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Figure 4: AC vs DC Circuit
You will be most familiar with AC circuits as those that deliver 120 or perhaps 240 volts, AC, to power
the appliances, lights and electrical outlets in your home.
The direction of electron flow in Direct Current (DC) circuits is constant at all times. DC voltages are
always trying to find a path to ground.
Telephone Circuit Electrical Requirement
Your phone’s components, including the DTMF key pad, tone generator, and transducers, require
that a Direct Current (DC) voltage be present. To be precise, the CO must provide a DC voltage of
anywhere between -48V and -52V, at a minimum current of 23 milliamps (mA), in order for your
phone to function.
The -48 to -52 volts are supplied on the ring side of the circuit, and the tip side is at ground potential
(0V DC).
There is one other electrical component of the telephone system that needs to be mentioned. It is
called
Ring Voltage
, which is an AC signal, and causes your telephone to ring. This ring voltage is
approximately 85V AC (must be no less than 40V), with a frequency of 20 Hz. When your phone
senses this particular voltage, it causes the ringer to activate, indicating that you have a call.
The reason that the ring voltage is AC in nature is illustrated in the following diagram.
Figure 5: Telephone electrical Circuit
As you can see, when the telephone is in the on-hook state, and waiting for a call to come in, there is
no connectivity on the cable pair (between Tip and Ring). Therefore, the DC signal path is
incomplete.
However, the ringer can still be operated by an AC signal, which does not require continuity through
Tip and Ring on the cable pair.
