I was curious about the actual current needed to blow my newly purchased se t of 250V 20mm fuses. Half a dozen experiments with 0.5A, 1A and 2A samples indicated a multiplier of 3.0-3.5. For example, 3.4A to blow a 1A fuse. Is that typical?
P.S: While googling about fuses I came across this scary incident on a rath er larger scale:
I?d recommend watching from the start for atmosphere, although the incident is at 08:49.
Terry, East Grinstead, UK
Depends on the type of fuse and to what standard it was designed to comply.
You didn't indicate a fuse type or test time value.
RL
Nothing surprises me with fuse ratings any more. It's vital to have them, of course, but many types can be extraordinarily forgiving of extended periods of over-current.
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Polyfuses are really erratic too. Especially the surface-mount ones.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
RTFDS -- and if there's no datasheet, run like hell!
e.g.
Note the time current curves: it can take very long indeed for a 1A fuse to open at, say, 1.9A (hours) -- down in the range where ambient temperature and vibration matters.
The response gets fast when the element melts, slumps and vaporizes faster than the blink of an eye, which may give closer to your estimated 3x multiplier (< 0.1s say).
Note also those curves are average. The min/max are given on the first page. 1A at 275% says max 2s, min 50ms.
Fuses are most effective at stopping fault currents, typically hundreds of amperes for residential mains. Potentially, hundreds of kiloamperes for industrial and distribution applications! Moreover, fuses that blow too quickly are seen as a nuisance, not a safety feature -- can you imagine! Often, handling a 100A inrush surge for a couple cycles is a requirement.
Keep this in mind when designing fuses and surrounding circuitry.
One final note, remember the transistor dies in a hundred microseconds. Ten thousand microseconds later, the fuse begins to melt. The transistor protects the fuse, never the other way around.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/ wrote in message news:e45d5436-af22-4bea-9c3a-baad00a1dd90@googlegroups.com... I was curious about the actual current needed to blow my newly purchased set of 250V 20mm fuses. Half a dozen experiments with 0.5A, 1A and 2A samples indicated a multiplier of 3.0-3.5. For example, 3.4A to blow a 1A fuse. Is that typical? P.S: While googling about fuses I came across this scary incident on a rather larger scale: https://m.youtube.com/watch?v=GPqcPoZgPMk&t=529s incident is at 08:49. Terry, East Grinstead, UK
====snip====
Hence the (half a century or older) engineer's definition:-
"Transistor:- A ten dollar device designed to protect a ten cent fuse."
-- Johnny B Good
set of 250V 20mm fuses. Half a dozen experiments with 0.5A, 1A and 2A sampl es indicated a multiplier of 3.0-3.5. For example, 3.4A to blow a 1A fuse. Is that typical?
normal yes. Curves here:
NT
ther larger scale:
e incident is at 08:49.
** Were the fuse value marked with the letters "T" or "F" ?? Egs T1A or F1A ? Clear glass, ceramic, sand filled ?
Single thin wire, spiralled wire or single with a lump or two ?
.... Phil
Thanks all, appreciate those replies.
To answer the questions raised, these fuses (from Amazon UK) are described as 'Ginsco 110pcs 5x20mm Quick Blow Glass Tube'. An example of the hard-to read markings is 'F240VL5A'. I see 'F' means 'Fast Acting'. That's why I was rather surprised that over three times the rated current was needed before the thin fuse wire glowed red and (after several minutes) finally expired.
I read that 'L' means 'Low Breaking Capacity', which seems unduly vague.
Terry, East Grinstead, UK
** IME they are bog standard, low cost, medium/fast acting, low breaking c apacity fuses. ** That last bit tells me the fuse wire is very likely tin plated copper.
Low melting point metals like pure tin do not glow red before failing.
** Low breaking capacity means that the fuse cannot be relied on to break h undreds or thousands of amps of fault current - fuses fitted to the AC supp ly of most appliances are not required to do this and generally cannot.Under such conditions, the fuses you have will likely explode as the fuse w ire instantly turns to metal vapour - the tube glass will blacken and shatt er as a high current arc jumps from one end to the other. The arc will cont inue for a second or so UNTIL a higher current capacity fuse or breaker in the AC supply opens.
Fuses that can break high currents have ceramic bodies and are filled with fine sand. Some use multiple fuses wires too. Known as HRC types they cost quite a bit more.
... Phil
Thanks Phil, understood.
BTW, one other thing I meant to add was that during one test as I gradually increased the current, probably around the 3X rating point, the fuse glass turned black. It remained like that for as long as my patience lasted and would apparently have done so indefinitely if I hadn?t hastened it to its final expiry with more amps.
(I?m almost sure this was from the same set, although there? ?s a slim chance I was trying another 5x20mm on the bench, with perhaps a different rating.)
This set was from China, particularly cheap.
Terry, East Grinstead, UK
Ever watched one under a microscope as you hand solder it?
It's hard to describe they go all soft and jelly like and then firm up as they cool.
ally increased the current, probably around the 3X rating point, the fuse glass turned black. It remained like that for as long as my patience las ted and would apparently have done so indefinitely if I hadn?t ha stened it to its final expiry with more amps.
?s a slim chance I was trying another 5x20mm on the bench, with perhaps a different rating.)
Right - cheap knock-off unrated products.
I made a simple fuse reference page for folks in my industry:
Note that unless the fuse is rated by a electrical safety rating company
such as CE, UL, or CSA then chances are it will not perform as expected and may be hazardous if used where fire or shock can be a likely outcome
of fuse not doing its job properly.
John :-#(#
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Makes sense: it is a polymer. The trip current is a radical function of the PCB layout, namely the thermal conductivity of the end caps to the world. We generally use thru-hole radial-lead polyfuses: they are a lot better behaved than the surface-mount ones.
They'r still not very predictable. Good for protecting PCB traces, gross stuff like that.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
I mean, not that "fuse" goes together with anything other than "gross".
The only thing equally gross that semiconductors can offer is diodes and SCRs, and even then they're pretty marginal against the energy let-through of polyfuses.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/
One of my guys here likes the TI eFuse parts, like TPS26600. The upside is that they are resistor-programmable and accurate and report the current and do over/under/reverse voltage protections. The downside is using a semiconductor as a fuse.
He's been successful at 12 and 24 volts; I blew some up at 48, so designed them out.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
I've been using ~$2 opamps as fuses for years, dip. it's a feature not a bug. George H.
Semiconductors can't pass enough current to start capacitors, at least not big ones. They're fine for POL stuff.
The amount of power and energy needed to start, say, a 48V 5A load and all its capacitance, is staggering: like, a dozen D2PAK transistors-worth.
The amount of power and energy a single transistor and diode can handle, with an inductor and a little switching, is staggering. You can even dump the charging energy into the source, rather than burning it (in a TVS or MOV or resistor or whatever). It's a nice application because you don't have to be precise or stable about controlling the current, or power dissipation, and you can do it for a relative eternity.
My first inline current limiter, I built for 30V 20A capacity and 150ms fault time. An absolute eternity for anything linear and solid state. You could do that with tubes I guess. It even handles ten cycles of this in rapid succession, then locks you out (thermally, until it cools back down again).
I somehow doubt a Seven Transistor Labs instrument box is going to make an appearance any time soon in your instruments, but any switching reg that supports 100% duty cycle (i.e., the switcher equivalent of an LDO) and output current limiting, will do nicely.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/
Even for nominally fast blow fuses on a bad day they can carry twice their rated current almost indefinitely - certainly for several minutes. The envelope for a fast blow looks something like:
Slow blow fuses will allow considerably more transient current through for a very short period of time without any trouble.
Fuses protect well against dead shorts but they don't protect against total stupidity like 2x 13A kettles plugged into one 4 way adapter. The fuse in the mains plug will run mad hot but will take a long while to blow!
-- Regards, Martin Brown
ly increased the current, probably around the 3X rating point, the fuse gla ss turned black. It remained like that for as long as my patience lasted an d would apparently have done so indefinitely if I hadn?t hastened i t to its final expiry with more amps.
?s a slim chance I was trying another 5x20mm on the bench, with perhaps a different rating.)
** Next time, look out for fuses made by " Littelfuse " of the USA.They carry agency approval logos and work as expected.
.... Phil
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