Re: Re: Optoisolation for a modem

> From: "Jim Fackert" <jfackert@...>
> Subject: Re: Re: Optoisolation for a modem
>

> 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.

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

Good idea, just more work.

> 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.

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

> From: "Harrison, Doug" <dharrison@...>
> Subject: RE: Re: Optoisolation for a modem
>
> The 170 volt MOV is what is used in power surge protectors. But isn't this
> clamp voltage too high for a modem? (Speaking, of course, with total
> ignorance.)

Your modem should have it's own protection in addition to this. A
working protector in front of this circuit is also helpful/required by
law.

> Like Jim, I also wonder about the choice of composition resistors. Why
> composition?

See answer above.

From: Jon Elson <jmelson@...>

>The FCC part 15 is entirely about EMI. Is it part 29 that deals with

telecommunications
subscriber equipment?

It's been 15+ years, your probably right.

>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.

Outright failure is a valid option, the only problem with some of the
films is their tendency to arc over.
3-terminal gas tube supressors. Your supposed to have one anyway! At
least in the areas that we were testing in. I agree, get an extra one if
you don't trust your local telcos.

> 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.

They still catch the very short transients passed by the tubes to a
certain degree. There are probably better, if more expensive devices
today.

>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.

True, if your equipments plugged into the wall you need 3 MOVs. If
electrical storms are common in your area, your local telco might have
some equipment for sale that can help. From past experience (a while ago
I admit) some companies are very helpful and others are almost useless.

Good luck,

Jim Eckman
Former designer of telephony equipment in the distant past