Re: Re: Optoisolation for a modem

----- Original Message -----
From: James Eckman <fugu@...>

> 10 ohm carbon COMPOSITION resistors in series with the tip and ring and
> followed by a 170 volt MOV, about quarter sized, across tip and ring.
> This combination can take multiple FCC part 15 test strikes.


I don't believe this is correct. carbon comps, all but extinct now, have a
solid block of carbon composition encapsulated in some plastics material.
When they fail they short and burn.

Carbon film, on the other hand, which are literally less than a dime a dozen
these days, consist of a ceramic rod whit a thin firm of carbon stuff on the
surface, cut spirally to form resistive element, adn coated with some
lacquer. When they fail, they open quickly like a fuse, because the carbon
is vaporized.

I would use carbon fills for fuse resistors. In fact I do this all he time
to protect the outputs of products we make...


Demonstrate it yourself. Take a 1/4 to 1/2 watt resistor, about 10 ohms, of
each type adn clip them across a 12 volt, 1 amp supply... adn watch what
happens. Be careful, use remote switch and eye protection, etc. Do this at
your own risk! fire and explosion may result.
Jim Fackert