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Fiber Optics in Sensors and Contr ol Systems
                          3.19.2 Fiber-Optic Ammeter                                   189
                          In many applications, including the fusion reactors, radio frequency
                          systems, and telemetry systems, it is often necessary to measure the
                          magnitude and frequency of current flowing through a circuit in
                          which high DC voltages are present. A fiber-optic current monitor
                          (Fig. 3.70) has been developed at the Princeton Plasma Physics Labo-
                          ratory (PPPL) in response to a transient voltage breakdown problem
                          that caused failures of Hall-effect devices used in the Tokamak fusion
                          test reactor’s natural-beam heating systems.
                             The fiber-optic current monitor measures low current in a con-
                          ductor at a very high voltage. Typical voltages range between tens of
                          kilovolts and several hundred kilovolts. With a dead band of approx-
                          imately 3 mA, the circuit derives its power from the conductor being
                          measured and couples information to a (safe) area by means of fiber
                          optics. The frequency response is normally from direct current to
                          100 kHz, and a typical magnitude range is between 5 and 600 mA.
                             The system is composed of an inverting amplifier, a current regu-
                          lator, transorbs, diodes, resistors, and a fiber-optic cable. Around an
                          inverting amplifier, a light-emitting diode and a photodiode form an
                          optical closed feedback loop. A fraction of the light emitted by the
                          LED is coupled to the fiber-optic cable.
                             As the current flows through the first diode, it splits between the
                          1.5-mA current regulator and the sampling resistor. The voltage
                          across the sampling resistor causes a small current to flow into the
                          inverting amplifier summing junction and is proportional to the cur-
                          rent in the sampling resistor. Since photodiodes are quite linear, the
                          light power from the LED is proportional to the current through the
                          sampling resistor. The light is split between the local photodiode and
                          the fiber cable. A photodiode, located in a remote safe area, receives
                          light that is linearly proportional to the conductor current (for current
                          greater than 5 mA and less than 600 mA).
                             To protect against fault conditions, the design utilizes two back-to-
                          back transorbs in parallel with the monitor circuit. The transorbs are
                          rated for 400 A for 1 ms. The fiber-optic ammeter is an effective tool for
                          fusion research and other applications where high voltage is present.



                     Further Reading
                          Berwick, M., J.D.C. Jones, and D. A. Jackson, “Alternating Current Measurement
                             and Non-Invasive Data Ring Utilizing the Faraday Effect in a Closed Loop
                             Fiber Magnetometer,” Optics Lett., 12(294) (1987).
                          Cole, J. H., B. A. Danver, and J. A. Bucaro, “Synthetic Heterodyne Interferometric
                             Demodulation,” IEEE J. Quant. Electron., QE-18(684) (1982).
                          Dandridge, A., and A. B. Tveten, “Phase Compensation in Interferometric Fiber
                             Optic Sensors,” Optics Lett., 7(279) (1982).
                          Desforges, F. X., L. B. Jeunhomme, Ph. Graindorge, and G. L. Baudec, “Fiber Optic
                             Microswitch for Industrial Use,” presented at SPIE O-E Fiber Conf., San Diego,
                             no. 838–41 (1987).
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