Re: Fuses

[Chuck Britton <britton@ODIE.NCSSM.EDU>]

> the bulbs dim each time another is added and I must wave my
> hands about the fact that in your living room this doesn't happen.

        Well, it DOES happen in MY living room, when the freezer or
refridgerator compressor kicks in with a BIG startup current surge.

Are you SURE that the dimming is caused by the iron heating up? Might the
power supply or batteries have a limited amount of current capacity. Check
this easily by repeating the experiment with the fuse 'shorted out'. If the
bulbs still dim as each is added, you need a 'beefier' power supply or
fresher 'D' cells etc.

If it ONLY dims when the 'fuse' is installed, your analysis is correct and
you might try using a shorter piece of 'iron'. This will have the same
current carrying capacity but less resistance.

I've noted that aluminum foil strips (about 25.4 mm wide ;-) get real warm
when more than 10 amps is put thru them. A narrower strip might work for
you. I KNOW that aluminum WON'T glow. It will oxidize (burn out) at a lower
temperature.

> I'm curious to know if anyone has a suggestion for a better material,
> and also,  if anyone can comment on what is actually used in real
> fuses.  Obviously the melting point has to be right,  but from what I
> see in this circuit,  a real fuse must also have a low resistance.

Real fuses come in many varieties, but yes, they must have low resistance
and therefor a fairly low melting point. Perhaps lead or even an alloy such
as 'Wood's metal would work. Gallium would work in cold environments!

Check the shelves at Radio Shack and you'll find both Slow Blow and Fast
Blow fuses.  The Slow Blow variety allows the large 'inrush' currents (like
with compressor motors) but won't burn out unless the high current doesn't
decrease quickly. (I suspect that these slow fuses have a larger 'Thermal
Mass' so they heat up more slowly.

Fast blow fuses would have a minimal thermal mass and are designed to blow
AS SOON AS the current goes over the disign level. The fuses would be used
to protect delicate electronic circuits as opposed to the more rugged
motors that need Slow Blows.

(Don't try describing these fuses to adolescent males who are fascinated by
White House interns!)