Is 48 V through 5 Megohms enough power for you? That's the limit for steady state.


"
On-Hook Impedance Limitations

Dc on-hook resistance is required to be greater than 5 megohms for an
applied voltage of up to 100 V and greater than
30 kilohms for an applied voltage of up to 200 V (see Figure 3).

[This would explain the neon bulb and series resistor I find across
my lightning arrestor. Apparently the telco jacks up the line
voltage to 200 V to fire the bulb and confirm line continuity to the
arrestor]

The telephone companies use an automated insulation
test system as part of their maintenance routine. If the dc on-hook
resistance of an attached piece of equipment is too
low, the line is interpreted as being damaged, and repair procedures
are initiated. The dc resistance requirement
prevents false service alarms.
"

So at most you get 10uA. Might be enough for LP mode, but LCDs
draw 2mA plus so you'll need a big cap, and high enough impedance that
you don't trip the ring circuit with the charging current. (>200 ohms).

David VanHorn wrote:
>
> At 09:19 AM 9/16/2003 -0500, michael brown wrote:
>
> >With all this talk about the CMX chips, I've been looking their line-up
> >over.  I was thinking that I could use a PIC, a CMX602B and an LCD
> >display without battery power to show who's calling yet never change
> >batteries.  Does anyone have any idea how much power you could tap from
> >the line without causing it to go off hook?  I know the number would
> >probably be small, but I have yet another idea if that won't work.  ;-)
>
> very little.
>
> this page will help you a lot.
> http://www.ce-mag.com/99ARG/Gubish233.html
>
> >I know the DC loop is limited to small amount of maximum current, but
> >the ringing signal seems to have quite some oomph behind it.  I'm
> >wondering how hard it would be to capture the first ring to charge up
> >some capacitors, and then use that to power up the PIC, LCD and CID
> >chip, capture the data and then display it till the caps run down.
> >Would it work?
>
> current drawn during ring is how the co tells that you're off hook.
>
> have a look at the link above, it will answer many questions.

Yes, I learned quite a bit about POTS (plain old telephone service).
Thanks.

R

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