Electrical resistors – Mechanically variable – Resistance value varied by removing or adding material
Patent
1982-02-18
1984-04-17
Albritton, C. L.
Electrical resistors
Mechanically variable
Resistance value varied by removing or adding material
29620, H01C 1000
Patent
active
044437820
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method for adjusting the value of a thick film resistor to a desired value and to the corresponding resistor.
It is known that, in order to produce electronic circuits at a reduced cost price one is lead to use components, the characteristics of which are not very exact, such that a regulating element, which is adjusted in order to regain all the tolerances of the components, must be incorporated in the said circuit in order to bring the output values of the circuits to the desired level. This regulating element is generally a thick film resistor and, in a known manner, the regulating is performed by making a slot between the connection terminals of the resistor through the resistor film, the length of the slot enabling the electrical resistance of the element to be brought to the desired value. A slot of this type is produced either by means of a grinding machine, or by means of a laser beam, and it is naturally desired that the degree of accuracy of the regulation as a function of the length of the slot is as great as possible.
In the conventional type of thick film resistors, such as that which is illustrated in FIG. 1 of the drawing attached to the present application, the resistance increases with the length of the slot 3, the said slot being disposed in the resistor film between the two connection terminals 1 and 2. If the slot extends over the entire length of the element it is evident that the resistance will be infinite; it is therefore understood that the variation of the resistance R of the element as a function of the length 1 of the slot is represented by a curve of the type shown by way of example in FIG. 2 of the attached drawing. The degree of accuracy of the regulation which may be obtained by means of an element of this type is the greater, the less the inclination of the curve in FIG. 2: it is thus clear that, if the degree of accuracy is suitable for the lengths of slots 1 reduced with respect to the length L of the resistor, the said degree of accuracy becomes insufficient when the length of the slot is increased; the result of this is that an element of this type can only be used with a limited variation of the electrical resistance of the element if it is desired to maintain a suitable degree of accuracy. Thus it is evident that if the components of a circuit have high tolerances as regards their characteristics, it is necessary to be able to vary considerably the regulating element if it is desired to return the output value to the nominal value. In general it is considered necessary to be able to vary the resistance of a regulating element of this type in a ratio of 1 to 10. The resistors of the type shown in FIG. 1 of the attached drawing may only vary within a ratio of 1 to 4 approximately if it is desired to maintain a suitable degree of accuracy. In the prior art, for example in the German Offenlegungsschrift No. 2 337 466, it has been proposed that this disadvantage be overcome by using thick film resistors which have been shown in FIG. 3 of the attached drawing and which, in addition to the connection terminals 11 and 12 disposed on the edge of the film, likewise comprise a connecting bar 14 disposed on a region of the edge of the film which is not occupied by the connection terminals. According to this technique it has been proposed to produce the slot 13, which enables the resistance value to be regulated, parallel to the connecting bar: the slot thus progressively separates the connecting bar from the resistor film and, given that the connecting bar comprises a shunt conductor, it is clear that increasing the length of the slot enables the resistance of the element to be increased. In FIG. 4 the curve of the variation of the resistance of the element of FIG. 3 has been illustrated as a function of the length 1 of the slot 13 which is provided therein parallel to the connecting bar 14. In view of the fact that the two extreme values R.sub.0 and R.sub.1 of the resistance of the element are both finite values, the degree of accuracy of the regulatio
REFERENCES:
patent: 3662317 (1972-05-01), DiGarolamo et al.
patent: 3889223 (1975-06-01), Sella et al.
patent: 4100525 (1978-07-01), Denes
patent: 4163315 (1979-08-01), Neese
H. S. Hoffman, IBM Technical Disclosure Bulletin, "Quick Trim High Aspect Ratio Resistor," vol. 22, No. 5, p. 1805, Oct. 1979.
Albritton C. L.
S.E.V. Societe pour l'Equipement de Vehicules
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