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The Ringer
Simply speaking this is a device that alerts you to an incoming call. It
may be a bell, light, or warbling tone. The telephone company sends a ringing
signal which is an AC waveform. Although the common frequency used in the
United States is 20 HZ, it can be any frequency between 15 and 68 Hz. Most of
the world uses frequencies between 20 and 40 Hz. The voltage at the subscribers
end depends upon loop length and number of ringers attached to the line; it could
be between 40 and 150 Volts. Note that ringing voltage can be hazardous; when
you're working on a phone line, be sure at least one telephone on the line is
off the hook (in use); if any are not, take high voltage precautions. The telephone
company may or may not remove the 48 VDC during ringing; as far as you're concerned,
this is not important. Don't take chances.
The ringing cadence - the timing of ringing to pause -varies from company to
company. In the United States the cadence is normally 2 seconds of ringing
to 4 seconds of pause. An unanswered phone in the United States will keep
ringing until the caller hangs up. But in some countries, the ringing will "time out" if
the call is not answered.
The most common ringing device is the gong ringer, a solenoid coil with
a clapper that strikes either a single or double bell. A gong ringer is
the loudest signaling device that is solely phone-line powered.
Modern telephones tend to use warbling ringers, which are usually ICs powered
by the rectified ringing signal. The audio transducer is either a piezoceramic
disk or a small loudspeaker via a transformer.
Ringers are isolated from the DC of the phone line by a capacitor. Gong
ringers in the United States use a 0.47 uF capacitor. Warbling ringers in
the United States generally use a 1.0 uF capacitor. Telephone companies in other
parts of the world use capacitors between 0.2 and 2.0 uF. The paper capacitors
of the past have been replaced almost exclusively with capacitors made of Mylar
film. Their voltage rating is always 250 Volts.
The capacitor and ringer coil, or Zeners in a warbling ringer, constitute
a resonant circuit. When your phone is hung up ("on hook") the
ringer is across the line; if you have turned off the ringer you have merely
silenced the transducer, not removed the circuit from the line.
When the telephone company uses the ringer to test the line, it sends a
low-voltage, low frequency signal down the line (usually 2 Volts at 10 Hz)
to test for continuity. The company keeps records of the expected signals
on your line. This is how it can tell you have added equipment to your line.
If your telephone has had its ringer disconnected, the telephone company
cannot detect its presence on the line.
Because there is only a certain amount of current available to drive ringers,
if you keep adding ringers to your phone line you will reach a point at
which either all ringers will cease to ring, some will cease to ring, or
some ringers will ring weakly. In the United States the phone company will
guarantee to ring five normal ringers. A normal ringer is defined as a standard gong
ringer as supplied in a phone company standard desk telephone. Value given
to this ringer is Ringer Equivalence Number (REN) 1. If you look at the
FCC registration label of your telephone, modem, or other device to be connected
to the phone line, you'll see the REN number. It can be as high as 3.2, which
means that device consumes the equivalent power of 3.2 standard ringers,
or 0.0, which means it consumes no current when subjected to a ringing signal.
If you have problems with ringing, total up your RENs; if the total is greater
than 5, disconnect ringers until your REN is at 5 or below.
Other countries have various ways of expressing REN, and some systems will handle
no more than three of their standard ringers. But whatever the system, if you
add extra equipment and the phones stop ringing, or the phone answering machine
won't pick up calls, the solution is disconnect ringers until the problem
is resolved. Warbling ringers tend to draw less current than gong ringers,
so changing from gong ringers to warbling ringers may help you spread the
sound better.
Frequency response is the second criterion by which a ringer is described.
In the United States most gong ringers are electromechanically resonant.
They are usually resonant at 20 and 30 Hz (+&- 3 Hz). The FCC refers
to this as A so a normal gong ringer is described as REN 1.0A. The other
common frequency response is known as type B. Type B ringers will respond to
signals between 15.3 and 68.0 Hz. Warbling ringers are all type B and some United
States gong ringers are type B. Outside the United States, gong ringers appear
to be non-frequency selective, or type B.
Because a ringer is supposed to respond to AC waveforms, it will tend to
respond to transients (such as switching transients) when the phone is hung
up, or when the rotary dial is used on an extension phone. This is called "bell
tap" in the United States; in other countries, it's often called "bell
tinkle." While European and Asian phones tend to bell tap, or tinkle,
United States ringers that bell tap are considered defective. The bell tap
is designed out of gong ringers and fine tuned with bias springs. Warbling
ringers for use in the United States are designed not to respond to short
transients; this is usually accomplished by rectifying the AC and filtering
it before it powers the IC, then not switching on the output stage unless
the voltage lasts long enough to charge a second capacitor.
Click on one of the items below to learn
more.
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