Current sensors are devices used to measure the flow of current in an electric circuit. They can also be referred to as current transducers or current sense transformers. They are available in a variety of types for measuring AC and DC current flows, including: Hall effect, Rogowski coils and transformers. Current sensors do not have a direct electrical connection to the current being monitored allowing for galvanic isolation to the measurement circuit.
Current sensors operate in either open or closed-loop circuits. Open-loop sensor circuits are typically lower cost than closed-loop alternatives but less accurate. Open-loop sensors use the detection voltage to directly obtain the current flow. This can lead to inaccuracies caused by non-linearity in the sensor, saturation effects and temperature drift. Closed-loop sensors use the detection voltage from the primary coil to feed an opposing current into secondary coil wound on the same core. The current is adjusted by the feedback loop so as to balance out the flux generated by the primary loop current. Balancing in this way eliminates the effects of temperature drift and saturation and allows for more accurate measurements to be made through monitoring the secondary current. The added complexity leads to higher cost. Closed-loop detectors are also referred to as zero flux sensors or null detectors.
A Hall effect sensor detects magnetic flux caused by the current in the conductor being measured inducing a current flow in a sensing coil. The device is placed in a gap in the sensing coil core and outputs a Hall voltage proportionate to the coils current flow in the following relationship:
Vh = Rh * (I/t * B)
VH is the Hall voltage in volts
RH is the Hall effect co-efficient
I is the current flow through the sensor in amps
t is the thickness of the sensor in mm
B is the magnetic flux density in Teslas
The output signal for linear (analogue) sensors is taken directly from the output of an operational amplifier attached to the sensor. An open-loop system uses the Hall effect voltage output to directly measure current. A closed-loop system will convert the Hall effect voltage into a current driving the secondary coil to create a flux balance as previously described.
A Rogowski coil is a special coreless helical coil wrapped around a straight wire conductor returning from the end of the coil itself that encircles the current carrying conductor. Current is derived by integrating the induced voltage across the wires ends of the assembly. They are a low cost way to measure AC. They can measure high speed current pulses due to their low inductance. They are also capable of measuring high currents due to not having a core that would otherwise saturate. Special ADCs are available with integrators to measure this value. Rogowski coil sensors are used in applications like: high performance welding systems, short circuit testing and high current metrology.
Shunt resistors can be placed in series with a power supply to sense current flow. Current flowing through a shunt will create a voltage potential directly proportional to the product of the current and the shunt resistance. This can be tapped off and measured by an analog to digital converter (ADC). Special accurate, low impedance (mOhms) shunt resistors are available for this function.
Transformer current sensors (also called current clamp meters) are used to detect and measure AC. They are typically used in applications like test equipment, where a split ferrite or soft iron ring encircles a conductor with a current to be measured. Wire is coiled around the core material creating a current transformer with the single wire it encircles. These are excellent for high currents but are not very good for measuring small alternating currents.
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