Battery boosted dial memory

A telephone subset has a dial circuit (2) which can be powered from the exchange line (11, 12) via a regulated voltage terminal (VDD) of an associated transmission circuit (3) when the subset is OFF-HOOK and the line voltage is above a threshold voltage. When the subset is ON-HOOK, the dial circuit (2) is powered from a battery (B1) via a first switching circuit (TR2, TR3, R3). A hook-switch (HS) hold the first switching circuit OFF when the subset is OFF-HOOK to isolate the battery (B1) from line voltage. If the line voltage falls below the threshold when OFF-HOOK, a current sensing resistor (R4) switches a second switching circuit to connect the battery (B1) to the dial circuit (2) while the regulated voltage terminal presents a high impendance to isolate the battery from line voltage.

TECHNICAL FIELD 
This invention relates to telephone subsets incorporating an integrated 
circuit for producing dial signals, sometimes called a pulse/tone 
repertory dialler circuit. 
The dialler circuit contains a volatile memory in which to store re-dial 
numbers and repetory numbers. This type of memory must therefore be 
maintained by a sufficient source of power both in the ON-HOOK and 
OFF-HOOK modes. 
BACKGROUND OF THE INVENTION 
Where low voltage subsets are used the memory of the dialler circuit is 
conventionally maintained when the subset is in the ON-HOOK mode by an 
internal battery in the form of a dry cell usually of the lithium type. 
The battery is disconnected from the dialler circuit's power terminals 
when the subset is brought into the OFF-HOOK mode, the power terminals 
then being connected to the exchange battery via the subset's operated 
hook switch. 
It is imperative that the internal battery terminals are electrically 
isolated from the exchange battery in order to avoid detrimental charging 
of the internal battery where the exchange battery voltage exceeds the 
internal battery voltage, and, where the internal battery voltage exceeds 
the exchange battery voltage, to avoid discharging of the internal 
battery. 
A problem arises when a low voltage subset is connected in a parallel 
arrangement with one or more other subsets in that, in the event that the 
low voltage subset is in the OFF-HOOK mode and its dialler circuit's 
memory is being powered by the exchange battery, and another subset is 
brought into the OFF-HOOK mode, the voltage at the line terminals of the 
subsets drops to a level insufficient to maintain the memory in the 
dialler circuit. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a circuit 
arrangement in a low voltage subset which, in the event of the voltage at 
the line terminals of the OFF-HOOK subset dropping to a level insufficient 
to maintain the subset's dialler circuit memory, connects an internal 
battery having sufficient power, and at the same time electrically 
isolating the internal battery from the exchange battery. 
According to the invention there is provided a telephone subset arrangement 
comprising line terminal means across which is connected, via the 
switching path of a controllable semiconductor line switch, a loop current 
circuit of a transmission circuit means, said loop current circuit 
including a load resistance means; a dialler circuit means including a 
memory means and power terminal means connected to a regulated voltage 
source derived from normal line current in said loop current circuit by 
regulator means; a hook switch means capable of being rendered into an 
OFF-HOOK state or an ON-HOOK state; wherein said arrangement further 
comprises a first controllable semiconductor switch means, a control 
element of which is coupled to a circuit which includes said hook switch 
means, and a switching path of which couples said battery power supply 
means to said power terminal means of the dialler circuit means when said 
hook switch means is in the ON-HOOK state, and decouples said battery 
power supply means therefrom when the hook switch is in the OFF-HOOK 
state; and a second controllable semiconductor switch means, a control 
element of which is coupled across said load resistance means, and a 
switching path of which couples said battery power supply means to said 
power terminal means of the dialler circuit means when line current in 
said loop current circuit falls to a predetermined magnitude, said battery 
power supply means being decoupled from the line current by said regulator 
means. 
In order that the invention may be readily carried into effect, an 
embodiment thereof will now be described in relation to a drawing showing 
a schematic circuit of part of a low voltage subset incorporating the 
present invention.

DESCRIPTION OF THE INVENTION 
Referring to the drawing, the circuit comprises a line switch 1 comprising 
a semiconductor switching arrangement (not shown) whose controllable 
conductive path is connected between terminals "a" and "b" and whose 
control element is coupled to terminal "c"; a line switch control switch 
in the form of an NPN transistor TR1; an internal 3 volt battery B1, 
preferably of the lithium type; a first battery switching arrangement 
comprising an NPN transistor TR2 and a PNP transistor TR3, controlled by 
hook switch HS; a second battery switching arrangement comprising NPN 
transistors TR4 and TR5, PNP transistor TR6 and associated bias resistors 
R1, R2 and R3; a dialler circuit 2 which includes a dial pulse output DP, 
power terminals VDD and VSS; an integrated transmission circuit 3, for 
example a Motorola MC34014 Telephone Speech Network, including a positive 
line voltage terminal V+ connected to the subset's line terminal L1 (not 
shown) via "a"-"b" terminals of line switch 1, a negative line voltage 
terminal V- connected to the subset's line terminal L2 (not shown) via a 
common rail 4, a regulated voltage output VDD connected to power terminal 
VDD of dialler circuit 2. This regulated voltage is the output of a known 
shunt type regulator (not shown) with a nominal voltage, typically about 
3.3 volts. The nominal output current is increased when dialling. The 
transmission circuit further includes a DC load resistor terminal LR which 
is connected to the negative line voltage terminal V- by a load resistor 
R4 which determines the DC resistance of the subset, and removes power 
dissipation from the transmission circuit. 
A capacitor C1 is connected across the emitter/collector of transistor TR4 
and an associated filter resistor R5 is connected between the collector of 
transistor TR4 and the junction of terminal "b" of line switch 1 and the 
positive line voltage terminal V+. 
A capacitor C2 is connected across VDD and the common rail. 
In operation, firstly in the case of the low voltage subset being ON-HOOK 
and no parallel subsets OFF-HOOK: being in the ON-HOOK mode line switch 1 
is in a non-conducting state and hence there is no voltage on line voltage 
terminals V+ and V-. Battery B1 renders transistors TR2 and TR3 conducting 
thereby connecting the positive pole of battery B1 to terminal VDD of 
dialler circuit 2. Terminal VSS of dialler circuit 2 is connected to the 
negative pole of battery B1 via the common rail 4. The dialler circuit's 
memory is thereby maintained. As line switch 1 is not conducting, exchange 
battery is isolated from line voltage terminals V+ and V- and consequently 
isolated from battery B1. 
Secondly, in the case of the above subset being brought into the OFF-HOOK 
mode and no parallel subsets OFF-HOOK: Transistors TR2 and TR3 are 
rendered non-conducting by a shunt circuit closed by hook switch HS. 
Transistor TR1 is rendered conducting by a signal on terminal DP of the 
dialler circuit 2 and thereby renders line switch 2 conducting. As a 
consequence exchange battery is connected to line voltage terminals V+ and 
V- causing current to flow through resistor R4 via LR terminal of the 
transmission circuit. In this case the current is such that a voltage is 
produced across resistor R4 which is sufficient to turn on transistor TR4 
which thereby prevents transistor TR5 and TR6 from turning on. The 
positive pole of battery B1 is thereby electrically isolated from terminal 
VDD of dialler circuit 2 and the regulated voltage provided at terminal 
VDD of transmission circuit 2. 
Thirdly, in the case of the subset being in the OFF-HOOK mode and a 
parallel subset is brought into the OFF-HOOK mode: the line current drops 
accordingly, typically to less than 8 .ma. The voltage at the positive 
line voltage terminal V+ falls and when the voltage is below a threshold 
voltage, typically 1 volt, the VDD terminal of the transmission circuit 
switches to a high impedance state in a known manner to electrically 
isolate battery B1 from line voltage terminal V+ and prevent it from 
discharging. The voltage produced across resistor R4 is not sufficient to 
turn on transistor TR4 via resistor R1 and consequently transistors TR5 
and TR6 are turned on. The positive pole of battery B1 is connected to 
terminal VDD of dialler circuit 2 via the emitter/collector junction of 
transistor TR6 maintaining the dialler circuit's memory. 
Capacitor C1 prevents TR5 from turning on while dial signals are applied to 
the subset's line terminals with no parallel subsets OFF-HOOK. 
If the subset is not connected to an exchange line, the internal battery 
cannot discharge because transistor TR6 is not conducting. 
While the present invention has been described with regard to many 
particulars it is understood that equivalents may be readily substituted 
without departing from the scope of the invention.