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Topic: Fuse not burning when expected (Read 278 times) previous topic - next topic

Jas15

I want to add a fuse to my project to protect against a short.  I am using glass fuses and this is my first time using fuses so I decided to experiment with them.  I am putting 1w resisters in parallel so I can see what the fuse does at different amperages, and measuring amperage of the circuit with my multimeter.  Here is what I'm seeing.

For my 0.2A 250v rated fuse running at 5v DC

200mA: No change to the fuse
400mA: No change to the fuse (I would have expected the fuse to burn up at this point)
600mA: No change to the fuse
800mA: The fuse starts glowing, but does not burn up

After this point adding more resistors increases the amperage only a small amount.  However the fuse still did not burn.

I then just shorted my power supply directly through the fuse (My power supply will output up to 3A which is 15x the fuse's rating), and the fuse still did not burn up.  The amperage raised to 1A and just stayed there, I left it for about 10 minutes, and the fuse stayed glowing, the glass became discolored, but still allowed 1A to flow.  The fuse seems to be acting more as a resistor than a fuse.  It also began to heat up, this is not really what I was hoping for.

I've tried this with several fuses, and half of them share this behavior with peeking at 1A and staying there.  The other half just burn up quickly as I would expect.

The project that I want to add a fuse to is going to should draw a maximum 300mA, I was going to put in a a 0.5A fuse, but after my experiments, I think that a 0.2A fuse would be safer, even though it is rated at less then the maximum current the project will draw.  Is this a reasonable plan?




BJHenry

Can you post a photo or schematic of your wiring, and a link to the fuse you've got? There's almost certainly something more going on here.

Jas15

#2
Aug 02, 2019, 04:05 am Last Edit: Aug 02, 2019, 04:07 am by Jas15
Can you post a photo or schematic of your wiring, and a link to the fuse you've got? There's almost certainly something more going on here.
It's pretty simple, but sure, its just power going into my multimeter and then through the fuse which I short.

larryd

Many fuses need 200% their rating to burn.

Also there are 'slow blow' and 'fast blow' fuses.



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Jas15

Many fuses need 200% their rating to burn.
This fuse is getting 3A and allowing 1A to flow apparently indefinitely though.  Considering the glass is discolored, I'm sure that it would burn up eventually.

Also there are 'slow blow' and 'fast blow' fuses.
Good point, I'm not sure if these fuses are fast blow, they aren't labeled as either.  I just ordered some fast blow fuses, and will repeat with those.

larryd

#5
Aug 02, 2019, 04:22 am Last Edit: Aug 02, 2019, 04:44 am by larryd
If there is a spring pulling on a thin fusible link it is probably a fast blow.

If it has a taut fusible link it's probably a fast blow.




No technical PMs.
If you are asked a question, please respond with an answer.
If you are asked for more information, please supply it.
If you need clarification, ask for help.

BJHenry

It's pretty simple, but sure, its just power going into my multimeter and then through the fuse which I short.
A couple of things:
1) I wouldn't be using little DuPont wires and a solderless breadboard for this. Solderless breadboard is notorious for not making good connections and those wires won't carry much current.
2) That photo doesn't really show much at all. Where do the orange and blue wires go? There are two grey wires, both of which go out of the frame of the photo.
3) Why are there two wires on each end of the fuse? There should only be one.
4) That reading on the multimer is suspiciously exact. Can you take a photo of the whole front of the meter so that we can see exactly what settings you're using?

prairiemystic

Look at some fuse curves in the datasheets Littelfuse Fuses, to see when they actually blow.

Their amp rating is what they will pass forever and typically 200% to blow- sometimes taking several minutes. So you design for a fault current 5-10X the operating current, and ensure the power supply can actually provide enough current to clear the fuse. Otherwise, it can just sit there and gets hot due to its high resistance and never blow.

I don't know your part's parameters but it's behaving like a typical fuse.

WattsThat

Quote
I want to add a fuse to my project to protect against a short.
Why would something short?

Perhaps you should tell us you exactly what you're trying to protect. With low voltage, low current circuits typically found with an Arduino project, there isn't much that can be protected with a fuse. Thinking you're going to prevent against component failures is not something that will occur. Fuses clear only after something has failed, not before.

When properly selected, in most cases, fuses are there to typically protect things from exploding or catching fire. Chances are good that the 3AG glass fuse you're using is rated at 250 volts so applying one in a low voltage dc circuit changes the clearing behavior. There are 32 volt class fuses used for low voltage and automotive applications which would be more appropriate.

You'll find that low voltage, low current fuses are quite expensive. The very, very thin wire required for such a fuse is difficult to manufacture and handle. I've seen wire down to 75 microns used in low current fuses which is approximately the thickness of a human hair. That's 56 gauge for those that use AGW sizing.
Vacuum tube guy in a solid state world

DrAzzy

#9
Aug 03, 2019, 06:46 am Last Edit: Aug 03, 2019, 06:48 am by DrAzzy
Well, anything can end up with a short - so with beefy power source (ex, lithium batteries) one might want a fuse even if a short seems unlikely - a stray blob of solder that fell off the iron and landed on desk shorting the underside of pins..

Though personally I usually don't use them unless the result of the short would be dangerous or destroy more than like, a couple bucks in parts that I have spares of.

Also consider polyfuses (self-resetting fuse)
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