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Resettable fuse (PTC)

A polyfuse heats up and cuts the current when a short or overload pulls too much, then recovers when it cools. Typical on USB VBUS.

What it does#

A PTC (positive temperature coefficient) fuse is a few hundred mΩ in normal use. Overload heats it, its resistance jumps and the current drops to a trickle until the fault is gone and it cools. Two currents matter:

  • Hold current: carried indefinitely (500 mA for the catalog parts).
  • Trip current: guaranteed to trip, eventually (1 A).

Between the two it may or may not trip; at the trip current it can take seconds. It protects wiring and the host port, not delicate chips.

Example circuit: 5 V from a screw terminal through F1 (500 mA hold, 1 A trip) to the board's 5V rail.
5 V from a screw terminal through F1 (500 mA hold, 1 A trip) to the board's 5V rail. Open the fuse_input example from this page in PCB Canvas: “Insert into my schematic”.

Choosing#

  • Hold current above your normal peak current.
  • Voltage rating above the supply: the 1206 part is 16 V, the 0805 only 6 V (5 V rails only).

Pitfalls#

  • Voltage drop: ~0.1–0.3 V at the hold current; LED strips at 3 A need a much bigger fuse.
  • Slow. A PTC does not save a chip from a short on its own pins.

Footprint#

1206 (C70075, 16 V) and 0805 (C20978, 6 V), both JLCPCB extended parts, both easy to hand solder.