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

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.