Surface mounted four terminal resistor

Abstract

An electrical resistor has a surface mounted four terminal current sensor of a very low resistance value and capable of handling short pulses of high power. It comprises a flat metal late, 1 to 50 mils thick, of an alloy of high electrical resistivity, to which are welded, on two opposite sides, two flat metal plates of very high electrical conductivity which serve as terminations for electrical interconnection. A slot is cut, from the outside edge toward the center, into each of the two termination plates which divides them into a wide pad for connection of current carrying wires and a narrow one for voltage sensing. The depth of the slots is optimized to get the best stability of resistance readings with changing ambient temperature and under influence of the self-heating effect.

Description

[0001]This application is based upon the applicants' provisional application Ser. No. 60 / 074,570 filed Feb. 13, 1998.BACKGROUND OF THE INVENTION[0002]The present invention relates to a surface mounting four terminal current sensing resistor of very low ohmic value and high stability.[0003]Surface mounted current sensing resistors have been available for the electronic market for many years. Their construction comprises a flat plate made of a resistive alloy like the Cu—Mn—Ni alloy onto which are plated lands of high conductivity metal forming the four terminals. The voltage-sensing node is set in the resistive alloy.[0004]When applied to ohmic values in the range of a few millohms or less, this construction introduces additional Joule losses due to the resistance between the point of connection of the current carrying wires and the a / m nodes. This leads to an additional temperature rise and results in drifts of the measurements.[0005]The primary object of this invention is to provid...

AI Technical Summary

Benefits of technology

[0012]A surface mounted resistor is formed by welding to each side of a resistive strip of Ni—Cr alloy two strips, one narrow and another wide, of a Ni plated high conductivity copper. The thickness and width of the resistive strip are chosen to form a resistance value below but close to the requested target, and therefore to minimize the extent of posterior laser trimming. This composite strip is punched to form individual resistors in a way described in the U.S. Pat. No. 5,604,477, but with an additional slot in the terminations in order to divide them into distinct current and sense pads, the current pad being at least twice as long as the sense pad. The depth of the slots is optimized to get the best stability of resistance readings with changing ambient temperature and under influence of the self-heating effect. The punched resistors remain attached to the wide copper strip by one current pad. This configuration permits four terminal (Kelvin) measurements of resistors on a continuous strip during subsequent trimming operation.

Problems solved by technology

When applied to ohmic values in the range of a few millohms or less, this construction introduces additional Joule losses due to the resistance between the point of connection of the current carrying wires and the a / m nodes.

This leads to an additional temperature rise and results in drifts of the measurements.

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