US2478903A - Stroboscope - Google Patents

Description

Aug. 16, 1949. H. E. EDGERTON STROBOSCOPE Original Filed Aug. 16, 1933 INVENTOR.

HAROLD E.EDGERTON BY 7 x ATTORNEY Patented Aug. 16, 1949 UNITED STATES PATENT OFFICE Original application August 16, 1933, Serial No. 685,501. Divided and this application August 5, 1946, Serial No. 688,405

4 Claims. 1

The present invention, though having fields of more general usefulness in electric systems, is particularly related to stroboscopes and to the production of intermittent or flashing light. The present application is a division of application, Serial No. 685,501, filed August 16, 1933.

An object of the invention is to provide a new and improved electric system and a new and improved stroboscope of the "above-described character.

Another object is to produce stroboscopic light of the same frequency as the alternating source of power from which energy is derived for charging a flash condenser the discharge of which produces the stroboscoplc light flashes.

A further object is accurately to time the flashes at the same relative position of each cycle.

Other and further objects will be explained hereinafter and will be particularly pointed out in the appended claims.

The single figure oi the accompanying drawing is a diagrammatic view of circuits and apparatus arranged and constructed to produce stroboscopic light according to a preferred embodiment of the invention. The stroboscopic light source is illustrated as operated at the frequency of the alternating-current supply and accurately timed to flash in phase with the applied voltage. Stroboscopic light of this nature is useful for such purposes as examining induction and synchronous motors, for accurately determining speed, and for other purposes.

A mercury-arc gaseous-discharge lamp is illustrated as provided with two internal principal electrodes 4 and 6 in a glass-tube envelope 2. The principal electrodes 4 and 6 define a principal-current path. The internal electrode 4 is in the form of a liquid pool of mercury, used for a cathode; the internal electrode 6 serves as the anode.

Lamps of this nature are normally non-conductive, ineffective or deionized, because the mercury gas therein is normally not ionized. The remarks throughout this specification concerning mercury-arc tubes or lamps apply equally well to tubes or lamps filled with other gases, with or without mercury vapor.

An external metal-band grid or control condenser electrode I is shown situated around the glass of the envelope 2, outside the mercury pool 4, opposite to the meniscus of the mercury. The electrode I00 serves as a starting band for initiating current flow in the principal path of the principal electrodes 4 and 6 to facilitate the starting of the tube. This starting is effected 2 through the medium of a starting-current path extending from the starting electrode I00. Other types of starting bands may be employed. It is also possible to employ an internal grid or control electrode.

The starting electrode I00 or its equivalent may be excited from the secondary winding 29 of a transformer of any desired type, such as a flashing high-ratio step-up transformer 30. The secondary winding 29 of the transformer 30 is shown connected in the input circuit of the tube 2, between the cathode 4 and the external electrode I00, to produce the hereinafter-described highvoltage triggering action to start the hereinafter described cathode spot on the mercury pool 4. Th primary winding 38 of the transiormer 30 is of relatively low impedance.

A source of direct-current voltage is shown as supplied from a thermionic-rectifier circuit comprising a thermionic or gaseous-discharge halfwave rectifier tube 20, connected with any desired source I44 of alternating energy of suitable voltage and frequency. The connection of the rectifier 20 to the alternating-current source I44 may be effected in any well known manner, as by means of a secondary winding I41 of a transformer I48. The alternating-current source I44 is shown as the primary winding of this transformer I48, and the transformer I48 is shown provided also with a further secondary winding I60. One end of the secondary winding M1 is shown connected to the anode of the rectifier tube 20. The cathode of the rectifier tube 20 is shown connected by a wire conductor I0 to the anode 6 through a current-limiting series-connected charging impedance 35. The anode 6 is thus connected to the said end of the secondary winding I41 through the rectifier tube 20 and the charging impedance 35.

A main discharge flash capacity or condenser 26 is normally charged with energy from the source of direct current to the full potential of the rectifier 20 through the impedance 35 when the secondary voltage of the winding I41 of the transformer I48 is positive. This may be effected by connecting the condenser 26 to the secondary winding I4'I through the rectifier 20 and the impedance 35 in any desired manner, as by means of a wire conductor 8 and the wire conductor I0. A difference of potential is thus created between the cathode 4 and the anode 6. The condenser 26 becomes thus charged by the voltage of the said direct-current source. The stroboscopic flashes are produced by discharging the condenser periodically through the lamp 2, between the cathode 4 and the anode 6, in a series circuit comprising the condenser 26 and the lamp 2. The cathode 4 and the anode 6 of the tube 2 are shown connected in the output circuit of the tube 2, directly across the condenser 26,

The impedance 35, which may be constituted of a wire conductor, is usually a combination of resistance and inductance, large enough to hold back the current until, after a flash has been produced, the lamp has again become deionized, but small enough to allow the condenser 26 to charge in time for the next flash.

Before each flash, the condenser 26 is charged from the direct-current source so that the anode 6 is positive. The usual voltage to which the condenser is charged is from 200 to 2000 volts. If the mercury-arc lamp 2 has been exhausted adequately, it will not conduct current when the condenser 26 is charged, notwithstanding the difference of potential between the cathode 4 and the anode 6.

The quantity of stroboscopic light is determined by the amount of energy in the condenser 26 and by circuit conditions. The capacity of the condenser 26 is increased until there is suflicient average light for the particular frequency of flashing and the extraneous illumination. Slow speeds require a larger amount of light per flash than fast speeds to give the same average illumination.

In order to obtain sudden surges of current through the primary winding 36 of the flashing transformer 30, a timing relay is employed. The timing relay is preferably in the form of a normally non-conducting mercury-vapor thyratron trigger tube I40, but other gaseous-discharge devices may also be employed, such as gas-filled hot-cathode thermionic tubes or grid-controlled cold-cathode arc-discharge tubes. As the tube I40 conducts in one direction only, it serves as a check valve. It is a gaseous-conductor device having at least three electrodes, namely, a cathode 48, a control-grid electrode 50 and an anode or plate 52, and it may be of the type in the maximum potential which can be established between the two main or principal electrodes 48 and 52 without appreciable current flow therebetween. The starting instant may be controlled by controlling the potential between one of the main electrodes, namely, the cathode 48, and the third electrode 50. The current between the anode 52 and the cathode 48 may be controlled, and the electric discharge between them may be initiated, by varying the potential of the third electrode 50 with respect to the cathode 48.

The thyratron M is shown connected with the transformer 30 in such manner that, when it operates, the energy .in a small second condenser 28 is discharged into the transformer 30, at selected intervals, whenever the tube 2 is to be set into operation. These intervals, according to the present disclosure, are determined by the frequency of the alternating current of the alternating-current source I44.

The primary winding 36 of the transformer 30 is connected, in series with the condenser 28, in the power-output or control-input or plate circuit of the thyratron 40, between the cathode 48 and the plate or anode 52. This power-output circuit, while normally open, because of a normally negative bias on the grid 50, becomes essentially short-circuited whenever the grid 50 is subjected to a suitable stimulus.

The anode of a second thermionic or gaseous discharge half-wave rectifier tube 2| is connect- .4 ed to a secondary tap ||i| of the transformer secondary winding M1. The cathode of the second rectifier 2| is connected in series with an adjustable impedance 3|, shown as a resistor, to charge a second condenser 28. The impedance 3| may, however, be inductive, or a combination of resistance and inductance. The connection of the condenser 28 to the cathode of the rectifier 2 I, through the resistor 3| is similar to that of the cathode of the rectifier 20 through the impedance 35 to the condenser 26. The rectifier 2| thus charges the condenser 28 from the same direct-current source of voltage as the condenser 26, but through the impedance 3|. The condenser 28 may, however, be charged from any other suitable voltage supply.

The cathode of the rectifier 2| is shown connected also through the impedance 3| to the anode 52 of the thyratron I40, similarly to the connection of the cathode of the rectifier 20 to the anode 6 of the lamp 2. The grid 50 of the thyratron I40 is connected through a resistor M2 to thatend of' the further secondary winding I60 of the transformer I48 that is negative during the half-cycle while the condensers 26 and 28 are being charged.

The direct current supplied by the rectifier 2| is converted by the condenser 28 into alternating pulses in the primary winding 36 of the transformer 30. The thyratron does not conduct current except when there are impulses in its input or grid circuit. A timing impulse coming into this input or .grid circuit, to'stimulate the grid 50, trips the thyratron 140, establishing a high-potential gradient between the starting electrode I00 and the cathode 4, thereby causing starting of the lamp 2. The time of starting is controlled by the potential on the grid 50. The resistor I42 serves to limitany current that may tend to flow when the grid 50 becomes positive, or when there is any ionization in the lamp 2.

The half- wave rectifiers 20 and 2| charge the respective condensers 25 and 28 during alternate half-cycles of the alternating current supplied by the alternating-current source I44. The starting electrode I00 is thus intermittently energized during alternate half-cycles only. It is during these alternate half-cycles that the condenser 26 discharges to produce the light flashes.

At the instant that the grid potential reaches the critical value, positive with respect to the cathode 48, in response to the stimulus applied to the grid 50, the output circuit of the thyratron 40 is completed from the anode 52 to the cathode 48, through the primary winding 36 and the condenser 28. The energy stored in the condenser 28 is then suddenly discharged through the output circuit of the thyratron tube I40, between the anode 52 and the cathode 48, and through the low-impedance primary winding 36 of the step-up transformer 30. This very quickly and suddenly induces magnetically a high voltage for a brief interval of time in the secondary winding 29. A high voltage is thus suddenly applied to the electrode I00. A surge of current is caused to flow through the resistor I42. The cathode 48 is thus raised to nearly the potential of the anode 52 during the discharge, and the grid 50 is given a sudden and very strong bias.

This suddenly applied relatively high voltage produced by the potential on the grid 50 across the terminals of the secondary winding 29 will cause a bright cathode spot to form on the surface of the mercury cathode 4 and at the junction between the mercury and the inner wall of the glass tube. The gas in the tube becomes thereupon ionized. The bright spot constitutes a source of electrons upon the mercury, in the vicinity of the cathode 4, that supplies electrons for ionizing the normally un-ionized gas in the lamp 2, to render the lamp conducting. The main discharge condenser 26 thereupon discharges its energy violently into the lamp 2, to produce an arc discharge between the cathode 4 and the anode 6 through the lamp 2. Part of this energy is transformed into a pulse of useful light. A brilliant flash of light is, therefore, produced by the lamp 2 when the condenser 26, after being charged to a certain value, discharges its energy violently and quickly into it at the desired instant, in response to the stimulus produced upon the band electrode I00 by the impulse surge in the secondary winding 29 cf the flashing transformer 30. The potential upon the grid 50 controls the time of starting of the violent electrical transients that are transformed into useful light.

At the instant of discharge, the current surge is naturally very large, over one thousand amperes, and the flash of light through the tube is very intense and quick. The apparent speed of a moving object is thus effectively reduced or stopped, since the object moves an inappreciazble distance during the time that the light is on. The duration of the flash discharge is short, the flash of light from the tube 2 being about 10 microseconds or less. the exact time being a function of the size of the capacity 26, the voltage to which it is charged, the dimensions of the tube, the temperature of the tube, the impedance of the leads a and i0 connecting the condenser and the tube, the volt-ampere characteristics of the tube 2, and other factors. Under some conditions, the duration of the flash is less than one microsec- 0nd.

While the grid 50 is negative, therefore, the thyratron is inefi'ective and, therefore, the condensers 28 and 26 are permitted to accumulate a charge. On the succeeding half-cycle, the grid 50 swings positive and trips the thyratron M0 which, in turn, causes the main condenser 26 to discharge through the mercury-arc tube 2.

The condenser discharge through the mercuryvapor lamp 2 would be oscillatory except for the fact that the tube 2 is a rectifier. This oscillatory tendency is useful, however, since it assists in preventing a continuous flow of current through the lamp 2 from the before-described direct-current power supply. The condenser 26 is charged with a potential of an opposite polarity after a surge of current flows through the tube 2. A negative voltage is thus put on the anode 6, which helps to deionize the tube.

As the thyratron H0 is a rectifier, the current in the circuit comprising the condenser 28 and the transformer 30 cannot oscillate, although there is a tendency to do so.

The thyratron trip circuit puts a high voltage on the primary winding 36, which is transmitted to the external starting band I00, in a sudden manner that makes the lamp 2 start reliably. The lamp 2 may be started by a few microamperes of current in the grid circuit of the thyratron-control tube I40, and there is a negligible time delay between the current impulses to the grid and the starting of the light flashes. The use of the starting-tube thyratron I40 thus makes the stroboscope a practical and useful arrangement.

Modifications will occur to persons skilled in the art, and all such are considered to fall within the spirit and scope of the invention, as defined in the appended claims.

What is claimed is:

l. A stroboscope having, in combination, a gaseous-discharge lamp having an anode, a cathode and a starting electrode, a condenser connected between the anode and the cathode, means including a circuit having a half-wave rectifier for charging the condenser from a source of alternating current during alternate halfcycles, and means for intermittently energizing the starting electrode to produce intermittently a source of electrons at the cathode during the remaining half -cycles, whereby the condenser will become enabled to discharge intermittently during the said remaining half-cycles between the cathode and the anode to produce intermittent flashes of light.

2. A stroboscope having, in combination, a normally deionized gaseous-discharge lamp having an anode, a cathode and a starting electrode, a condenser connected between the anode and the cathode, means including a circuit having a half-wave rectifier for charging the condenser from a source of alternating current during alternate half-cycles, and means for intermittently energizing the starting electrode intermittently to ionize the gaseous-discharge lamp during the remaining half-cycles, whereby the condenser will become enabled to discharge intermittently during the said remaining halfcycles between the cathode and the anode to produce intermittent flashes of light.

3. A stroboscope having, in combination, a normally deionized gaseous-discharge lamp having an anode and a cathode, a condenser connected between the anode and the cathode, means for charging the condenser with directcurrent energy from a source of alternating current during alternate half-cycles, and means for intermittently ionizing the gaseous-discharge lamp during the remaining half-cycles, whereby the condenser will become enabled to discharge intermittently during the said remaining halfcycles between the cathode and the anode to produce intermittent flashes of light.

4. A stro-boscope having, in combination, a normally deionized gaseous-discharge lamp having an anode and a cathode, a condenser connected between the anode and the cathode,

No references cited.

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