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A new principle for fast DC-sensitive potential-independent current comparators

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1 Author(s)
Marek, /A/. ; Brown Boveri Research Center, Switzerland

A new principle of operation makes feasible small, cheap potential-independent current comparators whose performance reaches or exceeds the best properties of bulky magnetic amplifiers of the second-harmonic type. Connection of a ferrite-core coil in parallel with a suitable negative resistance generates relaxation oscillations g(t) . Inherently, at large oscillations the coil ( i,\phi )- characteristic is perfectly symmetric about the origin: \phi(-i) = -\phi(i) . Provided that the negative-resistance ( i,u )- characteristic has the same symmetry: i(-u) = -i(u) , the oscillations g(t) are perfectly symmetrical in the sense that g(t+T/2) = -g(t) . Here g represents either the coil flux φ, the current i or the voltage u . The time is t and T the oscillation period. External magnetomotive force shifts the coil ( i,\phi )-characteristics, invalidating (1) and therefore (3). Consequently, the deviation from 50% duty-ratio of the oscillating q(t) is a sensitive and extremely stable measure of this MMF. Neither core temperature, pressure nor magnetic creep cause any zero drift. Orders of magnitude reached experimentally are: Short-time zero instability and noise: \leq 10^{-5} ampere-turn, time resolution: \leq 10^{-4} s, zero drift from -70 to +100°C: < 10^{-4} ampere-turn.

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Magnetics, IEEE Transactions on  (Volume:13 ,  Issue: 5 )