Episode ideas -- The Fuse Tutorial! - Page 1
Forum section: electronics / general

author: Harrkev
author_rank: Contributor
timestamp: March 23, 2015, 07:51:45 pm
content: How about this for an episode -- exploring FUSES!!!Yes, the humble fuse.I has a MSEE degree and do all things digital. I admit that I do not understand fuses.If you have a 1A fuse, will it carry 1A forever? How about 1.1A? How long would it take 2A to blow the fuse? 5A? 10A?For a humble 1A fuse, how much current will actually cause the fuse to physically break the glass?Does ambient temperature have any effect on the current capability of the fuse (0C vs. 125C, for example).Some fuses are "slow-blow" How slow is slow?Plus, lots of opportunity for wonderful slow-mo video of things blowing up!

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 23, 2015, 07:59:24 pm
content: Wow, that are a lot of questions!Perhaps, this one graph can answer them allBasically, its time versus current.Quoteforever is quite a long timeI would say not forever, but very long.QuoteA bit shorter then forever, but still very long. Depending on construction of the fuse, size, filling and ambient temperature.QuoteHave a look at the graph for those:A 5A BS3036 fuse can withstand 13A for 5 seconds.Important again is the construction of the fuse, size, filling and ambient temperature and all other stuff that I forgot.

author: Harrkev
author_rank: Contributor
timestamp: March 23, 2015, 08:33:34 pm
content: Wow. Fuses are crappier than I ever thought. A 5A fuse can handle 13A for 5 seconds? It also appears to handle 8A pretty much forever. Horrible performance.Still, thanks for the graphs.It still might make a good episode -- especially if Dave's microscope camera can handle a higher frame rate. It would be amusing so see the fuse wire get red-hot and melt in slow-mo.I would also like to see a glass fuse explode, and then compare that to the big nothing from a HRC fuse.

author: retiredcaps
author_rank: Super Contributor
timestamp: March 23, 2015, 09:45:53 pm
content: Quote from: Harrkev on March 23, 2015, 07:51:45 pmThese two videos were responses to multimeter input protection. #376 talks about some of the questions you asked.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 23, 2015, 09:56:40 pm
content: In the early stages of my careerI was working at Philips, on color TV with vacuum picture tubes.There was this poster in the lab about murphy's law:A $600 picture tube will protect a 6 cent fuse by blowing first.

author: Kjelt
author_rank: Super Contributor
timestamp: March 23, 2015, 10:09:24 pm
content: Quote from: Harrkev on March 23, 2015, 08:33:34 pmPerhaps it were you,re thoughts that are at fault. If you need a fuse to blow within 1 second at 1Amp you certainly should not choose a 1AT fuse. You can look for fast blow fuses.Anyway check the datasheet as you would do for all components and other stuff.At least the normal fuses are perfectly specked and tested.If you want to have a good laugh take a look at audiophile fuses with gold and platina wire but without datasheet and the manufacturer dies not even know or give a good answer to those questions.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 24, 2015, 03:34:05 am
content: Yes, fuses are terrible as protection devices. You must always keep in your mind:The fuse protects the WIRE UPSTREAM, not the device downstream!The fuse is to prevent a fire, when the stuff down below decides to take a crap and die shorted. Or crossed wires, or...A fuse is most certainly not intended to save transistors! For that, you need a very much fasterprotection device. But even that has its problems; the amount of surge current drawn by ordinary capacitors will usually cause nuisance trips on devices like this. So you can't just throw a bunch of junk together and hope it will work; a good engineer's job is never done so easily!To use a (conventional) fuse to save transistors, you must use additional protective hardware, like a crowbar circuit which is capable of drawing the entire supply's fault current. Typically, large SCRs are used for this purpose, triggered by an overvoltage condition, or the onset of some sort of failure.Tim

author: pickle9000
author_rank: Super Contributor
timestamp: March 24, 2015, 04:15:27 am
content: Best answer:The fuse is to prevent a fire.

author: necessaryevil
author_rank: Regular Contributor
timestamp: March 26, 2015, 07:24:10 pm
content: QuoteI think those graphs aren't from a datasheet. The title of the graph is 'BS 3036', BS does not stand for bullshit but for British Standard: it defines how fast a fuse should blow in England.And apparently it takes some time. And no, it can't take 8A forever, the graphs only go up to 10 000 sec (< 3 hour). I have to admit, more than I thought. But this won't be a problem, as long as the fuse melts before the wires do.

author: calexanian
author_rank: Super Contributor
timestamp: March 27, 2015, 06:06:36 am
content: As said before. Fuses are not protection for the appliance. They are protection for the rest of the electrical system and for cataclysmic things like overload caused fires. They are good for certain kinds of loads that may have surge or inrush currents as part of their normal use. Electric motors for example may draw large surge currents while coming up to speed over a few seconds. Thermal circuit breakers or fuses work well in those applications. Additionally they work great on the primary of transformers as the transformer itself will limit the surge until the fuse opens. As far as protecting electronics even fast blow fuses are not fast enough to save semiconductor devices. They are toast long before the fuse can do anything. Active circuit protection is about your only option, and that is not guaranteed. What a fuse can do however is lessen the risk of fire should a sustained fault occur. They are great in the proper application. Terrible in others.