US2083387A - Temperature compensation - Google Patents

Description

ELECTROMAGNETIC RESPONSIVE DEV/CE Inventor Frank W Mew-Til I,

His Attorney.

F. W. MERRILL TEMPERATURE COMPENSATION Filed May 11, 1935 I lfIlIiflI/Iffflfff/ll ill!!! llllllrvllitlllllflllflllllllltll' June 8, 1937.

NEGATIVE TEMPERA'TURE CO FFICIENT 0 RES/STANCE iatented June 8, 1937 UNITED STATES TEMPERATURE CODIPENSATION Frank W. Merrill, Fort Wayne, !nd., assignor to General Electric Company, a corporation of New York Application May 11, 1935, Serial No. 21,061

Claims. (Cl. 175-335) My invention relates to temperature compensation and more particularly to resistors used for the temperature compensation of electro-magnetically operated electro-responsive devices.

5 Most electro-magnetically operated electroresponsive devices, such for example as meters, instruments, relays and regulators are provided with an operating winding or windings of copper wire. Copper has an appreciable positive temperature coeflicient of resistance which means that changes in ambient temperature change the electrical resistance of the. windings thereby changing the effective calibration or setting of the device. One way to compensate for this is to connect in series with the copper winding a resistor of the proper value and having the proper negative temperature coefiicient of resistance. Theoretically, as the temperature changes the increase in resistance of the copper Winding will just be offset by the decrease in resistance of the compensating resistor so that there is no net change in the resistance of the circuit as a result of the temperature change. I have found, however, that such resistors have such a relatively small surface area that the self-heating in the resistor seriously affects its value as a temperature compensator. By selfheating, I mean the heating produced in the resistor as a result of the current flow therethrough, as distinguished from the heating of the resistor as the result of an increase in ambient temperature. Thus, due to the relatively small surface of the resistor, relatively high temperatures are attained by the self-heating before this heat can be radiated and a heat balance obtained.

Obviously, therefore, the temperature of the compensating resistor does not follow the ambient temperature so that its change in resistance does not complement the change in resistance of the copper winding.

In accordance with my invention I overcome this difficulty by in effect, greatly increasing the heat radiating surface of the resistor while at 5 the same time preventing direct radiation from the surface of the resistor and also preventing heat loss due to convection currents at the surface of the resistor. A preferred way of doing this is to embed the compensating resistor in a thermally conducting electrically insulating medium such for example, as dry sand. If desired, the sand may be confined by a metal container in the center of which is placed the compensating resistor so that the whole forms a sub-' stantially unitary assembly.

My invention is equally applicable to temperature compensating resistors having temperature coefficients which are either less than and greater than zero, that is to say, either positive or negative coeflicients. With zero temperature coeflicient resistors there is no need for my in-- vention as the resistance is independent of the temperature. Such resistors are some times used for rough or partial temperature compensation. In such cases their action may be described as diluting the over-all temperature coeflicient of the circuit.

An object of my invention is to provide a new and improved temperature compensating resistor assembly.

My invention will be better understood from the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

Referring now to the single figure of the accompanying drawing, which is a diagrammatic showing of an embodiment of my invention, I is an electric circuit to which is connected an electro-magnetically operated electro-responsive device 2, of any type, which is provided with an operating winding 3 composed of a conductor having either a positive or a negative temperature coefficient of resistance. In practically all cases the winding will be made of copper having a positive temperature coeflicient of resistance and this temperature coefficient of the compensator will be negative as indicated. If the device 2 is a voltage regulator employed to regulate the voltage of an automobile battery charging generator, (which it may be assumed to be) the winding 3 will be subjected to temperatures varying from 30 degrees below zero F. to degrees F., under the hood of the automobile. Such a wide temperature range would produce a resistance change in the copper winding which would vary the setting of the regulator by an amount which could not be tolerated.

For'compensating for this temperature error I connects in series with the winding 3 a negative temperature coeflicient of resistance resistor 4 which may be of any suitable type, such for example as a rod of the carborundum mixture which is known to the art as "Globar. The ends of this rod are coated with metal at 5 so as to make them highly conducting and connected or clamped thereto are electrical terminals 6 which are insulated by suitable insulating means I from a metal casing or cylinder 8 which may be constructed of steel. Filling the space in the cylinder 8 not occupied by the resistor 4 and terminals 6 is any suitable thermally conducting electrically insulating material, such for example as a quantity of dry sand 9. This assembly has been shown on the drawing on an exaggeratedly large scale so as more clearly to illustrate its details.

With the above construction it is impossible to have any convection air currents at the surface of the resistor l and also it is impossible to have any heat radiation from this surface and all the heat is transferred to the sand by conduction and the heat loss in the assembly is practically entirely due to radiation from the surface of the cylinder 8. This surface is so much larger than the surface of resistance 4 that the self-produced heat can be radiated at low temperature. As a result, the temperature of the'rod more nearly reflects the change in the ambient temperature surrounding the cylinder than it would if the rod were simply exposed to the air.

Another advantage of this construction is that the thermal capacity of the device is increased so that the cylinder as a whole will heat up slowly at a rate comparable to that of the heating of the coil 3.

A further advantage of this construction is that the thermal capacity of the device may be so adjusted by a choice of the size of the external tube 8, and the thickness of the thermally conducting, electrically insulating medium 9, as to bring the temperature of the compensating resistance due to self heating to just the right point so that the decrease in resistance of the rod 4 P will be closely equal to the increase in resistance of the coil 3 due to its self heating.

For these two reasons the voltage calibration of the circuit as a whole will be unaffected by the -self heating during the preliminary warming up of the coil and the compensating rod and both coil and rod will at all times respond slowly to changes in the ambient temperature at approximately the same rate and in proportionally corresponding amounts to maintain constant circuit resistance and thus insure proper temperature compensation of the device under all conditions.

While I have shown and described a particular embodiment of my invention, it will be obvious to those skilled in the art that changes and modifications can be made without departing from my invention and I, therefore, aim in the appended claims to cover all such changes and modiflca tions as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. A temperature error compensating arrangement of the type in which a resistor having a temperature coeflicient of resistance other than zero is connected to compensate a winding having a temperature coefflcient other than zero of an electro-responsive device for errors produced by variations in ambient temperature characterized by means for minimizing errors in said compensation produced by self heating in said resistor comprising heat conducting material for increasing the heat radiating area of said resistor to a point where the temperature of said resistor follows substantially the ambient temperature.

2. An ambient temperature compensating resistor assembly comprising, in combination, a rod of resistance material having a negative temperature coeiiicient of resistance, and means for making the temperature of said rod follow closely the ambient temperature of said assembly comprising a metal container surrounding said rod, and a quantity of dry sand filling the space in said container not occupied by said rod.

3. A temperature error compensating element for an electro-responsive device having an operating magnet with a winding having a positive temperature coefllcient of resistance comprising a negative temperature coeflicient resistor adapted to be connected in series with said winding for compensating said device for ambient temperature changes, and a quantity of thermally conducting electrically insulating material placed around said resistor for causing its temperature to be relatively independent of its own heating eifect.

4. A temperature error compensating element for an automobile voltage regulator having a copper operating winding whose resistance changeswith changes in ambient temperature adversely aifects the calibration of said regulator comprising a rod of resistance material having a negative temperature coeflicient of resistancesuch as to eliminate said adverse effect adapted to be connected in series with said winding,a metal cylinder axially surrounding said rod, and a quantity of dry sand filling the space between the outer surface of said rod and the inner surface of said cylinder.

5. An ambient temperaturecompensating device for a voltage regulator which has a copper operating coil comprising, in combination, a rod of resistance material which has a negative temperature coeflicient of resistance adapted to be connected in series with said winding, metal coating on the ends of said rod, electric terminals connected to said coating, a metal cylinder axially surrounding said rod and terminals, and dry sand filling the space in said cylinder not occupied by said rod and terminals.

FRANK W. MERRILL.

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