Testing M205 fuses

[hamster_nz]

Wanted to test some 2.5A M205 fast blow fuses I got from Ali Express today for a 12V lighting project.

I hooked up to PSU. Wouldn't blow with 3A for a few seconds (not unexpected), but my 10 Ohm / 9 W series resistor didn't like seeing 30V @ 3A for long before it started smoking  

So tested again using just the test leads and crocodile clips - took a little over 4A for a few seconds to blow,

Not bad at $2.35 for a box of 100

[sleemanj]

Fuse opening times can be surprisingly long.  If you look up some datasheets for reputable brands, you might even find that at only 200% rated current you are talking minutes, or even 10s of minutes, and that's for a "fast" fuse!

[Jwillis]

Kind of interesting
Fuses have a melting I2T.this is expressed as A*A*seconds.To find the seconds a fuse will blow you can divide the I2T by amps of the fuse squared.Very low valued fuses are designed the blow very fast were as higher values blow much slower. For instance your 2.5 amp fuse has a I2T of 8.1300. 8.1300/(2.5*2.5)=1.3 seconds. That means that your fuse of 2.5 amps should blow with 6.25 amps in 1.3 seconds.
Now if you take a 20amp fuse with a I2T of 2100 it would take 5.25 seconds to blow at 400 amps. A 100mA fuse with a I2T of 0.0018 would take 0.18 seconds.
Now I understand that they have these I2T to help best match a fuse to a construction but why do they square the amps and not a standard percentage over the rated amperage.

[dmills]

Because power dissipated is proportional to current squared!
Energy dumped into the fuse element is proportional to I^2 multiplied by time if you assume the fuse is a resistor (Not a safe assumption, but close enough).

Note that there are at least two implicit assumptions here, first that the resistance remains constant as the fuse heats (Ha) and secondly that the losses are negligible both to radiation at higher temperatures and via conduction to the end caps, neither are really safe assumptions.

Fuses are actually surprisingly complicated, and some have fascinating metallurgy where the alloy composition actually changes before failure. 

Regards, Dan.

[hamster_nz]

Kind of interesting
Fuses have a melting I2T.this is expressed as A*A*seconds.To find the seconds a fuse will blow you can divide the I2T by amps of the fuse squared.Very low valued fuses are designed the blow very fast were as higher values blow much slower. For instance your 2.5 amp fuse has a I2T of 8.1300. 8.1300/(2.5*2.5)=1.3 seconds. That means that your fuse of 2.5 amps should blow with 6.25 amps in 1.3 seconds.
Now if you take a 20amp fuse with a I2T of 2100 it would take 5.25 seconds to blow at 400 amps. A 100mA fuse with a I2T of 0.0018 would take 0.18 seconds.
Now I understand that they have these I2T to help best match a fuse to a construction but why do they square the amps and not a standard percentage over the rated amperage.

Very interesting - I wasn't aware of the I2T values.

I'm glad I'm putting in six 2.5A fuses (one for each 12V/20W lamp) rather than a single 10A fuse for all six. With a single 10A fuse it sounds like there is a good chance that a faulty lamp or short won't blow a fuse promptly due to wire and connector resistances.