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PICList Thread
'Phone line power'
1995\08\13@205743 by Kalle Pihlajasaari

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Hi,

Does anyone have any hints or tips on powering a PIC from a phone
line (yes I know, one would have to get type aproval to sell them).

Does one just use the same sort of diode chain clamp that is found in
IC telephone circuits.

I seem to recall someone asked about a similar problem but am not sure
if it was to do with phone lines.

Some of the applications I have in mind are dialtone checker, ring-counter
and perhaps even a call metering device.  Power is available in
large quantities when off hook (10 V at 50-60mA) but for the ring
counter I would like to load the line as little as possible, Is there
a 10-48V to 5V switching converter IC from maxim or the like that is
used.

Cheers Kalle
--
Kalle Pihlajasaari     spam_OUTkalleTakeThisOuTspamdata.co.za
Interface Products     Box 15775, Doornfontein, 2028, South Africa
+27 (11) 402-7750      Fax: +27 (11) 402-7751

1995\08\13@222651 by Mike Keitz

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>Hi,
>
>Does anyone have any hints or tips on powering a PIC from a phone
>line (yes I know, one would have to get type aproval to sell them).
>
>Does one just use the same sort of diode chain clamp that is found in
>IC telephone circuits.
>
>I seem to recall someone asked about a similar problem but am not sure
>if it was to do with phone lines.
>
>Some of the applications I have in mind are dialtone checker, ring-counter
>and perhaps even a call metering device.  Power is available in
>large quantities when off hook (10 V at 50-60mA) but for the ring
>counter I would like to load the line as little as possible, Is there
>a 10-48V to 5V switching converter IC from maxim or the like that is
>used.

If such a project will eventually be offered for sale, the first step would
be to research the FCC or other regulations to see how much power you're
allowed to draw from the phone lines.  Most phones seem to use a 10 M
resistor to keep the RAM alive while on-hook, that is only 4.8 uA, but a PIC
in standby mode could survive on that.  I don't think the switching
converter would justify its complexity, and is likely to couple switching
noise out to the phone line that could cause trouble with certification.  If
you need more than a few uA continously while in standby a battery is
probably going to be required.

Plan on SLEEPing the PIC a lot, and supplying brief bursts of power to it
from a large capacitor when it is actually running.  It may also be feasible
to use a really slow RC oscillator, which is speeded up by switching in a
smaller resistor from a port pin when more computing power is required.  A
simple Zener shunt should do, unless the device is to take over the line
completely.  The "line in use" detector could include a bypass of the
resistor to boost available current while the line is in use.  When the
phone rings, it is also permissible to draw a relatively large amount of
power.  This can be accomodated by AC-coupling the line to another rectifier.

Phone lines can be an excellent source of surges, so sending the raw line
inputs through small value resistors then a shunt MOV is a real good idea to
protect the circuit.

-Mike
Reverse-Enginnering the Future.

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