US2350604A - Telephone instrument circuit - Google Patents

Description

June 6, 1944- J M N 2,350,604

TELEPHONE INSTRUMENT CIRCUIT '7 Filed siSt. 3,1942

INVENTOR EMANUEL FRYDMAN .i ZZ

ATTORNEY Patented June 6, 1944 TELEPHONE INSTRUMENT CIRCUIT Emanuel Frydman, Liverpool, England, assignor, by mesne assignments, to Automatic Electric Laboratories, Inc., Chicago, 'IlL, a'corporation of Delaware 1 Application September 3, 1942, Serial'No. 457,119 In Great Britain September 6, 1941 5 Claims. ('Cl..179-81) The present invention relates to telephone substation instrument circuits suitable for use in systems which follow the usual practice of *employing low frequency alternating, current for signalling a Wanted party by the ringing of a bell or the like.

The invention finds particular application to two-party line circuits for use in automatic telephone systems in which the general arrangement is that the bells of the two parties sharing a line are earthed, one bell being'connecte'd to one leg of the line and the other bell to the "other leg. In order to avoidthe'completion' of direct current earth biassing circuits by way of the telephone bells, it is then desirable to connect a condenser in the earth circuit of each of the bells. With these arrangements, however, the changes in distributed exchange battery voltage at the substation end of a party line due to one of the parties setting up a call by dialling cause tinkling of the bells both to the calling party and of the other party sharing the line and it is possible for the tinkling of the second bell to be mistaken for ringing by the second party.

In telephone instrument circuits involving a single subscriber per line, the signalling bell is usually connected in series with a condenser across the line and arrangements are made to provide a low resistance shunt path for the bell during dialling, so as to preventtinkling during this time. In this case, however, it is possible for bell tinkling to be produced by branches of trees making intermittent contact with open'wire lines and also by maintenance "men testing or working on the line. The invention has for its main object the provision of simple and efficient means for preventing the annoyance of bell tinkling'due-to the above mentioned and similar conditions.

According to the invention in a telephone system employing low frequency alternating current for signalling purposes, the circuit for-= the bell or the like in a substation instrument circuit includes a non-linear resistance element arranged to present a high resistance to the exchange battery voltage but to break down at the signalling current voltage.

The invention will be better understood from the following description of one method of carrying it into efiect, reference being had to the accompanying drawing which shows circuits of the substation end of a two-party line assumedto extend to an automatic telephone exchange; The

circuits of one of the party line stations =A- i s shown in full, while as regards'the other station B, only the bell circuit of this ha been shown, since the equipment thereat is identical With that of the first station, the only difieren'ce being that the line conductors are reversed'so 7 that the bell circuit connects with the'opposite leg of the line. Considering now the equipment at the first party line station-A, this comprises the usual transmitter T, receiver R, three-winding antia side tone induction coil comprising windings ICI,

I02 and I03, resistors YA and YB and switchhook contacts SHI and SHZ which close when the receiver or handset, as the case may-beyis lifted. The subscribers dia'l impulse contacts are designated IMF and other contacts ofthedi'al (notshown) are closed when the dial is cirnor mal and serve in well-known manner to shore circuit the transmitter T and receiverR during dialling. As regards the bell ringing circuitjthis comprises the usual condenser l0 and bell H,

but'in accordance with the invention "the addi tional condenser through which the bell is normally connected to earth is now replaced bya non-linear resistance element l2. Similar-"Ic mark apply in connection with the second party line station B with the difierence'that'the bell 2| which is earthed via non-linear resistance ele ment 22 is connected via condenser 20 to the 'opposite leg of the line compared with the bell' l I.

In accordance with usual automatic exchange practice, alternating current at a voltage -'offth! order of volts is assumed to be used for ring ing' purposes, while as will be better appreciated later in the description, the voltages which ordi narily produce bell tinkling cannot -exceed 'the voltage of the exchange batterywhichisnormally in the neighbourhood of say 50 volts. 'The function of the non-linear resistance de vices l2 and 22 is essentially to discriminate 'between ringing and tinkling voltages so that on the one hand a low resistance will be offered: to ringing currents and on the other hand 'avery high resistance will be presented to currents 22 take the form of small solid units "ofnonlinear resistance material comprising amixture" of silicon carbide with carbon and/or tungsten or molybdenum'which is agglomerated under pressure and subsequently baked. Such units ing difliculties which are encountered when the instrument bells are connected to earth via con-i:

densers.

e ing now the telephone instrument cir: cult of party line subscriber A, the condenser I will normally be charged up to. the exchange bat tery voltage of 50 volts in series with the line relay associated with the party line in question,

but the bell does not tinkle'when this circuit is first closed owing to the slow rate of charging of condenser I0 due to the high resistance of unit I2. telephone instrument is looped across the line via the switchhook contacts SH2 and condenser IQ, is discharged over contacts SHI, resistance YA, induction coil'winding 1C2, resistance YB,

I transmitter T, and dial impulsing contacts IMP. 5 At the main exchange the loop thus completed operates the line relay and causes a first selector to be seized in well-known ,manner, whereupon battery is connected to the negative leg of the linevia one windin'gof the selector impulsing relay and earth is connected to the positive leg of the line via' the other winding of the relay.

When subscriber A dials the first digit of the When the subscriber initiates a call, the" itive line to earth at the main exchange, but the unit 22 again functions to limit the current flow to a very small value and so prevents tinkling of the bell. When the dial contacts re-make, the potential is reconnected to the line side of condenser 2D whereupon the unit 22 functions as already described. 2

' Similar considerations apply in the case of a call originated by party B, the units 22 and I2 serving to eliminate tinkling of the bells of either of, the parties on the line.

' It will'be appreciated that owing to the normal high resistance of the units I2 and 22 the unbalancing effect of the earth connection made to the party line during dialling and subsequent conversation is negligible.

Considering now incoming calls to the party line subscribers, when it is desired to ring party A an earthed ringing current generator is connected by the final selector of the automatic switch train to the negative leg of the party line, whereupon the full '75 volts ringing voltage is applied across condenser I0, bell II and non-linear resistance I2. The unit I2 being normally of very high resistance absorbs practically all the applied voltage, whereupon it breaks down on the firstpeak and rapidly lowers its resistance. The breakdown condition persists until the applied A. C. voltage reahces a low value, whereupon the unit restores to its normal high resistance and it then subsequently breaks down on requirednumber, the dial impulse contacts IMP,

in opening on thefirst break impulse, removethe shunt circuit from condenser Ill which now pro ceedsfto charge up over the following circuit: battery atthe main exchange, negative line,',con-

denser III, contacts SHI, resistance vYA, induction coil winding I02, resistance YB, induction coil'winding ICI, contacts SHE, and positive line toearth at the main exchange. A charging path for the condenser is also completed via bell II andunit I2 to earth, but owing to the shunting effect of the previously mentioned comparatively low resistance earth circuit via contactsSHI, the voltage developed across unit I2 will be well below that required: to produce breakdownthereoffend consequently it maintains its very high resistancewhich prevents tinkling of the bell. When jthedial impulsing contacts reclose, the

I condenser Iqis again shunted down and on the nextpopening of the contacts IMP' the cycle of operations already described is repeated but the. bel1"I'I] again fails to tinkle owing to the resist anCeofunitIZ. r As regards the bell'2 I I of party line subscriber B, the condenser .20 is normallyfin a discharged condition since it is connected to the earthed positive line and remains so until the A partys receiver is lifted when a potential with respect to earth of approximately half the exchange battery voltage appears on the line side of condenser,

due .to the current flow from earth at the ex'-} change, positive line,. A. partys telephone loop,

andnegative line to battery at the exchange.

This potential is oicourse insufficient tojbrea'k down the non-linearfresistance.unit 22' and the charging current "through the condenser. 20 will therefore be so small that bell 2| will not tiiikleQ when the 'A party dials thel first "digit r the se qired number the dial springs in openin'g re-e move the potential from the line side of con denser,- 20 and if' any charge has accumulated thereon, it. now; tends to discharge over the p cs-1 is desired to ring the bell of party line subs'criber B, ringing current is applied at the exchange to the positive leg of the line, whereupon unit 22 functions to allow bell 2| to operate satisfactorily in the same manner as unit I2 for party A.

The invention is not limited in scope to the earthed bell party line system described as it may also be applied with advantage to party line systems using different ringing frequencies for the various parties on a party line. In this case differently tuned bells are each connected in series with a condenser acrossthe party line and the insertion of a non-linear resistance device in series with each condenser and bell will serve to avoid tinkling .due to dialling or extraneous connections and will also improve the immunity of each'circuit to ringing current of frequencies to which it is not tuned.

The invention also has a sphere of usefulness for ordinary substations employing a single telephone instrument per line, particularly where the signalling bell is permanently. connected across the line byway of a condenser and a separate condenser is employed for anti-side tone'and spark ouench purposes. In this case thefibell condenser may be replaced by asuitable, nonlinear resistance device, since this will perform all the direct current isolating functions of the condenser and at the same time prevent anypossible bell tinkling due ;to the causes described, 7} What I claim as new and, desire to secure by Letters Patent is:

1. In a telephone substation, antincoming'line upon which aringing signal of certain voltage is impressed at times and upon which a .loweii voltage is impressed at other times; a condenser'andi said calling device, a solid body of non-linear resistance material in series with said ringer presenting a high resistance to the flow of charging and discharging current through said ringer thereby to prevent actuation of said ringer when said calling device is operated, the resistance of said body being effective to decrease responsive to said certain voltage being impressed upon said line, thereby to permit said ringer to respond to said ringing signal.

2. In a telephone system, a party line to which a certain potential is normally connected at the exchange and upon each side of which a ringing signal of a higher potential is sometimes impressed at the exchange, two subscribers stations on said line, one of said stations having a condenser and a ringer connected in series to one side of said line, the other of said stations having a ringer and condenser connected in series to the other side of said line, a calling device at one of said stations operated at times to transmit impulses over said line to the exchange, a charging circuit and a discharging circuit for the condenser at each station completed alternately over the ringer at the same station whenever the calling device at said one station is operated, a solid body of non-linear resistance material in series with each ringer presenting a high resistance to the fiow of charging and discharging current through that ringer, thereby to prevent actuation of both ringers when the calling device at said one station is operated, the resistance of each body being effective to decrease responsive to said certain potential being impressed upon the side of said line to which its associated ringer is connected, thereby to permit said associated ringer to respond to said ringing signal when same is impressed upon the side of said line to which that ringer is connected.

3. In a telephone substation, an incoming line upon which a ringing signal of certain voltage is impressed at times and upon which a lower voltage is impressed at other times, a transmitter and a receiver, means including a condenser and an induction coil for connecting said transmitter and said receiver to said line in an anti-side tone speech circuit, a solid body of non-linear resistance material, a ringer, a ringing circuit connected to said line and including in series said condenser, said body and said ringer, said body presenting a high resistance to the flow of current through said ringer whenever said lower voltage is applied to said line but reducing its resistance to a low value in the presence of said certain voltage, thereby to permit said ringer to respond to said ringing signal.

4. In a telephone substation, an incoming line upon which a ringing signal of certain voltage is applied at times and upon which a lower voltage is applied at other times, a solid body of nonlinear resistance material, and a ringer connected to said line through said resistance material, said material presenting a high resistance to the flow of current through said ringer whenever said lower voltage is applied to said line but reducing its resistance to a low value in the presence of said certain voltage, thereby to permit said ringer to respond to said ringing signal.

5. A substation as claimed in claim 4, wherein said non-linear resistance material comprises a mixture of silicon carbide with carbon and/or tungsten which has been agglomerated under pressure and subsequently baked.

EMANUEL FRYDMAN.

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