Two-terminal transistor PTC circuit protection devices

A two-terminal, Darlington bipolar transistor-PTC circuit protection device for protecting an electrical load against a predetermined overcurrent within a series network including the two-terminal device, the load and a direct-current power supply, the device comprising:

• a hybrid package having a first terminal and a second terminal, one said terminal being connected to the load and the other said terminal being connected to the power supply;
• a positive temperature coefficient (PTC) element within the hybrid package and having first and second ends and having a trip current and a trip state high resistance;
• and
• a Darlington bipolar transistor circuit within the hybrid package, selected to provide a pass current sufficient to pass said predetermined overcurrent through said two-terminal device, having a base terminal electrically connected to the first end of the PTC element, a collector terminal connected to the second end of the PTC element and to the first terminal of the hybrid package so that base bias current flows from the collector terminal to the base terminal through the PTC element, and an emitter terminal connected to the second terminal of the hybrid package;
• wherein said PTC element has a trip current selected to correspond to a base current of said Darlington bipolar transistor circuit resulting when said predetermined overcurrent passes through said two-terminal device and wherein said trip state high resistance reduces said base current and said pass current to a level significantly below said overcurrent such that said Darlington bipolar transistor circuit effectively amplifies a pass current transfer characteristic of the PTC element.