Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to apparatus for measuring a
current flowing in a conductor and more particularly to
apparatus which responds when inductively coupled to the
conductor.
There is frequently a requirement, particularly
in the testing of electrical systems of motor vehicles, to
measure heavy duty currents without disrupting electrical
cables, such as occurs in the insertion of an ammeter shunt.
Such a practice is inconvenient and sometimes can influence
the resulting analysis. Inductively coupled sensors have
been developed in the past which employ a two-piece ferro-
magnetic core member which can be attached to enclose the
conductor under test. This core carries windings which are
excited by an oscillator with the core sometimes forming
part of the oscillator resonant circuit. Asymmetry is
introduced into the oscillator circuit by induction from
the conductor which is detected and indicated on a meter.
Several electrical circuits are known which function as
the oscillator and it is conventional that an indication
of the current flow in the conductor is provided on the
scale of an ammeter which responds to current imbalance in
the excitation coils on the core.
One known type of ferromagnetic core is U-shaped
with a sliding or hinged I-bar pole piece to bridge the two
poles of the core. Such a core must be physically large
to avoid saturation at the high currents to be expected,
while the transmission of an output signal to a remote
indicator or data acquisition system may be difficult.
Such a prior device has been employed with a circuit with
reasonably low power consumption suitable for currents to
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be measured of up to lOO amperes. Such a circuit may have
an oscillator composed of two transistors feeding a constant-
voltage square-wave to two exciter coils~ with an ammeter
indicating the current imbalance in these coils. The coil
inductance in combination with the base resistance forms
the circuit time-constant. It can, therefore, be seen how
the above mentioned difficulty in transmission of the output
signal results from the low signal level available and the
fact that it is floating and not referenced to the zero-volt
; 10 supply line. The latter problems might be overcome by using
an oscillator circuit which saturates the core, and obtaining
the output as a high-level square-wave with modulated pulse
;width, but a very large core is still required and the power
consumption would be considerably higher.
It is the principal object of this invention to
provide current measuring apparatus which functions on rela-
tively low power consumption and which operates efficiently.
The invention in one general form, therefore,
provides apparatus for measuring current flow in a conductor,
comprising a magnetic core attachable about the conductor
for induction of a magnetic flux therein when current flows
in the conductor, said core being constructed with plural
, sections to provide a multiple path for the magnetic flux,
an electrical oscillating circuit functioning to saturate
;~ at least part of one of said sections, and means for deriving
from the waveform of oscillations of said oscillator a
signal indicative of the value of current flowing in the
conductor.
In accordance with the invention, a current measuring
device for measuring a current flowing in the conductor by
inductive coupling to the conductor comprises: an E-shaped
core of magnetic material having first and second outer legs
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and a center leg spaced from said second outer leg sufficiently
to provide a space therebetween for the reception of a conduc-
tor carrying the current to be measured, a bar movably asso-
ciated with said E-shaped core and movable between a first
position in which the bar is displaced from the end of said
second outer leg sufficiently to provide for the insertion of
a conductor therein to a normal position in which it is in
engagement with said first outer leg and said center leg but
is displaced from said second outer leg by an air gap, a
pair of windings disposed on said first outer leg, means for
alternately energizing said windings to cause flux flow in
opposite directions, and means for measuring any difference in
voltage across said windings resulting from the effect of
flux flowing in said center leg because of current flow in
said conductor.
Also in accordance with the invention, a current
measuring device for measuring a current flowing in a
conductor by inductive coupling to the conductor comprises:
a core of magnetic material having first and second legs, a
bar movably associated with said core and movable between a
first position in which the bar is displaced from the end of
one of said legs sufficiently to provide for the insertion
of a conductor into said core to a normal position in which it
forms a substantial part of the magnetic path of said core,
a pair of windings disposed on said first outer leg, including
a pair of transistors, each controlling the energization of
one of said windings to cause said windings to be alternately
energized to cause flux flow in opposite directions, a pair of
further transistors, one in series with each of the transistors,
to prevent clipping of the voltage peaks resulting from the
alternate energization of said windings, and means for measuring
any difference in voltage across said windings resulting from
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the effect of flux flowing in said center leg because of cur-
rent flow in said conductor, said last named means comprising
a further transistor having a control electrode thereof con-
nected to one of said transistors of the first named pair of
transistors.
Eor a better understanding of the invention,
reference will be made to the following description together
with the accompanying drawings in which:
Figure 1 illustrates one specific embodiment of a
ferromagnetic core constructed in accordance with this
nventlon;
Figure 2 is a schematic representation of a pre-
ferred form of electrical circuit to be utilized with the
ferromagnetic core of Figure l; and
Figure 3 shows a waveform to be derived at the
collector of certain transistors of Figure 2, under the
condition where no current flows in the conductor under test.
The present invention provides a pulse width
modulated output and has the advantages of reasonably low
power consumption with small core size. One additional ad-
vantage is that mating surfaces of the ferromagnetic core
are never separated and exposed to dirt, or other e~traneous
matter, when the core is opened to introduce the conductor
: under test.
With reference to Figure 1, it will be observed
that a small, easily saturated, "E" core 10 of ferrite
material is used, with a sliding I-bar 11 which is held
by spring pressure permanently in close contact with two
limbs 12 and 13 of the E-core 10. A large, preset air-gap
14 is left in series with the third limb 15. The conductor
16 carrying the unknown current induces a unidirectional
flux 17 in the third limb 15, which is limited by the size
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of the air~gap 14 so that it never saturates the core 10.
This flux 17 is further divided between the large center
leg 13 and the narrow leg 12 carrying the oscillator coils
18 and 19. The last leg 12 is alternately saturated in
opposite directions by the oscillator (Figure 2), but the
large area of the center leg 13, and the air-gap 14 in the
third leg 15, prevent these from also being saturated. The
imbalance flux in the oscillator leg 12 causes an unequal
mark-space ratio at the output of the circuit, and this is not
10 dependent on applied voltage or other circuit parameters to
any great degree, the overall calibration being controlled
by the air-gap 14.
The exciter circuit shown in Figure 2, in some
respects, resembles a conventional saturating core oscillator
of the type normally used for power supply inverters. This
portion of the circuit comprises the coils 18a, 18b, l9a and
l9b, resistors 21 through 27 and transistors 31 and 33. The
emitters of the latter would, in such a conventional satura-
ting core oscillator, be grounded directly. However, in the
20 present construction, complementary transistor switches 32 and
34 are added which avoid clipping of the voltage transient
peaks (Figure 3) as this would cause a loss of sensitivity.
Such clipping would be the direct result from normal break-
down of the transistors 31 and 33 under reverse voltage
condition. By the addition of transistors 32 and 34 a sym-
metrical switch is obtained.
The oscillator drives a simple transistor switch 35,
which provides a 5V peak-to-peak square-wave output between
its collector and emitter terminals, capable of being trans-
30 mitted over a great distance, without loss of accuracy. Aresistor 38 is connected between positive terminal Vcc and
the collector of transistor 35. A d.c. signal can easily be
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received from such a transmitted signal using any one of a
wide choice of circuits, depending upon the particular
application.
Zener diode 36, resistor 39, and capacitor 41
provide protection against any transients which may be
induced in the cable linking the probe to its associated
instrument. Resistors 23 and 26 are necessary to provide bias
current for transistors 32 and 34.
Figure 3 shows a typical waveform at the collectors
of either transistor 31 or 33. In the circuit shown, the
collector of transistor 33 is connected to the input of the
transistor switch 35 although switch 35 could be connected to
transistor 31. Where, as indicated by Figure 3, the ratio of
the integrated values of the peaks is substantially unity,
no current is flowing in the conductor 16. The waveform peaks
as shown and decays in a manner well-known in the functioning
of D.C. - A.C. converters.
When current flows in the conductor under analysis,
depending in sense upon the point of measurement connection
in the oscillator circuit, or the direction of current flow
in the conductor, the ratio of the integrated values of the
peaks of the waveform is other than unity. The value of the
current flow will be revealed by this ratio. Therefore, the
pulse on-off ratio at the output of the switch 35 provides an
accurate indication of the value of the current flow in the
conductor under analysis.
One simple manner in which this output may be
indicated upon a linear scale of an ammeter would be to con-
nect a potentiometer across the voltage supply and connect
the output lead from the switch 35 through an ammeter to a
center tap of the potentiometer. The meter would have zero
calibration when the peak ratio is unity.
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Whereas the foregoing description relates to measure-
ment of a D.C. current in the conductor under analysis, it is
also applicable to the measurement of an A.C. current pro-
viding the frequency thereof is low comparable with the
frequency of the saturating oscillator. There are many
applications for measuring apparatus of the present type, for
example automotive electrical system analysers and D.C. or
A.C. power distribution systems.
While I have shown a specific embodiment of my
invention, it is to be understood that this is for purposes
of illustration only and that the scope of the invention is
limited solely by the appended claims.
1 ,h
