Abstract: A circuit is described which maintains the current flowing toward a main circuit within a prescribed limit, regardless of instantaneous voltage or current changes. The circuit includes elements that maintain the current flow in a path toward a main circuit within a prescribed limit by selectively changing the impedance presented in response to any instantaneous change in the voltage flowing toward the main circuit and by introducing into the path a voltage which opposes any such instantaneous change in the current flowing toward the main circuit. Elements may also be provided to protect the main circuit by disconnecting same from a cable leading thereto after signals on the cable have been analyzed and a determination has been made that a signal outside of predetermined upper and lower limit values has been coupled to the cable, for example, from a high altitude electromagnetic pulse.