🚀 go-pugleaf

henry@utzoo.uucp (Henry Spencer) writes:
> an electrical fire is *hot*.

	This is getting a bit far afield, but what the heck?  We've got a
rack of MultiTech modems.  Supposedly the cards are hot swappable (i.e. you
can pull and insert cards with the power on) but if you get them misaligned
in just the right way when you insert them, you short the +12V and -12V
power rails in the card cage.  The modular power supply has 2 1-ohm,
2(5?)-watt current limiting resistors.

	The first time we shorted a card like this one of the resistors
blew (you would think the power rails would be fused, wouldn't you?).  We
decided to fix it outselves but couldn't find any suitable resistors
around, and Radio Schlock doesn't carry that size and value either.
However, we did come across some 1-ohm/foot nichrome precision resistance
wire.  We just measured out a foot of it, coiled it up into neat open-wound
coil and soldered it in place.  Worked like a charm until the next time we
shorted a card.

	Care to guess how much power 1 foot of 20-or-so gauge nichrome wire
can dissapate without melting?  A lot!  If I ever want to build a toaster
to run off my car battery, I know what to do.  It took 10-20 seconds or so
for us to notice something was glowing inside the rack; by the time I had
the door open, the coil was glowing bright cherry red, and the PC board was
starting to burn (I didn't even know PC board material was flamable, but
there it was, burning away even after we cut power).  The amazing thing is
that after I removed the short and wiped away most of the soot, the power
supply still worked, charred PC board and all!  In fact, right now, I'm
dialed up via a modem in that rack, with that very power supply, with the
nichrome resistor still in place.  One of these days I gotta see about
ordering a spare supply....
-- 
Roy Smith, System Administrator
Public Health Research Institute
{allegra,philabs,cmcl2,rutgers}!phri!roy -or- phri!roy@uunet.uu.net
"The connector is the network"