Resettable Fuse (PTC / Polyfuse)
Fuse
A thermally-sensitive resistor that increases resistance dramatically when overheated, acting as a self-resetting circuit breaker.
Schematic symbol
Fuse
How resettable fuse (ptc / polyfuse) appears in a circuit diagram.
Types & variants
Key specs
Hold current
Ampere (A)Max current it can pass without tripping.
Trip current
Ampere (A)Current at which it switches to high-resistance state.
Max voltage
Volt (V)Maximum voltage it can safely interrupt.
Resistance
Ohm (Ω)Low resistance in normal state, high resistance when tripped.
Time-to-trip
Seconds (s)How quickly it responds to overcurrent.
Laws that govern it
- Laws of resistanceR = ρ × L / A
A PTC polymer's resistivity leaps by orders of magnitude at a transition temperature — R = ρL/A with ρ as the switch.
- Joule's law of heatingH = I² × R × t
Self-heating from I²R is what trips it, and why hold current falls as ambient temperature rises.
- Ohm's lawV = I × R
It never fully opens: it settles at high resistance passing a trickle, which is exactly the leakage that keeps it latched until you remove power.
Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.
How it compares
- Fuse vs Resettable Fuse (PTC)
Side by side with Fuse (glass/blade/SMD/thermal) — specs, variants and which one the job calls for.
Markings
Usually marked with hold current (e.g., '250' = 0.25A) or trip current. SMD parts may have numeric codes or markings for current rating.
Standard values
Common hold currents: 0.1A, 0.15A, 0.2A, 0.25A, 0.3A, 0.5A, 1A, 2A, 3A, 5A. Voltage ratings: 6V, 12V, 24V, 30V, 60V, 250V.
How to choose
1) Determine normal operating current. 2) Select hold current 20-50% above normal. 3) Ensure trip current is below max circuit current. 4) Match voltage rating to circuit.
Pinout & package
2 terminals (non-polar). Packages: axial (through-hole), radial lead, SMD (0805, 1206, 1812, etc.).
Example circuits
- Power protection for USB ports
- Battery protection circuits
- Motor overload protection
- Surge protection for power supplies
- Protection for sensitive electronics
Common failures
Fails in high-resistance state (won't reset) if subjected to extreme overcurrent or repeated tripping. Physical damage or discoloration may occur.
How to test
Measure resistance with multimeter (should be low in normal state). Simulate overcurrent by applying voltage through current-limited source; resistance should increase significantly. Cool down and recheck for reset.
Substitutes
Conventional fuse (but not resettable), electronic circuit breaker (more complex, controllable), current-limiting IC.