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.)

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

Doug

> -----Original Message-----
> From: Jim Fackert [SMTP:jfackert@...]
> Sent: Tuesday, February 01, 2000 12:53 PM
> To: CAD_CAM_EDM_DRO@onelist.com
> Subject: Re: [CAD_CAM_EDM_DRO] Re: Optoisolation for a modem
>
> From: "Jim Fackert" <jfackert@...>
>
>
> ----- Original Message -----
> From: James Eckman <fugu@...>
>
> > 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.
>
>
> 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.
>
> Carbon film, on the other hand, which are literally less than a dime a
> dozen
> these days, consist of a ceramic rod whit a thin firm of carbon stuff on
> the
> surface, cut spirally to form resistive element, adn coated with some
> lacquer. When they fail, they open quickly like a fuse, because the
> carbon
> is vaporized.
>
> I would use carbon fills for fuse resistors. In fact I do this all he time
> to protect the outputs of products we make...
>
>
> 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.
> Jim Fackert
>
>
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