> Just out of curiosity, I worked out the numbers for a simple buck
> regulator that would convert 50V at 10uA to 5V at 100uA, just to see if
> it is doable.
>
> I chose a switching frequency of 100 kHz for a good balance between
> being easy to filter out of the audio and good efficiency. I came up
> with a coil value of 100mH, which is rather large. In fact, I couldn't
> find any 100mH coils that didn't have a self-resonant frequency that
> was less than 100kHz!

I assume you were trying to keep the maximum inductor current low.  I
would put an RC filter on the line with a big enough reservoir cap so that
the energy for each pulse comes almost entirely from the cap and not the
line.  The line would then see an average current draw, which of course is
desirable anyway.  In that case, the peak inductor current can be much
higher.  There would be short pulses with large spaces inbetween.  This
will be a bit less efficient, but should make the problem tractable.

For example, with a 1mH inductor (those are quite available), a 10uS pulse
at 50V builds up the current to 500mA.  That's an average current of 250mA
for 10uS from the reservoir cap.  This would only cause a 25mV drop on a
100uF cap.

A more serious problem is how to power the control electronics without
eating into a substantial portion of the power budget.

Again, this is an interesting intellectual exercise, but in the end trying
to draw any real power from the phone line is a bad idea.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu