.3 will keep the transistor V.sub.4 cut off.
The transistor V.sub.7 is saturated periodically at the clock pulse rate by a standard unijunction relaxation oscillator which comprises a unijunction transistor V.sub.8, coupled in a relaxation oscillator circuit including a resistor R.sub.15
and a capacitor C.sub.5.
The transistor V.sub.4 is turned ON by the integrating unijunction transistor V.sub.5. This will cause a negative "turn off" pulse to appear at the gate of the switch V.sub.2 to turn off the arc lamp after the preset integral of current is
The integration is performed in the capacitor C.sub.3. The charging current, which is proportional to the arc current, is derived from the inverting transistor V.sub.3. The collector current in the transistor V.sub.3 is proportional to the
emitter current, which in turn is proportional to the voltage drop across the resistor R.sub.1. Saturation at the clock pulse rate of the transistor V.sub.6 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.sub.4 and the diode D.sub.5.
The above described embodiment of the present invention was successfully operated using the following values for the components of the circuit.
Since the present invention relates to arc lamps, the following definition thereof will be helpful. Arc lamps are two or three electrode devices operating at gas pressures above one atmosphere, whose emission process is due to continuous cathode
heating by the arc itself. Arc lamps are low voltage devices (20 - 50 V). Arc and flash lamps are basically identical except for electrode spacing (small for arc lamps) and plasma temperature (higher for flash lamps). Most arc lamps are operated at a
plasma temperature corresponding to peak output near 1 .mu., while flash lamps are optimized for about 0.4 microns. The higher plasma temperature of flash lamps is due to higher currents. Stability in flash lamps is no problem.