US2375178A - Variable electrical resistor - Google Patents

Description

May l, 1945. s. RUBEN 2,375,178

VARIABLE ELECTRICAL RESHISYTOR Filqd oct.""1,'1941 2 sheetsshet' 1 fffw I N VEN TOR.

cfazfme/ uen TTORNE Y May l, 1945. RUBEN 2,375,178

VARIABLE ELECTRICAL RESISTOR Filed oct. 1, i941 2 sheets-sheet 2 90A/00d /v/ yynsssyJ BY .Samuel Enkel@ H TT URN E Y resistors.

their constructionare` stable in operation and Patented May 1, 1945 UNITED STATES PATENT FFIG'LE 2,375,178 A VARIABLE nLEormoAL nnsls'ron samel Ruben, New Rochelle, N. Y. Application october 1, 194,'1, serial No. 413,135' s claims. (c1. aol-o) '.lnis invention relates resistors.

An object of the invention is the provision of avariable resistor of new and improved construction which can be conveniently and economically manufactured, which is stable land which can be used to replace variable wire wound resistors of the prior art as well as variable re sistors of' the carbon pile type. A further object is the provision of an improved variable resistor of the compression type. Other objects will be apparent from the disclosure and from the drawings, Fig. 1 of which to variable electrical is a view partly in section, of a variable resistance device made according to this invention; Fig. 2 is a detail of one section of the resistance element not under compression; Fig. 3 is a detail of the same section oi the resistance element under compression and Fig. 4 is a graph illustrating the performance of the resistor in respect to pressure versus resistance, rotation versus resistance and temperature rise watt dissipation. f

The` art discloses two general types of variable One is the wire wound type utilizing the variation in resistance obtained by contacting the resistance element at various points along its length by' a conductor in the form of a. rotating sliding contact, or by bringing out connections from various points to the resistor element, contact to which is made by a sliding arm contact or lever. Variable resistance elements of this type are' also made utilizing a carbon base resistor instead of wire. The other type is the compression type which functions by reason of the contact resistance lvariation between materials having negative pressure resistance coeiicients such as carbon or which function by the compression of a mixture of powder or ilake carbon or graphite with an insulatingmaterial such as mica powder.

Each of these types has an inherent limitation` that is, inability to directly dissipate its heat without a high temperature gradient from the hottest portion to the heat radiating surfaces.

Ihe resistor of this invention allows the dissipation of heat more effectively from the resistor element andy permits the construction o! units having a much higher power capacity with volume. Also it allows the construction ci a resistor having a much longer life than the present compression type due to the fact that unlike carbon compression types which lose their surface, the units may be adjusted or repeatedly compressed at the same pressures with approximately the same resistance values.

Another factor of importance with the present resistoris that a large resistance range can be obtained'with a small angular change of the control pressure.

The invention comprises the impregnation into and onto the fibres of a glass mat or cloth of a resistor] material such as colloidal graphite, and the heating up of the impregnated glass mat to the point where the resistance material is rmly and permanently bonded to the glass. The preferred impregnating material is colloidal graphite. Other resistor materials such as platinum may be used, in which case the glass mat is impregnated with a platinic chloride solution and then baked'to the required tempera-` ture, such as 350 C., thus bringing about decomposition of the chloride and deposition of a thin platinum lm. It is also possible to utilize a very thin depositsoi nickel as the resistance material by heating the glass mat to 250 C. in a nickel carbonyl atmosphere.

In carrying out the invention, the glass fibre mat, preferably about 20 mils thick, -a'nd containing a large number of thin glass bres, is placed in a container in which is a solution of 20% by weight of colloidal graphite (Aquadag). The aquadag is in an aqueous solution to which a small amount oi ammonium hydroxide has been added to ins-ure colloidal suspension (l c. c. of 28% N'HiOI-l per liter of water). A low pressure is applied to'allow the air between the fibres to be exhausted and after a period of about iiiteen minutes the glass bre'mat having thereon a deposit of colloidal graphite is taken out of the impregnating solution and passed through a rubber/roller under pressure so as to eliminate -excess material. It is then baked for thirty minutes at-a vtemperature of about 300 C. As a result oi' thisbaking the colloidal graphite is firmly bonded to the glass fibres and the glass mat may be then punched into discs or squares with mounting holes 'of a .size dependent upon 'the application. By variation of the colloidal graphite content various ranges of intial resistance values canbe obtained For some applications an ammonium sheilac solution in the order of 5% of the graphite content may be added. This is a clear red solution completely miscible with the dolloid. It decomposes during the baking, adds to the bonding `of the graphite to the glass and increases the resistance to water vapor ell'ects.

In the construction of the variable resistance units electrodes/ of good heat' conductivity are positioned on both sides of the resistor discs, preferably at the minimum pressure required to secure good contact.v The resistivity is high due to the long resistance 'paths along the fibres. When compressed the variousbres are brought together to .provide an increasing number of multiple paths (due to contacting together of the libres) which lowers the resistance inv proportion to pressure.

In order to illustrate the construction of a typical unit and the performance of such a unit, reference is made to the drawings.

In Fig. 1 is shown a variable resistance device of this invention in which the resilient springlike glass fibre mat resistor element 3, separated by brass heat-radiating plates 8', is contacted by.

end plates i and 2 of #5 thickness aluminum or brass. A ceramic tube l0, is riveted to the back end plate l, serving as a guide and to keep the plates from rotating. Centrally located ceramic tube 5 is placed on thread screw 6, which is attached to the back end plate l. At the thread end the knob 'l containing a threaded metal insert B and further insulated from the end plates by glazed ceramic washer 9 is used to apply the pressure to the resistor elements. This glazed washer also reduces the turning friction of the knob.

By the use of spacer washers of various thick- -ness to increase the space between the radiator plates the amount of heat radiation and loss can be regulated.

In the unit described, the radiator plates have an areaJ 3" square and are maintained Ik" apart -by use of spacer washers. The impregnated glass mat resistor elements are 21/2 in diameterand a 32 pitch compression screw has been use d in order to regulate the operation.

In Fig. 2 is shown a detail of one of the resistor sections of the device. The impregnated glass The graph of Fig. 4 illustrates the operating characteristics of a resistor built according to the design illustrated in Fig. l.

The three curves, pressure against resistance, rotation against resistance and watts against temperature rise are self explanatory and illustrate the wide range of utility of the device.

The variable resistance device of this invention allows a Wide range of control, inasmuch as the sections may be connected in series, in parallel or in series parallel, and still have a control throughout the entire'range of adjustment. This is not possible with wire wound variable resistorsor compression type resistors.

Having described my invention what Iv claim as new and desire to secure by Letters Patent, is: l. The method of making an electric resistance element which comprises immersing a resilient glass 4fibre mat containing a large number of criss-crossing thin glass iibres in an aqueous ammoniacal suspension of colloidal graphite to deposit colloidal graphite on said libres, removing said mat from saidsuspension and extracting excess liquid therefrom and then baking said mat to bond said colloidal graphite thereto.

2. The method of making an electric resistance element which comprises immersing a resilient glass fibre mat containing a large number of criss-crossing thin glass bres in an aqueous ammoniacal suspension of colloidal graphite to deposit colloidal graphite on said bres, removing said mat from said suspension and extracting excess liquid therefrom and then baking said mat to bond said colloidal graphite thereto, and then .inserting said mat between parallel plate electrodes in contact therewith.

3. 'I'he method of making an -electric resistance element which comprises immersing a resilient mat l I is shown housed between contactor plate I2 and i3. The mat is not under operating pressure and the long current path length can be noted.

In Fig. 3 the same resistance section can be shown under compression and it is obvious that the current path has become much shortened due to parallel paths andcontacting together of the libres lla.

glass bre mat containing a large number of ,criss-crossing thin glass bres in an aqueous ammonium resin solution containing a suspension of colloidal graphite, to thereby deposit colloidal graphite and resin'on said fibres, removing said mat from said suspension and extracting excess` liquid therefrom and then baking said mat to decompose said resin and bond said graphite to said glass fibres.

SAMUEL RUBEN.

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