US 3198984 A
Description (OCR text may contain errors)
Aug. 3, 1965 A. FRANKE 3,198,984
REPEATING FLASH CIRCUIT FOR AN ELECTRIC LAMP Filed Dec. 5, 1959 2 Sheets-Sheet 1 E2 A ./m
-'- vvvv\, 1|
E 1 NC,
I if G G) -1 '\/vvv\ JI F E G 52 I L] AI f D B, a; 5 L] B2, A?
INVENTOR ANTON FRANKE BY;
ATTORNEY REPEATING FLASH CIRCUIT FOR AN ELECTRIC LAMP Filed Dec. 3, 1959 A FRANKE Aug. 3, 1965 2 Sheets-Sheet 2 Qi/n/fl avg/w? Aa' Vail/E731 Mir/C1100 1/10/7 INVENTOR.
Alt/TOM F RAM/(E W KW'M/ United States Patent 3,198,984 REPEATING FLASH CIRCUIT FOR AN ELECTRIC LAMP Anton Franke, Danvers, Mass, assignor to Sylvania Electric Products Inc., a corporation of Delaware Filed Dec. 3, 1959, Ser. No. 857,205 2 Claims. (Cl. 315-171) This invention relates to methods and apparatus for producing an electric flash of short duration. Such flashes are very useful in photography and can be used also for other purposes.
The customary and usual means of producing such flashes is by the rapid combustion of a metal foil or wire in an oxygen atmosphere in a sealed glass bulb. Such devices are commonly called flashbulbs and havebeen very effective for photographic purposes. They do, however, have the disadvantage that a new flashbulb is required for each flash, thus necessitating the carrying about of a large number of flashbulbs if a number of pictures are to be taken and requiring a time delay while the bulb is replaced. Moreover, the use of a new bulb for each flash is expensive.
Accordingly, the use of a gaseous discharge lamp, actuated by a condenser discharge has been suggested, the gaseous discharge lamp being used over and over again for a large number of flashes. Such a device generally requires bulky awkward and expensive equipment, however, and the discharge lamp does not have as complete a color spectrum as does the flashbulb.
The use of a condenser discharge to flash an incandescent lamp has also been suggested. Such an arrangement overcomes many of the above difficulties and is useful with cameras of slow shutter speeds, but the flash is of too longa duration to be effective at the high shutter speeds which are commonly in use.
I have discovered, however, that if the filament of the incandescent lamp is preheated to a low temperature, one at which there will be no appreciable light emitted, or in any event not enough light for the taking of the desired photograph, and the filament is then flashed by the condenser discharge, the period of the flash is extremely short and the time required to reach the maximum value is such that the flash can be used at higher shutter speeds and with shutters designed for lamps with short peak times (M2, AGl, etcl). In addition, the use of preheating will increase the brightness of the eventual flash.
However, the preheating battery, if connected directly across the lamp filament to energize the same, acts as a short-circuit on the condenser when the latter discharges and absorbs most of the energy of the discharge, making the flash ineffective unless the battery is cut out before the flash.
A high resistance can be used in series with the battery, but then the battery voltage must be raised greatly to take care of the voltage drop in the resistance. Moreover, if the current loss through the battery is to be cut to as small as even one-tenth that through the lamp, the series resistance would have to be ten times that of the lamp filament. This gives an inefficient bulky and costly arrangement.
The preheating current can be taken from the same battery that charges the condenser by means of a suitable dropping resistor, but the extra drain on the battery will shorten its life.
Since the current through the lamp during the flash is a transient one, an inductance coil might be used to pass the steady, direct current from the preheating battery while preventing the passage of the transient flash. However, an inductance large enough to do that would also be bulky and expensive.
I have discovered, however, that all the foregoing dis- "ice.
advantages can be eliminated and a very eflective flash produced by placing a rectifier in series with the preheating battery, the battery and rectifier being connected to send current therefrom in a direction opposite to that in which the condenser discharge would send it. In that Way, the condenser discharge is prevented from entering the preheating battery circuit, and the consequent shotcircuiting of the condenser discharge is eliminated.
I have further discovered that a semi-conductor diode will be effective as a rectifier in such a circuit thereby eliminating the need for an additional battery to heat the filament, as would be necessary with a vacuum tube rectifier.
Other objects, advantages and features of the invention will be apparent from the following description taken in conjunction with the accompanying drawing, in which:
FIGURE 1 is a schematic diagram of one circuit according to the invention;
FIGURE 2 is a schematic diagram of another such ircuit;
FIGURE 3 is a schematic diagram of such a circuit including a rectifier; and
FIGURE 4 is a graph showing the intensity against time for various types of flashes.
In FIGURE 1, the lamp L is the light-emitting element and can be, for example, a No. 428 incandescent flashlight lamp, that is, one which normally operates without flashing at 12.5 volts and 0.25 ampere. The lamp, of course, has a coiled tungsten filament.
The lamp L is connected in series with a battery B which can be a so-called B-battery of about 45 volts such as used in radio work. A resistor R condenser C and switch S are also included in the series circuit. The resistor R and condenser C in series are shorted by the resistor R and switch A in series. A switch G is connected across condenser C and lamp L in series for discharging the condenser through the lamp L In operation, the switch S is closed to allow condenser C to charge. Switch A is closed then, or earlier, to allow a preheating current to flow through lamp L For the lamp specified, the resistor R is of a value allowing about milliamperes to pass if the lamp is the No. 428 previously mentioned. That value of current may cause a slight glow from the filament. The resistance R is high enough to prevent any harmful short-circuit of the battery when the switch G is closed, but not high enough to make the charging time for the condenser to be so long as to be inconvenient.
When the lamp L is to be flashed, the switch G is closed, thereby discharging the condenser through the lamp. The closing of switch G is synchronized with the opening of the camera shutter by means well known in the art of flash photography. In some cases, it may be desirable to actuate switch G through a relay, transistor or the like in order to permit the use of a smaller current a in the triggering circuit for the flash.
If switch A is left closed during the flash, some of the condenser discharge current will be lost through the resistor R Nonetheless, the maximum current, through lamp L will be increased about 50% with a condenser C In FIGURE 2, the resistor R is again in series with condenser C lamp L and battery B through one arm A of double pole switch S Battery B is connected across lamp L through switch arm S which meets contact E and through arm G of switch S The contact F connected between resistor R and condenser C is in position to be contacted, when desired, by switch arm S In operation, the double-pole switch S is closed and switch arm S set on contact E. The battery B is then connected across the lamp L to preheat it, and the main battery B is connected to charge condenser C Since battery B is across the lamp L and smaller in voltage than B it would take most of the condenser discharge current if it were left in circuit during the discharge.
However, the switch arm S must be disconnected from contact E and connected to contact F to discharge the condenser, so battery B is automatically disconnected during discharge. The time consumed by the switch arm S in going from E to F should be small enough so that the lamp filament does not cool greatly during the switching.
In FIGURE 3, the battery B is'again connected in series with switch arm A, resistor R condenser C and lamp L Switch S is connected to discharge condenser C across lamp L as shown. But now battery B is connected in series with a diode D, which may be a germanium diode capable of withstanding high inverse voltages, and with arm G of switch S the series combination of the three being connected across the lamp L or, as in the figure, across lamp L and switch arm A.
Battery B is connected to be opposite in polarity to the polarity of charged condenser C when taken around a series circuit including them both. The diode D is connected to pass current in the direction of current flow from battery B and to block current in the reverse direction.
In one embodiment of the invention according to FIG- URE 3, the following values were used:
B 278 volts B :1.5 volts C :150 microfarads R :l000 ohms L was a General Electric No. 432 lamp rated at 18 volts and 0.25 ampere.
D is a Sylvania germanium diode No. SR-776.
The resultant circuit had a peak time, that is, time to reach the peak light intensity of 15 milliseconds with preheating and 17 without preheating, that is, without the preheating battery in circuit. Under the same conditions, the light output was 336 peak lumens without preheating and 500 with preheating, a gain of about 50% with preheating.
If the condenser value is reduced with preheating to give merely the same total light output as without preheating, there will be a much greater difierence in peak time with and without preheating.
Similar values can be used for B R C and lamp 1.; in the other figures. R in FIGURE 1 can also be about 1000 ohms.
Other suitable values can be used but those given are especially elfective.
In FIG. 4, the instantaneous light'output in arbitrary but linear units is plotted against time in milliseconds. Curve H in this figure is for the discharge without preheating of the filament and Curve I is for the same discharge with preheating. In the latter case, the circuit of FIG. 3 was used. A condenser of 300 microfarads was used in each case, and a voltage of 45 volts. The peak of the curve is seen to be shifted from about 17 to about 14 milliseconds, and a larger proportion of its energy is contained below the 19 or 20 second point which is the closing time of the usual shutter, known in the trade as Class F, second. With a smaller condenser the curve would be even sharper, and the difference between preheating and not preheating even more marked. In fact, with a microfarad condenser, the difference is about 4 to 1.
What I claim is:
1. An electric flashing circuit comprising: a lamp filament; a condenser; a resistance; a battery for charging said condenser connected to said condenser through said resistance; a switch connected to discharge said condenser across said filament when closed; and a circuit in shunt to said filament to preheat the same prior to discharging said condenser across it, said circuit including a separate battery connected across said filament.
2. An electric flashing circuit comprising: a lamp filament; a condenser; a resistance; a battery for charging said condenser connected to said condenser through said resistance; a switch connected to discharge said condenser across said filament when closed; and a circuit in shunt to said filament to preheat the same prior to disharging said condenser across it, said circuit including a separate battery and a diode connected in series opposition to the voltage of said condenser when the latter is charged by the first-mentioned battery, and in which the diode is con nected to pass current in the direction of current flow from said separate battery but not from said first-mentioned battery.
References Cited by the Examiner UNITED STATES PATENTS 1,939,332 12/33 Bouwers et a1 315-240 X 2,290,264 7/42 Wuerfel 315241 2,407,113 9/26 Tuck 315-291 2,609,523 9/52 Stein et al. 3l524l 2,656,488 10/53 Gray et a1. 315-173 RALPH G. NILSON, Primary Examiner.
GEORGE N. WESTBY, Examiner.