Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Magnetic saturation
Reexamination Certificate
2003-08-26
2004-12-28
Karlsen, Ernest (Department: 2829)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Magnetic saturation
C324S126000
Reexamination Certificate
active
06836107
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to AC coupling circuits and more specifically to a constant input impedance AC coupling circuit for a current measurement probe.
Current probes measure the flux field generated by the movement of electrons through a conductor. The flux field surrounding the conductor is converted to a linear voltage output that can be displayed and analyzed on a measurement test instrument, such as an oscilloscope. One type of current probe is an AC only probe. AC only probes are configured with either a solid core or a split core and are passive devices that do not require external power. AC/DC current probes generally have a split core configuration and include a Hall Effect device for producing a voltage output in response to a DC generated flux field.
FIG. 1
illustrates a simplified AC/DC current probe system
10
based on the A6312 current probe, the AM503B Programmable Current Probe Amplifier and TM500 Power Module manufactured and sold by Tektronix Inc., Beaverton Oreg. The TM500 Power Module provides electrical power to the AM503B Programmable Current Probe Amplifier. As shown in
FIG. 1
, the current probe
12
has a split core
14
of magnetic material defining an aperture
16
through which a conductor
18
carrying a current to be measured extends. A multi-turn winding
20
is wrapped around one leg of the core
14
. A thin film semiconductor Hall Effect device
22
is disposed within the magnetic core
14
. A bias source
24
housed in the current probe amplifier
26
provides power for the Hall Effect device
22
via a multi-conductor cable
28
. The Hall Effect device
22
provides a differential input signal to a Hall pre-amplifier
30
in the current probe amplifier
26
via the multi-conductor cable
28
. The output of the Hall pre-amplifier
30
is applied to a power amplifier
32
that is provided with a feedback resistor
34
. The output of the power amplifier
32
is connected via the multi-conductor cable
28
to one end of the multi-turn winding
20
and the opposite end of the winding
20
is connected via the multi-conductor cable
28
to a low input impedance scaling circuit
36
. The input to the scaling circuit
36
is terminated by resistor
38
having a value of 25 ohms. An AC/DC switching circuit
40
is positioned between the non-inverting input terminal of a differential scaling amplifier
42
and the terminating-load resistor
38
. The switching circuit
40
selectively couples an AC coupling capacitor
44
into the input line of the scaling amplifier
42
. The output of the scaling amplifier
42
is into a 50 ohm environment which is coupled via a coaxial cable
46
to the 50 ohm input resistor
51
of the measurement test instrument
48
, such as an oscilloscope. The front panel
50
of the current probe amplifier
26
includes buttons, knob, LEDs, numerical readout and input and output connectors for controlling the operation of the amplifier and coupling the current probe
12
and measurement test instrument
48
to the amplifier
26
. Depressing the appropriate buttons on the current probe amplifier
26
apply signals to a controller
52
that selectively couple the DC or AC signal path the input of the scaling amplifier
42
and generates a digital output to a digital-to-analog converter
54
to vary the gain of the scaling amplifier
42
.
The oscilloscope is set to DC coupling and 10 millivolts per division scale and coupled to the current probe amplifier
26
via the coaxial cable
46
. The current probe
12
is coupled to the current probe amplifier
26
via the multi-conductor cable
28
. An operator selects AC or DC coupling and the gain for the scaling amplifier
42
using the front panel
50
controls. The gain of the scaling amplifier
42
varies in a 1-2-5 sequence from 1 to 500 and is displayed on the numerical readout as current per division. The current carrying conductor is inserted through the aperture
16
of the split magnetic core
14
. The high frequency component of the current in the primary conductor
18
results in a current being induced in the secondary winding
20
in a direction such as to generate a magnetic field in the core
14
that is opposed to the field created by the current in the primary conductor
18
. The low frequency or DC component of the current in the primary conductor
18
is less effective at inducing current in the secondary winding
20
, but generates a potential difference across the Hall Effect device
22
, and the amplifier
32
provides a corresponding current in the winding
20
. The direction of the current supplied by the amplifier
32
is such that the magnetic field created in the core by the current flowing through the winding
20
is opposite to the direction of the magnetic field created by the current in the primary conductor
18
. Over a wide range of frequencies, the voltage developed across the load resistor
38
is representative of the current in the primary winding
20
.
The voltage developed across the load resistor is coupled to the high impedance input of the scaling amplifier
42
. The scaling amplifier
42
amplifiers the input voltage by the amount of gain set by the operator. The output signal of the amplifier
42
is coupled to the low input impedance input of the oscilloscope. The oscilloscope processes the signal from the current probe amplifier
26
and produces a trace on the oscilloscope display representing the current signal in the primary conductor
18
. To determine the amplitude of the current signal, an operation estimates the amount of vertical deflection of the signal in vertical divisions of the oscilloscope, for example 1.5 divisions. The vertical division number is divided by scale setting of the oscilloscope (i.e. 10 mv/div) and multiplied by the current per division setting of the current probe amplifier
26
(e.g. 20 ma/div) to produce the amount of current flowing through the primary conductor
18
.
What is needed is a current probe amplifier that allows the scaling circuitry of the measurement instrument to provide the current per division scaling for current measurements. This requires a current probe amplifier that couples the current output of the current probe directly into the low input impedance input of the measurement instrument while maintaining a constant input impedance.
SUMMARY OF THE INVENTION
Accordingly, the present invention is a constant input impedance AC coupling circuit for a current probe measurement system. The current probe measurement system has a current measurement probe generating a current output signal via transformer action with a current carrying signal conductor and a Hall Effect device disposed in the core of the transformer providing a DC or low component of the current carrying signal conductor. The constant input impedance AC coupling circuit couples the current output signal from the current measurement probe to a resistive terminating element of a low input impedance measurement instrument. The constant input impedance AC coupling circuit has a capacitor coupling the current output signal of the current measurement probe to the low input impedance measurement instrument. The capacitor forms part of a resistive-capacitive network that includes the resistive terminating element. The resistive-capacitive network has a low frequency cutoff, typically less than 10 hertz, and a RC time constant. A resistive-inductive network is coupled to the resistive-capacitive network and receives the current output signal from the current measurement probe for terminating DC and low frequency signal components of the current output signal below the low frequency cutoff of the resistive-capacitive network in the same low input impedance of the measurement instrument. The resistive-inductive network provides a current path for shunting the DC and low frequency signal components to prevent transformer saturation of the current measurement probe. The resistive-inductive network has a synthesized inductor with a high inductive value, large current carrying capacity and an L/R t
Bucher William K.
Karlsen Ernest
Tektronix Inc.
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