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Only partially true.

In well-designed microelectronics, they will.

The standard circuit involves a fuse, a fast Zener clamp, and sometimes a small resistor (e.g. 1 ohm) and/or capacitor. The design parameter is that, with the current limit from the resistor, the Zener should not blow out before the fuse.

The resistor needs to be small enough to not lose a lot of voltage under normal operations, but to still protect the Zener during the short surge during which the fuse blows. For most microelectronics, that's not hard. A 0.5W USB device might have 100mA of current max, which across 1 ohm is 100mV, so negligible for most purposes.

With high-power devices, it gets more complex.

Of course, consumer devices (a) will never be fixed (b) don't sell on this (c) every penny counts, so there's no market pressure to do things right.

But that's how we used to do it, and how it's still done many places where things count. If I'm building a one-off or few-off, it definitely will have proper protection.

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That's a good design for input overvoltage protection (assuming I've drawn the correct schematic in my head). But it doesn't help against most other fault conditions, particularly anything to do with the load/downstream going short.

Really, if you care beyond "blow the fuse if something shorts", you need active current limiting. For common cases like USB ports, you can buy chips that do it cheaply and efficiently. There are also some textbook circuits, though they each have their pros and cons. No matter what, if you've got a pass transistor or switching transistor that's about to go seriously overcurrent, you have to do something about it with active parts -- fuses aren't going to get it done.

But defense in depth is always a good strategy, and fuses play a key role there. The active circuitry saves the rest of the design; if it can't get the job done, the fuse makes sure nothing burns.

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Yeah, fuses are more of a overload protection kind of stuff. For cases where the load is trying to kill itself and you with it you need current limitation circuits, either on a converter or a latch current limiter.

Orrr you can design your circuit to survive a short condition for a bit longer than a fuse takes to blow. For old cars this was common, let whatever is short take all the current it wants for a tenth of a second and forget about it.the battery may lose a bit of useful life but those things were gonna fail early either way.

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Just want to mention how much I appreciate this discussion and the opportunity to learn from it. This is what I come to HN for (nowadays there are also really interesting YouTubers who do informative teardowns of power electronics and other devices, too e.g. Labo de Michel, Watch Wes Work, etc.)
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It's a scale problem; fuses can be set up protect components or machines in a car or factory context; and often this is what is taught (or what people have first hand familiarity with). It just doesn't scale down to semiconductor level.
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Even in a car, fuses are there for safety. A car battery will easily start a fire if there's a short anywhere in the wiring or accessory devices.
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> It's a scale problem; fuses can be set up protect components or machines in a car or factory context;

Induction motors use overload relays to protect the motor, fuses are strictly for overcurrent protection to stop the wire from catching fire. A VFD will have a fused switch or circuit breaker on the line side for overcurrent protection and a set of overload relays after the contactor to protect the motor itself.

Switchgear uses ground fault protection relays to protect the equipment, not fuses (or humans for that matter, GFP relays are set around 30mA IIRC)

Some motors also have phase loss protection relays that will power down the motor if a utility phase is lost.

A car is the same, fuses are to protect the wiring and prevent fires. Fuses do not act quickly enough to protect equipment, even fast-acting ones.

Relays act fast enough and deterministically enough to protect equipment, fuses aren’t fast or accurate enough to do so.

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