US 3517361 A
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June 23, 1970 H. REIFEL ET AL 3,517,361
SHIELDED TRANSFORMER Filed Jan. 19, 1968 F|G.|. 25 2e 27 2e BRAID 23 32 BRAID 27' ANCHOR 20 ANCHOR 2| CONDUCTIVE CONDUCTIVE COATING 30\. \/COATING 5| ENCAPSULATIONI?" Q Q EENCAPSULATIONIS TAPEI4/ Q E :\TAPE l6 NYLON BOBBINIO' S Q E-NYLON BOBBIN u Q Q Q PRIMARY Q SECONDARY WINDING|3- Q Q I "WINDINGIS i CORE I2 I 1 F|G.2 39 I2 3| 24 r'"-' |"---'I 27 I l I I I l5 l 1 28 J l l |NVENTORS= HARRY REIFEL,
VITO JMDI BENEDETTO,
ATTORNEY United States Patent 3,517,361 SHIELDED TRANSFORMER Harry Reifel, Waltham, and Vito J. Di Bendetto, Melrose, Mass., assignors t0 Stevens-Arnold Inc., South Boston, Mass.
Filed June 19, 1968, Ser. No. 738,292 Int. Cl. H011. 15/04 US. Cl. 33684 12 Claims ABSTRACT OF THE DISCLOSURE A transformer has primary and secondary windings on nylon bobbins which extend around a core. For common mode rejection, the bobbins, and the windings except where the lead wires extend therefrom, are completely enclosed by conductive coatings. The coating around the primary winding and its associated bobbin is connected to one of the lead wires of the primary winding. The coating around the secondary winding is connected to the transformer core. For preventing the coatings from acting as short-circuit turns, they have electrical resistances many times larger than copper having the same thicknesses. The lead Wires are within conductive braids which the respective coatings contact.
BACKGROUND OF THE INVENTION The field of the invention is transformers such, for example, as transformers which couple choppers used to change weak DC signals such as from strain gauges or thermocouples, to AC amplifiers. Such transformers are subject to common mode voltages. Such a voltage is a spurious voltage between one input lead of a transformer and its core. The output of the transformer is not zero because of the finite value of distributed capacity between the primary winding and the core which allows common mode current to flow through the primary winding, and to induce a voltage in the secondary winding. An electrostatic shield around the primary winding and connected to one of the input leads of the latter will reduce the capacitance between the primary winding and the core, but a spurious voltage may be induced in the secondary winding because of the capacitive coupling between it and the shield around the primary winding. A second electrostatic shield around the secondary winding and grounded to the core will reduce the latter voltage. The reduction in such spurious voltages is termed common mode rejection.
Previously, common mode rejection has been accom plished by enclosing such primary and secondary windings within copper shields. The resistance of each of such shields has been in the order of 0.001 ohm, requiring, in order to prevent the shields from acting as short-circuit turns, slits in the shields. The largest common mode rejection that has been accomplished with such shields is 170 decibels.
We have determined that such slits in such shields admit common mode voltages, and have found that we can greatly increase common mode rejection by using as shields, conductive coatings such as graphite in a carrier of alkyd resin, with no slits, the resistances of such coatings being suflicient to prevent them from acting as shortcircuit turns. Preferably, the coatings have resistances in the range of one ohm-per-square to one thousand ohmsper-square. With such a coating having a resistance of one ohm-per-square, its shunting effect reduces the true signal voltage only by about one percent. Using our coatings, the common mode rejection is increased from 170 decibels to 200 decibels, a very substantial increase considering that decibel gains are logarithmic.
3,517,361 Patented June 23, 1970 Summary of the invention The primary and secondary windings of a transformer are wound on nylon bobbins which extend around the core of the transformer. The windings are encapsulated between the peripheral edges of the bobbins with insulation which may be an epoxy or a urethane. The bobbins and the encapsulation form insulation and protection completely around the windings except Where their lead wires extend through the encapsulations. The bobbins and the encapsulations are then coated with an adhesive liquid acting as a carrier of conductive material such as graphite. The lead wires of the windings are within conductive braids which are soldered to conductive anchors embedded within the encapsulations, and which extend through the conductive coatings in contact therewith.
Description of the drawings FIG. 1 is a side view, partially in section, with only a portion of the transformer core shown, of a chopper coupling transformer embodying this invention, and
FIG. 2 is a diagrammatic view showing how the shields are electrically connected.
Description of the preferred embodiment of the invention Referring first to FIG. 1 of the drawing, nylon bobbins 10 and 11 extend around a transformer core 12. A centertapped, primary winding 13 is wound on the bobbin 10. An insulating tape 14 such as Mylar extends around the winding 13, and holds the outer turns of the latter in place. Secondary winding 15 is wound on the bobbin 11. A tape 16 similar to the tape 14 extends around the winding 15, and holds the outer turns of the latter in place. Between the peripheral edges of the sides of the bobbins 10 and 11 are encapsulations 17 and 18 respectively, of insulation such as an epoxy resin or urethane such as Wothane H110 with hardener Wothane 300, manufactured by the Woburn Chemical Company. A brass anchor 20 is embedded within the encapsulation 17, and a similar anchor 21 is embedded within the encapsulation 18. A conductive braid 23 extends around lead wires 24, 25 and 26 of the winding'13, and is soldered to the anchor 20. A similar braid 27 extends around lead wires 28 and 29 of the winding 15, and is soldered to the anchor 21. The encapsulations 17 and 18 may be formed by placing the bobbins with the windings thereon within molds containing the encapsulation material.
A conductive coating 30 is in contact with and extends completely around the bobbin 10 and the encapsulation 17 except where the braid 23 extends through the coating 30. A similar coating 31 is in contact with and extends completely around the bobbin 11 and the encapsulation 18 except where the braid 27 extends through the coating 31. The conductive coatings can be brushed on, or sprayed on, the bobbins and the encapsulations. It is preferred, however, that they be applied by dipping the bobbins into a liquid carrier containing the conductive material, up to the line 32 on the braid 23, and up to the line 33 on the braid 27. The braids can serve as connections to the conductive coatings. Another conductive coating could be applied on each conductive coating to decrease the shield resistance if desired.
Suitable coatings are particles of graphite or other forms of carbon, or mixtures of such, dispersed within liquid carriers such as alkyd resin, alcohol, or a lacquer. The preferred coating consists of graphite interspersed within alkyd resin. As previously mentioned, the resistance in ohms-per-square of each coating should be within the range of one ohm to one thousand ohms. A resistance of one ohm-per-square is many times that of copper having the same thickness. The publication Materials Selector of Materials Engineering published by Reinhold Publishing Corporation of New York, N.Y., n page 43 of the mid-October, 1967 issue, gives the resistances in micro-ohms-cm. of copper as being between 1.71 and 2.05; of graphite as being between 820 and 1300, and of carbon as being between 3500 and 4600. For a given thickness of each, the ohms-per-square would be proportional to the listed resistances. Thus, the ohms-persquare resistance of a coating containing carbon or graphite, or a mixture thereof, is many times that of copper having the same thickness.
The bobbins, after being assembled on the core, would be potted and encased as is usual, the pottings and casings not being shown by the drawings since forming no part of this invention.
Referring now to FIG. 2 of the drawings, the shield coating 30 around the primary winding 13 is electrically connected to the lead wire 25 which is connected to a center-tap of the winding 13, although it could be connected to one of the other lead Wires 24 or 26. The shield coating 31 around the secondary winding 15 is electrically connected to the transformer core 12. The shields so connected, greatly reduce common mode voltages.
1. In a transformer having a core, and having around said core, a primary winding on a first bobbin of electric insulation, and a secondary winding on a second bobbin of electric insulation, the improvement comprising:
a first encapsulation of electric insulation around said primary winding, and between and on edge portions of said first bobbin;
said primary winding having lead wires extending through said encapsulation;
a first, electrically conductive coating extending in contact with all surfaces of said first bobbin except those contacted by said encapsulation, those between said encapsulation and the central, outer surface of said first bobbin around the window of the latter, and said central outer surface, and extending around the outer surface of said encapsulation except adjacent to where said lead wires extend therefrom;
a second encapsulation of electric insulation around said secondary winding, and between and on edge portions of said second bobbin;
said secondary winding having lead wires extending through said second encapsulation; and
a second, electrically conductive coating extending in contact with all surfaces of said second bobbin except those contacted by said second encapsulation, those between said second encapsulation and the central, outer surface of said second bobbin around the window of the latter, and said last mentioned surface, and extending around the outer surface of said second encapsulation except adjacent to where 2. The invention claimed in claim 1 in which the resistance of each of said conductive coatings in ohms-persquare is between one and one thousand.
3. The invention claimed in claim 2 in which each of said conductive coatings comprises graphite.
4. The invention claimed in claim 1 in which each of said conductive coatings comprises graphite.
'5. The invention claimed in claim 1 in which:
a tape of electric insulation extends around and in contact with said primary winding between the latter and said first encapsulation, and
a tape of electric insulation extends around and in contact with said secondary winding between the latter and said second encapsulation.
6. The invention claimed in claim 5 in which the resistance of each of said conductive coatings is between one and one thousand ohms-per-square.
7. The invention claimed in claim 6 in which each of said conductive coatings comprises graphite.
8. The invention claimed in claim 5 in which each of said conductive coatings comprises graphite.
9. The invention claimed in claim 1 in which:
an electrically conductive anchor is secured to said first encapsulation;
an electrically conductive braid extends around said lead wires of said primary winding, and is connected to said anchor;
a second, electrically conductive anchor is secured to said second encapsulation;
a second electrically conductive braid extends around said lead Wires of said secondary winding, and is connected to said second anchor;
said first conductive coating extends around and in contact with a portion of said braid around said leads of said primary winding, and
said second conductive coating extends around and in contact with a portion of said second braid.
10. The invention claimed in claim 9 in which the re-" sistance in ohms-per-square of each of said conductive coatings is between one and one thousand.
11. The invention claimed in claim 10 in which each of said conductive coatings comprises graphite.
12. The invention claimed in claim 9 in which each of said conductive coatings comprises graphite.
References Cited UNITED STATES PATENTS 1,740,458 12/ 1929 Hebrew 336-84 XR 2,170,048 8/1939 Dunning et al. 33684 XR 2,790,944 8/1957 Sifii' et al. 33684 XR 2,896,096 7/1959 Schwarzer 33684 XR 3,173,115 3/1965 Peuron 33684 3,244,960 4/1966 Stevens et al 33684 XR 3,277,416 10/1966 Barr 336-84 3,292,127 12/1966 Dormaier 336--84 3,312,919 4/1967 Ross 33684 3,393,388 7/1968 Young 33684 THOMAS J. KOZMA, Primary Examiner US Cl. X.R. 1743 5