Electronics notes/Fuses
A fuse protects against large currents flowing by breaking the connection.
Fuses can be a good idea
- ...when such a current is not unthinkable
- ...when this is good for safety, in that a fuse burning is a lot more controlled than many other components burning out
- ...when the fuse costs less than the components cost they protect (and/or the maintenance cost) - which is probably a lot, more often than fuses are used in practice.
Basic (melting) fuses
A very simple fuse in household items is a thin resistive wire which will simply heat up and burn through at some current.
These are often in glass or ceramic, sometimes plastic.
Automotive fuses are more often plastic and often more exposed to air.
Both glass and ceramic should encompass the heat of the fuse burning - in particular high current may have the wire vaporize into the air.
Glass fuses are often just a wire in air. They will heat up and disconnect themselves, but these are less ideal for high voltage or high current, in that they could vaporize the fuse and continue conducting some current, and/or arc after blowing.
In these cases ceramic fuses are much safer, because aside from the ceramic walls they also contain more sand than air.
Fuses come in various speeds
- fast-blow fuses may take perhaps a tenth of a second to a second.
- Useful when regular fuses are too slow to actually protect something delicate/expensive.
- Still don't protect quite everything, though
- slow blow fuses / time-delay fuse may take seconds to tens of seconds to blow. They are used when short spikes are to be expected and acceptable.
- useful to not blow from short inrush current
There are more variants, but consumer generally don't have to care about them(verify)
There is such even within a single type, in part because it depends on how much overcurrent they are. High-current short circuits should be fast with both, the more interesting question is more what they do when crossing the rating listed on the thing itself.
Note that when replacing a fuse, even with the correct rating, if the speed characteristic is different you may still get fuses burning when everything is okay (if you replaced a slow with a fast) or not protecting components as well (if you replaced fast with slow). You absolutely have some leeway, but be careful using it.
In situations where they fuses are slow-blow to deal with inrush currents,
replacing them with fast is more likely to burn out regularly without there being any fault.
Generally, slow-blow don't blow without a reason.
Thermal fuses
Thermal fuses are a variation that break in reaction to heat external to them (rather than from their own resistance), seen for example in hairdryers. (though people also use it to refer to other things)
Fusible resistor
Resettable fuses / polyfuses / semifuses / PTC fuses
...and also known under at least half a dozen brand names.
Resettable fuses refers to any design that cuts (or just significantly lowers) the circuit at some amount of current,
but will then conduct again later without replacement, e.g. once power is removed and/or restored to the an acceptable level.
Used where fuse replacement is hard or annoying, say, space stations and USB ports. USB specifies that ports should be able to take short circuits without having the USB port be broken forevermore - which is nice because motherboard USB ports would be hard to replace, particularly in laptops. (In practice, resetting these fuses might sometimes still require complete removal of wallpower or laptop battery
Often a non-linear, polymeric PTC (PPTC (a specific design of thermistor).
When it sees more current than it is rated for, heat causes it to become a high-resistance element (on the order of a few kilo-ohms), which in most applications (in series with the real load) is equivalent to reducing the current to low amounts.
(Note: much of the below is more specific to PTCs, which are not the only type - e.g. eFuses fit the description but should probably be hadled separately)
While speccing, you probably care about
- Ihold - hold current; the current that you should be able to pass without tripping
- Itrip - trip current; the current that . Might e.g. be twice IHold
- Vmax and IMax - Maximum rated voltage and current
- If you manage Vmax or IMax, you may actually damage this resettable fuse, so you may wish to have some secondary reason that is hard.
- these should be rather higher than Ihold and ITrip, but are still worth thinking about
- resistance
- e.g. PPTCs have a higher resistance than simple burn-through fuses. This can matter to power efficiency of your ovreall design (may be order of half a watt?(verify))
In part due to the slowness, you may want to avoid "just to be safe" overspeccing of Itrip and Ihold,
- because all you're really doing is slowing an already not-fast fuse, meaning larger currents are allowed to flow for longer.
- At the same time, overspeccing may mean the regular resistance (and implied heat loss) is lower, yet the trip current will be higher, so it's a balance you may wish to tweak to context.
Being temperature-operated, it has some behaviour you should be aware of.
- current in tripped state isn't zero.
- This is by design - the triggering is heat-based, and this current keeps it warm enough to stay tripped.
- Say, a polyfuse triggering at 1A may then still conduct a hundred mA.
- That should definitely protect the thing it's protecting, yet it can matter that this isn't actually off.
- After tripping, PPTCs may also take a while to go to their pre-trip resistance.
- Polyfuses do not trigger very fast, so high current will be allowed for a short time.
- Arguably, polyfuses are primarily for protecting components against (heat damage from) long-term overcurrent, and then primarily clunkier things like big loads and batteries, but not necessarily much sensitive ICs.
- It may also not be good enough against damage from short voltage spikes (there are definitely better ways of protecting against transients).
- These fuses are also somewhat slow at their at rated current -- just like glass fuses, even the fast-blow type.
- Yes, glass fuses burn fast under extreme-current short circuits, but when only a little over a their rated current,
- both may take order of a dozen seconds.
See also:
Tangentially related: MOVs
Circuit breakers
A long time ago, circuit breakers referred to large switches you would manually switch, such as knife-style switches.
Meant primarily to, well, intentionally and only manually break the circuit.
These days, circuit breakers often refer to switches that, on top op being manually operated, are also overcurrent protection devices: they trip when they sense too much current passing. Which protects your house's wiring from overheating.
(The most common design uses a solenoid to pull on a trip mechanism. Other designs exist, in part because different needs are better served by different designs)