Choosing fuses and protection devices
Polyfuses, ordinary fuses, TVS diodes, varistors. Decide first what you are protecting against and the part follows.
Fuses
| Type | Characteristics | Where used |
|---|---|---|
| Fast-acting fuse | Blows quickly | Protecting semiconductors |
| Time-delay fuse | Withstands inrush current | Motors, inputs with large capacitors |
| Polyfuse (PTC) | Resets automatically, high resistance | USB, low power |
Choose the current rating at 1.25 to 1.5 times the normal maximum current, and the breaking capacity above the largest current that can flow in a fault. A battery's short-circuit current is hundreds of amps, so a fuse with a small breaking capacity can burst from the arc as it blows.
TVS diodes
They react within nanoseconds and absorb ESD and short surges.
- Working voltage (VRWM): above the normal signal voltage
- Clamping voltage (VC): below the absolute maximum rating of the chip being protected
- Unidirectional / bidirectional: bidirectional if the signal also goes negative
Varistors (MOV)
They absorb high-energy surges (lightning, inductive loads) but react slowly and degrade with repetition. Used mainly on AC mains inputs.
Combination
Place them in the order input fuse → TVS → reverse-polarity protection, and when a large overvoltage persists, the TVS fails short and blows the fuse, saving the board.