EP0001520A1 - Flat high-precision electrical resistor having an adjustable ohmic value and method of adjusting this ohmic value - Google Patents

Abstract

1. A high-precision flat electric resistor having an adjustable resistance value and comprising a sheet of metal or metal alloy deposited on or bonded to a support comprising several electric, resistive, sinuous and interconnectable circuits, each of them being constituted by a series of electrical resistance filaments separated by grooves, some of the said resistive circuits being short-circuited by conductive strips of the metal or metal alloy of the sheet, these circuits being interconnectable by means of cut-out portions made in the conductive strips, the resistance value of a resistor having n electric circuits interconnected in serie being RTn and that of a resistor having n-1 electric circuits interconnected in series being RTn-1, characterized in that the ratio DELTA R = RTn - RTn-1 to the resistance value RTn-1 follows the relation : DELTA R/RTn-1 = 1.3...2n-1/2.4...2n K**2n where K is an adjustable numerical coefficient having a value smaller than 1.

Description

The present invention relates to a planar electrical resistance of high precision and adjustable ohmic value.

The invention also relates to a method for adjusting the ohmic value of the above-mentioned electrical resistance.

High precision resistors are already known which are obtained by etching, by electrolytic machining or ion bombardment, a sheet of metal or of alloy so as to obtain a multitude of electric resistant filaments separated by grooves or slots. Such resistors and their manufacturing process are described in French patent applications No. 76 07889 filed March 18, 1976 and No. 76 17269 filed June 8, 1976, both in the name of the Applicant.

The aforementioned methods make it possible to obtain a resistive circuit of sinuous contour and of great length. The electrical resistance filaments etched in the sheet, of metal or of alloy have a thickness which does not exceed a few microns, so that these resistors have a very high ohmic value per unit of surface.

After mass production, these resistors must be adjusted to a precise resistance value which is sometimes 50 to 150% higher than the initial resistance value.

In known methods, this value is adjusted by making, between the edge of the resistor and the grooves which separate the electric resistant filaments from the cutouts intended to switch on a predetermined number of these filaments, so as to increase the ohmic value. This adjustment of the resistance by cutting is very delicate. Indeed, this operation must be carried out under a microscope by scraping, sand blasting or laser. Furthermore, this operation is very long, because it is often necessary to make a very large number of cuts, taking into account the high number of electric resistant filaments contained in each resistor. Thus, for example, to adjust to 42 Kiloohms the resistance value of a resistor having an initial value of 23 Kiloohms, it is necessary to make 102 cuts on the sides of the resistor.

The object of the present invention is to provide a planar electrical resistance, the ohmic value of which can be adjusted in a much more convenient and rapid manner than in the case of the above-mentioned resistors.

The planar electrical resistance targeted by the invention comprises a metal or alloy sheet deposited or glued on a support comprising a series of electrical resistant filaments separated by grooves so as to define a sinuous electrical resistance circuit, this resistance comprising a series electric resistant circuits short-circuited by conductive strips made of the metal or the sheet alloy.

où Un est le nième terme d'une série absolument convergente au sens de Cauchy et

According to the invention, this resistor is characterized in that the ratio between the increase 0 R of the ohmic value RTn when one has connected n resistive auxiliary circuits in series, and the ohmic value RTn-1 of the resistance when we connected n-1 resistant auxiliary circuits, follows the relationship:

where Un is the nth term of an absolutely convergent series in the sense of Cauchy and

To adjust the final ohmic value of the resistance, cutouts are made in the sheet intended to put in series with the rest of the circuit, the additional electric resistance circuit (s) whose value ohmic corresponds to the ohmic adjustment value desired for the resistance.

It was found that when the increase ΔR follows the above-mentioned relationship, it was possible to considerably reduce the number of cuts to be made in the sheet to adjust the ohmic value of the resistance.

où K est un coefficient numérique ajustable inférieur à 1.According to a preferred version of the invention:

where K is an adjustable numerical coefficient less than 1.

The invention also relates to a high precision electrical resistance as defined above, in which the metal or alloy sheet is deposited or sealed on a substrate having a coefficient of thermal conductivity, generally equal to or higher than that alumina, sufficient to avoid a too high temperature gradient between the resistive parts in circuit and those still short-circuited.

The invention also relates to the method for adjusting the ohmic value of the above-mentioned electrical resistance.

In this process, cuts are made in conductive strips which short-circuit the auxiliary electrical resistance circuits. This process is characterized in that such a cut is made first in the conductive strip corresponding to an additional circuit such that the increase in the ohmic value is as close as possible to the desired adjustment value, then one refines the adjustment by successively switching on additional circuits giving rise to increasingly small increases in the ohmic value.

In this way, you only roll one die cutting by additional resistive circuit, so that the maximum number of cuts to be made does not exceed that of the number of these circuits. In addition, since each annex circuit is generally constituted by several parallel resistant filaments which extend over a relatively large width, the cutting requires a precision which is much less than in the known method.

It is also advantageous to engrave in the aforementioned bands, at the same time as the various electrical resistance circuits, a reference number as well as a slot extending between this reference number and the corresponding additional resistance circuit and, to adjust the final ohmic value of the resistance, said cutting is carried out between the side of the sheet and the aforementioned reference number.

These numbers considerably facilitate the identification of the additional circuits to be circuited and the making of the cuts.

• - la Figure 1 est une vue schématique en plan d'une résistance électrique obtenue selon l'invention,
• - la Figure 2 est une vue à grande échelle, d'une partie de la résistance de la Figure 1, montrant un circuit résistant électrique annexe connecté avec le reste de la résistance, après découpe effectuée sur le bord de la feuille.

Other features and advantages of the invention will appear in the description below. In the appended drawings, given by way of nonlimiting examples:

• FIG. 1 is a schematic plan view of an electrical resistance obtained according to the invention,
• - Figure 2 is a large-scale view of part of the resistor of Figure 1, showing an additional electrical resistance circuit connected with the rest of the resistor, after cutting performed on the edge of the sheet.

In the accompanying Figures, we see a planar electrical resistance obtained from a square sheet 10, of a metal alloy having a thickness of a few microns, such as nickel-chromium. The electrical resistance circuit of this resistance is obtained in a manner known per se, preferably by electrolytic machining or by ion bombardment through a mask comprising windows defining the outline of the electrical circuit to engrave on sheet 10.

This sheet 10 is deposited or sealed on an electrical insulating support, not shown, having a coefficient of thermal conductivity equal to or greater than that of alumina of purity equal to 96%.

For the clarity of the representation, the electrical resistant filaments 11 are shown in FIG. 1 between parallel lines which designate the grooves or slots 12 of the resistor. An actual representation of part of the electrical resistance circuit is shown in Figure 2.

As can be seen in FIG. 1, the resistor comprises a multitude of parallel filaments 11 which define a sinuous electric circuit of great length and consequently of great ohmic value per unit of surface.

où Un est le nième terme d'une série absolument convergente au sens de Cauchy et où :

RTn étant la valeur ohmique de n circuits résistants annexes et RTn-1 étant la valeur ohmique de n-1 circuits résistants annexes.The sheet 10 comprises a series of additional resistant circuits A ₁ , A ₂ , A ₃ ; B ₁ , B ₂ , B ₃ ; C ₁ , C2, C ₃ , ..... C ₈ ; D ₁ , D ₂ , ..... D ₉ whose increase Δ R in intrinsic ohmic values follows the relationship:

where Un is the nth term of an absolutely convergent series in the sense of Cauchy and where:

RTn being the ohmic value of n additional resistive circuits and RTn-1 being the ohmic value of n-1 additional resistive circuits.

We see, in Figure 1, that the circuit A ₁ has a number of filaments 11 which is greater than that of the circuit A ₂ which itself has a greater number of filaments 11 than the other circuits A ₃ , B ₁ , B ₂₁ etc.

où K est un coefficient numérique ajustable inférieur à 1.The converging series can be such that:

where K is an adjustable numerical coefficient less than 1.

This converging series can be a geometric progression.

As can also be seen in FIG. 1, during manufacture, between the sides A, B, C, D of the sheet 10 and each of the auxiliary circuits A ₁ , ...., B1, is left. , C1 ......, D ₁ bands such as 13, 14, 15, 16, 17, 18 which short-circuit these additional circuits with respect to the rest of the resistor.

Consequently, the resistance obtained after manufacture has an initial resistance RT _o constituted by the ohmic value of all the electrical resistant filaments which do not belong to the annex circuits A1, ......, B1, .... .., C ₁ , ......, D ₁ .

To adjust the ohmic value of the resistance to the desired value, we start by first switching on the annex circuit A ₁ , ......, B ₁ , ......, C ₁ , or D ₁ which presents the ohmic value closest to the desired adjustment value, then the adjustment is refined by switching on additional circuits having increasingly lower ohmic values.

This operation is carried out by making in the conductive strips such as 13, 14, 15, 16, 17 or 18, a cutout 19 (see Figure 2) which extends between the side A or B, C, D of the sheet 10 and the corresponding annex A ₃ circuit. These cuts such as 19 are made in a manner known per se, for example by scraping by means of a point, by sandblasting or by laser.

To be able to adjust the resistance according to a fairly wide range of ohmic values, we will generally choose a multiplier coefficient M equal to the ratio RTn (RTn being the ohmic value obtained when all the RTo additional circuits are connected) close to 2.

As seen on the other hand in Figures 1 and 2, we engraved in the bands 13, 14, 15, 16, 17 or 18, in same time as the various electrical resistance circuits A ₁ , ..... B ₁ , ...... C ₁ , ...... D ₁ , ......, reference numbers 1, 2, 3, ......, 9 as well as a slot such as 20 (see Figure 2) which extends between these numbers and the corresponding annex circuit A ₃ .

Consequently, to adjust the resistance, it suffices to locate the number of the annex circuit A ₁ , ...... B ₁ , .... C ₁ , ...... D ₁ , ..... intended to be switched on, then to react. read a cutout, as designated in 19 in Figure 2, between side A (or B, C, D) of sheet 10 and the reference number of the corresponding annex circuit.

In the embodiment of Figure 1, the electrical resistive circuits annexes A ₁ , ..... B ₁ , ...... C ₁ , ..... D ₁ , ......, are grouped in four rectangular blocks 21, 22, 23, 24, arranged along each of the sides A, B, C, D of the sheet 10. This arrangement further facilitates the identification of the auxiliary circuits.

Some numerical examples are given below.

Example I.

This nonlimiting example shows how to create a resistance adjustment law according to the method according to the invention.

This resistance is characterized by a number of adjustment points equal to 23 and a multiplying coefficient M equal to 2. The multiplying coefficient is the ratio between the value of the resistance when all the points have been adjusted and its initial value. We pass the law:

We therefore have the following table, for a value RTo = 15 kiloohms.

When all the resistors are switched on, i.e. when 23 cuts such as 19 have been made, the maximum adjusted resistance is equal to 14,212 ohms + 15,000 ohms (initial value) = 29,212.11 ohms , i.e. a multiplier of 1.9474.

Example II.

The resistance, according to this example, has an initial ohmic value RTo equal to 15 kiloohms. We want to adjust this value to 25,000 ohms ± 0.1%. The initial value must therefore be increased from 66.51% to 66.83%, in order to to be within tolerance + 0.1%.

To this end, we first choose the circuit (s) whose ohmic value (s) is (are) the closest in% of the value that we want to get.

In this example, points A ₁ and A _{2 have} respective weights equal to 33.6% and 22.6%. A cut such as 19 is therefore carried out, as indicated above.

• . Pour B₁ qui représente 9
• · Pour C₂ qui représente 1,4 %.
• · Pour D₁ qui représente 0,07%.

We then proceed in the same way successively:

• . For B ₁ which represents 9
• · For C ₂ which represents 1.4%.
• · For D ₁ which represents 0.07%.

66.67% are therefore obtained. This result is therefore achieved by making only 5 cuts as designated in 19, in Figure 2.

If it were necessary to further refine the ohmic value of the resistance, it would be necessary to carry out other cuts to put in circuit additional resistances whose ohmic value is increasingly low.

Example III (comparative).

In the case of a resistance produced using a conventional adjustment method, having the same initial ohmic value as in the case of Example II, to obtain a final resistance equal to 25 kiloohms ± 0.1 %, it is necessary to make 27 cuts.

Of course, the invention is not limited to the examples just described, and many modifications can be made to them without departing from the scope of the invention.

• · soit d'indiquer à l'opérateur au moyen d'un micro-afficheur digital, la découpe à réaliser, l'image de la résistance étant observée dans une binoculaire ;
• . soit de commander une table XY portant un dispositif automatique de découpe connu en soi (jet de sable ou laser), et piloté par l'ordinateur. Ceci peut être réalisé grâce au fait que les chiffres de repérage 1, 2, 3, ......, des circuits annexes sont relativement peu nombreux et suffisamment grands pour qu'il n'existe aucun problème mécanique pour guider le dispositif de découpe.

Thus, the resistance can be adjusted by incorporating it in a known manner into a Wheastone or Kelvin bridge and using a digital voltmeter. It is also possible to use a computer comprising in memory the ohmic values corresponding to each additional circuit. A ₁ , ...... B ₁ , ...... C ₁ , ..... D ₁ , ...... A simple calculation program thus allows:

• · Or to indicate to the operator by means of a digital micro-display, the cut to be made, the image of the resistance being observed in a binocular;
• . either to order an XY table carrying an automatic cutting device known per se (sand jet or laser), and controlled by the computer. This can be achieved thanks to the fact that the locating digits 1, 2, 3, ..., auxiliary circuits are relatively few and large enough that there is no mechanical problem to guide the device. cutting.

Furthermore, thanks to the tracking numbers and the metal bands existing between these numbers and the sides A, B, C, or D of the resistance, the adjustment of the resistance can still be carried out by a simple scraping between the number in question and the adjacent edge of the resistor.

The invention therefore makes it possible to considerably facilitate the adjustment of the ohmic value of a resistor of the kind described.

The invention also makes it possible to limit the risks of error so that the reliability of the precision obtained is statistically much greater than in the case of the known method.

Claims (6)

1. Planar electrical resistance of high precision and adjustable ohmic value comprising a metal or alloy sheet deposited or glued on a support comprising a series of electrical resistant filaments separated by grooves so as to define a sinuous electrical resistant circuit, this resistor comprising a series of electrical resistant circuits short-circuited by conductive strips formed by the metal or the alloy of the sheet, characterized in that the ratio between the increase Δ R of the ohmic value when Ion is connected in series the resistant auxiliary circuits, and the ohmic value RTn-1 of the resistance when n-1 connected auxiliary circuits is connected follows the relationship:

where Un is the nth term of an absolutely convergent series in the sense of Cauchy and

2. Resistor according to claim 1, characterized in that:

where K is an adjustable numerical coefficient less than 1. 3. Resistor according to any one of claims 1 or 2, the sheet being square or rectangular, characterized in that the additional electrical resistance circuits are grouped in four distinct rectangular blocks arranged along each of the sides of the keel. 4. Resistor according to claim 3, characterized in that the resistive circuits of each block are identified by a number less than or equal to 9 etched in a conductive strip located between the annex circuits and the adjacent side of the sheet. 5. Resistor according to claim 1, ca characterized in that the support on which the resistant sheet is deposited or sealed has a coefficient of thermal conductivity equal to or greater than that of alumina of purity equal to 96%. 6. Method for adjusting the ohmic value of the electrical resistance according to claim 1, in which cutouts are made in the conductive strips which short-circuit the auxiliary electrical resistive circuits, characterized in that such a cutout is practiced first in the conductive strip corresponding to an additional circuit such that the increase in the ohmic value is as close as possible to the desired adjustment value, then the adjustment is refined by successively switching on additional circuits giving rise to increasingly small increases in ohmic value.

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