Electricity: power supply or regulation systems – Including a transformer or an inductor – With compensation
Reexamination Certificate
1999-07-30
2001-01-23
Han, Jessica (Department: 2838)
Electricity: power supply or regulation systems
Including a transformer or an inductor
With compensation
C324S11700H
Reexamination Certificate
active
06177791
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a current sensor according to the compensation principle, in which the magnetic field produced by a primary winding through which the current to be measured flows is compensated by the compensation current in a secondary winding, and in which for the controlling of the compensation current at least one sensor that is influenced by the magnetic field acquires deviations from the zero current and supplies this measurement value to a driver circuit for the production of the compensation current, whereby at the output of the driver circuit the secondary winding is connected in series to a terminating resistance, and a voltage that is proportional to the current to be measured is present at the terminating resistance.
A current sensor of this type according to the compensation principle is for example known from European Reference EP 356 248 and from European Reference EP 691 544, and is shown in
FIG. 5
of the drawing. The current i
1
that is to be measured thereby flows through the primary winding
1
of a current transformer, which for example comprises a magnet core
2
as well as a sensor
3
that measures the magnetic flux in the magnet core
2
.
The sensor
3
consists for example of a transformer driven to saturation, with a rectangular magnetization characteristic. The output voltage of the sensor
3
is processed in a subsequently connected evaluation circuit
4
; a driver circuit
5
is in turn connected downstream therefrom. The output of the driver circuit
5
is connected with a reference potential G via the secondary winding
6
of the current transformer and a terminating resistance
7
.
The current to be measured now produces, via the primary winding
1
, a magnetic flux in the magnet core
2
, which flux is acquired by the sensor
3
. The evaluation circuit
4
connected after the sensor
3
supplies a signal—which is dependent on the magnitude and direction of the magnetic field in the magnet core
2
—to the driver circuit
5
, which signal drives a compensation current i
2
through the secondary winding
6
. The compensation current i
2
is oriented in such a way that its magnetic field compensates the magnetic flux in the magnet core
2
. The current in the secondary winding
6
is altered by the sensor
3
in connection with the evaluation circuit
4
, the driver circuit
5
, and the secondary winding
6
until the magnetic field in the magnet core
2
goes to zero. The current i
2
in the secondary winding
6
is thereby a measure for the momentary value of the current i
1
to be measured in the primary winding
1
, whereby both direct and alternating currents can be acquired. In addition, the current i
2
flows via a terminating resistance
7
at which the output voltage Ua of the current sensor falls off, which output voltage thereby corresponds in magnitude and phase position to the current i
1
to be measured in the primary winding
1
.
The maximum measurable current i
1max
of the compensation current sensor is thereby:
i
1max
=w
2
·(
U
V
−U
B
)/
R
1
+R
a
),
whereby w
2
is equal to the number of secondary windings, U
V
is equal to the supply voltage, U
B
is equal to the voltage drop in the driver stage
5
, R
i
is equal to the internal resistance of the secondary winding
6
and R
a
is equal to the resistance value of the terminating resistance
7
.
The known compensation current sensors operate predominantly with a terminating resistance that is connected in series with the secondary winding against a reference potential. By this means, for the compensation of a positive current a positive (or, respectively, negative) supply voltage is required, and for the compensation of a negative current a negative (or, respectively, positive) supply voltage is required. As a consequence, a bipolar voltage supply must be provided. If no such voltage supply is available, two bipolar voltages can be obtained from a unipolar voltage source by voltage halving; however, the individual voltage values thereof are thereby also halved. However, due to the halved voltage values the maximal measurable voltage is also reduced according to the above equation.
SUMMARY OF THE INVENTION
The object of the invention is to indicate a current sensor that operates according to the compensation principle, with a unipolar voltage supply, and that does not have these disadvantages.
According to the invention, the current sensor named above is developed in such a way that two amplifiers in a bridge circuit, which are controlled by the measurement value and which supply output signals in phase opposition to one another, are used as a driver circuit, whereby the series circuit of the secondary winding and terminating resistance is connected between the outputs of the amplifiers.
Preferably, two push-pull output stages operated in a bridge circuit are provided. The measurement value of the sensor is thereby supplied mediately or immediately to the two push-pull output stages that are operated in a bridge circuit. The outputs of the two push-pull output stages between which the series circuit of the secondary winding and the terminating resistance is connected thereby supply signals that are in phase opposition to one another. The phase opposition can for example be achieved in that both push-pull output stages are driven by the same signal, respectively representing the measurement value, whereby one of the push-pull output stages is constructed so as to be inverting and the other is non-inverting. Alternatively, both push-pull output stages can have transmission characteristics with the same phase, but can be driven in phase-opposed fashion. The advantage of this is that, given a unipolar supply voltage, both positive and negative compensation currents can be produced. It is true that the output voltage proportional to the current to be measured is not voltage to ground, but the further processing of what are called floating voltages is unproblematic with a difference amplifier.
In a development of the invention, it is provided that the amplifiers respectively have an operational amplifier, that the non-inverting inputs of the operational amplifiers are connected with a reference potential, and that the supply of voltage to the operational amplifiers respectively takes place with the intermediate connection of a first or, respectively, second resistance of a positive supply potential or, respectively, a reference potential. In addition, two output stage transistors are respectively provided whose base terminals are connected to the supply terminals of the respective operational amplifier, and are thus coupled with the supply potential or, respectively, with the reference potential via the first resistance or, respectively, the second resistance.
The emitter of each output stage transistor, which is of one conductivity type, is connected to the supply potential, and the respective other output stage transistor, which is of the other conductivity type, is connected to the reference potential. The collectors of both output stage transistors—forming the output of the respective amplifier, and thereby an output of the driver circuit—are thereby coupled with one another, whereby the secondary winding and the terminating resistance connected in series thereto are connected, on the one hand, between the coupled emitters of the output stage transistors of the one amplifier, and the coupled emitters of the output stage transistors of the other amplifier on the other hand. Finally, the inverting input of one of the operational amplifiers is connected to the output of the evaluation circuit, and the inverting input of the other operational amplifier is connected to the input or output of the other operational amplifier. The advantage is that by means of the quiescent current of the respective operational amplifier a base-emitter bias voltage already arises for the associated output stage transistors at the correspondingly dimensioned resistances. By this means, the takeover region is already strongly reduced, without a
Han Jessica
Hill & Simpson
Vacuumschmelze GmbH
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