Constant power dynamic focus coil

摘要

A dynamic focus coil produces different selected ampere-turns while consuming constant power and therefore maintaining constant temperature. The focus coil comprises a set of coils that all consume the same power. A selected subset of coils (e.g. two or three) are energized at any given time, the subset being selected to produce the desired focus adjustment. Illustratively, the coils all have the same resistance and are driven by current having the same magnitude and selectable polarity. An alternative embodiment contains a "coarse" set of coils having selected values of ampere turns and a "fine" set of coils having values of ampere turns that fill in between the values of the coarse set.