05/377294

01/07/1975

07/09/1973

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Lumedyne, Inc. (Commack, NY)

315/241P

331/111, 363/21.170

H05B41/32; H05B41/30; H02P13/22; G05F1/20

323/22Z 321/2,11,12 315/241P 331/111,112

3564393		February 1971	Williamson
3569779	HIGH VOLTAGE POWER SUPPLY FOR A FLASH DISCHARGE LAMP	March 1971	Luursema
3639826	ELECTRONIC CONTROL CIRCUIT	February 1972	Grundberg
3697854	REGULATED DC-DC POWER SUPPLY	October 1972	Berger
3740639	TRANSFORMER COUPLED SWITCHING REGULATOR	June 1973	Easter
3764849	ELECTRONIC FLASH CHARGING AND TRIGGERING CIRCUITRY	October 1973	Ohta

Beha Jr., William H.

Kenyon & Kenyon Reilly Carr & Chapin

What is claimed is

1. A direct current voltage converter, comprising:

2. A direct current voltage converter, comprising:

3. A direct current voltage converter, comprising:

4. A voltage converter, comprising:

5. A voltage converter, comprising:

6. A voltage converter, comprising:

7. A voltage converter as in claim 6 wherein said semi-conductor oscillator means further comprises capacitive means connected to charge during said first portion of a cycle of said semi-conductor oscillator means, and also connected to said transistor switching means in said semi-conductor oscillator means to energize said transistor switching means to prevent the commencement of said first portion of said cycle after the completion of the first portion of a prior cycle and before said inductive feed-back means is operative.

8. A voltage converter, comprising:

9. An electronic photoflash apparatus, comprising:

10. A photoflash apparatus as in claim 9, wherein said semi-conductor oscillator means further comprises capacitive means connected to charge during said first portion of a cycle of said semi-conductor oscillator means, and also connected to said transistor switching means in said semi-conductor oscillator means to energize said transistor switching means to prevent the commencement of said first portion of said cycle after the completion of the first portion of a prior cycle and before said inductive feed-back means is operative.

11. An electronic photoflash apparatus, comprising:

12. A photoflash apparatus as in claim 11, wherein said semi-conductor oscillator means comprises variable resistance means to control the duration of said first portion of a cycle of said semi-conductor oscillator means.

13. A photoflash apparatus as in claim 12, wherein said flash tube means comprises a flash tube and switching means adapted to be attached to a camera so that energy is provided from said storage capacitor means to cause said flash tube to flash synchronously with operation of the camera shutter.

14. A photoflash apparatus as in claim 13, further comprising second zener diode means, the cathode of said second zener diode means being serially connected to the positive terminal of said battery to prevent current from being drawn from said battery when the voltage of the battery falls below a certain value.

BACKGROUND

This invention relates to DC voltage conversion and regulation, and particularly high power conversion for charging energy storage capacitors. Such a voltage converter and regulator can be used in photographic flash equipment, resistance welders, lasers, portable power supplies, signaling devices, and a variety of other battery powered equipment and energy discharge applications.

Many different types of electrical equipment of the general type to which the invention pertains are presently commercially available. However, existing voltage converters are unable to efficiently deliver the level of power required to quickly charge the energy storage capacitors necessary for use in, for example, high power, light weight, battery operated, portable equipment such as electronic flash power supplies.

It is impracticable to use circuits employing high voltage dry cell batteries because such batteries are extremely heavy, making them inconvenient for use in portable equipment. Also, the battery voltages drop as the batteries are used, resulting in non-uniform amounts of power being delivered from one electronic flash to the next, thereby interferring with picture quality.

Vibrator converters, which can be used with low voltage, rechargable batteries, are also known. However, such converters are not particularly reliable because vibrators are mechanical devices which make and break electrical contacts and are therefore susceptible to failure. Converters of this type generally have a low efficiency, and in addition, are not very light-weight.

Currently in more common use are transistor converter circuits which use electronic rather than mechanical switching. Many of these circuits use mechanical relays to shut off the circuit when the maximum desired charge voltage across the storage capacitor is reached, thereby reducing the reliability of the circuits as a unit. In addition, they often have no means to limit the current drain from the batteries or the magnitude of the charging current into the storage capacitors, except through the losses in the circuit components themselves, thus limiting the maximum efficiency of such circuits. Since the power losses are dissipated as heat in the components, heavier components must be used, thereby increasing the weight and volume of such converters. Moreover, the high peak currents which may be drawn from the batteries reduce the ampere-hour capacity of the batteries, further reducing efficiency.

Some converters, such as that shown in U.S. Pat. No. 3,541,420, issued to L.T. Rees, employ neon lamps or other types of glow lamps to control the voltage across the storage capacitors. Because neon lamps change their characteristics with age, ambient light variations, and ambient temperature changes, the regulated voltage will fluctuate depending upon these conditions. Moreover, neon lamps have a rather large voltage hysteresis (their turn-on voltage is significantly greater than their turn-off voltage) resulting in additional fluctuations in the regulated voltage. Thus, in an electronic flash unit where neon lamps are used as the sole voltage regulation control, flash power may undesirably differ from flash to flash,

Such converter circuits also contain no mechanism for adjusting either the frequency of the transistor oscillator in the circuit, or the "on" time portion of the oscillator cycle in a production unit. Since each different unit produced necessarily exhibits somewhat different characteristics because of variations in the performance of components within their inherent tolerances, production units will differ in efficiency and maximum power output unless the frequency or "on" time can be conveniently adjusted.

Another disadvantageous feature of converters such as that shown in Rees, in that the feed-back control which senses the current through the storage capacitors to affect the frequency of the oscillator prevents the isolation of the low (input) voltage from the high (output) voltage, as would be desirable in many power supply applications.

THE INVENTION

It is therefore an object of the present invention to overcome some of the disadvantages of the available portable voltage converter circuits. More specifically, an object of the present invention is to provide a more efficient, lighter, more reliable portable voltage converter and regulator having greater control over the operation of the circuit and allowing isolation between the input and output voltages.

These and other objects of this invention, which will become apparent from the detailed disclosure and claims to follow, are realized in several direct current voltage converter circuits, each of which is comprised of a direct current voltage source, a semi-conductor oscillator driven by the voltage source, a storage capacitor and voltage sensing and regulating means. The oscillator is in energizing relation with the storage capacitor to produce charging current to charge the capacitor, and the voltage sensing and regulating means is responsive to the voltage across the capacitor and regulates the voltage within a narrow range. The voltage sensing and regulating means comprises a glow lamp such as a neon bulb, a zener diode, and a resistor connected in series, which combination is connected across the storage capacitor. When the storage capacitor reaches its desired charge, the neon bulb and zener diode conduct energy away from the storage capacitor until its charge falls to a predetermined lower level, at which time the neon bulb and zener diode cease to conduct, and the charging current is once again supplied to the storage capacitor. The capacitor's charge is thus maintained within narrow limits.

Each circuit also comprises inductive coupling means coupled between the oscillator and the storage capacitor, and arranged so that during the "on" portion of each cycle of the oscillator, current is drawn from the voltage source and energizes the primary winding of the inductive coupling means, but no charging current is provided during that portion of the cycle to the storage capacitor. During the other, "off" portion of each cycle of the oscillator, current from the voltage source is no longer provided to the primary winding of the inductive element, but charging current to the storage capacitor is provided by the secondary winding of the inductive coupling means.

The oscillator is provided with a variable resistance circuit to control the duration of the "on" portion of the oscillation cycle, and also includes a switching circuit to control the frequency of oscillation. The switching circuit in one illustrative embodiment is operable by switching the resistance in a portion of the oscillator circuit, and in another illustrative embodiment, is operable by switching the capacitance in a portion of the oscillator circuit.

Each of the converter circuits also includes inductive feed-back from the inductive coupling means in the form of a further secondary winding. This winding is connected to a transistor circuit so that the commencement of the "on" portion of the oscillator cycle is prevented so long as current is being supplied from the inductive coupling means to the storage capacitor. The circuit also includes feedback from the voltage sensor and regulator to the oscillator so that, by transistor switching, the commencement of the "on" portion of the oscillator cycle is prevented whenever the glow lamp and zener diode in the voltage regulator circuit are conducting excess energy away from the storage capacitor.

So that the commencement of the "on" portion of a cycle is prevented after the completion of the "on" portion of the prior cycle of the oscillator, and before the transistor feedback circuit which is responsive to the supplying of current to the storage capacitor can operate to independently prevent the commencement of the "on" portion of the cycle again, a capacitor circuit is provided in conjunction with the oscillator, connected so that the capacitor charges during the "on" portion of the cycle, and uses that charge to prevent the commencement of the "on" portion of the next cycle until the inductive feedback prevention means can operate.

Each of the voltage converter circuits is provided with a second zener diode (in addition to the one in the voltage regulation circuit) whose cathode is connected in series to the positive terminal of the voltage source so that when the voltage of the source falls below a value determined by the parameters of the zener diode, current cannot be drawn from the voltage source to operate the circuit and damage the source.

A feature of the voltage converter circuit of the invention is thus the provision for a zener diode in series with a neon lamp for greater voltage regulation, so that output voltage variations are reduced to a level which is inconsequential for photographic electronic flash applications. Another feature of the circuit is the provision for an adjustable resistor for adjustment of the "on" time of the circuit oscillator, thereby permitting the output power delivered by the circuit to be controlled. In photographic electronic flash circuits, the recycle time for the storage capacitors to have sufficient charge after one flash to permit the next flash is controlled by controlling the output power level. Thus, when used in an electronic flash unit, the circuit of the present invention permits precise adjustment of the flash recycle time for each unit. In addition, a further feature of the circuit of the present invention is the provision for a recycle time switch which can be operated by the user of the flash unit so that the user may choose and switch between a more efficient but slower recycle time and a less efficient but faster recycle time. This capability permits the optimum utilization of the stored battery energy.

A further feature of the circuit of the present invention is the provision for maintaining the circuit oscillator in the "off" condition while charging current is flowing into the storage capacitor, by means of a transformer secondary winding coupled to the transformer providing power to the storage capacitor, thus allowing for isolation between the low input voltage and higher output voltage of the circuit when required. A still further feature of the invention is the provision for a zener diode in series with the rechargable battery which provides the input voltage for the circuit, to prevent damage to the battery which might be caused by drawing current from the battery after it has run down and needs recharging.

FIG. 1 illustrates one embodiment of the present invention in block diagram form.

FIG. 2 illustrates a circuit embodiment of the present invention in schematic form. The circuit units shown in the block diagram of FIG. 1 are indicated by dotted lines superimposed on the schematic diagram of FIG. 2.

FIG. 3 illustrates another circuit embodiment of the present invention.

FIG. 4 illustrates still a further circuit embodiment of the present invention.

With reference to FIG. 1, there is shown in block diagram form a voltage converter and regulator circuit such as can be used in a photographic flash unit. The "on" time starting current generator 42 produces a starting current for the circuit which is applied to the "on" time generator 43, which generates a square wave signal which is applied to drive circuit 51 and also to the "on" time sensor and "off" time control 44. Drive circuit 51 translates the square wave signal to a lower voltage, higher current form suitable for driving power primary circuit 46. During the duration of the "on" time of the circuit, the drive circuit energizes the power primary circuit 46. The "on" time sensor and "off" time control 44 applies a signal to start prevent circuit 45, which is in turn connected to the "on" time starting current generator 42, to end the "on" time of the circuit and begin the "off" time of the circuit, and to maintain the "off" condition until the secondary voltage presence detector 48 can take over as described below.

The power secondary circuit 47 is energized from the power primary circuit 46, and delivers power to the load 50, which may be, for photographic applications, a xenon flashtube, or for other applications, a laser, or continuous load, as may be required. The secondary voltage presence detector 48, which is also coupled with the power primary circuit 46, applies a signal to the start prevent circuit 45 so long as the power secondary circuit 47 is delivering power to the load. The start prevent 45 therefore prevents the "on" time of the circuit from starting again so long as the power secondary circuit 47 is delivering power to the load. When the power secondary circuit 47 ceases to deliver power to the load, the secondary voltage presence detector 48 senses the absence of secondary voltage and ceases to provide a signal to start prevent 45, which in turn permits the "on" time starting current generator 42 to supply starting current to the "on" time generator 43.

The "on" and "off" cycles are repeated until the power secondary circuit 47 output voltage reaches the point se by the voltage detector and regulator 52, at which point the voltage detector and regulator 52 applies a signal to start prevent circuit 49, which in turn prevents "on" time starting current generator 42 from providing a starting current to "on" time generator 43, thereby commencing the "off" portion of the cycle of the circuit. The circuit will remain in the "off" condition until the output voltage from power secondary circuit 47 reduces sufficiently for voltage detector and regulator 52 to remove the signal it is applying to start prevent 49, thereby permitting the "on" time starting current generator 42 to provide starting current to begin the "on" time of the circuit once again.

The above described operation and "on" and "off" cyclical operation of the circuit is repeated until power is removed from the "on" time starting current generator 42.

Referring now to the complete schematic diagram of the circuit contained in FIG. 2, there is shown a direct current voltage source of battery 40. The positive terminal of the battery 40 is connected to one pole of a switch 41. The voltage source may be DC or rectified AC providing a voltage over a wide range, from about 6 volts to about 450 volts. For an electronic flash unit, the input voltage is preferably in about the 9 to 15 volt range for battery operation and in approximately the 130 to 190 volt range when the circuit is operated from rectified and filtered power line voltages. In certain power supply applications, higher input voltages may be desirable. The circuit decreases in efficiency as lower input voltage sources are used.

An "on" time starting current generator 42 is connected to the second pole of the switch 41. The "on" time starting current generator 42 comprises a resistor 1, one of whose terminals is connected to switch 41, a zener diode 100, whose cathode is connected to other terminal of resistor 1, and a diode 2, whose anode is connected to the anode of zener diode 100.

Diode 2 provides starting current to the "on" time generator 43. The cathode of diode 2 is connected through a resistor 3 to the negative terminal of the battery 40, and is also connected to the base of a transistor 4 so that when current flows through diode 2 and resistor 3, the voltage drop across resistor 3 will be sufficient to turn on (saturate) transistor 4. The collector of transistor 4 is connected through divider resistors 6 and 5 to switch 41, and the emitter of transistor 4 is connected to the negative terminal of battery 40. Transistors 7 and 12 are also provided in the "on" time generator 43. The base of transistor 7 is connected to a point between divider resistors 6 and 5, its emitter is connected to switch 41, and its collector is connected to the anode of a diode 8, whose cathode is connected through a resistor 18 to the negative terminal of battery 40. Connected between diode 8 and resistor 18 is a variable resistor 9, which is connected in series with a capacitor 10 and the anode of a diode 11, whose cathode is connected to the base of transistor 4. The cathode of a diode 19 is connected to a point between capacitor 10 and diode 11, and its anode is connected to the negative terminal of battery 40. Between variable resistor 9 and capacitor 10 is connected one terminal of resistor 38, which is serially connected with one terminal of switch 39, the other terminal of switch 39 being connected to a point between diode 8 and variable resistor 9. The base of transistor 12 is connected to the collector of transistor 7; the collector of transistor 12 is connected to swtich 41, and its emitter is connected through divider resistors 20 and 21 to the negative terminal of battery 40.

Drive circuit 51 is comprised of a transistor 22, a transformer 23, and a resistor 24. The base of transistor 22 is connected to a point between divider resistors 20 and 21 of the "on" time generator 43. The emitter of transistor 22 is connected to the negative terminal of the battery 40, and the collector is connected through the primary winding of transformer 23 to switch 41. One end of the secondary winding of transformer 23 is connected to the negative terminal of the battery 40, and the other end of the secondary winding is serially connected with resistor 24 which is in turn connected to the negative terminal of battery 40.

The power primary circuit 46 is comprised of a transistor 25 and a primary inductive winding 26, which are connected in a manner similar to transistor 22 and the primary winding of transformer 23 in drive circuit 51. Hence, the base of transistor 25 is connected to a point between the secondary winding of transformer 23 and resistor 24; the emitter of transistor 25 is connected to the negative terminal of the battery 40, and its collector is connected through the winding 26 to swtich 41.

Coupled with primary winding 26 is secondary inductive winding 27, of opposite polarity, contained in the power secondary circuit 47. One end of secondary winding 27 is connected to the negative terminal of the battery 40 and the other end is serially connected with the anode of a diode 28. Connected between the cathode of diode 28 and the negative terminal of the battery 40 is storage capacitor 29.

Connected across storage capacitor 29 is the voltage detector and regulator 52, which consists of, connected in series, a neon bulb 33, a zener diode 34, and divider resistors 35 and 36. One terminal of the neon bulb is connected to the terminal of the storage capacitor common to the cathode of diode 28. The other terminal of the neon bulb is connected to the cathode of the zener diode 34, whose anode is connected through resistors 35 and 36 respectively, to the negative terminal of the battery 40.

Also connected across storage capacitor 29 is the load 50, which in photographic applications would consist of a xenon flashtube and the associated camera sync shutter switch and firing circuit. The "on" time sensor and "off" time control 44 comprises a diode 13, resistors 16 and 14 and a capacitor 15. The anode of diode 13 is connected to the emitter of transistor 12 in the "on" time generator 43. The cathode of diode 13 is connected to a point between resistors 16 and 14, the opposite terminal of resistor 14 being connected through capacitor 15 to the negative terminal of the battery 40, and the opposite terminal of resistor 16 leading to the start prevent 45.

Start prevent 45 consists of a transistor 17, the base of which is connected to resistor 16 in the "on" time sensor and "off" time control 44. The emitter of transistor 17 is connected to the negative terminal of battery 40, and its collector is connected to a point between the anode of zener diode 100 and the anode of diode 2 in the "on" time starting current generator 42. Start prevent 49 consists of a transistor 37, whose base is connected to a point between divider resistors 35 and 36 in the voltage detector and regulator 52, to provide feed-back from that portion of the circuit to control the duration of the "on" and "off" times of the circuit as a function of the voltage across the storage capacitor 29. The emitter of transistor 37 is connected to the negative terminal of the battery 40, and its collector is connected to a point between the anode of zener diode 100 and the anode of diode 2 in the "on" time starting current generator 42.

The secondary voltage presence detector 48 consists of a secondary inductive coil 30, a resistor 31 and a diode 32. Secondary coil 30 is inductively coupled with primary coil 26 of the power primary circuit 46. One terminal of winding 30 is connected to the negative terminal of the battery 40, and the other terminal is connected with one terminal of resistor 31 whose other terminal is connected to the cathode of diode 32. The anode of diode 32 is connected to the negative terminal of battery 40. Connected to a point between resistor 31 and the cathode of diode 32 is the base of transistor 17 in start prevent 45.

OPERATION OF THE CIRCUIT IN FIG. 2

For explanatory purposes, assume initially that switch 41 of the circuit is open, that the circut is at rest, that all capacitors are discharged and all transistors are in an off, non-conducting condition. To start the operation of the circuit, switch 41 is closed. If the voltage on the positive terminal of battery 40 is below a certain level, zener diode 100 is chosen so that it will prevent the passage of current, and the circuit will not operate. This will prevent a "deep discharge" which may damage the battery if current is drawn from the battery when the battery voltage has fallen to too low a value. For an electronic flash unit, battery 40 may conveniently be a 12 volt nickel cadmium battery, and zener diode 100 may have a cut-off voltage at about 10 volts.

When the circuit is first turned on by the closing of switch 41, the current flowing through resistor 1 and zener diode 100 will not flow through either of transistors 17 or 37, which are initially in an off condition. Similarly, there will be no flow of current through transistors 7 or 12, or through resistors 6 and 5 and transistor 4, all three transistors 4, 7, and 12 being initially non-conductive.

Current will flow from the positive terminal of the battery 40, through the closed switch 41, through resistor 1, zener diode 100 and diode 2 in the "on" time starting current generator 42, and through resistor 3 back to the negative terminal of battery 40. The current through resistor 3 in the "on" time generator 43 will cause a voltage drop across resistor 3 and hence between the base and emitter of transistor 4, thereby turning on transistor 4 so that current can flow from the positive terminal of the battery 40 through resistors 6 and 5 and transistor 4. The current flowing through resistor 5 to the base of transistor 7 will cause transistor 7 to conduct, with current flowing through transistor 7, diode 8, variable resistor 9, capacitor 10 and diode 11. Concurrently, the current flowing through transistor 7 to the base of transistor 12 causes transistor 12 to conduct, with current flowing through transistor 12 and into divider resistors 20 and 21, which in turn turns on transistor 22 whose base is connected between divider resistors 20 and 21. Transistor 22 drives transformer 23 in the drve circuit 51, with transformer 23 stepping down the battery voltage and stepping up the current flow, to provide a suitable drive signal for transistor 25 in the power primary circuit 46. The voltage drop across resistor 24 and hence between the base and emitter of transistor 25 causes transistor 25 to conduct, thereby applying a square wave voltage to the transformer primary winding 26. The transformer contains a nonsaturable core, in which is established a magnetic field as current flows in the primary winding 26. No current flows however, in transformer secondary winding 27 because of the reverse biasing of diode 28 in series with winding 27. The voltage induced in transformer secondary winding 30 in the secondary voltage presence detector 48, which is also coupled with primary winding 26, has no effect at this stage, because its polarity is opposite to that which would be necessary to turn on transistor 17. Diode 32 is provided to clamp the voltage level resulting from the current flowing through winding 30 and resistor 31 so that transistor 17 will not be damaged by excess reverse voltage being applied between its base and emitter.

The condition of the circuit at this point, with transistors 4, 7, 12, 22 and 25 conducting, and current being drawn from the battery 40, will be considered the "on" time of the circuit.

During the "on" time of the circuit, the current flowing through transistor 7, diode 8, variable resistor 9, capacitor 10 and diode 11 is charging capacitor 10. As the capacitor is charged, the current level through it decreases until the level is reduced below the value necessary to maintain a voltage drop between the base and emitter of transistor 4 sufficient to keep transistor 4 in a conducting condition.

During the "on" time of the circuit, the current flowing through transistor 12 flows through diode 13 and splits between resistors 16 and 14 in the "on" time sensor and "off" time control 44. The current flowing through resistor 14 charges capacitor 15, arming the "off" time control. The current in resistor 16 flows into the base of transistor 17, causing it to conduct. With transistor 17 conducting, the starting current flowing through resistor 1 and zener diode 100 in the "on" time starting current generator 42 is shunted through transistor 17 to the negative terminal of the battery, and away from the "on" time generator 43.

The "on" time of the circuit is determined by the time it takes until the current through variable resistor 9 and capacitor 10 has dropped sufficiently that transistor 4 is no longer conducting. This time is a function of the time constant of the combination of resistor 9 and capacitor 10. The "on" time of the circuit can therefore be controlled by simply adjusting the variable resistor 9.

When transistor 4 is no longer conducting, current no longer flows through resistor 5, and transistor 7 no longer conducts, therefore shutting off also transistor 12, and since current no longer flows through resistors 20 and 21, transistor 22 becomes non-conducting and transistor 25 becomes non-conducting. This condition, in which transistors 4, 7, 12, 22 and 25 are non-conducting will be considered the "off" time of the circuit.

Transistor 17 is initially turned on by current flowing through resistor 16 from diode 13 and transistor 12. So long as current flows through transistor 17, shunting away the starting current, the "on" time of the circuit will not begin again. When the "on" time of the circuit terminates as determined by the values of variable resistor 9 and capacitor 10, the current flowing through transistor 12, diode 13 and resistor 16 ceases. However, the charge that has been built up in capacitor 15 from current flowing through transistor 12, diode 13 and resistor 14 provides drive current through resistors 14 and 16 for the base of transistor 17 to hold it in a conducting condition for the time period necessary for the other circuit elements to be able to control the "off" time of the circuit as described below.

When the circuit changes to the "off" condition, so that current ceases to flow through the primary winding of transformer 23 or the primary transformer winding 26, the corresponding transformer secondary voltages reverse polarity. Thus, current flows through transformer secondary winding 27 in the power secondary circuit 47, and through diode 28 and into storage capacitor 29, thus providing energy to the storage capacitor and partially charging it during the "off" portion of the circuit's cycle.

Simultaneously with the current flowing through transformer secondary winding 27, the voltage polarity of transformer secondary winding 30 (which is also inductively coupled to transformer primary winding 26) is reversed, and current flows through winding 30 in the secondary voltage presence detector 48, through resistor 31, and (because diode 32 connected between resistor 31 and the negative terminal of the battery 40 is reverse biased) into the base of transistor 17, causing transistor 17 to remain conducting and the starting current flowing through resistor 1 and zener diode 100 to remain shunted back to the negative terminal of the battery 40 so that the "on" time of the circuit cannot begin.

As the "off" time of the circuit continues, the transformer magnetic fields and secondary voltages gradually collapse, current ceases to flow through transformer secondary winding 27 and diode 28 into storage capacitor 29, and current ceases to flow through transformer secondary winding 30, resistor 31 and into the base of transistor 17. Meanwhile, the charge on capacitor 15 has decreased below the voltage required to hold transistor 17 in a conducting condition. This time is determined by the time constant of the capacitor 15 and resistors 14 and 16. Those elements are chosen so that the transistor 17 is held in a conducting condition between the beginning of the "off" time of the circuit, and until the voltage induced in secondary transformer winding 30 provides sufficient current to maintain transistor 17 in a conducting condition without the charge from capacitor 15. With capacitor 15 discharged below the value required to maintain transistor 17 in a conducting condition, and the current ceasing to flow through winding 30 and resistor 31 into transistor 17, transistor 17 shuts off, and the start prevent 45 no longer shunts starting current away from the "on" time generator 43. Thus, current may again flow through resistor 1 and zener diode 100 into diode 2 and resistor 3, turning on transistor 4 and in turn the other transistors, 7, 12, 22 and 25, thereby once again beginning the " on" portion of the cycle.

The "on" and "off" portions of the cycle repeat until storage capacitor 29 has charged to a voltage level at which neon lamp 33 and zener diode 34 in the voltage detector and regulator 52 will conduct. That voltage level, and the parameters of neon lamp 33 and zener diode 34 are chosen depending on the output voltage that it is desired to provide for the load 50. It is possible to regulate the voltage on storage capacitor 29 at virtually any voltage. For example, in a photgraphic flash unit, with the proper choice of components, for capacitor 29 to recycle to an energy level of about 100 watt seconds after a photogtaphic flash discharge will require between about 5 and 6 seconds of operation of the circuit, with the circuit completing a complete "on" and "off" cycle between 15,000 and 20,000 times per second. However, other desirable operating frequencies and recycle times may be obtained by varying the circuit components as is well understood by those skilled in the art. Longer recycle times after a discharge result is higer circuit efficiency, and shorter recycle times result in lower efficiency and also require larger and heavier transformer components. Lower frequencies require larger and heavier transformer components, while higher frequencies result in greater energy losses and hence lower efficiency of the circuit.

When the charge on storage capacitor 29 reaches the value necessary to cuase neon lamp 33 and zener diode 34 to conduct, current flows through these two elements and also divider resistors 35 and 36, and also into the base of transistor 37 in start prevent 49, which is connected between the two divider resistors. This current will cause transistor 37 to conduct, and will shunt the starting current through resistor 1 and zener diode 100 through transistor 37 and back to the negative terminal of the battery 40, thereby preventing the "on" time of the circuit from beginning again. The circuit will be held in the "off" mode until the voltage on storage capacitor 29 drops to a level at which zener diode 34 and neon lamp 33 no longer conduct, so that current will cease flowing through resistors 35 and 36 and into the base of transistor 37, causing transistor 37 to become non-conductive and permitting the starting current to once again be applied to the "on" time generator 43 to being the "on" time of the circuit again.

When isolation between the low input voltage and the high output voltage of the circuit is required (as it may be in certain power supply applications), the connection between the voltage detector and regulator and the base of transistor 37 to provide the feedback signal to the start prevent circuit 49 can be replaced by an optical isolator, consisting of a light emitting diode connected across resistor 36 and a photo transistor responsive to light emitted from the diode in place of transistor 37, as is known in the art. The negative terminal of the battery 40 must be disconnected from the negative voltage terminals of the power secondary circuit 47, the storage capacitor 29, the voltage detector and regulator 52 and the load 50 to complete the isolation. Such isolation is possible because the secondary voltage presence detector circuit 48 is already isolated from the power secondary circuit.

So long as power is applied to the circuit by switch 41 remaining closed, once the charge on storage capacitor 29 is built up to the desired value, the voltage regulation provided by the series combination of neon lamp 33 and zener diode 34 contained in the voltage detector and regulator 52 maintains the voltage across capacitor 29 within rather narrow limits so that uniform power can be obtained each time it is desired to operate the electronic flash. For example, if it is desired to regulate the voltage across capacitor 29 at about 365 volts, a zener diode may be chosen with a breakdown voltage of about 285 volts, and a neon lamp may be used having a turn-on voltage of about 80 volts and a turn-off voltage of about 65 volts. Thus, the zener diode and neon lamp would start to conduct when the voltage across capacitor 29 reached about 365 volts, and would cease conducting when the voltage dropped to below about 350 volts.

Without zener diode 34, the charge on capacitor 29 would vary much more widely, and hence, at different times, widely varying amounts of power would be supplied to a flash unit. For example, if several neon lamps were provided in series to provide a regulated voltage high enough for photographic flash applications, the resulting hysteresis would provide for poor voltage regulation, because the turn-on and turn-off voltages of the voltage detector and regulator would be the sum of the turn-on and turn off voltages respectively, of the several serially connected neon lamps, the total hysteres is consequently being the sum of the individual hystereses of the several neon lamps. If, instead, one or more neon lamps were provided in conjunction with voltage dividers to furnish regulation at a required voltage, the hysteresis of the neon lamp or lamps would likewise be multiplied. With zener diode 34 provided in series with neom bulb 33, most of the voltage regulation is performed by the zener diode to prevent such wide fluctuations in power output, and also because the characteristics of neon bulbs vary with surrounding temperature and light conditions and with the age of the bulb. The one neon bulb is necessary because a zener diode has almost no hysteresis, so if a zener diode along were used to regulate the output voltage, the circuit would be on almost continuously, thus wasting a considerable amount of power.

When power is provided to the load 50, such as by the electronic flash being fired, the effect on the circuit is simply to reduce the voltage across storage capacitor 29 below the desired value controlled by the voltage detector and regulator 52. The circuit is thus in the same condition as it is during the initial charging of the storage capacitor and before the desired voltage is achieved. The neon lamp 33 and the zener diode 34 do not conduct, transistor 37 and start prevent 49 do not shunt the starting current away from the "on" time generator 43, and the "on"-"off" cycles of the circuit repeat until storage capacitor 29 is again charged to the desired value.

The circuit is provided with a switch 39 which permits the user of the circuit to switch the circuit frequency. Closing switch 39 places resistor 38 in parallel with variable resistor 9 between the cathode of diode 8 and capacitor 10. When the circuit is used to power an electronic flash unit, switch 39 permits the user to swtich the circuit between a lower level power operation with a slower recycle time and a higher frequency when the switch is closed, and a higher power level operation with a faster recycle time and lower frequency when the switch is open. The maximum power output of the circuit in either position of switch 39 is adjusted by varying the "on" time of the circuit, by means of variable resistor 9. In power supply use for the circuit, the provision of variable resistor 9 permits a "power output limit" adjustment, and in electronic flash units, permits precise adjustment of the circuit recycle time, as opposed to switch 39 which merely provides for two different and preset recycle time modes.

Because of the manner of operation of the circuit, its efficiency is not limited as are many of the prior art circuits. Some loss of efficiency will occur at lower input voltages. Any output voltage is possible, and for example, if a lower output voltage is desired for a low voltage power supply, and/or if tighter output voltage control is desired, circuits which are well known in the art can be used to replace the voltage detector and regulator 52.

OPERATION OF CIRCUIT SHOWN IN FIG. 3

With reference to FIG. 3, there is shown a somewhat simplified version of the circuit shown in FIG. 2. The components shown in the schematic circuit diagram of FIG. 3 corresponding to similar components in FIG. 2 have been identified with the same reference numerals.

For the modified circuit shown in FIG. 3, resistor 1 and zener diode 100 are connected in series as in the circuit in FIG. 2. However, diode 2 has been eliminated, so that the anode of zener diode 100 is connected directly to the base of transistor 4. Transistors 17 and 37 are connected in a manner similar to FIG. 2, with the emitter of each transistor being connected to the negative terminal of the battery 40, and the collector of each transistor being connected to the base of transistor 4 and the anode of zener diode 100. Divider resistors 6 and 5 are connected between switch 41 and the collector of transistor 4 as in FIG. 2. Also as in FIG. 2, the base of transistor 7 is connected to a point inbetween divider resistors 6 and 5, its emitter is connected to switch 41 and its collector is connected through diode 8 to variable resistor 9. As in FIG. 2, in parallel with variable resistor 9 is the series connection of resistor 38 and switch 39, for switching the circuit frequency if desired. Transistor 12 has been eliminated, and divider resistors 20 and 21 are now connected directly to the collector of transistor 7.

Connected to the terminal of variable resistor 9 opposite to the terminal connected to the cathode of diode 8 is capacitor 70. Connected to the opposite terminal of capacitor 70 are a resistor 62, which is in turn connected to the switch 41, and the anode of a diode 63, whose cathode is connected to the collector of transistor 4. Capacitor 70 replaces and performs the functions of capacitors 10 and 15 contained in the circuit shown in FIG. 2. To a point between variable resistor 9 and capacitor 70 is connected one terminal of a resistor 61, whose other terminal is connected to the base of transistor 17. Also connected to the base of transistor 17 is a resistor 60, whose other terminal is connected to the negative terminal of battery 40. As in FIG. 2, a resistor 31 and the secondary transformer winding 30 are respectively serially connected between the base of transistor 17 and the negative terminal of battery 40, but in addition, a diode 71 is connected between those two elements with its anode connected to one end of the winding 30 and its cathode connected to one terminal of the resistor 31.

Diodes 11, 13, 19 and 32, and resistors 3, 14 and 16 are eliminated from the circuit shown in FIG. 2. The rest of the circuit is the same as shown in FIG. 2. Thus, the power primary circuit 46, the power secondary circuit 47, the voltage detector and regulator 52, the drive circuit 51, and the load 50 are unchanged from the circuit shown in FIG. 2.

The circuit of FIG. 3 operates in basically the same manner as the operation heretofore described for the circuit shown in FIG. 2. The primary difference is in the "on" time generator and the "on" time sensor and "off" time controls. When power is applied to the circuit by closing switch 41, and if the voltage of battery 40 is sufficient for zener diode 100 to conduct, current flows through resistor 1 and zener diode 100 into the base of transistor 4 turning on transistor 4. With transistor 4 conducting, current flowing through resistors 6 and 5 and transistor 4 causes transistor 7 to conduct. Current flowing through transistor 7 is provided to divider resistors 20 and 21, and drives transistor 22 in the drive circuit 51, which in turn provides drive to the power primary circuit 46 during the "on" time of the circuit, and the power secondary circuit 47 provides energy to the storage capacitor 29 during the "off" time of the circuit.

Current flowing through conducting transistor 7, diode 8, variable resistor 9 and resistor 61 into the base of transistor 17 will turn on transistor 17, causing it to shunt the starting current through resistor 1 and zener diode 100 away from the base of transistor 4, and back to the negative terminal of the battery 40, causing transistor 4 to become non-conducting and beginning the "off" time of the circuit. During the "off" time of the circuit, current stills flows through resistor 62, into capacitor 70 and through resistor 61 into the base of transistor 17 maintaining transistor 17 in a conducting condition and the circuit in the "off" mode.

Unlike the circuit in FIG. 2, in which the "on" time must precede the "off" time when switch 41 is closed, because there is no path for current to flow into the base of transistor 17 until transistors 4, 7 and 12 are conducting, in this circuit the "off" time could precede the "on" time, because the current path through resistor 62, capacitor 70 and resistor 61 into the base of transistor 17 is available even when transistors 4 and 7 are non-conducting. Which portion of the cycle begins first when power is applied to the circuit is of no consequence, and the "on"-"off" cyclical operation will occur in either event.

When, during the "off" time of the circuit, capacitor 70 becomes sufficiently charged that the current through it and through resistor 61 into the base of transistor 17 is reduced sufficiently so that transistor 17 is turned off, the circuit once again changes to the "on" condition with transistors 4 and 7 conducting. Thus, the "off" time of the circuit is determined by the time constant of the combination of capacitor 70 and resistors 61 and 62.

During the "on" time of the circuit with transistor 4 conducting, current flows through resistor 62 and diode 63 into the collector of transistor 4, causing capacitor 70 to apply a negative voltage across resistors 60 and 61, maintaining transistor 17 non-conducting. The current flowing thorugh conducting transistor 7, diode 8 and variable resistor 9 reduces the negative charge across capacitor 70, until the positive voltage across resistors 60 and 61 is sufficient to turn on transistor 17, ending the "on" time of the circuit and beginning its "off" time. Thus, the "on" time of the circuit is determined by the discharge time of the capacitor 70, which depends on the current flowing through diode 8 and variable resistor 9.

Switch 39 and resistor 38 perform the same function as in FIG. 2, enabling the user to swtich the frequency of the circuit by closing switch 39.

Secondary transformer winding 30 operates as in FIG. 2. During the "on" time of the circuit, during which transformer primary winding 26, to which winding 30 is inductively coupled, is being energized, the voltage induced in winding 30 is opposite to that necessary to turn on transistor 17, and is blocked by diode 71. Thus, diode 71 serves the same function as diode 32 performed in the circuit shown in FIG. 2.

When the "on" time of the circuit ends and the "off" time of the circuit begins, and the voltage reverses in the primary transformer winding 26, a voltage of opposite polarity is induced in secondary winding 30, which provides current through diode 71, resistor 31 and resistor 60 to maintain a sufficient voltage drop between the base and emitter of transistor 17 to keep it conducting and maintain the "off" condition of the circuit while power is being supplied to storage capacitor 29.

It can be appreciated that the circuit shown in FIG. 3, instead of having separate circuitry for the "on" time generator 43 and the "on" time sensor and "off" time control 44 as does the circuit shown in FIG. 2, combines those functions using fewer components. An advantage of the circuit shown in FIG. 3 over that shown in FIG. 2 is thus that because there are fewer components it would be cheaper to manufacture, and would also be more reliable.

OPERATION OF THE CIRCUIT SHOWN IN FIG. 4

An even more simplified form of the circuit than that shown in FIG. 3 is shown in FIG. 4. The functions of the start prevent circuits 45 and 49 have been combined, and instead of two separate transistors 17 and 37 as shown in the circuits in FIGS. 2 and 3, there is provided a single transistor 72 whose collector is connected to the base of transistor 4 and whose emitter is connected to the negative terminal of the battery 40 as were each of the transistors 17 and 37 shown in FIG. 3. The base of transistor 72 (rather than the base of transistor 37 as in FIGS. 2 and 3) is connected to a point between divider resistors 35 and 36 to receive the feed-back signal from the voltage detector and regulator 52.

Transistor 22 in the drive circuit 51 as shown in FIGS. 2 and 3 is eliminated, with one end of the primary winding of transformer 23 being connected directly to the collector of transistor 7, and the other end of the winding being connected to the negative terminal of the battery 40. The emitter of transistor 7 is connected to the switch 41. Resistors 20 and 21 which provided current to drive now eliminated transistor 22 in FIGS. 2 and 3 are also eliminated. Resistors 60, 61 and 38, and diode 8 are also eliminated. Thus, one terminal of capacitor 70 is connected directly to the base of transistor 72, and no resistor is provided between the base of transistor 72 and the negative terminal of the battery 40. Thus, also, variable resistor 9 is connected directly between the emitter of transistor 7 and the base of transistor 72, diode 8 and resistor 61 having been eliminated. Instead of resistor 38, a capacitor 73 is provided in series with switch 39 which combination is in turn connected in parallel to capacitor 70.

The operation of the circuit in FIG. 4 is basically the same as the circuit shown in FIG. 3. The only differences are, first, that the frequency of the circuits oscillation, which may first, that the frequency of the circuits oscillation, which may be switched by closing switch 39, is controlled by means of capacitor 73, which is placed in parallel with capacitor 70 when switch 39 is closed, thus varying the "on" and "off" times of the circuit. In the circuit shown in FIGS. 2 and 3, frequency was switched by means of switch 39 by providing a change in resistance, rather than capacitance. The second change in the circuit operation of FIG. 4 from that in FIG. 3 is that when the charge on storage capacitor 29 becomes large enough to cause neon lamp 33 and zener diode 34 to conduct, the circuit is maintained in the "off" condition by means of transistor 72, which is the same transistor used to control the "off" time of the circuit while the charge on capacitor 29 is being built up in normal cyclical operation, Combining the functions of transistors 17 and 37 into transistor 72 is less efficient, and although it results in a savings on components, will result in more power loss when the circuit is in the "off" condition.

It is to be understood that, while the specific embodiments of the invention described hereinabove have been shown and described in detail to illustrate the application of the principles of the invention, the invention may be embodied in other ways without departing from these principles in light of the teachings herein.