Spark gap switch

A flashlamp unit for individual and sequential firing of flashlamps includes a pair of flashlamps connected in an electrical circuit with a radiation-responsive N/O arc gap switch in series connection with one of said pair of flashlamps and including an arc gap having a radiation-responsive material thereover and therebetween.

TECHNICAL FIELD 
This invention relates to switches for multilamp photoflash arrays and more 
particularly to a normally open (N/O) switch for a sequentially activated 
multilamp photoflash unit. 
BACKGROUND ART 
Generally, photoflash units may be classified as either a low voltage or a 
high voltage unit. The low voltage photoflash units usually employ a 
battery or a charged capacitor whereby a voltage in the range of about 1.5 
to 15.0 volts is provided. The high voltage photoflash units ordinarily 
employ a piezoelectric element and provide a pulse voltage in the range of 
about 2000 to 3000 volts. 
Also, it is a common practice to employ sequencing circuitry wherein a 
plurality of photolamps are sequentially activated by a voltage from 
either a low or high voltage source. Moreover, this sequential activation 
of the photolamps is usually controlled by a plurality of radiation 
switches connected on circuit with the photolamps and the voltage source. 
The radiation switches may be either of the normally closed type or the 
normally open (N/O) variety, and the normally open switch appears to be 
the more common. As is known, the N/O type radiation switch is 
positionally located adjacent a photolamp and has a relatively high 
resistance prior to radiation impingement. However, activation of the 
nearby photolamp serves to provide the necessary radiation whereupon the 
radiation-responsive switch is converted from a high resistance or open 
circuit condition to a relatively low resistance substantially short 
circuit condition. 
Ordinarily, the N/O radiation-responsive switches include a pair of 
terminals spaced about 0.04" to 0.08" apart and covered over with an 
insulating material which becomes electrically conductive upon exposure to 
radiant energy from a nearby lamp. Examples of such radiation-responsive 
switches and materials are provided in U.S. Pat. Nos. 3,969,065 and 
3,951,582 wherein copper and silver salts are employed with a plurality of 
different combustible binders. 
Silver salts are currently used in most of the radiation-responsive 
switches employed in sequentially operable multilamp photoflash arrays. 
However, the silver used in such switches must have a relatively high 
level of chemical purity which adds greatly to the already relatively high 
cost of silver salts. Moreover, the silver salts have a relatively limited 
range of activation as compared with a photolamp which has a temperature 
range which is both wider and less controllable than the activation range 
of the N/O switch. As a result, it has been found that a switch sensitive 
enough to be activated by a low temperature lamp will exhibit "burn off" 
or inactivation when energized by a high temperature lamp. On the other 
hand, a switch insensitive enough to resist "burn off" by a high 
temperature lamp will not be activated by a low temperature lamp whereupon 
an open circuit will result. 
At present, it is a common practice to design a N/O radiation-responsive 
switch such that exposure to a low temperature lamp is sufficient to 
activate the switch. The problem of "burn off" due to an excess of radiant 
energy is compensated for by making the N/O switch larger than the 
activating aperture of a reflector as illustrated in FIG. 1. In this 
manner a switch activation gradient is provided between a completely 
vaporized area 5 and a completely inactivated area 7. However, a 
relatively large switch area tends to introduce problems of cracking 
during the switch drying process and, if the cracks are large enough, 
results in an undesired lamp failure. Moreover, a relatively large switch 
is undesirably expensive of materials. 
Additionally, it is known that arc gaps may be used in a sequential 
photolamp array as evidenced by U.S. Pat. No. 3,742,298. However, the 
above-described structure is dependent upon the breakdown of the arc gap 
whereupon conduction across the arc gap is achieved. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an enhanced flashlamp 
unit. Another object of the invention is to improve the operational 
capabilities of a sequentially operable multilamp photoflash unit. Still 
another object of the invention is to provide an enhanced 
radiation-responsive switch for a flashlamp unit. A further object of the 
invention is to provide a sequentially operable multilamp photoflash unit 
with an improved radiation-responsive switch which includes a spark gap 
capability. 
These and other objects, advantages and capabilities are achieved in one 
aspect of the invention by a flashlamp unit having at least a pair of 
flashlamps with a radiation-responsive switch in series connection with 
one flashlamp and adjacent the other flashlamp and the 
radiation-responsive switch having an arc gap with a radiation-responsive 
material thereover and therebetween the arc gap whereby radiation from a 
flashlamp renders the radiation-responsive switch conductive. 
In another aspect of the invention the radiation-responsive material of the 
switch may be of the type which is rendered conductive in response to 
radiation and thus provides not only an electrical conductive path by way 
of the radiation-responsive material but also by way of the arc gap. 
Alternatively, the radiation-responsive material may be of a readily 
vaporized material such that the radiation from a flashlamp evaporates the 
non-conductive material and an electrical path is provided by the arc gap.

BEST MODE FOR CARRYING OUT THE INVENTION 
For a better understanding of the present invention, together with other 
and further objects, advantages and capabilities thereof, reference is 
made to the following disclosure and appended claims in connection with 
the accompanying drawings. 
Referring to the drawings, FIG. 1 diagrammatically illustrates a 
radiation-responsive switch of the prior art. Therein, a radiation 
responsive material 4 includes a vaporized area 5 wherefrom the 
radiation-responsive material may be completely removed whenever an 
adjacent flashlamp provides an excess of radiation. Also an inactivated 
area 7 provided by the shielding adjacent a limited aperture 9 provides a 
conductive path should the excessive radiation render the vaporized area 5 
non-conductive. However, It has been found that severe cracks sometimes 
develop in the inactivated area 7 during switch fabrication. Moreover, 
these undesired cracks, if severe enough, prohibit development of a 
conductive path by way of the inactivated area 7 and also prohibit 
conversion of the switch from a non-conductive to a conductive state. 
Also, the material required to provide both a vaporizable area 5 and an 
inactivated area 7 is both excessive and expensive. 
Referring to FIG. 2, a printed circuit board 11 has affixed thereto a 
radiation-responsive spark gap switch. This radiation-responsive spark gap 
switch includes a first and second electrical conductor 13 and 15 affixed 
to the printed circuit board 11 and having an arc gap 17 therebetween. A 
radiation-responsive material 19 is positionally located over a portion of 
the electrical conductors 11 and 13 forming the arc gap 17 as well as 
within the arc gap 17. 
Normally, the radiation-responsive material 19 is in an electrically 
non-conductive state until exposed to radiation, usually from a nearby 
flashlamp. Thereupon, the electrically non-conductive radiation-responsive 
material 19 is converted to an electrically conductive state. Thus, there 
is provided a radiation-responsive arc gap switch wherein the 
radiation-responsive material 19 provides an electrically conductive path 
and the arc gap 17 also provides a path for electrical conduction when a 
relatively high potential source is utilized. 
Alternatively, the radiation-responsive material 19 may be substantially 
removed or "burned off" by excessive radiation from a nearby flashlamp. 
However, removal of the radiation-responsive material 19 does not negate 
the effects of the radiation-responsive switch since the arc gap 17 
provides a path for electrical conduction and conversion of the switch 
from a non-conductive to a conductive state. 
As to materials, a preferred radiation-responsive material 19 for an arc 
gap switch includes silver coated glass spheres in a binder such as 
polystyrene, for example. Such a material is not only converted to a 
conductive material in responsive to radiation from a nearby photolamp but 
also utilizes the silver coated spheres to reduce the arc gap 17 
intermediate the electrical conductors 11 and 13. Thus, the potential 
necessary to overcome the arc gap 17 and provide a conductive path is 
reduced while the reliability of the conversion of the switch from a 
non-conductive to a conductive state is enhanced. Preferably arc gap 17 
has a spacing in the range of about 0.005 to 0.015 inch and is converted 
from a voltage breakdown level of about 2000 to 3000-volts to a level of 
about 300 to 400-volts by radiation from a nearby flashlamp. 
Alternatively, an easily vaporizable radiation-responsive material 19, such 
as acrylic, waxes, or organic polymers is suitable for a 
radiation-responsive arc gap switch. As mentioned before, radiation from a 
nearby photolamp "burns off" the easily vaporizable material whereupon the 
arc gap 17 provides the electrical path whereby a subsequent photolamp is 
energized. 
As to the sequential operation of a multilamp array, reference is made to 
the diagrammatic illustration of FIG. 3. Therein, a first flashlamp 21 is 
directly connected to a high voltage source 23. A first 
radiation-responsive arc gap switch 25 is positionally located adjacent 
the first flashlamp 21 and in series connection with a second flashlamp 
27. Also, a second radiation-responsive arc gap switch 29 is located 
adjacent the second flashlamp 27 and in series connection with a third 
flashlamp 31. Similarly, a third radiation-responsive arc gap switch 33 is 
adjacent the third flashlamp 31 and in series connection with a fourth 
flashlamp 35. Moreover, the series connected flashlamps and 
radiation-responsive switches may be continued so long as such sequential 
operation is desired. 
In operation, a high voltage pulse from the high voltage power source 23 
activates the first flashlamp 21 which, in turn, provides radiation in an 
amount sufficient to convert the first radiation-responsive arc gap switch 
25 from a non-conductive to a conductive state. A second high voltage 
pulse is transmitted via the first arc gap switch 25 to the second 
flashlamp 27 to effect energization thereof. The energized second 
flashlamp 27 provides radiation which impinges the second 
radiation-responsive arc gap switch 29 and converts the switch to a 
conductive state. Moreover, the procedure is repeated for each series 
connected switch and flashlamp. Thus, the flashlamps are sequentially 
activated in conjunction with the conversion of the individual 
radiation-responsive arc gap switches. 
While there has been shown and described what is at present considered the 
preferred embodiments of the invention, it will be obvious to those 
skilled in the art that various changes and modifications may be made 
therein without departing from the invention as defined by the appended 
claims. 
INDUSTRIAL APPLICABILITY 
Thus, there has been provided a unique flashlamp unit suitable for use with 
a relatively high voltage potential source. The flashlamp unit includes a 
radiation-responsive arc gap switch having an arc gap covered with a 
radiation-responsive material. The radiation-responsive material, in 
response to energization by an activated flashlamp, is converted from a 
non-conductive to a conductive state to provide a first electrical path by 
way of the radiation-responsive material and a second electrical path via 
the arc gap from a potential source to a following flashlamp. Thus, 
sequential operation of the flashlamps is effected. 
In another aspect of the invention, the switch remains workable even though 
an excess of radiation is present. In such cases, the excess radiation 
tends to "burn away" the radiation-responsive material. However, removal 
of the radiation-responsive material still leaves the arc-gap and an 
electrically conductive path from a voltage source to a following 
flashlamp. Again, sequential operation of the flashlamps is effected.