Fuse
A fuse breaks the current path if too much current flows for too long. Cartridge fuses blow once; resettable PTC fuses heat up, go high-resistance, and reset when they cool.
A thin metal link or a polymer slug heats up as current passes. Past a calibrated threshold the link melts (or the polymer expands and breaks contact), opening the circuit.
In plain terms
A weak link deliberately put in the chain so the rest of the chain survives the strain.
Also called: PTC, polyfuse, TVS-paired.
Why designers use it
- Stop a fault from setting fire to the board.
- Protect a USB host from a short downstream.
- Comply with safety standards that require overcurrent protection.
- Reset itself after a transient short — handy for end-user gear.
Best for
- Mains protection
- USB and battery rails
- Hot-swap input feeds
When not to use it
- When the fuse's voltage or interrupting rating is inadequate for the fault it may need to clear.
- When its time-current characteristic cannot protect the circuit before damage occurs.
Common mistakes
- Picking a fuse that just trips at normal inrush current.
- Using a fuse instead of proper short-circuit current protection on mains.
Where you will find it
- A polymer PTC can limit fault current by switching to high resistance. Removing the fault and cycling power lets it cool and recover; it should not be described as instantaneous or as fitted to every USB port.
- A fuse is one part of a protection design. It does not remove shock hazards or stored energy, and does not make a mains-powered appliance safe to repair.
A short history
Overcurrent protection comes in different forms. A conventional fuse interrupts current by melting an element; a polymer PTC resettable device instead changes to a high-resistance state when heated. Its recovery requires removal of the fault and enough cooling, often with power cycled. Neither type makes an unsafe circuit automatically safe.
Good to know
- A resettable PTC does not operate like a tiny mechanical circuit breaker. Its material changes resistance dramatically as it heats.
- Trip time is part of the specification, not a universal property of every fuse. Check the time-current curve and operating conditions.