A coil arrangement supplies energy or control signals to, or provides information from, a medical device implanted in a human or animal patient. The coil arrangement includes an external coil that is larger than an implanted coil so as to reduce the risk of damage to circuitry in an energy transmitter attached to the external coil that results from a low impedance at the external coil when a switch in an over-voltage protection circuit is opened to protect circuitry in a medical device attached to the implanted coil from being damaged by a too high voltage induced across the implanted coil.
