FR2516738A1 - METHOD AND DEVICE FOR DISTRIBUTING ENERGY TO POWER LIGHT TUBES - Google Patents

Abstract

The invention relates to electronic-flash lamps such as are used, in particular, in professional photography, in which a single supply source, called generator, supplies a plurality of electronic-flash lamps with energy, each lamp being used as a light source. In order to vary the distribution of the energy between these electronic-flash lamps, which are fed in a parallel circuit from one and the same capacitor block, it is provided that the lamps are fired successively at points (ta, tb) in time that are slightly offset. The instant of firing (tb) of any lamp apart from the lamp firstly fired is determined by a comparison between a fraction of the initial useful voltage (Ua) of the capacitors and their instantaneous useful voltage (Ub). The capacity of the generators can be substantially increased thereby, and it is possible to use circuits which are less costly and designed for only a low power level.

Description

 The present invention relates to electronic flash units in which a single power supply provides energy to several flash tubes, each flash tube being used as a light source.

 We know that a flash of electronic flash is obtained by the passage of current in a gas flash tube. Electrical energy sent to the tube is then transformed into light and cha parasite. Since it is desired that lightning be fast and deliver a lot of energy, the instantaneous power is very high. This power can not be supplied directly by the sector or by accumulators. To divert this difficulty, energy is first charged in a block of high-voltage capacitors connected to the flash tubes. In their normal state, the tubes are stable. flash are insulators, but a trip circuit is always ready to send them simultaneously a brief pulse THT which ionizes their gas and makes them conductive; Figure 1. They then have a resistive behavior fixed at the manufacturing stage. We will say that the tubes are lit.

A current of the form I = + passes through each tube, and the total of these currents discharge the flash capacitors according to an exponential law of time, -fig. 4 and 5-. The lit tubes remain on as long as the capacitors provide power to them. They are extinguished only at the complete discharge, or almost, of the capacitors, or if one manages to interrupt the supply line of the tubes by an electronic switch which will generally be a thyristor, -fig. 11- and 14
To clarify this presentation, we will neglect certain secondary phenomena whose incidence is low. We can therefore consider: -1 that the lit flash tubes have exactly one behavior
resistive.

-2 that the ignition of a tube is instantaneous when it receives a
pulse THT

-3 that at the end of each discharge the residual voltage Ur at
flash capacitors terminals Cf is weak enough to be neglected. This approximation will be kept whenever it brings great clarity and little error. It will be removed for the detailed development of the electronic circuits.

By way of example, here are some of the values encountered for an average industrial flash:
Capacity of capacitors Cf in microFarads 10 maximum charge voltage ................................. 500 Volts available energy in joules: ... W = 1 / 2C.U .......... 1 250 internal resistance of a tube ........................... 0.8 # resistance of 4 tubes in parallel ..................... 0.2 # peak theoretical current: r st 2500 A power of epoxy: P = UI ............. 1 250kWatts lightning time constant: t = RC 0.002 s.

 In photography it is necessary to regulate the light energy emitted by each tube at each trigger. For this purpose, the electrical energy sent to them is regulated because these two values are substantially proportional. It is then necessary to distinguish the adjustment of the total energy of a fasb, and the distribution of this energy between the different tubes of the same flash. An equal energy on all the tubes is called symmetrical distribution, otherwise we speak of an asymmetric distribution.

the means currently used to regulate the electrical energy sent into the flash tubes are: -1 the interposition of a variable resistor or an induc
variable flow in series with each tube.

-2 shortcircuiting of Cf during normal discharge, by
a thyristor or spark gap, -fig.9-. The discharge curve
corresponding is given fig. 10.

-3 the disconnection of the power supply during discharge, by a
thyristor or other electronic switch interposed on
the current line between capacitor and tube, -fig. 11-.

The discharge curve is given in figure 12. This device
called computer is very used on portable flashes.

de l'un à l'autre.-4 the use of different internal resistance tubes

from one to the other.

These first four methods have drawbacks that make them unusable on powerful multi-tube flashes.

It remains: -5 the use of multiple capacitor blocks, -fig. 2-,
charged to different voltages by sta circuits
S, and each connected to a tube. The inconvé
are: the need for several stability circuits
tion, the interference between the distribution settings and
of total energy, the loss of dissymmetry when the energy
total of the flash is at most known to the minimum, and after
every change of setting a waiting time is required
so that capacitors can charge or
unload to the new value. Response curves
in figure 4.

-6 the use of multiple capacitor blocks, -fig 3-,
charged at the same voltage but switchable according to plusl-
their combinations. Disadvantages: switching on lines
high intensity, heavy, expensive and cumbersome,
step response, number of limited combinations, ris
high explosions if capacitors charged to
different voltages are connected by mistake
Response curves are given in Figure 5.

 the present invention consists in varying the distribution by successively lighting each of the tubes. Response curves in Figure 6 for n tubes and Figure 7 for 2 tubes.

the tubes can be unlimited and powered directly in parallel by a single block of capacitors, -fig.

 13-.

 the tube that is to supply the most energy is ignited first, and the others follow in descending order of the energy that is assigned to them. In fact, the moment of extinction being substantially the same for all the tubes, the more we wait to light a certain tube, the less energy it retains to share with the others. Nevertheless, all the tubes must be well lit before the end of the discharge.

 This offset is in microseconds and milli-seconds. The possibility of making this shift is offered by the observation that a flash duration of a few milliseconds is both long enough to be broken down electronically, and short enough for its different steps to be integrated on a photographic pellicle. as a single event during the opening of a lens shutter.

 This process can work with both equal and unequal internal resistance tubes, but it will be studied with equal resistances, which is the most logical case. When triggered by an operator who wants a global flash, for a symmetrical distribution, all the tubes are lit simultaneously when the same usa voltage is present at their terminals. They are then in parallel during the entire discharge and distribute the total energy equi- tably.

 For an asymmetrical distribution, let's examine in detail the process for 2 tubes: Ea and Eb. It is desired, for example, that the energy Wa of the tube Ea be greater than the energy of the tube Eb in the ratio defined by the equation (1).

At the moment ta, -fig. 7-, the operator asks for a global flash.

The total energy available for the 2 tubes is then WÂ (equation 2) and the electronic circuit only triggers the tube Ea. At the moment tb, -fig. 7-, the energy still available is given by equation (3). Now after this moment the 2 tubes are lit and behave like 2 parallel resistors in equitable distribution of the available energy, hence equation (4). equations (2) and (4) make it possible to know the value of Wa, equation (5).

equations (1), (4) and (5) allow to know in (6) the value of the coefficient k which we note that it depends only on the ratio J between the initial voltage Ua and the instantaneous voltage of the second trigger Ub .

 This calculation is exact in practice if one considers in all the equations values of useful voltages. The actual flash voltage of the instantaneous flash capacitor voltage reduced by the residual voltage Ur will be referred to as the flash voltage.

The average value of Ur is about 65 Volts. All calculated theoretical values will therefore be increased by 65 volts for a practical application.

The law obtained in equation (6) is also verified in principle with any number of tubes. The energy of the different tubes is always distributed in proportions
J fixed only by the ratio / between the ignition voltages.

It follows that such a procedure will work preferentially but not exclusively by determining the instant of each ignition by comparison between the initial voltage and the instantaneous operating voltage across Cf. one of these values will be affected. a coefficient adjustable and specific to each tube, and it is the variation of this coefficient, by means of a potentiometer for example that will adjust the distribution of energy.

 It also follows from equation (6) that the distribution is not modified by the value of the initial charge voltage or by the capacity of Cf. This distribution method is therefore compatible with a setting of the total energy. by variation of initial voltage and by variation of capacity.

 The distribution setting will always be independent of the total energy, the dissymmetry performance will be fully preserved in the extreme cases of global energy regulation, the distribution setting will act immediately, the distributed electronic circuit will include low power components low knives. These benefits had never been brought together.

 Depending on the needs, the order of successive ignitions can also be determined by the absolute or relative value of the following variables: current, electrical energy, electrical power, light energy, light power.

résistance en série.Comme la chute
de tension ainsi obtenue doit être memorisée pendant
toute la décharge des condensateurs, on complétera ce
montage par un condensateur en parallèle, -Fig.18 et 19- -2 interrompre la décharge de l'ensemble des tubes lorsque la
tension aux bores de Cf approche de la valeur présumée de
Ur, afin de ne pas utiliser la zons incertaine. Cette
interruption se fera au moyen d'un thyristor,-fig.14 et 15
ou autre commutateur électronique .In the quest for extreme performance with this process, the residual voltage will be particularly affected. This difficulty can be diverted by three means: -1 compensate the variaton of Ur according to its parameter
the best known. It has been noticed that Ur is not
really a constant of 65 volts. It is an ini value
about 45 volts, which increases when the
of Cf increases. In the circuits ot Ur would have been compensated
by a simple zener diode ensuring my voltage drop
constant, we replace this zener of 65 V. by a zener of 45 V with w-ie

resistance in series.As the fall
voltage thus obtained must be stored during
all the discharge of the capacitors, we will complete this
parallel capacitor mounting, -Fig.18 and 19- -2 interrupt the discharge of all the tubes when the
the boron tension of Cf approaching the presumed value of
Ur, so as not to use the uncertain zons. This
interruption will be by means of a thyristor, -fig.14 and 15
or other electronic switch.

-3 remove the range of use of the flash capacitors from
the uncertain area of Ur so that the error caused dar
a variation of Ur is small. For this we will use
fractionated capacitors in: -Cfo Cf2 Cf3 fig.6
of increasing capacity. Starting from a low energy
asked the flash set, Ct will first load
partially, and then to increase the overall energy
requested will correspond to a rapid increase of its load.

When it reaches its maximum voltage, Cf2 begins to
load in turn quite quickly, and so on,
as shown in FIG. On such a load curve
we can easily choose operating points
always distant from uncertain low voltage, while
keeping a wide range of variation of overall energy,
It should also be noted that the diagrams shown in FIGS. 13 and 14 must be completed by a well-known preignition circuit and represents FIG. 21. It consists in artificially increasing the voltage across each tube by a pre-charge capacitor Cp charged to several hundred volts and separated. of the main circuit by a power diode Dp.

Such an arrangement is intended to improve the ignition of the tuges.

tant à la figure 21 on remarquô/Cp est en opposition avec Cf.
Il s'en suit

Only the layout of this preignition circuit has been studied to render a maximum of services. In re-or-

as in figure 21 we note / Cp is in opposition with Cf.
It follows

et qui tendrait à rechargrCf, la tension aux bornes de Cf reste nulle pendant tout le tempe où le tube correspondant est éclairé.<tb> that <SEP>('ignition
<tb> of a tube, the discharge current starts to pass in the loop: Cf, E, Cp. As this current is several hundred amperes and Cp has a low capacity of about 1 microfarad, Cp will be discharged very quickly, in about 2 to 3 microseconds. It can therefore be considered that at the beginning of a flash, the capacitors Cp corresponding discharges. During the rest of the flash, the current goes into the loop: Cf, E, Dp. This current being very fart compared to that which comes from the resistance

and which would tend to recharge, the voltage across Cf remains zero throughout the temple where the corresponding tube is illuminated.

We therefore have electronic information on the presence or absence of a lit tube. This information will be used in the circuits, it will have 3 destinations: -1 setting up an individual operating witness of
each tube by a neon light and a resistance to
Cp terminals, -fig21-.

-2 cut off the charge of flash capacitors Cf during early
the time when at least one tube is illuminated. This avoids
overload the stabilization circuits and to have
tubes that remain illuminated.

-3 intervene in the distribution circuit as we will see
further.

 The distribution circuit can be analog, logical or digital. It will mainly comprise the memorization of a fraction of the initial useful voltage of Cf and the comparison of this value with a fraction of the instantaneous operating voltage. Equality or a well-chosen inequality equation between these two values causes the corresponding tube to be ignited by the appearance of a signal at the output of one of the comparators O which then sends the tripping current to the thysistor Q corresponding. -fig.20 and 21-.

In the example embodiment, the fraction of the instantaneous operating voltage is present at &alpha; 11 and a lower fraction of a few millivolts is present at &alpha; 10. This fraction of the useful voltage is obtained by the divider bridge R10, R11, RZZ, which divides the voltage present between the points &alpha; 12 and "OV", -fig.20 - by a constant coefficient. the voltage of Cf reduced by a constant value by the Zener diode Z13, which zener is equal to the value of the residual voltage, is therefore in the presence of the useful voltage. If the residual voltage is assimilated to a constant is considered too coarse we will replace this zener by the device described in lines 23 to 33 of page 5,

when at the instant t0 the operator requests flashing a voltage appears at point a7 It follows that the fraction of the initial voltage is then stored in C 5 and sent by follower follower 04 at the common point &alpha;; 8 potentiometers

figure

et que la tension présente au point

soit à coup sùr inférieure à celle du point

The main tube is then lit by its own tripping circuit if the corresponding comparator becomes on to send current to the gate of its thyristor. For that, it is enough that one of the potentiometers is in maximum position, as represented on the potentiometer

figure

and that the tension present at the point

or certainly less than the point

This second condition is obtained thanks to the resistance R11 whose value will be about 200 times lower than that of R {o. From the moment when the first tube is lit, the voltage at point a8 remains unchanged, while the fraction of the instantaneous operating voltage gradually decreases, -f ig.

22 and 23 -, to achieve in turn the voltage of the middle points Ma to Mn potentiometers, triggering abacus tube at the desired time via comparators oa to OM., Thyristors Qa to Qn, capacitors
CI7a to CI7n and pulse coils THT la to Tn.

The voltage of the point 7 can then fall to zero, which has the particular effect of discharging through the high impedance of R3 the storage capacitor C5, -fig. 24-.

This capacitor will then recharge to the new value of the useful voltage which is sent to it by the amplifier at low output impedance and mounted in rectifier sana threshold. The voltage slot sent to the point a7 comes from the timer constituted by CI and R2, controlled by a pulse on the trigger of IQ and whose @o rtie is done by the inverting gate P2. The delay time of a few milliseconds will be set to exceed the presumed duration of the flash.

 In this version represented in a simple way, if the precision is considered insufficient it can be added to all the improvements in detail described using the diagram Figure 21.

 The device described below has its diagram Figure 21, it is also given by way of non-limiting example.

 When an operator requests a global flash, it sends, via a conventional circuit mp not shown, a pulse on the trigger, -aI-, the thyristor QI, This pulse generates a voltage cbute point a2 that lasts 200 to 300 microseconds. At this time all the pre-ignition capacitors are charged with their negative voltage, and the point aI6 is at the low level. Your logic gates PI, P2, P3, and P4 then intervene to change the voltages of the points a3 and a4 from their initial low state to the high state, + 10 volts. The storage capacitor C5 will then be charged in 200 microseconds by the diode D4 and the resistor R4.

For purposes of simplification, R4 is represented as a conventional resistor, but it will be preferable to place here a rectilinear ramp generator, or better an exponential ramp generator which will control the OS load as shown in the diagrams of FIGS. and 26.

 At the instant T0, the voltage of the middle point M of the potentiometer which is in the strongest position relative to the others reaches the value of the voltage of the point aI0.

The corresponding tube is then lit, which has the effect of discharging the corresponding pre-ignition capacitor.

by
This voltage variation is then transmitted a diode
DI4 and the resistance RI5 at the door PI then at the doors
P2, P3 and P4. The voltage of the point a4 then becomes low, which stops charging the capacitor C5, but the voltage of a9 remains low, leaving the OS loaded. A certain fraetion of the initial flash voltage is then memorized in C5 and transmitted to point a8 by the follower amplifier 04. The fraction of the instantaneous operating voltage present in the antenna fades gradually to reach in turn the points Ma to Mn. which triggers the tubes at the desired moments.

 There comes a time when all the tubes flashed and went out. At this extinction corresponds a cessation of passage of current in each tube oe which allows all the capacitors Cpa to Cpu to resume their negative voltage thanks to RI4. This phenomenon is transmitted to the logic gates that drop the voltage of a3 and unload OS through D3.

Some particularities of this assembly are to note:
I- a delay of 200 microseconds maximum exists between the moment when the operator requests a flash, and the ignition of the strongest tube. This delay is negligible in photo.

2- the assembly works @ even if no adjustment potentiometer is at its maximum osisition
3- it is the ignition of a tube which controls the end of the load of C5. for very large tubes whose ignition time is no longer negligible we can control the end of charging C5 by the appearance of a signal at the output of one of the comparator amplifiers O @ to On,
4- the diagram is shown with a block of flash capacitors separated in two to increase the gamma of global energy variation. In this case, the useful voltage isntantaneous point aIO will be obtained through a divider bridge that will have as many branobes high voltage than elementary blocks of capacitors. In obacune of these branobes, the main resistance, RI2, R2I2, will have a value inversely proportional to the capacitance of the corresponding capacitors. This is how the voltage of the aIO point will be the best image of the available energy.

 In all the configurations of this assembly, it is possible to connect the middle points Ma to Mn to a correction circuit comprising active or passive components, in order to conceal the mathematical laws that are sought to be found in the distribution.

 The voltage of the capacitors p will be chosen to be compatible with a technology of chemical capacitors.

 The prototypes have shown that this device mounted with a single block of capacitors easily makes it possible to have an overall energy variation of .1 to 10 and a distribution of this energy in the ratio of I to 10. In the case of an I000 joules flash, this makes it possible to obtain less than 10 joules on a tube. This resustat is also very agreable in the photo.

 It will be noted a similarity between the method and the device described, on the one hand, and the short-circuit method during lightning described line 22 page 2, on the other hand. The resemblance will appear better when it is known that the short-circuit device represented, -fig.9-, by a thyristor, is supping a gas switching tube.

The total difference between the invention and the known short-circuit process becomes clear if the characteristics of the short-circuit process are well noted, namely:
I- very sudden drop in voltage during the short-circuit 2 - the unused energy in the tube is lost from the point
from light, and transformed into heat and possibly non-visible radiation.

3- it is the regulation of the light emitted by a single tube, and not the regulation of the distribution of a global energy between several gubes.

4- The short-circuit process can only be used for
very low energies, whereas the described invention is universal without any upper limit of energy.

 The present invention may be used in all cases where it is desired to distribute with a wide range of variation, the light energy emitted by several flash tubes connected to the same power supply. The tubes can enter in unlimited number and the identical re-partition circuit whatever the total energy involved.

Claims (7)

 I. Feeding method for flash tubes, characterized in that the different tubes are switched on at slightly offset times to vary the distribution of energy sent to them by a block of flash capacitors.  2. Method according to claim I, characterized in that the moment of ignition of each tube is controlled by the ratio between the useful instantaneous voltage of the flash capacitors and the initial voltage before the start of the discharge.  3. Process according to claims I and 2, characterized in that the useful voltage is reconstituted from the actual reduced voltage of the natural or artificial residual voltage.  4. Process according to claims I, 2 and 3 characterized by the reconstitution of the average natural residual voltage of a member creating a constant voltage to which is added a variable voltage. the sum of these two voltages being stored for the duration of a flash, and the variable voltage being a function of the actual voltage. These different parameters are defined from experimental results and vary with flash tube technology and flash capacitors.  5. The method according to claim 1, characterized by the creation of an artificial residual voltage by breaking the flash at a fixed voltage. above the value of the natural residual voltage.  asked for flash.  others as the total energy increases  flash (Cf1, Cf2, Cf3 figure 16) are loaded one after the other  4, characterized in that the elementary blocks of capacitors  6. Process according to all of claims 1, 2, 3 and  7. Device for implementing the following procedure  1, characterized in that it comprises an electrical connection  between the pre-ignition capacitors (Cpa, Cpb, Cpn, FIG. 21), and the electronic circuits for charging and distribution.