GB2054972A - Thick film variable resistor - Google Patents

Abstract

A variable resistor (1) of the type comprising a layer (2) of resistive material deposited on an insulating support base (3) and connected at its opposite ends (4, 5) to respective conducting terminals (6, 7) is described. The main feature of said resistor is that it comprises at least along a peripheral edge (17) of said layer (2) joining said opposite ends, at least one portion (21) of conducting material electrically connected to a peripheral portion of said edge (17) of said layer (2) and extending outside thereof to enable the resistance presented by said layer (2) between said portion (21) of conducting material and at least one of said opposite ends (4, 5) to be determined. The resistor may then be trimmed, during manufacture, to the desired value while providing a track, for the sliding contact 15, which is free from irregularities. <IMAGE>

Description

SPECIFICATION Thick film variable resistor This invention relates to a thick film variable resistor.

In particular, this invention relates to a resistor of the type comprising a layer of resistive material deposited on an insulating support base and disposed on said base between a first and second end, at which it is electrically connected to respective conducting terminals.

Resistors of the aforesaid type are often used as precision resistors, and to this end they are suitably calibrated by a method which consists essentially of determining the resistance presented progressively between a point mobile along the layer and one of the conducting terminals, and correcting this value by removing part of the resistive layer, for example by using a laser beam. More particularly, in order to automate and standardise the checking of the resistance, it is known to dispose a plurality of conducting tracks which extend suitably spaced apart below the entire resistive layer and transversely thereto.Although they allow totally automated and rapid checking of the resistance presented by the resistor at a number of points on the resistive layer, such transverse conducting tracks give rise to certain drawbacks of a mechanical and electrical nature, which negatively influence the accuracy of the resistor. In this respect, as said tracks are generally disposed on the support base before depositing the layer of resistive material, they consequently give rise to a slight variation in the thickness of said layer.In such positions, there is consequently an increase in the mechanical resistance offered to the sliding of a mobile contact brush over the surface of the resistive layer, so that the brush moves in a discontinuous manner in both the directions in which it traverses the track, with an inefficient mechanical contact which leads to a transducing error between the movement of the brush and the resistance correspondingly presented by that portion of the resistive layer concerned.

With the passage of time, the brush tends to wear as said thickness variations in the layer lead to high friction between the brush and the layer.

There is also an electrical drawback which arises when the brush is moved along the resistive layer on a transverse conducting track during normal use of the resistor. In particular, when the brush is upstream of the track, the current which flows towards the brush from a conducting terminal disposed upstream has lines of flux which substantially converge from said terminal towards the point of contact between the layer and the brush. When this latter is downstream of the track, the track gives rise to an equipotential line of voltage distribution in a transverse direction, so that said lines of flux flow substantially parallel from the conducting terminal to the conducting track, so correspondingly causing a reduction in the equivalent resistance presented between the brush and conducting terminal relative to the case in which the brush is disposed upstream.

Consequently, any movement of the brush around each transverse resistive track leads to a resistance variation of sign opposite that which should be obtained, thus giving rise to a zone of indetermination which negatively influences the accuracy of said resistor.

The object of the present invention is to provide a thick film variable resistor which has the advantage of connections to the resistive layer intermediately along its length, but avoids the problems discussed previously of such resistors.

This object is attained according to the invention by a variable resistor of the type comprising a layer of resistive material deposited on an insulating support base and connected at its oppoiste ends to respective conducting terminals, characterised by comprising at least along one peripheral edge of said layer joining said opposite ends, at least one portion of conducting material electrically connected to said layer, this being connected to the layer only at a peripheral portion thereof, and extending outside the layer to enable the resistance presented by said layer between said portion of conducting material and at least one of said opposite ends to be determined.

The present invention will be more apparent from the description given hereinafter by way of non-limiting example of a preferred embodiment with reference to the accompanying drawing, which shows a plan view of a variable resistor constructed according to the present invention.

Said drawing shows a variable resistor indicated overall by the reference numeral 1 and comprising substantially a'layer 2 of resistive material of the thick film type deposited on an insulating support base 3.

In particular the layer 2 is in the form of a sector of a circular strip, with its opposing end portions 4 and 5 resting on relative conducting zones 6 and 7 deposited on the base 3 before depositing the layer 2.

Each zone 6, 7 has a first portion 8, 9 on which said ends 4 and 5 rest, this first portion being constructed of a metal material having good adhesion towards the resistive material constituting the layer 2, and a second preferably tin-plated portion 10, 1 to which end terminals 12, 1 3 of the resistor 1 are connected.

A brush 14 carrying a contact element 15 arranged to slide on the resistive layer 2 is sliable on said layer 2, and is connected to an external terminal 16.

According to the present invention, conducting tracks 1 9 and 20 respectively are provided along the internal peripheral edge 1 7 and external peripheral edge 18 of the layer 2, these tracks extending partly below the layer 2 and partly outside the layer 2. As described with reference to the zones 6, 7, each plate 19, 20 comprises a portion 21, 22 in contact with the strip 2 and constructed of a material having good adhesion twoards the material constituting the layer 2, and a preferably tin-plated portion 23, 24 which offers minimum contact resistance towards a point shaped supply terminal, not shown on the drawing. Finally, with reference to the drawing, each track 20 extends below the layer 2 along a radial extension of a corresponding track 1 9.

The operations involved in calibrating the resistor 1 are as follows.

Firstly, before depositing the resistive layer 2, the dimensions (width, thickness and length) of the layer 2 and the type of constituent resistive material are chosen such that the resistance per unit of length offered by the layer is slightly less than the required value.

The resistor 1 is then supplied by a voltage generator while maintaining one pole of this latter permanently connected to one of the zones 6, 7, for example the zone 7, and connecting the opposite pole either to the portion 23 of the track 19 closes to the zone 7, or the portion 24 of the track 20 disposed along the radial extension of said track 19. Because of the different arrangement of the lines of flux of the electric current flowing in the portion occupied by the layer 2, due firstly to a different geometrical configuration of the transverse sections traversed by the current, and possibly also due to lack of uniformity in the thickness of the deposited layer 2, two different values of said current intensity are obtained.By partly removing the edge 17 and/or 18, the read intensity values are reduced until the average value between said two values, which coincides practically with that obtained by moving said opposite pole of the generator to the centre of the layer 2 between the tracks 19 and 20 under consideration, assumes the required calibration value.

The same procedure is adopted for the remaining conducting tracks 19, 20, each time removing part of the resistive material of the layer 2 lying between the track under examination and that immediately adjacent on the side facing the zone 7.

From an examination of the characteristics of the resistor 1 constructed according to the present invention, it can be seen that it attains the said objects.

In this respect, the resistive layer 2 is distributed uniformly, so that the aforesaid drawbacks of a mechanical nature are totally overcome. The slight penetration of the conducting tracks 19, 20 below the resistive layer 2 influences the distribution of the current flux lines to an irrelevant extent during the use of the resistor 1, so that the aforesaid drawbacks of an electrical nature can also be considered to be obviated.

Finally, it is apparent that modifications can be made to the resistor 1 without leaving the scope of the inventive idea.

For example, if the brush 14 is subjected only to a traversing movement, the resistive layer 2 can be conveniently deposited in the form of a rectangle, and because of the symmetry which would be created it would be sufficient to provide conducting tracks only on one longitudinal edge of said rectangle.

Claims (6)

1. A variable resistor of the type comprising a layer of resistive material deposited on an insulating support base and connected at its opposite ends to respective conducting terminals, characterised by comprising at least along one peripheral edge of said layer joining said opposite ends, at least one portion of conducting material electrically connected to said layer, this being connected to the layer only at a peripheral portion thereof, and extending outside the layer to enable the resistance presented by said layer between said portion of conducting material and at least one of said opposite ends to be determined.

2. A resistor as claimed in claim 1, characterised in that each of said portions of conducting material comprises a conducting track deposited on said insulating base before depositing said resistive layer, such that said conducting track becomes partly covered by said resistive layer.

3. A resistor as claimed in claim 1 or 2, characterised by comprising pairs of conducting material portions disposed on opposite edges relative to a cross-section taken through said resistive layer.

4. A resistor as claimed in claim 3, characterised in that said resistive layer is in the form of a sector of a circular strip, and that each of said pairs of conducting material portions has a first and a second portion disposed on opposite edges of said layer and along a direction radial to said layer.

5. A method of producing a thick film resistor comprising forming at least one pair of portions of conductive material on an insulated support base; depositing a layer of resistive material on said base between conducting terminals so that said portions are electrically connected to said layer's opposing edges joining said terminals, the said portions being connected to the layer only at peripheral portions thereof and extending outside the layer; comparing the resistance presented between one of said terminals and each one of said pair of portions; and removing some of said layer adjacent the respective portion thereby to alter said resistance and so calibrate said resistor.

6. A variable resistor, substantially as described with reference to the accompanying drawing.