A single-use system for separating blood and producing platelet concentrates includes an elongated container for receiving blood from a patient. The coil device may include: a case having a reception space and varying a charging area upon a user’s selection; and a coil part, having, in the reception space of the case, a coil body formed of a wound conductor having a predetermined length. A conduction path may be engaged when the input power is of the proper polarity and the output voltage does not match a desired value.