Re: Re: Optoisolation for a modem

James Eckman wrote:

> From: James Eckman <fugu@...>
>
> > From: "Harrison, Doug" <dharrison@...>
> >
> > Does anyone know of a good way to protect a modem from spikes on the
> phone
> > line?
>
> 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.

The FCC part 15 is entirely about EMI. Is it part 29 that deals with
telecommunications
subscriber equipment?

> Of course, finding carbon comps may require a search of surplus
> stores,
> I think everything nowadays is carbon film or metal film.

No, you can get them from distributors, but they are expensive. they
also have the capability
to take heavy surge loads without failure. I WANT them to fail - blow
the film right off, leaving
a clean ceramic core - when there is a heavy lightning strike. The FCC
telecom specs for
lightning discharges is probably OK for a $15 phone, but just not enough
for a $150 modem
(or a $3000 phone system). Everybody who is serious about this uses
3-terminal gas tube
supressors. There must be a reason why. I don't have the facilities to
determine whether they
are right, but I have good reason to believe them. The 3-terminal gas
tube shorts BOTH
sides of the line to ground within 10 Ns of a substantial spike.
Supposedly, the MOVs can take
100+ uS to get up to full conduction. The gas tube devices are rated at
80,000 amps
(obviously for a very short time). I suspect the MOVs drop large
voltages if the peak
current exceeds a few thousand amps.

Also, just using one MOV across tip and ring does not protect against
common-mode
transients, which is what you'd expect a lightning-induced spike to be.
The voltage from
the phone line terminals to the power ground or computer data ground
would be allowed
to rise until something inside the modem arcs over, most likely in the
signal transformer.
You need to use 3 MOVs, one across the line, and one from each side to a
heavy ground
path. Suddenly, the $3 or so for the gas tube doesn't seem so bad.

Jon