Officially the voltage is not very important and can be anywhere between
24 and 48 volts DC (when the phone is on-hook)
When the phone goes "off hook" a 600 ohm impeadance completes the
circuit and draws current equal to the voltage drop divided by 600 ohms
(Ohms Law) ie 12v drop / 600 ohms = 0.02 amps (20ma)
This 20ma current is enough to tell the C.O. that the phone has gone
off hook and will either supply a dial tone or stop the "ringing"
and connect the caller
The "ringing" is about 90 Volts AC at 30 cycles per second (can vary
from country to country)
The phone can easily filter the AC ringing component to ring or trigger
the bell or ring tone.
When "off-hook" the impeadance is modulated by the voice signal at each
end of the line, thus the line is truly bi-directional
and any earphone/earpiece connected at any point along the line will
hear both sides of the conversation.
A simple capacitor can filter the DC component leaving just the audio.
This technology is over 100 years old ..simple yet very effective eh!.
Henry Coleman per VoIP-pBX.ca
im Van Meggelen wrote:
Michael Richardson wrote:
-----BEGIN PGP SIGNED MESSAGE-----
"Jim" == Jim Van Meggelen <[EMAIL PROTECTED]> writes:
>> Stupid question --- I thought that the voltage on the
PSTN was >> DC, not AC.
Jim> Not a stupid question at all; you are correct.
Nominal 48VDC on
Jim> an idle line (drops to about 6VDC when off hook).
Jim> Ringing voltage, however, is AC
oh. I thought it was -48VDC normally, and -96VDC to ring.
Nope. Battery voltage is DC, ringing voltage is AC.
Jim.
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