Re: Re: Optoisolation for a modem

A couple of notes ......

The FCC part governing phone line devices is Part 68 ...... and the "lightning
tests" required therin DON'T require the equipment to survive a 1500 volt
strike ..... the equipment must only fail gracefully (ie open) after the test
and not short the line (to itself, or ground). (you won't fail the test if the
equipment still operates after the lightning surge, however it isn't a requirement).

In the US, much of the lightning "protection" (on the phone company side) is
via "carbon blocks" that will conduct transients to ground (to some degree).

I personally like gas tubes, because they do react very fast, and survive many
strikes, however their downside is their firing voltage (being somewhat higher
than many MOVs).

We used "flameproof" resistors for the "fuses" on equipment we made for
the US market (at my last employer) that weren't supposed to burst into
flame (and certain ones would fail open) during the FCC lightnign surge.

The UK phone intefaces I designed didn't have any lightning protection, as
they (by design and component selection) would survive 5 KV dc line to
ground strikes without failure). (and at that time, there wasn't any surge
requirements, but because of the various "barrier" requirements we ended
up with a tough interface).

Regards,
George Potter
Placerville, California

Jon Elson wrote:

> From: Jon Elson <jmelson@...>
>
> 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
>
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