US3348304A

US3348304A - Method of making encapsulated resistors of high precision

Info

Publication number: US3348304A
Application number: US429693A
Authority: US
Inventors: Tassara Luigi
Original assignee: Consolidated Electronics Industries Corp
Current assignee: Consolidated Electronics Industries Corp
Priority date: 1961-03-28
Filing date: 1964-12-09
Publication date: 1967-10-24
Anticipated expiration: 1984-10-24

Description

METHOD OF MAKING ENCAPSULATED RESISTORS OF HIGH PRECISION Original Filed Marsh 28, 19611 INVENTOR Lu/ Tia $64 M .4 n-cmvevs United States Patent 3,348,304 METHOD OF MAKING ENCAPSULATED RESISTORS OF HIGH PRECISION Luigi Tassara, Milan, Italy, assiguor to Consolidated Electronics Industries Corp., New York, N.Y., a corporation of Delaware Original application Mar. 28, 1961, Ser. No. 98,861, now Patent No. 3,213,402. Divided and this application Dec. 9, 1964, Ser. No. 429,693

4 Claims. (Cl. 29-610) ABSTRACT on THE DISCLOSURE The invention is directed to a method of making high precision encapsulated resistors by connecting a plurality of resistors together and encapsulating them in a body having a hollow space to receive an additional resistor. After the initial resistors have been encapsulated, they are measured, and a compensating resistor having the proper resistance value to make the overall resistance correct is connected to the initial resistors and physically placed in a hollow space and encapsulated therein. Any deviation in the resistance of the final resistor due to the encapsulation procedure will then have only a minor effect on the overall resistance of the complete multi-part resistor.

This application is a division of my co-pending application Ser. No. 98,861, now Patent 3,213,402.

The present invention relates to encapsulated resistors of high precision, for example of the order of .1%, and to a method for making the same and retaining the high precision in spite of the change in resistance produced by the encapsulation process. The invention is particularly adaptable to resistors of high ohmic value.

A resistor constructed according to the invention comprises a plurality of individual resistors not previously encapsulated connected together end to end and arranged so as to be physically parallel to each other within a container. The location of the resistors within the container is such that one end of each of the resistors points in the same direction toward the open mouth of the container to extend above the surface of encapsulating material poured into the container. Also located within the container is a region dammed off from the remainder and of sufficient size to receive an additional resistor. The ohmic value of the series-connected resistors is predetermined to be slightly less than the desired ohmic value of the finished resistor, and after the encapsulating material has been poured around the series-connected resistors and allowed to set, the resultant resistance is measured and an additional resistor sufiicient to bring the total series resistance up to the desired value is connected to the other resistors and is inserted into the dammed-01f section. Thereafter, this section is also filled with encapsulating material and additional encapsulating material is added to cover the open ends of all of the resistors.

The composite resistor may be formed so as to be easily connected in series in similar composite resistors by providing a terminal at each end thereof, one of the terminals comprising an internally threaded nut and the other terminal comprising an externally threaded bolt.

The invention will be further described in connection with the drawings, in which:

FIG. 1 shows a cross-section of a high precision resistor constructed in accordance with this invention;

FIG. 2 is a schematic representation of the resistor of FIG. 1 at an intermediate stage of manufacture;

FIG. 3 is a plan view of the resistor of FIG. 2; and

3,348,304 Patented Oct. 24, 1967 FIG. 4 is a schematic representation of the electrical connections and measuring points of the resistor of FIG.

The resistor in FIG. 1 is enclosed within hollow container 1, preferably of synthetic resin, which is open at one end. A plurality of resistors 2, only one of which appears in the drawing, is located within the container 1 and physically positioned parallel to each other pointing, respectively, at the closed'end 3 and the open end 4 of the container 1. The resistors 2 may comprise any suitable form of resistor, such as an insulating body coated with a conductive layer of metal or other material. The resistors are connected as shown in FIG. 2 so as to be electrically in series, with alternate junctions between resistors adjacent the closed end 3 and the open end 4 of the container.

An opening 6 is provided in the closed end 3 of the container and a conductive metal bushing 7 having a locking flange 8 is placed in the opening to fill it up and serve as a closure therefor. The bushing is internally threaded, as indicated by reference numeral 9, and has a cylindrical appendage 10 extending into the container. The external surface of the appendage 10 is knurled or otherwise roughened to receive and to hold fast to one end of a tube 11 of insulating material. One terminal or lead 12 at one end of the series-connected resistors 2 is wound around and conductively joined to the appendage 10. A machine screw 13 together with a washer 14 may be used as an external fastening means to make electrical connection to the outside of the composite resistor in order to connect with the terminal 12 within the container 1.

The tube 11 serves to darn off a region within the container 1 when the latter is placed in the position shown in FIG. 2 and is partially filled with an encapsulating material 5 in liquid form. A compensating resistor 15 is chosen and placed within the dammed-off region and is connected by means of a lead 16 to the opposite end of the series-connected resistors from the lead 12. The selection of resistor 15 will be described in greater detail hereinafter.

The container 1 is thereafter filled with the encapsulating material which is allowed to set. Subsequently, a hole 17 is formed in the surface of the material 5 and a hollow bushing 18 having an extension 19 is forced into the hole 17. Prefereably, this bushing is coaxial with the bushing 7 at the other end of the container 1. The bushing 18 has a locking collar 20 to prevent it from turning and to increase the friction on the surface ofthe encapsulating material 5 and the outer end of the bushing is threaded, as indicated by reference numeral 21.. Preferably, this thread matches the internal thread of the bushing 7 so that similar composite resistors can be joined end to end, as indicated by the container 1 and the container 1a. The bushing 18 has an axial hole 22 through which extends a lead 23 from the compensating resistor 15. The protruding end of the lead may be cut off and welded to the bushing, as indicated by reference character 24, so as to seal oif the assembly.

The process of forming the composite resistor just described will now be considered in greater detail. Reference has already been made to the fact that alternate connections between the resistors 2 are adjacent the closed end 3 and the open end 4 of the container 1. If the resistors are very numerous, they may be arranged in a spiral, as indicated in FIG. 3.

The resistors 2 are chosen so that when connected in series their total ohmic value will be slightly less than the desired value. After the resistors have been introduced into the container 1 and after connection has been made between the lead 12 and the appendage 10 and the tube 11 has been attached to the appendage 10, the container 1 is placed in the position as shown in FIG. 2 and filled to the level 25 which, it will be noted, is lower than the open end of the tube 11. Therefore, the latter serves as a dam and prevents the encapsulating material, which may be an epoxy resin, for example, from spilling over into the region within the tube 11. The level 25 is such as to immerse completely the resistive element portion of each of the resistors 2, leaving only the conductive terminals 26 between resistors above the level of the encapsulating material. A measurement may then be made of the total resistance between the lead 12 and the lead 16 at the ends of the series-connected resistors. The compensating resistor 15 is then selected to have a value equal to the difference between the desired value of the composite resistor and the total value of the resistors 2. If a composite resistor of high ohmic value is to be made, it may be necessary to measure the resistance of one pair of resistors 2 at a time. Such measurements may be made at the points indicated by the reference letters A, B, C and D, as shown in FIG. 4, and it is for this reason that it is desirable to have the terminals 26 remain above the level 25 of the encapsulating material 5.

While each of the resistors 2 could be measured to any desired accuracy before being encapsulated, the process of encapsulation produces a change in the resistance, and this change cannot be predicted precisely. The change may not be large, for example it may only be approximately 1%. However, if it is desired to produce a-composite resistor to an accuracy of .l%, clearly a change of 1% would be intolerable, particularly where the change would vary from one composite resistor to the next.

After the compensating resistor 15 has been selected, one of its leads is connected to the lead 16 of the resistors 2 and the compensating resistor is placed within the tube 11. Thereafter, more encapsulating material is added to bring the level up to the open end 4 of the container 1. In so doing, encapsulating material spills over inside the tube 11 and encases the compensating resistor 15. This changes the value of resistance of the compensating resistor, but since the total resistance of the compensating resistor is only a small fraction of the total resistance of the composite resistor, any change in its value is relatively negligible. The change in value of the resistance of the composite resistor due to a change in value of the resistance of the compensating resistor is given by the equation a b=c, where c is the change in resistance of the composite resistor, a is the fraction of total resistance of the composite resistor represented by the compensating resistor and b is the change in resistance of the compensating resistor. To be specific, if the change in value of the composite resistor given by the letter c must be less than .1% and if the change in resistance of the compensating resistor is known to be not greater than 2% when it is encapsulated, then the compensating resistor must have a resistance no greater than 5% of the total value of the composite resistor.

Although this invention has been described in terms of a specific embodiment, it will be understood that modifications may be made therein without departing from the scope of the invention as measured by the following claims.

I claim:

1. The method of making a precision composite resistor comprising the steps of: connecting a plurality of individual resistors into a two-terminal network; placing said network inside a container having an interior wall separating one portion thereof from the remainder, said network being located in said remainder of said container; partially filling said remainder with encapsulating material; measuring the resistance of said network; connecting a compensating resistor thereto to adjust the overall resistance to a desired value; placing said compensating resistor Within said one portion; and subsequently filling said container and said one portion with encapsulating material.

2. The method of making a precision composite resistor comprising the steps of: connecting a plurality of individual resistors in series in a two-terminal network; placing said network inside a container having an interior wall separating one portion thereof from the re mainder, said network being located in said remainder with said resistors extending in the same direction as said interior wall; partially filling said remainder with encapsulating material to cover said resistors substantially entirely; measuring the resistance of said network; connecting in series with said network a compensating resistor having a resistance equal to the difference between a desired value and the value of resistance of said network; placing said compensating resistor within said one portion; and subsequently filling said container and said one portion with encapsulating material.

3. The method of making a precision composite resistor comprising the steps of: connecting a plurality of individual resistors in series in a two-terminal network; placing said network inside a container having an interior wall extending upwardly from the bottom of said container and separating one portion of said container from the remainder; said network being located in said remainder with said resistors extending substantially parallel to said wall, said resistors being shorter than said wall; partially filling said remainder with encapsulating material to cover said resistors substantially entirely except for the uppermost terminals thereof; measuring the resistance of said network; connecting in series with said network a compensating resistor having a resistance substantially equal in value to the difference between a desired resistance of said composite resistor and the measured value of said network; lacing said compensating resistor within said one portion; and subsequently filling said container and said one portion with encapsulating material.

4. The method of claim 3 in which the measurement of resistance of said network is accomplished by measuring the resistance between electrically adjacent ones of said uppermost terminals.

References Cited UNITED STATES PATENTS 7/1914 Smith 338-77 X 2/1925 Dominguez 338-319

Claims

1. THE METHOD OF MAKING A PRECISION COMPOSITE RESISTOR COMPRISING THE STEPS OF: CONNECTING A PLURALITY OF INDIVIDUAL RESISTORS INTO A TWO-TERMINAL NETWORK; PLACING SAID NETWORK INSIDE A CONTAINER HAVING AN INTERIOR WALL SEPARATING ONE PORTION THEREOF FROM THE REMAINDER, SAID NETWORK BEING LOCATED IN SAID REMAINDER OF SAID CONTAINER; PARTIALLY FILLING SAID REMAINDER WITH ENCAPSULATING MATERIAL; MEASURING THE RESISTANCE OF SAID NETWORK; CONNECTING A COMPENSATING RESISTOR THERETO ADJUST THE OVERALL RESISTANCE TO A DESIRED VALUE; PLACING SAID COMPENSATING RESISTOR WITHIN SAID ONE PORTION; AND SUBSEQUENTLY FILLKING SAID CONTAINER AND SAID ONE PORTION WITH ENCAPSULATING MATERIAL.