Re: Re: Re: Optoisolation for a modem

James Eckman wrote:

>
> > 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.
>
> They are pretty obsolete nowadays. However, in this application they
> have a tendency to stay together and will pass dozens of simulated
> part
> 15 lightning strikes. It will also pass at least a couple of real
> ones.
> Carbon films have a tendency to arc over and open up. Nothing wrong
> with
> that so long as you don't mind replacing them.

Well, I can say that my setup, with the film resistors, has survived
about 4 years in St. Louis, and
the 'Network Interface Unit' on the outside wall of the house has had
it's supressor blown out
at least once since I put mine in. (Of course, the Phone company's unit
gets to handle as much
of the surge as it can, FIRST!)
<snip>

> True, but instead hook them up to a lightning simulator, or field
> trial
> them in Colorado or parts of Texas for two years ;)

Well, I discovered this 'feature' of the film resistors when I was
working on a circuit
to start a mercury-arc lamp, with a 20 KV supply and a spark gap. I
wanted to limit
the peak current, but a 100 Ohm 2W film resistor blew apart, telling me
that my peak
current (especially without the resistor) was quite high.

Jon