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Multiple-wire ac corona chargers are designed to charge film to a final voltage which is well regulated with respect to parameter variations. These low-sensitivity designs are optimal in the sense that they minimize a performance index which penalizes high dc bias voltages. To obtain low sensitivity to film capacitance, velocity, and charging efficiency, the film must be charged so that its surface voltage "overshoots" as it moves past the charger. For low sensitivity to film leakage resistance, however, an "undershooting" response is required. In experimental tests the optimally designed chargers perform nearly as expected.