Low power circuit protection devices are important for making durable and long lasting electronic circuits. Resettable fuses, also called PPTC devices, are made of special PTC materials that maintain a low resistance up to a certain through current. Once that current level is reached, the PPTC begins to heat up and the resistance jumps to very high levels, stopping current flow and interrupting the circuit. The fuse then starts to cool and if the fault has been removed the circuit goes back to operating normally, but if the fault remains the PPTC settles at some high resistance that permits just enough current to heat the fuse and maintain the tripped state.
PPTC’s are handy for enclosed or sealed electronics where fuse replacement is too labor intensive or impossible. However, there is another option. N-channel depletion mosfets limit their drain to source current when the gate voltage drops to a certain level below the source voltage. Thus, by connecting the gate of a depletion mosfet to the other side of a source resistor, the transistor becomes increasingly more current limiting as the current from drain to source increases. Thus, the transistor permits current to flow freely until it reaches a trip level and then it clamps the current at the trip level. With their built in source to drain diode action, this transistor only permits current limitation in one direction. However by attaching the source of another n channel depletion mosfet on the other side of the resistor such that the current flows drain to source to resistor to source to drain, and the gates of the transistors are connected to the drains of the other transistor, a bidirectional automatic current limiting device can be created. Using this type of circuit protection requires special attention to how much power is being lost to heat in the transistors, which is a function of the voltage across the device. As such, voltage limiting circuitry may also be required for effective, reusable circuit protection.
