Fuse (glass/blade/SMD/thermal)
Fuse
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).
Schematic symbol
Fuse
How fuse (glass/blade/smd/thermal) appears in a circuit diagram.
Types & variants
Key specs
Current rating
AContinuous current it passes without blowing.
Voltage rating
VMax voltage it can safely interrupt (AC/DC).
Speed
-Fast-blow (F) vs slow-blow/time-delay (T).
Breaking capacity
AMax fault current it can safely clear.
Markings
Rating on the cap/body, e.g. 'T2A 250V' = 2A time-delay. Blade fuses are colour-coded by amps.
Standard values
0.5, 1, 2, 3.15, 5, 10A glass; 5/7.5/10/15/20/30A blade; PTC by hold current.
How to choose
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.
Pinout & package
2-terminal, non-polar. Cartridge in a holder/clips, blade in a fuse box, SMD on the PCB. PTC resets after cooling.
Example circuits
- Mains input protection before a power supply
- Battery/automotive line protection
- USB/port over-current (polyfuse)
Common failures
Blows (its job) on overcurrent; nuisance-blows if under-rated for inrush; corroded holder gives intermittent contact.
How to test
Continuity/ohms across it: ~0Ω = good, OL (open) = blown. Never bypass with foil - replace with the exact rating.
Substitutes
EXACT current/voltage/speed rating - never higher current. Match fast vs slow-blow; equal or higher breaking capacity.