Note: Descriptions are shown in the official language in which they were submitted.
1$1~
The invention relates to telephone line circuits in which
an energizing current for a telephone line is supplied via a winding
in a transformer and in an amount subs-tantially as determined by the
load characteristic of the telephone line. More particularly the
invention relates to telephone line circuits in which the transformer
includes a balance winding for reducing the direct current magnetic
flux associated with the telephone line energizing current.
The desirability of utilizing direct current balanced
transformers in telephone line circuits has been well known for some
time. The reduction in the physical size of the transformer accrues
economies in transformer cost and printed circuit board space utilization.
-; The design of telephone line circuits with current balancing transformers
has evolved into two different types, one being of the type in which
the load resistance of the telephone line, in the off-hook condition,
substantially determines the amount of energizing current drawn, and
the other being of the type in which a predetermined amount of
energizing current is supplied to the telephone line in the off-hook
condition. Examples of line circuits of the first type are given by
Max S. Macrander in United States patents 3,714,548 issued on
~-~ 20 November 17, 1971 and 4,064,449 issued on December 20, 1977, and by
-- Vincent Viacheslav Korsky in United States patent No. 4,103,112 issued `-
July 25, 1978. In each of these examples, a transformer includes a
balance winding through which a balancing current is caused to flow
in an amount appropriate to reduce or cancel the magnetic flux
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associated with the telephone line current. The balancing current is
typically obtained from the same source as that which supplies the
telephone line energizing current to the line circuit, however if need
be the balancing current could be supplied from some other suitable
source.
In a typical electronic private branch exchange (EPBX)
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using line circuits carried on printed circuit boards but without current
balancing windings, it has been found that the necessarily heavy
transformer tends to cause structural circuit board failure. This
failure usually occurs when the circuit board is subjected to heavy
shock loading, as for example during shipping. The solution to this
problem appeared to be to replace the existing line circuits having the
heavy transformer with line circuits having a miniature and relatively
lightweight transformer including a balance winding to reduce the
presence of direct current flux in accordance with the principles
exemplified by M.S. Macrander or Y.V. Korsky. This however requires
that the EPBX power supply must be capable of supplying additional
current for the operation of the balance winding. As the power supply
in any given PBX is typically no larger than that required for normal
operation of the PBX, this additonal balance current required by each
of the line circuits is of serious consequence. The extra current
requirement prevents practical retrofitting of line circuits with
balance windings into presently operating exchanges without the
accompanying expense of upgrading the exchange's power supply.
; A line circuit with a miniature transformer in accordance
with the invention includes a balance winding in the transformer through
which a fractional portion of the communication line energizing current
is caused to flow. As the overall operational current requirement of
the line circuit is similar to previous line circuits without a balance
winding, the requirements of the associated power supply are likewise
similar.
The present invention provides a line circuit for
supplying an energizing current to a communication line. In the line
circuit, a transformer has a split primary winding consisting of
n turns for conducting the energizing current and a balance winding
connected in series with the split primary winding. The balance winding
is poled opposite to the split primary winding. A current regulator
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is connected in series combination with the balance winding for
conducting a fraction of the energizing current through the balance
winding. A conducting means, for conducting the remainder of the
energizing current is connected in parallel across the series
combination of the balance winding and the current regulator.
In one arrangement, the turns of the balance winding are
wound counter to the n turns of the split primary winding. The split
primary winding consists of two portions, one portion being at least
a fractional turn greater than the other portion. The balance winding
is connected in series with said one portion of the split primary
winding. In this arrangement an improvement in longitudinal balance
is obtained.
In a further arrangement the conducting means is a
resistance connected in series with the one portion of the split
primary winding and conducts current through the split primary winding.
~ The current regulator conducts current serially through the balance
- winding and the split primary winding, in response to a direct current
potential developed across the resistance.
The invention also provides a method for reducing d-c
flux in a line circuit transformer having a split primary winding
consisting of n turns for supplying an energizing current to a ~ -
communication line, and a balance winding poled opposite to the split ~ -
primary winding and consisting of more than n turns. The method
includes the steps of conducting a first direct current serially via the
split primary winding and the line, in an amount proportional to the load
characteristic of the telephone line, and conducting a balance current
' serially via the balance winding, the split primary winding, and the
line, in an amount predetermined in proportion to the first direct
current. The balance current and the first direct current additively
combine to provide the energizing current and the d-c flux associated
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with the energizing current in the transformer is substantially
reduced.
Example embodiments of the invention will now be
described with reference to the accompanying drawings in which:
Figure 1 is a schematic drawing of a telephone line
circuit, and
Figure 2 is a simplified schematic drawing of an
alternate circuit arrangement in the telephone line circuit in
figure 1.
The telephone line circuit in figure 1 is intended for
connection between a typical private branch exchange (PBX) telephone
line, via tip and ring terminals 1, and an associated switching
facility via terminals 7. The switching facility may be of the
conventional metallic talking path type or one of the more recent time
division multiplex (TDM) types in which case a codec is usually used
to interface the line circuit with the switching facility. The line
circuit includes a transformer with a split primary winding 2 connected
across the tip and ring terminals 1 and a secondary winding 6 connected
across the terminals 7. A capacitor S is connected between the two
portions of the split primary winding. One portion of the split
primary winding 2 is connected via terminal 13 and a current conducting
' network to a power source terminal +V. The current conducting network
includes a resistor 11, a diode 16, a resistor 12, and a resistor 3,
all connected in series. A capacitor 18 is connected between the
terminal +V and the junction of the resistors 3 and 12. The transformer
-~ includes in addition, a balance winding 10 poled opposite to and
-~ having more turns than are present in the split primary winding 2. The
balance winding 10 is connected between the terminal 13 and a current
regulating network. The current regulating network includes a
resistor 14 connected between the terminal +V and the emitter of a
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transistor 15. A diode 22 is connected between the collector of the
transistor 15 and the balance winding 10 remote the terminal 13. The
base electrode of the transistor 15 is connected to the junction between
the diode 16 and the resistor 12. A capacitor 17 and a diode 24 are
connected in parallel between the terminal +V and tbe base electrode
of the transistor 15, the diode 24 being oriented in current opposing
relationship. A capacitor 21 and a diode 23 are connected between a
power source terminal -V and the junction of the diode 22 and the
balance winding 10. A resistor 4 is connected between the other portion
of the primary winding and the terminal -V.
An operational telephone line circuit is normally
associated with a d-c supervision circuit and a ringing circuit. Various
of these circuits and the use of same are generally well known to ~ -
persons normally skilled in telephony. Hence examples of these
circuits are not shown or described as they would not materially
contribute to the understanding of the invention.
In operation, a telephone set (not shown) associated with
the telephone line and in an off-hook condition completes a current
path across the power source terminals +V and -V via the telephone
line circuit. A first direct current flows from the terminal +V and via
the resistors 11, 12 and 3, and diode 16, substantially as determined by
the load characteristic of the telephone line and the telephone set.
The first direct current causes a voltage, lower than the potential at
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~ the terminal +V, to be de~e~ed at the junction of the diode 16 and
-~ the resistor 12. This causes the transistor 15 to conduct a direct
current, via the diode 22 and the balance winding 10, of sufficient
magnitude to develop a voltage across the resistor 14 which corresponds
to the voltage across the resistor 11. This current is hereafter
referred to as a balance current. The balance current and the first
direct current combine additively to provide an energizing current for
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the telephone line. The energizing current in the telephone line flows
via the split primary winding 2 and the resistor 4 to the terminal -V.
The balance current in the balance winding is determined by the ratio of
the ohmic values of the resistors 11 and 14. The values of these
resistors is chosen such that the balance current substantially cancels
the d.c. flux associated with the energizing current in the split
primary winding 2.
Alternating current signals are introduced at the
transmitter in the telephone set and also from the secondary winding 6
in a well known manner. The capacitor 5 provides a voice frequency
alternating current path between the two portions of the split primary
winding 2. Alternating current feed impedances are substantially
determined by the resistors 3 and 4, with the capacitor 18 providing
a low impedance a-c path between the terminal +V and the junction of
the resistorsl2 and 3. The capacitor ~4 provides an a-c ground at the
base of the transistor 15 so that the balance current conducted by the
transistor 15, is preferably void of any appreciable a-c component.
A line circuit constructed and operated in accordance
with the preceding description will advantageously utilize a miniature
: 20 transformer as compared to the more conventional line circuit without
a balance winding. However, in spite of the balance current requirement,
the former requires essentially no greater current supply than does
; the latter.
The balance winding 10 in any given transformer design has
been found to have an influence upon the longitudinal balance
characteristics of the line circuit. For example, in one design the
balance winding 10 was wound in the same direction as the split primary
winding 2 and connected to be poled opposite thereto in operation. In
another design the balance winding was counter wound relative to the
turns of the split primary winding. The one portion of the split
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primary winding, to which the balance winding is connected, includes
about an extra half turn more than the other portion of the split
primary winding. In this design a longitudinal balance improvement of
several dB was obtained. Yet further improvement was obtained with
the addition of the capacitor 21.
In one design of the transformer wherein it is required
to match the typical telephone line a-c impedance with an input
impedance in the switching facility of about 7000 ohms, the turns
ratio of the balance winding 10 to the primary winding 2 is 3:2
disregarding the extra half turn. The current flow through the
resistor 3 is about one-third of the energizing current and the current -
flow through the transistor 15 is about two-thirds of the energizing
current. This is achieved by having the ohmic values of the resistors 14
and 11 corresponding to a ratio of 2:3 with the diode 16 compensating
for the base emitter junction voltage drop in the transistor 15. During
tests for longitudinal balance, a value of 200 picofarads was found to
be satisfactory for the capacitor 21, however it is likely that the
optimum value will vary from one design to the next.
The line circuit in figure 1 is intended for use in
areas which are unlikely to be subjected to lightning strikes, as for
example in the case of PBX telephone lines. However, in this use, the
occurrence of high voltage hits from inadvertent or accidental power
line crosses remains a distinct possibility. The diodes 22, 23 and 24
are therefore included to protect the transistor 15 in the event of
; such occurrence.
The circuit shown in figure 2 can be directly substituted
between the terminals +V and 13 in place of the corresponding circuits
in figure 1. In this circuit, a resistor 33 carries the first current
- with the voltage developed tnereacross being divided across resistors 31
and 32. The resistor 31 provides a function similar to that of the
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resistor 11 in figure 1. A capacitor 37 provides an a-c ground at the
junction of the resistors 31 and 32. The balance winding 10, the
transistor 15 and the resistor 14 remain and operate as previously
described in reference to figure 1, however the base of the transistor
is now connected to and driven by the output of a differential amplifier 39
connected in a voltage follower configuration. Components to enhance
longitudinal balance and to provide overvoltage protection are not shown
as they are similar to the components already described in relation to
figure 1. As the differential amplifier 39 is a high input impedance
device, this alternate embodiment carries cost reduction advantages over
the line circuit in figure 1 in that the capacitor 18 is no longer
required. Also the resistors 31 and 32 are of ohmic values in the order
of tens to thousands of times greater than the resistor 33, and therefore
- the capacitor 37 is correspondingly very much reduced in size.
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