Secondary resonant
pickup coils (102) used in loosely coupled inductive power transfer systems, with resonating capacitors (902) have high Q and could support large circulating currents which may destroy components. A current limit or "
safety valve" uses an
inductor designed to enter saturation at predetermined resonating currents somewhat above normal working levels. Saturation is immediate and passive. The constant-current characteristic of a loosely coupled, controlled
pickup means that if the saturable section is shared by
coupling flux and by leakage flux, then on saturation the
current source is terminated in the saturated
inductor, and little detuning from
resonance occurs. Alternatively an external saturable
inductor (1101, 1102) may be introduced within the resonant circuit (102 and 902), to detune the circuit away from the
system frequency. Alternatively
DC current may be passed through a winding to increase saturation of a saturable part of a core. As a result, a fail-safe
pickup offering a
voltage-limited constant-current output is provided.