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CASR-50-NP Datasheet(PDF) 3 Page - LEM |
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CASR-50-NP Datasheet(HTML) 3 Page - LEM |
3 / 24 page RS I Compensation VOUT +5V GND Fluxgate Interface Filter Driver Diff Amp VREF Int. ref ICOMP2 ICOMP1 IP Primary conductor Magnetic Core Fluxgate Compensation Winding 3 CAS / CASR / CKSR Transducers Technology: Closed Loop Fluxgate technology Closed Loop current transducers measure current over wide frequency ranges, including DC. They provide contact-free coupling to the current that needs to be measured as well as safe galvanic isolation and high reliability. Their output signal is an accurate, high-resolution image of the primary current with a very short delay. In higher frequency ranges these transducers function exactly the same way as (passive) current transformers, where a relatively small induced voltage in the secondary winding is capable to drive the secondary current through the secondary winding and, most important, through the burden resistor. A low induced voltage equals low magnetic flux in the magnetic core, which is the cause for the good accuracy (low flux means a small difference between primary and secondary current linkage1, too). For DC and in low-frequency ranges, the induced voltage is too low to be able to drive the secondary current, and the error of simple current transformers will increase with decreasing frequency. In this domain, the magnetic flux density in the core is measured by a sensing element and a voltage is applied to the secondary circuit that in the end keeps the flux density near zero, effectively creating a closed control loop. The only basic difference between the CAS / CASR / CKSR transducer series and standard Closed Loop transducers of LEM is that the Hall element used for feedback is replaced by a Fluxgate detector. The driving force behind this choice is the need for a “better” feedback, which basically means more voltage per current linkage, a quantity that is called “Open Loop sensitivity”. Given an equal electronic circuit, the zero output of a current transducer (traditionally called “offset” in analogy to operational amplifiers) will be less influenced by changes in the electronics (e.g. offset variations of the amplifiers used) if the Open Loop sensitivity is higher. The complexity of a Fluxgate based current transducer is comparable to the one of a transducer based on a Hall effect IC (integrated circuit). Like there, some AC signal processing and synchronous rectifying is applied. In addition, the Fluxgate detector is needed. Fortunately, this Fluxgate is a very simple small solenoid with a tiny soft magnetic strip used as detector core. Because of the complexity of the signal chain, an IC is used to stay at a competitive cost level compared to Hall effect current transducers. A circuit in this IC forms an oscillator together with the Fluxgate, driving it into saturation each half cycle at a frequency of several hundred kilohertz. The effect that is used for the detection of a residual flux in the main transducer core is the fact that in such a configuration a change of the duty cycle of the driving voltage will occur when a magnetic DC flux is present in the fluxgate core. The signal processing stages in the IC comprise a duty cycle demodulation, frequency response compensation, an integrator and a bridge amplifier that provides the secondary current. This output architecture can provide a higher (doubled) voltage to the secondary circuit when compared to a single output stage with the other side of the circuit connected to a reference potential at typically 2.5 V. In this configuration, the burden (or measurement) resistor is floating, so in order to obtain an output signal referenced to a fixed voltage, a difference amplifier is used which is also part of the IC. Fig. 1. Closed Loop Fluxgate Technology used for the CAS / CASR / CKSR current transducers 1 Current linkage is the technical term for current multiplied by turns count |
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