Power & protection
Fuse & PTC fuse
A deliberate weak link. If too much current flows, the fuse heats up and breaks the circuit before the wiring or the parts catch fire. A PTC "resettable" fuse does the same, then recovers once it cools.
Think of it as: A safety valve that bursts before the pipe does.
Pins
- Either end
- : No polarity. It goes in series with the supply, as close to the source as possible.
Glass cartridge fuses let you see the wire; car blade fuses are colour-coded by amps; PTC fuses on circuit boards look like small yellow-brown chips or discs.
What to look for inside
- A 1 A fuse while the current slowly climbs past its rating.
- Like any resistor, its thin wire heats up as current flows; the heat grows with the square of the current.
- Past its rating, the wire gets hot enough to melt. The circuit opens and the current drops to zero: the fault is stopped.
- An ordinary fuse must then be replaced. A PTC fuse instead becomes very high resistance while hot, and lets current through again once the fault is gone and it cools.
The numbers that matter
| Spec | Typical |
|---|---|
Rated current Pick it above the normal current but below what the wiring can survive. | 100 mA to 30 A |
Speed Slow-blow fuses ride through switch-on surges (motors, capacitors). | fast (F) or slow-blow (T) |
Voltage rating The voltage it can safely break. Must be at least your supply. | 32 V to 250 V |
PTC hold / trip current Below hold it never trips; above trip it always does; between, it depends on temperature. | e.g. 0.5 A hold · 1 A trip |
Where you’ll use it
- Protecting a whole device at the power input.
- USB ports and battery packs (PTC fuses, so a short circuit doesn’t mean a repair).
Common mistakes
- Fitting a bigger fuse because the right one "keeps blowing". The fuse is telling you about a fault.
- Putting the fuse after the parts it should protect. It belongs right at the input.
Lessons that use it
Lesson 5.2 uses a PTC fuse in its protection stage.
Go deeper
A fuse blows when enough heat (I²t) builds up, so a big overload blows it much faster than a small one.