In electrical engineering, current sensing
is any one of several techniques used to measure
electric current. The measurement of current
ranges from picoamps to tens of thousands
of amperes. The selection of a current sensing
method depends on requirements such as magnitude,
accuracy, bandwidth, robustness, cost, isolation
or size. The current value may be directly
displayed by an instrument, or converted to
digital form for use by a monitoring or control
system.
Current sensing techniques include shunt resistor,
current transformers and Rogowski coils, magnetic-field
based transducers and others.
== Requirements in current measurement ==
Current sensing technologies must fulfill
various requirements, for various applications.
Generally, the common requirements are:
High sensitivity
High accuracy and linearity
Wide bandwidth
DC and AC measurement
Low temperature drift
Interference rejection
IC packaging
Low power consumption
Low price
== 
Current Sensing Techniques ==
The measurement of the electric current can
be classified depending upon the underlying
fundamental physical principles such as,
Ohm's Law
Faraday's Law of Induction
Magnetic field sensors
Faraday Effect
== 
Shunt resistors ==
Ohm's Law is the observation that the voltage
drop across a resistor is proportional to
the current going through it.
This relationship can be used to sense currents.
Sensors based on this simple relationship
are well known for their lower costs, and
reliability due to this simple principle.
The common and simple approach to current
sensing is the use of a shunt resistor. The
voltage drop across the shunt is proportional
to its current flow. Both alternating currents
(AC) and direct currents (DC) can be measured
with the shunt resistor.The high performance
coaxial shunt have been widely used for many
applications fast rise-time transient currents
and high amplitudes but, highly integrated
electronic devices prefer low-cost surface
mounted devices (SMDs), because of their small
sizes and relatively low prices.
The parasitic inductance present in the shunt
affects high precision current measurement.
Although this affects only the magnitude of
the impedance at relatively high frequency,
but also its effect on the phase at line frequency
causes a noticeable error at a low power factor.
The low cost and high reliability make the
low resistance current shunt a very popular
choice for current measurement system. The
major disadvantage of using the shunt is that
fundamentally a shunt is a resistive element,
the power loss is thus proportional to the
square of the current passing through it and
consequently it is a rarity amongst high current
measurements.
Fast-response for measuring high-impulse or
heavy-surge currents is the common requirement
for shunt resistors. In 1981 Malewski, designed
a circuit to eliminate the skin effect and
later in 1999 the flap-strap sandwich shunt
(FSSS) was introduced from a flat-strap sandwich
resistor. The properties of the FSSS in terms
of response time, power loss and frequency
characteristics, are the same as the shunt
resistor but the cost is lower and the construction
technique is less sophisticated, compared
to Malewski and the coaxial shunt.
=== Trace Resistance sensing ===
The intrinsic resistance of a conducting element,
usually a copper trace in Printed circuit
Board(PCB) can be used as sensing element
instead of a shunt resistor. Since no additional
resistor is required this approach promises
a low-cost and space saving configuration
with no additional power losses either. Naturally,
the voltage drop of a copper trace is very
low due to its very low resistance, making
the presence of a high gain amplifier mandatory
in order to get a useful signal.
There are several physical effects which may
alter the current measurement process: thermal
drift of the copper trace, initial conditions
of the trace resistance etc. Therefore, this
approach is not suitable for applications
that require a reasonable accuracy due to
the large thermal drift. In order to overcome
the problems associated with the temperature
drift, a digital controller can be used for
thermal drift compensation and calibration
of the copper trace.
A significant drawback of this kind of current
sensor is the unavoidable electrical connection
between the current to be measured and the
sense circuit. By employing a so-called isolation
amplifier, electrical isolation can be added.
However, these amplifiers are expensive and
can also deteriorate the bandwidth, accuracy
and thermal drift of the original current
sensing technique. For these reasons, current
sensing techniques based on physical principles
that provide intrinsic electrical isolation
deliver a better performance at lower costs
in applications where isolation is required.
== Current sensor based on Faraday's Law ==
Faraday's Law of induction – that states:
the total electromotive force induced in a
closed circuit is proportional to the time
rate of change of the total magnetic flux
linking the circuit – has been largely employed
in current sensing techniques. Two major sensing
devices based on Faraday’s law are Current
transformers (CTs) and Rogowski coils. These
sensors provide an intrinsic electrical isolation
between the current to be measured and the
output signal, thus making these current sensing
devices mandatory, where safety standards
demand electrical isolation.
=== Current transformer ===
The CT is based on the principle of a transformer
and converts a high primary current into a
smaller secondary current and is common among
high AC current measurement system. As this
device is a passive device, no extra driving
circuitry is needed in its implementation.
Another major advantage is that it can measure
very high current while consuming little power.
The disadvantage of the CT is that a very
high primary current or a substantial DC component
in the current can saturate the ferrite material
used in the core ultimately corrupting the
signal. Another problem is that once the core
is magnetized, it will contain hysteresis
and the accuracy will degrade unless it is
demagnetized again.
=== Rogowski coil ===
Rogowski coil is based on the principle of
Faraday’s law of induction and the output
voltage Vout of the Rogowski coil is determined
by integrating the current Ic to be measured.
It is given by,
V
o
u
t
=
−
k
N
A
μ
0
2
π
r
I
c
+
v
o
u
t
(
0
)
{\displaystyle V_{\rm {out}}=-k{\frac {NA\mu
_{0}}{2\pi r}}I_{\rm {c}}+v_{\rm {out}}(0)}
where A is the cross-sectional area of the
coil N is the number of turns,
The Rogowski coil has a low sensitivity and
is due to the absence of a high permeability
magnetic core, that the current transformer
can take advantage of. However, this can be
compensated by adding more turns on the Rogowski
coil or using an integrator with a higher
gain k. More turns increase the self-capacitance
and self-inductance, and higher integrator
gain means an amplifier with a large gain-bandwidth
product. As always in engineering, trade-offs
must be made depending on specific applications.
== Magnetic field sensors ==
=== Hall effect ===
Hall effect sensors are devices based on the
Hall-effect, which was discovered by Edwin
Hall in 1879 based on the physical principle
of the Lorentz force. They are activated by
an external magnetic field. In this generalized
device, the Hall sensor senses the magnetic
field produced by the magnetic system. This
system responds to the quantity to be sensed
(current, temperature, position, velocity,
etc.) through the input interface. The Hall
element is the basic magnetic field sensor.
It requires signal conditioning to make the
output usable for most applications. The signal
conditioning electronics needed are an amplifier
stage and temperature compensation. Voltage
regulation is needed when operating from an
unregulated supply. If the Hall voltage is
measured when no magnetic field is present,
the output should be zero. However, if voltage
at each output terminal is measured with respect
to ground, a non-zero voltage will appear.
This is the common mode voltage (CMV), and
is the same at each output terminal. The output
interface then converts the electrical signal
from the Hall sensor; the Hall voltage: a
signal that is significant to the application
context. The Hall voltage is a low level signal
on the order of 30 μvolts in the presence
of one gauss magnetic field. This low-level
output requires an amplifier with low noise,
high input impedance and moderate gain. A
differential amplifier with these characteristics
can be readily integrated with the Hall element
using standard bipolar transistor technology.
Temperature compensation is also easily integrated.
=== Flux gate sensors ===
Flux gate sensors or Saturable inductor current
sensors work on the same measurement principle
as Hall-effect-based current sensors: the
magnetic field created by the primary current
to be measured is detected by a specific sensing
element. The design of the saturable inductor
current sensor is similar to that of a closed-loop
Hall-effect current sensor; the only difference
is that this method uses the saturable inductor
instead of the Hall-effect sensor in the air
gap.
Saturable inductor current sensor is based
on the detection of an inductance change.
The saturable inductor is made of small and
thin magnetic core wound with a coil around
it. The saturable inductor operates into its
saturation region. It is designed in such
a way that the external and internal flux
density will affect its saturation level.
Change in the saturation level of a saturable
inductor will alter core’s permeability
and, consequently, its inductance L. The value
of saturable inductance (L) is high at low
currents (based on the permeability of the
core) and low at high currents (the core permeability
becomes unity when saturated). Fluxgate detectors
rely on the property of many magnetic materials
to exhibit a non-linear relationship between
the magnetic field strength H and the flux
density B.In this technique, high frequency
performance is achieved by using two cores
without air gaps. One of the two main cores
is used to create a saturable inductor and
the other is used to create a high frequency
transformer effect. In another approach, three
cores can be used without air gap. Two of
the three cores are used to create saturable
inductor, and the third core is used to create
a high frequency transformer effect. Advantages
of saturable inductor sensors include high
resolution, high accuracy, low offset and
gain drift, and large bandwidth (up to 500
kHz). Drawbacks of saturable inductor technologies
include limited bandwidth for simpler design,
relatively high secondary power consumption,
and risk of current or voltage noise injection
into the primary conductor.
=== Magneto-resistive current sensor ===
A magneto-resistor (MR) is a two terminal
device which changes its resistance parabolically
with applied magnetic field. This variation
of the resistance of MR due to the magnetic
field is known as the Magnetoresistive Effect.
It is possible to build structures in which
the electrical resistance varies as a function
of applied magnetic field. These structures
can be used as magnetic sensors. Normally
these resistors are assembled in a bridge
configuration to compensate for thermal drift.
Popular magneto resistance-based sensors are:
Anisotropic Magneto Resistance (AMR), Giant
Magneto Resistance (GMR), Giant Magneto Impendence
(GMI) and Tunnel Magneto Resistance (TMR).
All these MR-based sensors have higher sensitivity
compared to Hall-effect sensors. Despite this,
these sensors (GMR, CMR, and TMR) are still
more expensive than Hall-effect devices, have
serious drawbacks related with nonlinear behavior,
distinct thermal drift, and a very strong
external field can permanently alter the sensor
behavior (GMR). GMI and TMR sensors are even
more sensitive than GMR based sensors, but
still in the test phase and no commercial
products are available as of 2016-06.
== See also ==
Ammeter
Current sensor
Electric current
Electrical measurements
History of electrical engineering
