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Re: [Phys-L] ceramic fuses

I thought the reason for the ceramic was to contain the arc, not extinguish it. If the meter cost > 10X the cost of the fuse, I'd go w/ the ceramic. (both as suggested by BW)

This reminds me of H. Ford's answer to the question how can he afford (make a profit) on his inexpensive 'biles. "I make it up on the spare parts."

bc gets his DMMs free from Harbor Freight.

p.s. No lab fee or penalty after the first fused fuse?

p.p.s. discharging a cap. may result in a high peak current, but not last as long as connecting to the domestic supply or a HV P/S. Of course if a student is messing w/ a high capacity flash lamp storage cap, he should not be a HS student. I offered a bunch (>100) of caps to a CC and was refused as too dangerous.

On 2012, Aug 20, , at 10:01, brian whatcott wrote:

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

On Mon, Aug 20, 2012 at 10:38 AM, <> 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

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