Precision high-voltage resistor comprising a plane substrate of insulating material (1) on which there has been deposited by silk-screening at least one resistive film (3) of approximately rectangular, square or similar shape and a conductive film in the form of two parallel strips (2a, 2b) extending along two opposite edges of the resistive film and constituting terminals electrically connected to one another by said resistive film, the resistive film comprising rectilinear cuts (5a to 5f) made in its thickness, down to the insulating substrate, parallel to said opposite edges from a third edge (6) of the resistive film, characterized in that the rectilinear cuts (5a to 5f) are evenly spaced along the third edge (6) and have lengths that are larger the closer they are to the center of the third edge (6), the ends opposite the third edge (6) of the rectilinear cuts defining from the intersections (8a, 8b) of the third edge (6) with the terminals (2a, 2 b) a contour (7; 9) exhibiting an apex in its center part.

BACKGROUND OF THE INVENTION 
This invention relates to a resistive circuit element made according to the 
so-called "thick-film" technology by superposition of two films, one 
conductive, the other resistive, to obtain a resistance that withstands 
strong voltages and that is brought to a precise ohmic value by several 
laser cuts correctly distributed in the resistive film. 
"Thick-film" hybrid technology has as its principal base materials 
resistive, conductive and insulating inks. These inks are in the form of 
pastes that contain the following elements: special powdered glass, 
pulverulent precious metals, organic binder, diluent consisting of a 
mixture of solvents. These ingredients which are mixed to form a thick 
paste are deposited on ceramic plates called substrates, generally of 
alumina, by the process of silk-screen printing. Once the paste is 
deposited on the substrate, the piece is dried at 100.degree.-150.degree. 
C. to remove the solvents from it and fired in a furnace at 
500.degree.-1,000.degree. C., generally 850.degree. C. During the firing, 
three phenomena occur: breakdown of the organic binder, sintering of the 
glass particles on the surface of the substrate and vitrification of the 
unit. Thus, the elements that make up the circuit adhere very strongly to 
the ceramic. 
A resistor made in "thick-film" hybrid technology is shown in FIG. 1 of the 
accompanying drawings. It comprises two different films deposited on a 
substrate 1: the first 2a, 2b, made of a silk-screened conductive ink, 
dried and optionally fired, serves as a support and as terminals for the 
resistor; the second 3, made of a silk-screened resistive ink, dried and 
fired, is in itself the actual resistor. These two films, if the method of 
manufacturing allows it, can be co-fired, i.e., fired together. 
This technology makes it possible to make resistors in a very wide value 
range (10-10.sup.6 .OMEGA.) depending on the choice of the type of 
resistive ink used and on the variation of the geometry of the printed 
resistors. 
The materials going into the composition of the conductive ink for the 
conductive film have a base of metals or alloys such as silver, palladium, 
platinum, gold, copper, aluminum. The choice of these various metals rests 
on several criteria: solderability, resistance to aging, definition for 
printing, low resistivity, adherence to the substrate, compatibility with 
the resistive ink used and possibility of annealing. The thickness of the 
conductive film is generally between 5 .mu.m and 50 .mu.m. 
The most used materials going into the composition of the ink for the 
resistive film are metal oxides such as ruthenium oxide or pyrochlores 
such as thallium ruthenate, whose principal parameters are resistivity, 
heat variation coefficient, stability over time. The thickness of the 
resistive film is generally between 10 and 30 .mu.m. 
This "thick-film" hybrid resistor can be adjusted by means of a 
medium-power (0-5 watts) laser beam. This technology of laser cutting 
consists in vaporizing the resistive materials by creating high intensity 
coherent light pulses of short duration. A series of laser pulses that 
more or less overlap creates a narrow groove (on the order of 50 .mu.m) 
that goes through the resistive film to the substrate and thus cuts the 
resistor. This cut deflects the lines of current that go through the 
structure, thereby increasing its ohmic value, and the totality of the 
voltage applied to the resistor is found on both sides of the laser 
groove. 
The two major problems encountered with this type of cutting for 
high-voltage resistors are therefore the creation of one or more hot spots 
accompanied by microcracks at the top of the cutting or cuttings where the 
concentration of the lines of current are located, and the appearance of 
an electric arc while operating, from one edge to the other of certain 
laser cuts when the electric field exceeds a certain limit (on the order 
of 3,000 v/mm in dry air). 
FIGS. 2A and 2E of the accompanying drawings show several forms of cuts 
which were the object of experimental tests on small-sized "thick-film" 
hybrid resistors subjected to voltages of several hundred volts. These 
forms of cuts have proven unsuitable because there resulted either the 
creation of hot spots at 4a, 4c, 4d, 4g, 4h, 4i, 4j, or too strong a 
voltage gradient between the two edges of the laser groove marked 4b, 4e, 
4f, 4k, 4l, that could cause a poor stability or the destruction of the 
resistor, by appearance of an electric arc. 
With these forms of cuts, said problems can be solved only by oversizing 
the resistor, which is not always compatible with the installation 
capabilities offered and increases the manufacturing costs. 
SUMMARY OF THE INVENTION 
Accordingly, one object of this invention is to provide a novel 
high-voltage resistor made according to the "thick-film" hybrid 
technology, whose precise adjustment to the desired value is provided by 
way of cuts made in the thickness of the resistive film in a configuration 
which makes it possible to increase the voltage strength of the resistor 
despite the minimal sizing of the resistor. 
For this purpose, the invention has as its object a precision high-voltage 
resistor comprising a plane substrate of insulating material on which 
there have been deposited by silk-screening at least one approximately 
rectangular, square or similar resistive film and one conductive film in 
the form of two parallel strips extending along two opposite edges of the 
resistive strip and constituting terminals connected electrically to one 
another by said resitive film, the resistive film comprising rectilinear 
cuts made in its thickness, up to the insulating substrate, and parallel 
to said opposite edges from a third edge of the resistive film, 
characterized in that the rectilinear cuts are evenly spaced along the 
third edge and have lengths that are larger the closer they are to the 
center of the third edge, the ends opposite the third edge of the 
rectilinear cuts defining, from the intersections of the third edge with 
the terminals, a contour exhibiting an apex in its center part. 
Preferably, the contour is approximately symmetrical in relation to an axis 
parallel to the terminals and passing through the center of the third 
edge. 
According to a characteristic of this invention, the length of the 
rectilinear cuts is directly a function of the distance that exists 
between the adjacent terminals and the cut under consideration. 
According to another characteristic of this invention, said contour 
exhibits the shape of an isosceles triangle whose base coincides with the 
third edge and whose two equal sides extend from the intersections of the 
axis of symmetry. 
According to yet another characteristic, one of the cuts that imparts to 
the resistor its definitive value exhibits such a length that its end 
opposite the third edge does not necessarily coincide with said contour.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The "thick-film" hybrid resistor, as shown in FIG. 3, comprises a resistive 
film 3 of rectangular shape (a square shape or an approximately polygonal 
shape may also be employed) deposited on an insulating substrate (not 
shown), for example of ceramic with an alumina base. Two opposite edges of 
resistive film 3 overlap a conductive film that is in the shape of two 
parallel strips 2a, 2b constituting the terminals of the resistor. 
The precise adjustment of the resistor to the desired value is provided by 
cuts 5a to 5f made in the thickness of the resistive film 3 up to the 
insulating substrate. These cuts are directed parallel to terminals 2a and 
2b and extend from other third edge of resistive film 3 perpendicular to 
terminals 2a, 2b. Rectilinear cuts or grooves 5a to 5f have lengths 
gradually increasing from the terminals to the center of third edge 6, so 
that their ends opposite this edge 6 define an enclosure or contour 7 that 
extends from intersections 8a, 8b of reference edge 6 with films 2a, 2b 
while exhibiting an apex in its center part. Preferably, rectilinear cuts 
5a to 5f are evenly spaced along reference edge 6 and are made by a laser 
beam as previously described. 
As a result of this configuration of the cuts, a distribution of the 
equipotentials is obtained along the resistor so that, on the one hand, 
the voltage gradient on both sides of the end of each laser groove is less 
than a value allowing the creation of an electric arc and so that, on the 
other hand, the areas where the voltage gradients are strongest are 
located at the bottom of the longest laser grooves (those near the axis of 
symmetry of the resistor), the access paths to these areas being very long 
and consequently sufficiently resistant to prevent the feeding of current 
for an electric arc. 
FIG. 4 shows an alternate embodiment of this invention in which the 
enclosure or contour defined by the ends of the cuts and the reference 
edge is triangular. The lengths of the cuts are directly functions of the 
gap that exists between adjacent terminal 2a or 2b and the cut under 
consideration, so as to be nearly evenly and symmetrically decreasing from 
an axis that divides into two equal parts the length of resistive film 3. 
However, it should be noted that the last cut imparting the desired 
precision to the resistor can have a length that does not coincide with 
the outline of triangular contour 9. 
Experimental tests have been performed on a 10 k .OMEGA. resistor 2.9 mm 
long and 2.7 mm wide made with a resistive ink of 10 k 
.OMEGA./.quadrature.. This resistor was supposed to be adjusted with a 
precision of 1% and supposed to be able to withstand 400 v pulses for 100 
.mu.s between its two terminals. These tests showed that the configuration 
of the cuts according to the invention made it possible to meet these 
requirements whereas this was not the case with a resistor built according 
to the configurations of FIGS. 2A to 2E. 
The choice of a small-sized resistor with a cut configuration according to 
the invention rather than an oversized resistor with, for example, two 
straight cuts offers, among other things, advantages in production cost 
due to the low cost of material and of a miniaturization of the structure 
that is reflected by a savings in installation. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.