Re: [Phys-L] ceramic fuses

[brian whatcott <betwys1@sbcglobal.net>]

The full scale current reading of moving coil meters of respectable make 
was as
 sensitive as 50, even 35 micro amps full scale.

To read higher currents such meters used shunt resistors to carry the bulk
of the current, and it was this input current that was to be limited 
with a fuse.
A frequent cause of blowing this fuse was applying the meter set to a 
current scale
across a voltage source.    This is the region where a ceramic fuse can be
helpful - where the construction is better suited to extinguishing very 
high arcing
over-currents. This is the so called interrupt current rating.
If the meter $  value times its fuse blow frequency is attractively low, 
one would
 consider the glass fuse: which in passing a higher arc current might 
well damage
the meter's interior where a ceramic fuse would contain the damage 
likelihood.

The current crop of  Digital meters use much the same philosophy.

Brian W


On 8/20/2012 10:03 AM, George M Caplan wrote:
> I would use exactly the fuse specified.
> In addition to safety reasons, here's another reason:
> The resistance of the fuse determines (in part) the resistance of the ammeter.
> If the resistance changes with current as the fuse heats up, the
> resistance of the ammeter will not be constant.
> This problem exists even when using the proper fuse in at least one
> model of Extech multimeter.
> I spoke to a very nice fellow at Extech about this. He said he'd check
> with engineers.
> After I spoke to him twice, he stopped returning my calls.!
>
> George M. Caplan
> Senior Instructor in Physics Laboratory
> Wellesley College
> 106 Central Street
> Wellesley, MA  02481-8203
> USA
>
>
>
>
> On Mon, Aug 20, 2012 at 10:38 AM,  <DMathies@tulsacc.edu> wrote:
> > I have new multi-meters with, of course, different fuses than I have in
> > stock. The students have already blown the fuses provided as replacement.
> > The meter literature specifies ceramic 1/4 amp fuses. They run $3 each
> > Glass fuses are cheaper. So the question - Could I get away with using
> > glass fuses instead of ceramic? To complicate things I am "encouraged" to
> > order from specific venders so even if I find a vendor with real cheap
> > ceramics I could not order (ebay is NOT allowed)
> >
> > Don Mathieson
> > Tulsa Community College
> > dmathies@tulsacc.edu