Note: Descriptions are shown in the official language in which they were submitted.
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~OLD CIRCUIT
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CROSS REFERENCE TO RELATED APPLICATION
My co-pending Canadian application entitled
"Hookswitch Bounce Resistance Telephone Hold-Mute Circuit"
Serial No.470,678-1 filed on December 20,1984 is related
to the present application.
~AC
Fleld of ~he Inven~ion
The pr~ent inve~tion relates to hold cir-
cuitry for u~ in ~ub~crib~r's telephones, and more
particularly to an electronic telephone hold circuit
which provide~ ~xtremely reliable operation under
short and long loop condition~ is insensiti~e to
static discharge and presents a r~latively high AC
impedence characteri~tic.
Since the development of circuitry employing
active component~, i.e.~ solid state devices, ~tc.,
~any additional features have been very easily added
to conventional telephone cir~uitry. One such feature
has been the addition of the so called "~old~ cir-
~uit. With the provision o~ ~uch a circuit, a tele-
phone call ~ay be electronically latched onto an
a~tificial load allowing the hand~et to be returnedto the h~okswitch, until a call i9 picked up on an
associated extension telephone or again picked up
at the same or original phone, at which ~.ime the hold
condition i8 dropped au~om~tically~
Previou~ hold c1rcults made u~e of expen~ive
components such as latching relay~ and or integ~ated
circuit ~oltage comparator~ and r~lated circuitry
adding ~ubstantial co~t to the telephone. Sometimes
~uch h~ld circuitry had paor sen~itivity becau ~ of
the com~romi~e be~ween trigger ~on" sen~i~ivity and
the ability to drop compl~tely on a high resi~tance
exten~ion and/or in the presence of low central o~fice
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battery voltage. Some hold circuit methods require
the use of a microprocessor and related circuitry
to sense the line voltage change with conversion oE
the result to a digital signal. It is obvious that
all such techniques suffer to some extent from a great
degree o overcomplexity and attendant thereon a re-
duction in the reliability stemming from the increased
number of components required in such circuit design.
One particular problem with previous state
of the arts circuits which utilize silicon controlled
rectifier (SCR) components is sensitivity to rapid
change of the SCR anode voltage causing the device
to turn on (also called the DV-DT sensitivity~ in
the presence of transients such as a static discharge
or lightening surge. In those cases where there is
no time out for the hold circuit, a static discharge
or lightening surge turn on of the hold circuit would
be an operational failure mode, causing the telephone
line to be in a busy mode even though the telephone
handset is still on hook. Another problem with pre-
vious circuits is the inability to detect high im-
pedence extension telephones on a short loop or failure
to latch and hold on a long loop with the telephone
in the offhook condition~
Still another problem with some previous
SCR circuitry which utilize a high microfarad energy
storage capacitor to turn off the SCR on a drop out
condition, is that the circuitry presents a low AC
impendence to the telephone line. This low impedence
makes it difficult to use with music -on- hold appli-
catlons or two line telephones on conference where
it is desired to transmit and receive voice or data
on the line.
SUMMARY OF THE INVENTION
.
The present invention employs standard NPN
and PNP type transistors insteacl of an SCR for latch-
ing and holding loop current. Also employed in the
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circui-try are resistors, diodes, capacitors, met.alic
oxide varistors and a light emitting diode. In the
arrangement disclosed a two wa~t load resistor in
the collector circuit of a high voltage NPN transistor
is switched across the telephone loop by a PNP tran
sistor used to trigger the NPN transistor on. Also
in the collector circuit of the NPN transistor is
a diode and a low value resistor in series used to
tap off emitter and base current for the PNP transis~
tor and thus establish its biasing when the NPN tran-
sistor is in the on condition. The circuit is ar-
ranged so that a momentary switch closure brings the
base of the PNP transistor low (to common) turning
it on and allowing it's collector current to ~low
into the NPN transistor base turning it on. Once
the NPN transitor is turned on it's collector current
establishes the bias currents for the PNP transistor
thereby latching the circuit into the "on" state.
Also, included in the emitter circuit of the NPN tran-
sistor is a llght emitting diode (LED), a currentlimiting resistor and a protective zener diode. The
LED, indicates -the latched "on" condition. In order
to prevent "false latch on" on transients, the posi-
tive transient (Erom the input bridge rectifier) is
capacitively coupled direct to the base of the PNP
transistor, at the same time the base is capacitively
coupled to the emitter, thereby preventing transient
turn on of the device. The PNP device is either a
high voltage device or is protected b~ a low leakage
varistor of the metalic oxide type, connected Erom
collector to emitter (for lightening surge protection).
A resistor in series with the transient coupling capa-
citor is used for protection against excessive current
flow during a surge. It should be noted that a metalic
oxide varistor across the circuit provides the basic
surge protection.
Once the circuit is latched on the positive
voltage at the anode of the LED is also Eed to a second
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NP~ transistor which in turn switches on a second
PNP transistor. The second PNP transistor is forced
into saturation and conducts almost full loop voltage
(less the drop due the latched on load~ through cur-
rent limiting resistors to a capacitor~ This capaci-
tor is allowed to charge toward a voltage defined
by a voltage divider ~ormed by a parallel resistor
and current limiting resistors.
The capacitor charges until its voltage
lU is one diode junction potential greater than the vol-
tage at the base oE -the Eirst aforementloned PNP tran-
sistor at which time a diode connected between the
capacitor and the PNP transistor become forward biased.
The charge on the capacitor then stabilizes at the
desired voltage. The absolute value of the voltage
can be adjusted by changing the parallel resistor
to the capacitor allowing to discharge more or less.
This method of charging the capacitor keeps it from
shunting the loop in the off state. The base of the
aforementioned first PNP transistor is now held at
a reference voltage that changes only slowly with
changes in loop voltage due to a time constant of
approximately 200 milliseconds formed by the ~C value.
When an extension phone is taken off-hook and the
circuit has been previously latched up, the loop vol-
tage drops suddenly thereby instantly droppin~ the
emitter bias voltage while the base remains supported
by the capacitor charge. Therefore the eirst afore~
mentioned PNP transistor becomes biased to an off
3~ condition and the circuit goes to an off state with
a high impec~ence and low DC leakeage. The reEerence
capacitor discharges through it's parallel resistor.
A mute signal can be obtained erom the collector of
the second aforementioned PNP transistor during the
on state.
Ascordingly it is the object of the present
invention to provide a high reliability hold circuit
which is resistant to lightening surges or static
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discharge transients and also has high operational
sensitivity and AC impedence circuitry. It is also
compatable with previous state of the art techniques
relative to muting of the voice network in the on
hold condition as in the technique described in my
aforementioned co-pending application. Such technique
also provides hookswitch bounce resistance for the
present invention.
BRIEF DESCRIPTION OF TFIE DRAWINGS
The single sheet of accompanying drawings
shows a simplified schematic of a hold circuit accord-
ing to the present invention located in a typical
telephone instrument, where the basic telephone cir-
cuitry is shown in block form in as much as it does
not form a portion of the present invention. The
input polarity guard and protective varistor actually
are part of the telephone circuit but are shown Eor
a better understanding of the present invention.
DESCRIPTION OF THE PREFERRED FMODIMENT
Referring now to the accompanying drawing,
polarity guard BRl metalic oxide varistor MOV-l and
the hookswitch HS-l and part o the basic telephone
circuit are also ~hared by the hold circuit of the
present invention. The metalic oxide varistor MOV-
l may not be needed for the hold circuit protection
unless surges are expected to be greater than l,000
volts. It should be noted that in a practical embodi-
ment of the present invention using standard components
circuitry has been tested to withstand 1,000 volt
surges with a 10 mlcrosecond rise time to peak and
a 1,000 microsecond eall time using a 100 ampere cur-
rent limit. In the "off hold" state transistors Ql,
Q2, Q3 and Q4 are all biased of and it can be readily
seen that there are no low impedence paths from the
positive rail to common rail when the transistors
are in the off condition. ThereEore the high imped-
ence nature of the circuit is demonstrated for the
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oEf ~tate. Typical i~pedence m~asurem~nts in the
voiceband at 3 volts rm~, indicate greater than 200,000
ohms impedence at 3,200 hert~.
To ~hange the circuit to the on state ton-
hold) switch Sl is clo~ed momentarily when the tele=
phone is o~fhook to bia~ transistor Q~ to the on con
dition~ Since ~ran~istor Q2 has turned on txansistor
Ql i~ turned on by curr~nt forced into it's ba~e by
tran~i~tor Q2 with transistor Ql turned on, current
flow~ through diode CRl and re~i~tor ~2 as well a~
resistors Rl, R3, R4 and light emitting diode CR3.
Th~ voltage drop across forward biased diode CRl and
resis~or R2 controls the emi~ter to base bias voltage
for transistor Q2 in~uring sufficient bias to latch
lS transiqtor Q2 on even u~der long telephone loop con-
ditions ~ith the telcphone oEf-hook. Therefore the
cir~uit i~ now latched into th~ "on" state (on hold3.-
Re~i~tors R4 and R5 are ratioed to ensure that the
proper bia~ is ~aintained in the on-state. Resistor
R2 is a low value of approximately 10 ohm~, sufficient
~o add approxi~ately .2 volt~ to the forward voltage
drop of diode C~l, the total approximately equal to
.6 volts or ~nough to place trans.i~tor Q2 into the
ac~ive linear region of operationO
Resi~tor~ Rl and R3 with light emitting
diode C~3 now form~ the primary hoLd current load.
Light e~itting diod~ C~3 ~mit~ visible light to indi-
cate tbe hold condition to the instrument user~ When
the circuit goe~ into th~ ~on" state and is "latched",
transistor Q4 i~ turned on by current to it's base
~rom tra~ tor Ql through re3i~tor Rll. Transis~or
Q4 ln turns biases ~ran~istor Q3 to th~ on condition
and into a saturation model Capacitor C3 charges
to a steady state voltage determ.ined by input current
through re3istor R14 and dl~charge~ current through
diod~ CR5 ~which i~ forward bia~ed) and through resis-
tor R9 back to common. The operating point of transis-
tor Q2 i9 modi~E:Led by the di~charge current through diode
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CR5 but is not cut off as long as the emitter oE tran
sistor ~2 is at a sufficiently high voltage~
Capacitor C3 and resistor R9 form a reEer~
ence voltage with a relatively long time constant
of approximately 200 milliseconds for discharge~
Th~refore if the tip and ring voltage should suddenly
drop as when an extension phone or the master phone
goes off hook, capacitor C3 working with resistor
R5 will maintain a reference potential at the base
of transistor Q2 long enough for transistor Q2 to
cut off as it emitter voltage is dropping rapidly.
Capacitor C3 will slowly update it's voltage to ac~
comodate rising tip and ring voltages such as during
a change from off-hook to on-hook without dropping
the hold condition. Once the circuit changes to an
o~f state (transistor Q2 is cutoff, dropping the hold~
capacitors Cl and C2 will prevent retriggering of
the circuit due to rising voltage at the emitter of
transistor Q2, by capacitor Cl coupling the rising
voltage to the base of transistor Q2 and clamping
base and emitter to the same instantaneous voltage
through capacitor C2. Therefore the circuit is ex-
tremely resistant to transients r although a deliberate
low voltage applied to the base of transistor Q2 through
switch Sl will immediately trigger the circuit on.
Zener diode CR2 and metalic oxide varistor MOV 2 pro-
tect the LED CR3 and transistor Q2~ respectively from
high level transients such as lightening surge.
In the on state, a relatively high AC im-
pedence of approximately 600 ohms can be obtained
by ad~usting the values of resistors Rl and R3 while
resistors R7 and R8 are an order of magnitude higher
in values. Also in the on state DC current is regula-
ted by the combination of the values of resistors
R4 and R9 such that sufficient current is developed
to hold the central office relay or other offhook
detection circuitry in the central office. If neces-
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sary the zener voltage of diode CR2 can be set toprevent excessive current to light emitting diode
CR3 on short loop conditions. Another advantage of
the circuit is it's insensitivity to light emitting
diode parameters such as the forward voltage require-
ment making many different light emitting diode types
suitable for use in the circuit assuming that maximum
operating current is not exceeded. Resistors R6,
R13 and R10 are adjusted to prevent leakage of the
circuit in the "off" state and are sufficiently high
in value to providing nonsaturation of the transistors
when required.
It will be obvious to those skilled in the
art that numerous modifications may be made of the
present invention withowt departing from the spirit
which shall be limited only by the scope of the claims
appended hereto.
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