All component comparisons
Fuse vs Resettable Fuse (PTC)
Fuse (glass/blade/SMD/thermal) and Resettable Fuse (PTC / Polyfuse) compared — what each is better at, and which one the job actually calls for.
A fuse opens once and is replaced, which is exactly why it is the definite protection: near-zero resistance in normal use and an unambiguous break on fault. A PTC polyfuse heats into a high-resistance state and recovers when the fault clears, which suits a port that gets abused repeatedly or sits somewhere nobody can reach — but it carries real series resistance, trips slowly and imprecisely, and never quite returns to its original resistance.
What each one is
Fuse (glass/blade/SMD/thermal)
A deliberate weak link that melts and breaks the circuit when current exceeds a safe level, protecting wiring and devices from fire and damage. One-shot (or resettable).
Resettable Fuse (PTC / Polyfuse)
A thermally-sensitive resistor that increases resistance dramatically when overheated, acting as a self-resetting circuit breaker.
Key specs
Fuse (glass/blade/SMD/thermal)
- Current rating(A)
- Continuous current it passes without blowing.
- Voltage rating(V)
- Max voltage it can safely interrupt (AC/DC).
- Speed(-)
- Fast-blow (F) vs slow-blow/time-delay (T).
- Breaking capacity(A)
- Max fault current it can safely clear.
Resettable Fuse (PTC / Polyfuse)
- 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.
Types you will meet
Fuse (glass/blade/SMD/thermal)
- Glass cartridge (5x20mm)
- Ceramic
- Automotive blade
- SMD chip fuse
- Resettable PTC (polyfuse)
- Thermal fuse
Resettable Fuse (PTC / Polyfuse)
- Radial lead
- Surface mount (SMD)
- Disc-shaped
- Flat ribbon
- High voltage
- High current
How to choose
Fuse (glass/blade/SMD/thermal)
Rate just above normal operating current (≈1.25-2x), with voltage >= circuit voltage and adequate breaking capacity. Slow-blow for inrush loads (motors, supplies); fast for sensitive electronics.
Resettable Fuse (PTC / Polyfuse)
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.
How each one fails
Fuse (glass/blade/SMD/thermal)
Blows (its job) on overcurrent; nuisance-blows if under-rated for inrush; corroded holder gives intermittent contact.
Resettable Fuse (PTC / Polyfuse)
Fails in high-resistance state (won't reset) if subjected to extreme overcurrent or repeated tripping. Physical damage or discoloration may occur.
Full reference for each part
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