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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).