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What is the difference between closed loop and open loop ?

Open Loop Hall Effect current transducers

A current carrying conductor creates a magnetic field. This field is concentrated by a magnetic core. The core has a gap cut through it and a hall generator is used to sense the magnetic flux density in the gap. The control current, IC, and Differential amplification are supplied by electronics (Fig. 1 built into the transducer).

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Fig 1: Conversion of the primary current into an output voltage.

Closed loop Hall Effect current transducers

Compared to the open loop transducer, Hall Effect closed loop transducers (also called Hall Effect ‘compensated’ or ‘zero flux’ transducers) have a compensation circuit that dramatically improves performance.  While open loop current transducers amplify the Hall generator voltage to provide an output voltage, closed loop transducers use the Hall generator voltage to create a compensation current (Fig. 2) in a secondary coil to create a total flux, as measured by the Hall generator, equal to zero. In other words, the secondary current, IS, creates a flux equal in amplitude, but opposite in direction, to the flux created by the primary current.

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Fig 2: Operating principle of the closed loop transducer

Operating the Hall generator in a zero flux condition eliminates the drift of gain with temperature. An additional advantage to this configuration is that the secondary winding will act as a current transformer at higher frequencies, significantly extending the bandwidth and reducing the response time of the transducer. When the magnetic flux is fully compensated (zero), the magnetic potential (ampere-turns) of the two coils are identical. Hence: NP • IP = NS • IS which can also be written as IS = IP • NP / NS.  Consequently, the secondary current, IS, is the exact image of the primary current, IP, being measured. Inserting a „measurement resistor“, RM, in series with the secondary coil (Fig. 2) creates an output voltage that is an exact image of the measured current.

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