Peter Todd wrote:
> Anyway I'm going to send off some gerbers to AdvancedCircuits tomorrow
> for another project and I was planning to put some test fuses on them.
> Right now I'm thinking I'll make each fuse be two large traces, say
> 250 mils, with the smallest allowable trace joining them, 6 mils.
> I'll do a few different ones with different fuse trace lengths, I'm
> assuming shorter fuses will be easier to blow, but thermal
> heatsinking effects may of course make that quite wrong.

You want to make them long enough so that the center gets close to the
exepected temperature for an infinitely long fuse with the same current.  I
don't know what the length/width ratio needs to be, but I'd knee jerk pick
around 10:1 for starters.

Of course if you can deliver the energy very quickly, then shorter shouldn't
matter much since there won't be time for heat flow away from the heated
area.  You probably want to slowly charge up a large electrolytic cap (or
probably a bunch in parallel) and then discharge it as quickly as possible
accross the fuse.  In that case you want the fuse resistance to roughly
match the cap ESR.

In any case, making fuses from PCB copper is going to be rather
unrepeatable.  A "6 mil" track is probably anywhere from 4-8 mils, check the
specs.  Also most board houses only guarantee the minimum coppper width, not
the maximum since for most applications more is better.  I think you should
probably expect a factor of 2-4 to 1 in resistance accross production lots.

> I'm also
> thinking of trying two pointed triangles, with the tips touching each
> other.

That will be even more prone to resistance variation as a function of track
width variation.

> Testing this would then be a matter of connecting each fuse up to a
> low-impedance power source and and firing. I think a DSO measuring the
> voltage rise across a shunt resistor should get me my current, and
> integrating the current waveform and voltage should get me roughly the
> power used, I think...

You're going to have a tough enough time blowing them without a shunt
resistor in there.  For testing take a large cap and measure the before and
after voltage.  From that you can calculate the energy used in blowing the
fuse.  Since that will be how the real circuit will likely do it, it will be
a relevant measure of circuit power drain to blow one fuse.  Also make sure
you connect the cap to the fuse with a transistor or FET, not a mechanical
switch.  The characteristics of a switch will otherwise get in the way.  You
may also find that the transistor makes a better fuse than the copper trace.

How about a bunch of the smallest and cheapest surface mount diodes?  I
think they can probably take less abuse than a 6 mil copper trace.


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