jerry freedomev via EV wrote:
I've used for many EVs just a 2" length of solid copper wire sized
right for the load [as a fuse]. Higher voltage, use longer. Using SS bolt/nuts
as terminals and support with distance and something to catch the
molten metal if it blows.
That works! :-) A fuse is indeed just a piece of wire.
But if you make your own fuses, you need to *test* them to know the
current at which they will open. You can't rely on someone else's "#16
wire blows at 100 amps" to work, because there are many variables that
have a *drastic* effect on the fusing current:
- the length of the wire
- any insulation on it
- heat being dissipated by the connections at each end
- airflow around it
- the surrounding air temperature
- how long you expect it to carry the current before opening
But if you have the patience, you can do the tests and figure it out.
Another point Jerry mentioned. When a fuse blows, it scatters molten
metal around. This needs to be contained, so it won't short something
else or start a fire. Commercial fuses are packaged in glass, ceramic,
or phenolic for this reason.
Arcing is another concern. The arc in a DC circuit can stretch out a
surprising distance. This is especially true in a fuse, because the
melting metal provides conductive particles to help the arc. On DC,
you'll need at least an inch per 100 volts to break the arc.
Commercial DC fuses generally fill the inside with sand or some other
noncombustible insulator. When the fuse blows, the heat also melts the
sand, which flows into the space to help break the arc.
I've made my own fuses for battery boxes. For example, a pair of 5/16"
bolts on standoffs in a bakelite box. The fuse element is a strip of
sheet copper between the bolts. I trim the width until it blows at the
desired current and time.
--
I view this year’s failure as next year’s opportunity. Failures are
not something to be avoided. You want them to happen as quickly as
you can, so you can make progress rapidly. -- Gordon Moore
--
Lee Hart's EV projects are at http://www.sunrise-ev.com/LeesEVs.htm
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