Current measurement by use of Hall effect generato

Hood, R. T.

English

NASA Technical Reports Server

R. T. HOOD 393 
HALL CURRENT MEASURING APPARATUS HAVING A SERIES 
RESISTOR FOR TEMPERATURE COMPENSATION 
Fi l ed  J u l y  27, 1964 
I 
INVENTOR. 
RALPH T. HOOD 
BY 
https://ntrs.nasa.gov/search.jsp?R=19710013561 2020-03-17T03:01:08+00:00Z
3,365,665 
ALL CURRENT M ~ A S U ~ ~ ~ ~  APPARATUS 
IING A SlEWIES ~ E S ~ $ ~ ~ ~  FOR ~ E ~ ~ ~ E ~ A ~ ~ ~ E  
C ~ ~ ~ E ~ S A ~ ~ ~ N  
Ralph T. Hood, Cupertino, Caiif., assignor to the United 
States of America as represented by the A ~ ~ i ~ ~ s ~ a ~ Q r  
of the Nationall Aeronautics and Space A a ~ i ~ ~ s t r a i i o ~  
Piled July 27, 1964, Ser. No. 385,520 
3 Claims. (421. 324-117) 
A temperature compensating arrangement for compen- 
sating for changes in the Hall voltage of a Hall generator 
with changes in ,temperature of the generator, comprising 
a control circuit including a constant voltage source con- 
nected to the control electrodes of the Hall generator 
through a compensating resistor, such compensating resis- 
tor comprising the sole means for temperature compensa- 
tion of the Hall generator. 
The invention described herein may be manufactured 
and used by or for the Government of the United States 
of America for governmental purposes without the pay- 
ment of any royalties thereon or therefor. 
This invention relates to apparatus for measuring cur- 
rent and, more particularly, to the measurement of cur- 
rent by the use of a Hall-effect generator. 
Methods of sensing and reading out current flow in high- 
voltage conductors, such as the power lines to extremely 
high energy arc-jet facilities, have been limited either in 
frequency response or amplitude sensitivity. One such 
limited prior method is the classical method of inserting a 
low-resistance block (instrument-shunt) in series with the 
power line and reading the voltage developed across the 
resistance by means of a divider network. The shunt 
method is also limited by the amount of power that can 
be dissipated in the shunt. Voltage isolation of the shunt 
necessarily reduces the signal to such a low value that high 
sensitivity galvanometers are required. Since high-sensitiv- 
ity galvanometers cannot have good high frequency re- 
sponse one is forced to accept a compromise. Other prior 
approaches have also suffered from one or more of the 
enumerated problems. 
Accordingly, it is an object of the invention to provide 
apparatus for measuring current in a manner having im- 
proved frequency response and improved amplitude sen- 
sitivity. 
The solution provided by the invention employs a con- 
ventional Hall generator. As is well known in the art, such 
a device comprises a magnetic field responsive plate, pref- 
erably of a semi-conducting material. The plate is provided 
with two current supply terminals and with two output 
electrodes. When an input current is supplied to the ter- 
minals and the piate is properly oriented in a magnetic 
field an output voltage will be generated across the output 
electrodes. The output voltage is proportional to the mag- 
netic field and therefore is a measure of the strength of the 
field and of course a measure of the energy which estab- 
lishes the field, such as the current passing through a con- 
ductor. It has, of course, been known in the past that Hall 
generators can be used to measure current flow. One of 
the main problems, however, which has limited the use of 
the Hall generator for current measurement has been 
caused by the fact that the Hall generator is highly tem- 
perature responsive. In other words, the output from the 
Hall generator will vary considerably with variations in 
temperature even though the magnetic field remains con- 
stant. This problem has been solved according to the in- 
vention by utilizing the knowledge that the semiconductor 
Hall generator exhibits a negative temperature coefficient 
when a constant-current source is used and a positive tem- 
perature coefficient when a constant-voltage source is 
used. In other words, when the Hall generator is driven 
by a constant current source the output voltage from the 
generator decreases as the temperature increases. When 
the generator is driven by a constant voltage source the 
output from the generator increases with increase in tem- 
perature. It has been found, according to the invention, 
10 that if a resistor is inserted in series with the Hall generator 
driving circuit it is possible to arrive at a compromise be- 
tween the two types of power sources and achieve adequate 
thermal compensation. In addition, it has been found that 
the specific required resistance necessary for thermal com- 
15 pensation over a specified range of temperatures can be 
calculated from the basic equation for the output of a Hall 
generator. 
Accordingly, an additional object of $he invention is to 
provide measuring apparatus including a Hall generator 
20 wherein the thermal coefficient of the generator is reduced 
substantially to zero over an appreciable temperature 
range. 
A further object of the invention is to provide measuring 
apparatus comprising a Hall generator and a control cir- 
A further object of the invention is to provide an im- 
proved apparatus for measuring current with improved 
isolation characteristics. 
Another object of the invention is to provide measuring 
30 apparatus involving a Hall generator and comprising p- 
tentiometer circuitry for resistive nulling. 
Other and further objects and features of advantage will 
be apparent from the following detailed description 
wherein reference is made to the accompanying drawings 
FIGURE 1 is a schematic circuit diagram used in ex- 
plaining the calculation for obtaining the required magni- 
tude of resistance; 
FIGURE 2 is a cross section view showing the con- 
FIGURE 3 is a circuit diagram showing an actual cir- 
cuit employed in using the invention in practice and in- 
cluding a resistive nulling arrangement. 
Referring in more detail to the drawings, FIG. 1 dis- 
45 closes a schematic circuit diagram showing a conventional 
Wall generator 1 having an internal resistance R,. A con- 
stant voltage power supply V,, supplies the current for 
the Hall generator. The Ha11 generator includes output 
terminals 2 and 3 across which the output voltage V, can 
50 be measured. As will be hereinafter described in detail the 
control circuit includes a compensating resistor R,. 
A preferred physical arrangement for the apparatus is 
shown in FIG. 2. The apparatus comprises a box 6 of di- 
55 electric material such as Lucite. The box 6 is made of end 
walls 7 and 8, a bottom wall 9, and a pair of side walls 10 
(only 1 being shown in the drawing). The box comprises 
a removable top wall PI which is held in place by screws 
12. The end walls 7 and 8 are apertured to  receive a 
eo Lucite tube 14 having its ends bonded in the apertures in 
the end walls. Thus, a completely closed chamber is 
formed, and in practice the chamber is preferably filled 
with transformer oil. The apparatus includes a conven- 
tional Hall generator IS connected to the lower end of 
65 a Lucite supporting rod 86 which is in turn attached to 
the top 81. The wiring for the Hall generator can be car- 
ried in a cable 17 to a conventional plug and socket unit 
¶8 mounted in the top 11 and thence out to the associated 
apparatus. The Lucite supporting rod 16 is threaded in 
70 the top 11 so that the exact spacing of the Hall generator 
from the Lucite tube E4 can be adjusted. The plate of the 
Hall generator is positioned with the plane of the plate 
5 
25 cuit therefor containing a compensating resistor. 
35 in which: 
*' struction of apparatus for employing the invention; and 
3,365,665 
parallel to and passing through the axis of the tube 14. 
A metal rod 20 is positioned in the tube 10 and connected 
in the circuit whose current is to be measured. The current 
flow through the tube 20 creates a magnetic field which 
causes a voltage output from the Hall generator in a 
manner well known in the art. 
As previously explained, the reason why Hall generators 
have not been used more fully in current measurement is 
that they are sensitive to temperature changes. Such detri- 
mental temperature sensitivity has been overcome accord- 
ing to the invention by the inclusion of the compensating 
resistance R, shown in FIG. 1, and more specifically a 
resistance having a particular value. The reason for the 
resistance and its particular value are arrived at in the 
following manner. The plate of the Hall generator is made 
of a material, for example, Indium Antiminide, such that 
if the Hall generator is driven by a constant voltage source 
it will have a positive temperature coefficient, and if 
driven by a constant current source the generator will 
have a negative temperature coefficient. I t  has been found 
that if a particular value of resistance is inserted in series 
with the constant voltage source V, in FIG. 2 a com- 
promise between constant voltage and constant current 
results so that the output of the Hall generator will be 
independent of temperature. 
In more detail, the reason for using the resistance is as 
follows. The equation for the output of a Hall generator is: 
I,ChRb 
( 1 )  
vh=- A 
where: 
Vh=OutPut voltage (open circuit) 
I,=control current (driving current) 
ch=Hall coefficient of the generator material 
B=magnetic flux 
b=width of generator 
A=cross-sectional area of the generator. 
1 we can rewrite the equation as: 
In the constant voltage operation we see from Equation 
Eg ChB b vh=- 
in other words representing the control current I, by its 
voltage across the generator E, divided by its internal 
resistance R,. 
Now it is a characteristic of the material used that both 
the Hall coefficient and the internal resistance change with 
temperature. Furthermore, they both exhibit an inverse 
relationship to temperature but their rates of change dif- 
fer from one another. Thus, in Equation 2: E,, B, b and A 
all remain constant with changing temperature but R, 
and ch do not. It happens that R, changes more rapidly 
than ch so that increasing temperature will cause the quan- 
tity E,chBb/R,A to become larger since ch is in the 
numerator and will decrease less with increasing tempera- 
ture. Thus, when the Hall generator is driven with a con- 
stant voltage source it will have a positive temperature 
coefficient; that is, the voltage output of the generator will 
increase with increasing temperature, under a constant 
magnetic flux condition. 
Rg A (2) 
h the constant currect mode the equation 
now has three variables because in order to maintain a 
constant E,/R,=I,, E, will have to decrease as W, de- 
creases with tempeTature. The decrease in E, together with 
the decrease in ch will more than offset the decrease in R, 
and will decrease the entire quantity. Thus, when the Hall 
generator is driven with a constant current source it will 
have a negative temperature coefficient; that is, the volt- 
age output of the generator will decrease with increasing 
temperature, under a constant magnetic flux condition. 
Therefore, by inserting a series resistor with a constant 
4 
voltage supply we obtain the following beneficial results. 
As R, decreases, with temperature increase, more current 
will attempt to pass through the generator and therefore 
through the series resistor. The current increase through 
the series resistor will cause more voltage to be dropped 
across it and therefore the E, will be decreased. There- 
fore, if the value of the series resistor (R,) is properly 
selected it will cause the particular decrease in E, which 
will cooperate with the decrease in ch to cancel any change 
The equation for determining the specific resistance 
necessary for thermal compensation over a specified range 
of temperatures is derived from the basic Equation 1 for 
the output of a Hall generator. 
If Equation 1 is differentiated with respect to tempera- 
ture T, 
1o in output caused by a decrease in the resistance R,. 
15 
dVh-C;Bb d I ,  --+% I P b  - dCh 
dT A dT A dT (3 )  
20 For the desired condition where Vh does not change with 
temperature, Equation 3 becomes 
(4) 
25 The parameter dch/dT may be determined experimen- 
tally from the output voltage change with temperature for 
constant control current. From Equation 3 
30 ( 5 )  
or 
35 
The parameter dl,/dT may be determined from the Hall 
generator excitation circuit, shown in FIG. 1 having a 
constant voltage source V,, for which circuit 
I,=& 
(7) 
40 
Ra+Rg 
Differentiating Equation 7 with respect lo temperature 
45 where dR,/dT may be readily measured for any given 
Substituting Equations 1, 6, and 8 into Equation 4, we 
Hall generator. 
obtain 
50 
55 
The solution for (R, j -R,)2 is 
But, since V,/I,= (R,+R,), Equation 10 becomes 
6o merefore 
65 Where the values to be taken for Vh and R, are those 
which apply at the operating temperature sbout which 
compensation is io be made. 
Using Equation 12 the necessary ccmpensati!ig resist- 
ance may be computed for any particular Hall generator, 
70 using only easily measurable parameters characteristic of 
the particular Hall generator. 
FIG. 3 is a circuit aiagrrm similor to FIG. 1 but show- 
ir,g a potcrltiometx arrangcment for recictive nuiling. FIG. 
3, as in FIG. 1, discloses a constaqt voltage source V,, a 
75 Hall generator i hhving ontput terminals 2 and 3, and a 
3,365,665 
compensating resistance Rs. In addition, the practical em- 
bodimei?t shown in FIG. 3 includes a potentiometer com- 
prising a resistance 23, a fixed tap 24 connected to the 
center of the resistance and a movable arm 25. The cen- 
is connected to one side of a galvanometer 26, 
the other side of which is connected lo the terminal 2 
lhrough a damping resistance 27. The movable arm 25 
is conected to the terminal 3. A pair of nul!ing resistances 
30 and 31 are connected on opposite sides of the poten- 
tiometer resistance 23 to place the potentiometer resist- 
ance 23 in parallel with the Hall generator. The embodi- 
ment of FIG. 3 also preferably includes a voltmeter 32 
and an ammeter 33. The potentiometer is for resistive 
nulling in accordance with the following procedure. The 
mechanical zero of the galvanometer is located. Tnen after 
the Hall generator is energized the output of the galva- 
nometer is returned to its zero reading by adjusting the 
potentiometer. 
The sensitivity of the apparatus may be adjusted by 
changing the magnitude of the constant supply voltage V, 
or by changing the recording galvanometer 26. I n  a specific 
construction according to the invention an oscillograph 
galvanometer was used, and with a Hall generator supply 
voltage of about 3 volts, the Ha!l e3ect ammeter provided 
a sensiiivity of about one inch deflectiol? of galvanometer 
trace for each 1,000 amps. of current through the con- 
ductor 20. The distance from the center of the conductor 
to the Hall effect generator was approximately 1.9 inch. 
The conductor 20 was 1” diameter copper rod. Since the 
frequency response of the Hall generator WBS substantially 
flat the response of the system was limited only bj7 the re- 
cording galvanometer which, by reason of the invention, 
can be a relatively low-sensitivity, high-frequency-response 
type. The specific galvanometer used had a natural fre- 
quency of 225 c.P.s., as compared to a natural frequency 
of the order of 6 C.P.S. for the galvanometer required for 
use with the instrument shunt techniqae. 
Although preferred embodiments of the present inven- 
tion are shown and described herein, it is to be understood 
that modifications may be made therein without departing 
from the spirit and scope of the invention as set forth in 
the appended claims. For example, it should be under- 
stood that although the invention has been described with 
specific reference to measuring current, it is applicable to 
measurement of magnetic fields, per se, and any param- 
eters such as current which bear a known relation to a re- 
lated magnetic field. 
Having thus described the invention, what i3 claimed 
as new and desired to be secured by Letters Patent is: 
I. In measuring apparatus having a Hall generator, a 
control circuit comprising a substantially constant voltage 
power supply and a compensating resistor in series with 
said Hall generator, and the value of said resistor being 
equal to 
where 
R,=resistance of the Hall. generator, 
T= temperature, 
Vb=the output voltage of the Hall generator, 
dR,/dX=the temperature rate of change of the H2ll 
generator resistance, and 
5 
10 
15 
20 
25 
30 
35 
40 
45 
50 
55 
60 
65 
(dV~/dT)I,=03nstant=the temperature rate of change of 
the Hall generator voltage using a 
constant current power supply. 
2. A current mcasuring apparatus for measuiing large 
a current carrying conductor through which the large 
current to be measured is adapted to be carried, 
a Hall-effect generator a spaced distance from the con- 
ductor poqitioned in the magnetic flux field of the cur- 
rent carrying conductor, said generator including a 
pair of control termina!s and a pair of output ter- 
minals, 
values of current which aparatus includes, 
a power supply having a constant voltage output, 
a compensating resistor, 
means connecting the constant voltage output to the 
control terminals of said Hall-eEect generator 
through said compensating resistor for the sole com- 
pensation of changcs in Hall voltage output with 
changes in temperature of the Mall-effect generator, 
the value of the compensating resistor being substan- 
tidly equal to, 
dR&T I.7 - 
( d  VhIdT 1 I,-oonstant h- 
where: 
R,=resistance of the Hall generator, 
X= temperature, 
V,=the output voltage of the Hall-effect generator, 
dR,/dT=the temperature rate of change of the Hall- 
effect generator resistance, and 
( d V h / d T )  Ic=constant=the temperature rate of change of 
the Hall-effect generator voltage 
using a constant current power sup- 
a potentiometer including a resistance in parallel with 
said constant-voltage power supply, 
a low sensitivity, high frequency response type galva- 
nometer, and 
means conecting the galvanometer to the output ter- 
minals of the Hall-effect generator through a portion 
of the potentiometer resistance to compensate for 
the resistive null voltage of the Hall-effect generator, 
the galvanometer providing an indication of the cur- 
rent flowing through the current carrying condcctor. 
3. The current measuring apparatus as defined in claim 
2 wherein said galvanometer has a natural frequency of 
approximately 225 cycles per second. 
References Cited 
UNITED STATES PATENTS 
2,957,109 10/1960 White et al. _ _ _ _ _ _  324-45 XR 
FOREIGN PATENTS 
1,131,798 6/1962 Germany. 
Ply, 
OTHER REFERENCES 
Borneman, E. H.: Basic Course in Semiconductors and 
Technology, 1963, pub. by Westinghouse Semiconductor 
Div., pp. 9-6 and 9-7. 
Sauber, J. W.: “Audio Frequency Measurements Using 
the Hall Effect,” The Right Angle, February 1964, pp. 4 
and 5. 
RUDOLPH V. ROLINEC, Primary Emniner.  
E. F. KARLSEN, Assistant Examiner. 


Hall current measuring apparatus having a series resistor for temperature compensation Patent

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