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RINGER
Bell
Hammer
As shown in Figure 2, the ringer is connected across the tip and ring inputs
in series with a capacitor to block the 48VDC. The ringer consists of a
permanent magnet attached to an armature as shown in Figure 7. When
Armature
an alternating current of 20Hz is passed through the coils, the armature is
Permanent
Magnet
alternately attracted to one coil and then the other. The hammer attached
to the armature thus strikes one bell and then the other to produce the
ringing sound. In a newer electronic type telephone, the hammer and bell
ringer may be replaced by a piezoelectric buzzer used in an electronic
oscillator. A piezoelectric material changes its dimensions when a voltage
is applied. When the oscillator is running, it applies a 3 to 4kHz AC voltage
to the piezoelectric buzzer. The buzzer changes its dimensions and
produces a 3 to 4kHz sound. The oscillator is turned on only during 1/2 of
each cycle of the 20Hz ring signal. Thus, when a ring signal is received,
Coils
the buzzer produces a 3 to 4kHz sound switched on and off at a 20Hz rate.
Figure 7
INDUCTION COIL / BALANCE NETWORK
When transmitting and receiving is done over the same two wires, the problem arises that current from the
transmitter flows through the receiver. The speaker then hears his own voice from the receiver. This is called
sidetone. Too much sidetone may be objectionable to the speaker and cause him to speak too softly. A small
amount of sidetone is desirable to keep the telephone from sounding dead.
The induction coil and balance network limit the sidetone. The impedance of the balance network shown in
Figure 2 approximately matches the impedance of the balance network shown in Figure 2 approximately
matches the impedance of the local loop. Thus, about half of the current from the transmitter flows through L1
and the local loop and the other half flows through L2 and the balance network. The currents in L1 and L2
induce voltages in L3 of opposite polarity which limits the voltage across the receiver to an acceptable level.
When receiving a signal from the local loop, the currents in L1 and L2 induce voltages in L3 of the same polarity.
These voltages combine to drive the receiver.
The newer electronic telephones perform this function
electronically.
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