Re: Optoisolation for a modem

There are basically two failure modes:
1: The phone line is low impedance and a AC transient uses the phone line as return path to ground and fries the modem.

2: The AC line is low impedance and a transient comes in on the phone line and use the AC/ ground as low impedance return. Same
result!

Get an AC transient suppressor that is common to both the PC with its accessories and the phone line.

For example, I use an APC UPS with a built in phone transient protector. Then make sure all your equipment is plugged into that
protector. In other words, ALL power (and grounds) should go through that one common point.

By connecting the two transient protectors together at one common point that failure mode is avoided.
Bertho
==============================
"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.
>
> 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.
>
> We live on a lake, so lightning is a problem. Any suggestions, such as
> optoisolation?
>
> Doug