Current Probes, Sensors | Non-contact voltage probe,CAN sensors Principle of Current Sensors

This section describes the principles behind current sensor operation. You should choose a current sensor to suit the application in which you plan to use it, based on an understanding of the differences among the various methods of measuring currents, including the Hall element method, the Rogowski method, and the zero-flux method.

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01. Current sensor measurement principles

Measurement principles: CT current sensors

CT current sensors use the principle of converting the current being measured to a secondary current that is proportional to the turns ratio. 

Measurement principle:
An AC current that is proportional to the turns ratio flows in the secondary-side winding so as to cancel out the magnetic flux Φ produced in the magnetic core by the AC current flowing in the conductor being measured (the primary side of the circuit). 
This secondary current flows to the shunt resistor, producing a voltage across its terminals. This voltage is the measurement circuit’s output, which is proportional to the current flowing in the conductor being measured.

Features compared to other current measurement methods:
The CT method can only measure AC current. (It cannot measure DC current.)
CT current sensors are inexpensive. 
The CT method is used primarily with commercial power frequencies of 50/60 Hz. 
The CT method is used in instruments such as clamp power meters used in managing energy savings in buildings, among other applications. 
Since sensor operation depends on canceling out magnetic flux (a type of negative feedback operation), it is characterized by generally good linearity.

Corresponding Hioki current sensors (model numbers):

9675, 9657-10, 9661-01, 9695-03, 9695-02, 9694, 9669, 9661, 9660, 9132-50, 9018-50, 9010-50, 9650, 9651, 9298, 9291, etc.
*See individual product pages for more detailed specifications. 


 

Measurement principles: Hall element current sensors

Hall element current sensors use the principle of converting the magnetic field produced around the current being measured to voltage using the Hall effect.

Measurement principle:
When the magnetic flux Φ produced inside the magnetic core by the current flowing in the conductor being measured (the primary side of the circuit) passes through the Hall element that has been inserted into the gap in the magnetic core, the Hall effect results in a Hall voltage that varies in proportion to the magnetic flux.
Since this Hall voltage is small, it must be amplified by an amp to produce the output signal.
This output signal is proportional to the current flowing in the conductor being measured.

Features compared to other current measurement methods:
The Hall element method can measure DC as well as AC current (up to several kilohertz).
Hall element current sensors are inexpensive.
Due to the effects of the Hall element’s linearity and the magnetic core’s B-H characteristics, Hall element sensors do not generally have good precision.
Due to the characteristics of the Hall element, sensor operation is subject to drift caused by factors including temperature and variations over time, making the devices poorly suited to long-term measurement.
Because the magnetic core acts as a load, Hall element sensors cannot be used in the high-frequency band.

Corresponding Hioki current sensors (model numbers):
CT7636, CT7631, CT7642, CT7731, CT7736, CT7742, etc.

*The above products feature improved drift and precision.
*See individual product pages for more detailed specifications. 

 

Measurement principle: Rogowski coil current sensors

Rogowski coil current sensors make measurements by converting the voltage that is induced in the air-core coil by the AC magnetic field produced around the current being measured. 

Measurement principle:
A voltage is induced in the air-core coil by interlinking the magnetic field produced by the AC current flowing in the conductor being measured (the primary side of the circuit) and the air-core coil.
Since this induced voltage becomes the time differential value (di/dt) for the current being measured, it can be passed through an integrator to produce an output signal that is proportional to the current being measured.

Features compared to other current measurement methods:
Since there is no magnetic core, Rogowski coil current sensors can measure large currents without experiencing magnetic saturation. 
There is no heating, saturation, or hysteresis caused by magnetic loss.
Since the air-core coil serves as the sensor, the sensor can be thin and flexible. 
Rogowski coil current sensors have low impedance.
Rogowski coil current sensors can only measure AC current. They cannot measure DC current.
Since measurement precision is affected by the cross-sectional area and length of the air-core coil, Rogowski coil current sensors are susceptible to the effects of conductor position and external noise, making them poorly suited to high-precision measurement.
The coreless design of Rogowski coil current sensors makes it difficult for them to measure small currents of 10 A or less.

Corresponding Hioki current sensors (model numbers):
CT7046, CT7045, CT7044, CT9667-01, CT9667-02, CT9667-03, etc.
*The above products feature improved noise resistance.
*See individual product pages for more detailed specifications. 

 

Measurement principles: AC zero-flux (winding detection type) current sensors

AC zero-flux (winding detection type) current sensors improve on the low-frequency characteristics of the CT method.

Measurement principle:
A secondary current that is proportional to the turns ratio of the feedback winding on the secondary side of the circuit flows so as to cancel out the magnetic flux Φ produced inside the magnetic core by the AC current flowing in the conductor being measured (the primary side of the circuit). 
In the low-frequency region, the magnetic flux cannot all be canceled out, and some remains. 
The magnetic flux that remains is detected by the detection winding, and a secondary current is made to flow by the amp circuit so as to cancel out the magnetic flux Φ.
This secondary current flows to the shunt resistor, producing a voltage across its terminals.
This voltage is the measurement circuit’s output, which is proportional to the current flowing in the conductor being measured.

Features compared to other current measurement methods:
Since sensor operation depends on canceling out the magnetic flux in the magnetic core (a type of negative feedback operation), AC zero-flux sensors have excellent linearity and are not affected by the magnetic core’s B-H characteristics.
AC zero-flux sensors are well suited for use in power measurement since they are characterized by small phase error, even at low frequencies.
AC zero-flux sensors have low insertion impedance since they are characterized by low operating magnetic flux levels. 
Since AC zero-flux sensors operate as CT sensors in the high-frequency region, they are able to deliver a broad frequency band. 
Since detection is performed by a winding, AC zero-flux sensors can measure only AC current. They cannot measure DC current. 

Corresponding current sensors (models):
9272-10, etc.
*See individual product pages for more detailed specifications. 

 

Measurement principles: AC/DC zero-flux (Hall element detection type) current sensors

AC/DC zero-flux (Hall element detection type) current sensors are able to measure DC current as well because they combine the CT method with a Hall element.

Measurement principle:
A secondary current that is proportional to the turns ratio in the feedback winding on the secondary side of the circuit flows so as to cancel out the magnetic flux Φ produced in the magnetic core by the AC current flowing in the conductor being measured (the primary side of the circuit). 
In the low-frequency region starting at DC, the magnetic flux cannot all be canceled out, and some remains. 
The magnetic flux that remains is detected by the Hall element, and a secondary current is made to flow by the amp circuit so as to cancel out the magnetic flux Φ.
This secondary current flows to the shunt resistor, producing a voltage across its terminals.
This voltage is the measurement circuit’s output, which is proportional to the current flowing in the conductor being measured.

Features compared to other current measurement methods:
This type of AC/DC zero-flux current sensor delivers excellent linearity and maintains high precision down to low-level currents.
This type of AC/DC zero-flux current sensor also delivers a high S/N ratio across a broad frequency band.
This type of AC/DC zero-flux current sensor has low insertion impedance since it is characterized by low operating magnetic flux levels. 
Since this type of AC/DC zero-flux current sensor functions as a CT in the high-frequency region, it can deliver a broad frequency band. 
Since detection is performed by a Hall element, this type of AC/DC zero-flux current sensor can detect magnetism produced by DC current, and it can measure DC current.

Corresponding Hioki current sensors (model numbers):
3273-50, 3274, 3275, 3276, CT6700, CT6701, etc.
*See individual product pages for more detailed specifications. 

 

Measurement principles: AC/DC zero-flux (flux gate detection type) current sensors

AC/DC zero-flux (flux gate detection type) current sensors are able to measure DC current as well because they combine the CT method with a flux gate (FG) element.

Measurement principle:
A secondary current that is proportional to the turns ratio in the feedback winding on the secondary side of the circuit flows so as to cancel out the magnetic flux Φ produced in the magnetic core by the AC current flowing in the conductor being measured (the primary side of the circuit). 
In the low-frequency region starting at DC, the magnetic flux cannot all be canceled out, and some remains. 
The magnetic flux that remains is detected by the FG element, and a secondary current is made to flow by the amp circuit so as to cancel out the magnetic flux Φ.
This secondary current flows to the shunt resistor, producing a voltage across its terminals.
This voltage is the measurement circuit’s output, which is proportional to the current flowing in the conductor being measured.

Features compared to other current measurement methods:
This type of AC/DC zero-flux current sensor delivers excellent linearity and maintains high precision down to low-level currents.
This type of AC/DC zero-flux current sensor has low insertion impedance since it is characterized by low operating magnetic flux levels. 
Since the FG element that detects DC current has an extremely small offset across a broad temperature range for reasons related to its operating principle, the sensor is able to deliver high accuracy and high stability.

Corresponding Hioki current sensors (model number):
CT6841, CT6843, CT6844, CT6845, CT6846, CT6862, CT6863, CT6865, 9709, etc.
*See individual product pages for more detailed specifications.