why,why,why a fuse

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duikkie
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why,why,why a fuse

Post by duikkie » Fri Jan 20, 2017 3:36 pm

i wonder why some electronic things have protection things in the schematic

if the hole thing is NOT to be opened ?

a 12 volt 2AMP ADAPTOR from CHINA , sealed in plastic !!! , you have to break the black box , they melt the box

so with sewdriver and hamer open the box breaking stuff off

the fuse was broken with the ac/dc bridge :)

but why make a fuse ? the other fuse was a wire ??

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topcat96
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Re: why,why,why a fuse

Post by topcat96 » Fri Jan 20, 2017 6:44 pm

It's made as cheap and disposable. Once it breaks, most will go and buy another thus the cycle begins again ...
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Re: why,why,why a fuse

Post by duikkie » Fri Jan 20, 2017 8:24 pm

but why a fuse, the bridge is allso a sort of fuse (diode):
without fuse much cheaper :)
topcat96 wrote:It's made as cheap and disposable. Once it breaks, most will go and buy another thus the cycle begins again ...

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Re: why,why,why a fuse

Post by 1024MAK » Fri Jan 20, 2017 11:40 pm

Diodes can fail open circuit, or more likely, short circuit.

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Re: why,why,why a fuse

Post by duikkie » Sat Jan 21, 2017 7:34 am

that is what mostly happens , the only time the fuse blows if there is a fail AC/DC bridge.
but because the hole thing is in a black box melted together , the danger is not that great
that you can get to a open circuit.

what happens is the ac/dc bridge is broken , one of the diodes , then the fuse goes.

so why the fuse ? you can't fix it because the black box can not be opened.


1024MAK wrote:Diodes can fail open circuit, or more likely, short circuit.

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Re: why,why,why a fuse

Post by poink » Sat Jan 21, 2017 3:22 pm

duikkie wrote:that is what mostly happens , the only time the fuse blows if there is a fail AC/DC bridge.
but because the hole thing is in a black box melted together , the danger is not that great that you can get to a open circuit.
Safety...because there's actually significant danger. If it was a part that could safely have been removed, it wouldn't be there, even LED bulbs in Poundland have something to act as a fuse.

A mains fault is quite capable of blowing the casing of the PSU apart, or just producing enough heat that the casing melts - watch the results of a a safety test on a cheap multimeter, you'll have some idea of why it's needed.

And, of course, as you point out with the AC/DC bridge example, the fuse is virtually never the cause of why the power supply stops working - it's only blowing because something else has failed.

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Re: why,why,why a fuse

Post by duikkie » Sun Jan 22, 2017 8:24 am

why not use a cheaper resistor, is allso a wire, but maybe a am old school , a fuse you can replace, but in a sealed box you can't a resistor maybe cheaper ( about 0.1 to 1 ohm p=250mw?)
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duikkie wrote:that is what mostly happens , the only time the fuse blows if there is a fail AC/DC bridge.
but because the hole thing is in a black box melted together , the danger is not that great that you can get to a open circuit.
Safety...because there's actually significant danger. If it was a part that could safely have been removed, it wouldn't be there, even LED bulbs in Poundland have something to act as a fuse.

A mains fault is quite capable of blowing the casing of the PSU apart, or just producing enough heat that the casing melts - watch the results of a a safety test on a cheap multimeter, you'll have some idea of why it's needed.

And, of course, as you point out with the AC/DC bridge example, the fuse is virtually never the cause of why the power supply stops working - it's only blowing because something else has failed.

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topcat96
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Re: why,why,why a fuse

Post by topcat96 » Sun Jan 22, 2017 4:13 pm

Different countries have different rules and regulations in regards to electrical equipment. Maybe they used a fuse to comply with as many as possible.
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Re: why,why,why a fuse

Post by poink » Sun Jan 22, 2017 7:08 pm

duikkie wrote:why not use a cheaper resistor
You can, but at the expense of predictability. This means you can get away with it in a totally sealed product (like a light bulb - where you're using something like a capacitive dropper anyway), but you probably can't anywhere that you have to be accurate for the fault current.

In the case of a power supply, you want accurate response to overload - if you overheat the windings, you risk burning off the insulation, and having full mains voltage flow across the transformer. Yes, you could use a transformer that can handle the greater current, but that's a lot more expensive, bulkier and heavier than a fuse.

This also suggests why it's undesirable for people to be able to replace the fuse...once the fuse goes, you've almost certainly had a fault, and there's a distinct chance that various other components that the safety of the user relies on may also be compromised.

edit: The other thing I forgot to mention is that, particularly in 220/230/240V countries, a single resistor may not have the voltage rating for the supply - an 1/8W resistor is perhaps 200V, and a 1/4W 250V. Depending on where it is in the circuit, that might, or might not be acceptable.

Remember in all of this, that there's not necessarily someone watching the device.
topcat96 wrote:Different countries have different rules and regulations in regards to electrical equipment. Maybe they used a fuse to comply with as many as possible.
Sort of; the compliance regime differs (in both focus and rigidity of application), but safety of household appliances (for instance) is IEC 60335, so, the rules are, essentially, a worldwide requirement. The specific per country law usually just requires that it's 'as safe as it reasonably can be'[1] or points to 'relevant international standards'. Adherence to standards like IEC 60335 is, of course, an excellent way of demonstrating due diligence.

Examples of non-members/affiliates of the IEC are North Korea, Somalia and the Solomon Islands. However, it's likely there are equivalent requirements in the vast majority of non-members - for instance the Solomon Islands require designers, manufacturers, importers and suppliers, 'to ensure, so far as is reasonably practicable, that the article is so designed and constructed to be safe and without risks to health when properly used'. Again, compliance with international standards is a good way of demonstrating you did this.

[1] For instance, a table saw needs a spinning sharp blade (that could remove a hand) in order to work, however, it shouldn't allow sparks to ignite collected sawdust.

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Re: why,why,why a fuse

Post by duikkie » Mon Jan 23, 2017 9:16 am

but why even bother with a fuse, if you can not open the black box a melting wire is cheaper then a wire in a glass box , with two metal ends :) , if cheap, cheaper,cheapest.

i have opened a lot of broken 220/230 to dc adapters by now , some have fuse other not ( have a wire with F1 on the print)

if something fails it is mostly the transistor , with results in the ac/dc bridge get the full load of amps, and break, then the fuse at last. :)

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Re: why,why,why a fuse

Post by poink » Mon Jan 23, 2017 12:18 pm

duikkie wrote:but why even bother with a fuse, if you can not open the black box a melting wire is cheaper then a wire in a glass box
There are different types of fuses for different applications: fuse wire, fuse resistors, glass fuses, ceramic fuses etc.,. There's an increase in cost for the better ones, but they're also capable of coping with worse faults. What you specify here is an engineering trade off, so without if you have technical file (that the manufacturer has one is a CE requirement) you'd probably find the justification.
duikkie wrote:i have opened a lot of broken 220/230 to dc adapters by now , some have fuse other not ( have a wire with F1 on the print)
It may well be fuse wire. It used to be common in homes - at least until miniature circuit breakers were commonplace.

Obviously products move through a number of revisions, and, over time are refined. Where more that one type of fuse is allowed in a design, what actually gets used may change with what's was available/cheapest for a particular production run.

(There's also the possibility of a contract manufacturer saving a few pence and using ordinary copper wire (which is pretty dangerous); not that a fuse proves that this hasn't occurred - there are counterfeit fuses. However, trusting/auditing your supply chain is a whole different issue.)
duikkie wrote:if something fails it is mostly the transistor , with results in the ac/dc bridge get the full load of amps, and break, then the fuse at last. :)
That it's usually another component merely suggests that the presence of the fuse doesn't significantly affect the reliability of the product. It does not mean that the fuse is unnecessary - it's there to prevent failures like damage to other safety critical components and fire, which are much more of a concern than a phone not being charged in the morning.

Yes, it'll probably fail because the transistor failed, but the hazard of a power supply burning someone's house down whilst they sleep is an extreme hazard (someone may die) and it's almost certain it will occur to someone; hence it's mitigated with a fuse (and self-extinguishing enclosures etc.,) to bring the risk down to an acceptable level.

So, in short:
Why a fuse? So someone doesn't die or become seriously injured in the event of certain types of failure.
Why the exact form of fuse in a particular product? Someone decided that it was sufficient to meet the safety requirements and the cost was acceptable.

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Re: why,why,why a fuse

Post by duikkie » Mon Jan 23, 2017 12:25 pm

the CE label , is not a safety lable , something to do with europa :) , maybe next time we see a CBE label (bexit :) )

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Re: why,why,why a fuse

Post by poink » Sat Jan 28, 2017 11:55 am

duikkie wrote:the CE label , is not a safety lable , something to do with europa :)
"A CE[1] mark is a manufacturer’s claim that its product meets specified essential safety requirements set out in relevant European directives." (gov.uk).

The confusion about whether it's a safety mark seems to come from a handful of sources:
  1. There's no strict requirement of product testing (there is for some goods, but not for ordinary household items like televisions, printers and kettles).
  2. The CE mark indicates compliance with more than just very basic safety, it also includes electromagnetic compatibility (EMC), that you've provided printed instructions with the product in an appropriate language, that something dangerous isn't made to look like a toy etc.,
  3. There are some goods, like mains plugs, that aren't covered by the EC directives, so, a CE mark should not be affixed. This is similar to claims of BS 1363 compliance (UK mains plug standard) for a double glazed window, one would immediately have concerns about whether the manufacturer of the window had any idea what they were doing.
The 'new approach directives' (now more than 20 years old), see the manufacturer as the best placed to evaluate a product, and, accordingly, place liability for the safety (and other compliances) of that product on the manufacturer.

In practice, reputable manufacturers will apply standards from institutions like the IEC etc. and perform recommended testing, both because it reduces a far more costly recall, and affords a 'presumption of compatibility' with the requirements.

[1] CE just being 'Conformité Européenne'.
duikkie wrote:maybe next time we see a CBE label (bexit :) )
It'll be BS, for 'British Standard' and the kitemark (a British Standards Institution logo); it's mostly seen on UK mains plugs now, as they're a national concern. The BSI is really the UK national standards body, so it's largely the BSI who provide the UK input on the aforementioned standards from IEC etc.,

A big concern Brexit-wise (other than a paperwork headache and/or lengthy customs checks) is 'incompatible divergence', meaning you can no longer build one product, and sell it to 28 countries. Another is no mutual recognition of standards, so you have the expense of certifying products multiple times; and a potential return to the pre-single market experience of imported goods mysteriously failing compliance tests.

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