Re: Optoisolation for a modem

"Harrison, Doug" wrote:

> From: "Harrison, Doug" <dharrison@...>
>
> Does anyone know of a good way to protect a modem from spikes on the phone
> line? The Triplite surge protector works once, then the next jolt cooks the
> modem. We went through three modems and a fax machine last year. Bear in
> mind that we almost always unplug the modem after each use. It's those rare
> "almost's" that cause the problem.

I live in the St. Louis area, we definitely have big thunderstorms here.
I built my own protector, and it has done fine so far. I put a 1/2W
10 Ohm carbon FILM resistor in series with each side of the phone
line. I then connected it to a 3-terminal gas-tube supressor. The
phones are connected across the supressor. The center terminal of
the supressor is grounded with a heavy wire to the water pipe that
heads outside. I've had the outside protector blown out once (actually,
maybe twice, in 10 years) but the modem and phone gear has never been
harmed. (I not only have a modem to protect, but also a business
PBX phone system, which would cost a lot more than the modem
to replace.) I generally leave the modem plugged in.

The 1/2W 10 Ohm film resistors are specifically selected as a very fast
fuse, for line cross and direct lightning hits. I have heard the gas tube
fire on induced potentials due to ground strokes within a few hundred
yards. It sounds like a substantial pop. But, the resistors have never
been called upon to do the fuse thing.

A crude attempt at ascii art :

Phone line to phones

______^^^^^____________
Res |
__|__
| |
Ground | |
______________| |
| |
| |
|____|
|
Phone Line | to phones
_____^^^^^______|__________

> Our power is relatively surge free, probably because we have MOV's on three
> circuit panels and three wall outlets. Haven't blown one of these yet. The
> Triplite phone line protectors are just too expensive to replace as often as
> they go bad here. And they only get the first blast. The modem gets the
> rest.

Buy a better class of surge protectors! They cost $10 more, and carry a
lifetime warranty on the protector, and a $10,000 coverage on the computer
gear they are protecting.

> We live on a lake, so lightning is a problem. Any suggestions, such as
> optoisolation?

Opto-isolation won't work on phone lines or power lines. Too bad, it
would be nice.

Jon