CA1332752C - Telephone lines overvoltage protection apparatus - Google Patents

Abstract

TELEPHONE LINES OVERVOLTAGE PROTECTION APPARATUS
Abstract of the Disclosure Overvoltage protection arrangements are pro-vided for a pair of telephone lines effective for sup pressing noise and transient voltage signals occurring above data and voice signals carried on the telephone lines and that provides common mode signal rejection and avoids current flow between the telephone lines responsive to common mode voltage signals on the lines so that audible hum is avoided. The overvoltage protection arrangements include first voltage clamping circuitry for clamping voltage signals on the lines at a first predetermined voltage potential and second voltage clamping and filter circuitry for clamping voltage signals on the lines at a second predetermined voltage potential and for filtering noise and transient signals from the voltage signals only when the voltage signals exceed the second predetermined voltage potential. The second voltage clamping circuitry provides common mode signal rejection and substantially avoids current flow between the lines responsive to common mode voltage signals on the lines.

Description

~ 332 75~

1. Field of the _nvention This inventlon relates generally to an overvoltage protection circuit for a pair of telephone lines and more particularly, to circuitry for filtering noise and transient voltage signals above data signals on a pair of telephone lines.

2. Prior Art Various arrange~ents have been used ior prote~tlng telephone equipment against hazardous voltages due to llghknlng or power surges. However, conventional arrangements are eifective for clamping transient vol~ages at a rated potential above the ring signal of conventional telephone systems. The typical ring signal is about 170 volts AC peak with a irequency between 15 and 30 hertz so ~hat the conventional arrangements have a rated clamping voltage of approximately 200 volts. Consequently, such arrangements are ineffective for filtering noise and transient voltage signals occurring below this standard clamping voltage.

~ 3~22~C~2 Overvoltage protection circuitry that over-comes many of the disadvantages of the pr.ior art is disclosed in McCartney, United States Patent No. 4,758, 920, issued July 19, 198a and assigned to the same assignee of the present invention.
Known protection circuitry generally effec-tive for normal mode overvoltage noise and transient protection can produce a differential current flow in the lines in response to common mode induced voltages of the same phase and ~mplitude on both lines. Such differential current flow can result from a common mode dynamic unbalance in the protection circuitry and cause an audible hum. It is desirable to provide an over voltage protection circuitry that a~oids introducing any common mode dynamic unbalance dif~erential current flow in the lines in response to common mode induced voltages or audible huM.
SUMM~y OF THE INVENTIQN
It is an object o~ the present invention to provide overvoltage protection circuitry that overcomes many of the disadvantages of the prior art systems.
It is another object of the present invention to provide an improved overvoltage protection apparatus Por coupling a sub~criber's station to a telephone sy6-tem.
It is another ob~ect of ~he presen~ invention to provide such apparatus that is e~fective for pro-tecting subscriber's station equipment ~rom hazardous voltages due to lightnlng, noise or powqr surgee.
It is another object of the present invention to provide such appara~us that i5 effac~ive for sup-pressing or ~inimizing noise and other transient volt-ags ~ignals occurring below a predefined potential of a ring signal supplied by the telephon~ central of~ice.
It is ~nother object o~ the present invention to provide such apparatus further that provides common 13327~2 63C76~
mode signal rejection and subs~antially avoids curren~ flow between the lines responæive to common mode voltage signals on the lines so that audible hum is avolded.
Therefore, in accordance with the preferred embodiments o~ the invention, there are provided overvoltage protection arrangements used with a pair of telephone lines. The overvoltaga protection arrangements include flrst voltage clamping circuitry for clamping voltage signals on the llnes a~ a first predetermined voltage potential and second voltage clamping and filter circuitry ~or clamping voltage signals on the lines at a second predetermined voltaye potential and for flltering nolse and translent slgnals ~rom the voltage slgnals only when the voltage signals exceed the second predetermined voltage potential. The second voltage clamping circuitry provides common mode signal rejectlon and substantlally avoids current flow between the lines responsive to common mode voltage signals on the lines.
In accordance with an lmportant feature of the invention, the overvoltage protection arrangements are effective for suppressing nolse and translent voltage slynals occurring above data and voice signals aarried on the telephone lines and that avoids current flow between the telephone llnes so that audible hum is avolded.
In dlfferent embodiments the second volta~e clamping means ls con~lgured di~ferently. As an example, in one embodlment, the second voltage clamping means includes a diode brldge coupled to said lineæ, a pair of ~ransient vqltage suppressor devicles, said translent voltage suppressor devices belng aoupled be~ween a ~irst and seaoncl center connqction oi sald diode bridge and grounq, and ~ald filter means lncluding reslstance means connected in series between sald llnes and sald dlode brldge and capacitance means connected between sald transient voltage suppressor devices and sald diode bridge center connections.

133~7~

DESCRIPTION OF THE DRAWING
These and other objects and advantages o~ the present inventlon will become readily apparent upon consideration of the following detalled description and attached drawing wherein, FIG. 1 is an elec~rical schematic representation of an overvoltage protection apparatus arrang~d ln accordance with the principles of the present inventlon;

3a ;'.~ .

~ 3 ~ ~ 7 ~5 ~ ~4-FIGS. 2 ~ 8 are electrical schematic repre sentations of alternative overvoltage protection cir-cuitry arranged in accordance with the principles of the present invention.
DETAILE~ ~ESCRIP~ION OF THE PRÆFE~ EM~Q~
Re~erring now to F~G. 1, ther~ is illustrated an ov~rvoltage protection circuit for telephone lines according to the lnvention generally designated by the re~erence nu~eral lC. At its INPUT, ~he overvoltage pro~ection circuit lO is connec~ed to a pair of lin~s or TIP and RING conductors of a telephone system. A
subscribsr's station, such as a modem or telephone set is connected to the OUTPUT of the circuit lO.
Shown enclosed in dotted line of th~ over-voltage protection circuit 10 is first circuitry gener-ally designated by the reference nu~er~ or clamp-ing voltage signals on ~he TIP and RING conductors at a ~irst pred~t~r~ined voltage potential above thQ conven-tional ring sign~l. Voltag~ cla~ping circuitry 11 in-cludes a gas discharge tube 12 or si~ilar device that is connected between the TIP and RING conductors and to ground pot~ntial at ground line G. A resistor 1~ is connected in series wi~h the TIP line and a resistor 16 i~ conne~t~d in ~erie~ with the ~ING lin~ rAnsient volt~g~ ~uppre~sor 18 i~ connected be~ween the resi~tor 14 and ground potential and a tr~n~ien~ voltage 8Up-pressor 20 i5 connect~d be~ween the re~is~or 16 and ground potential.
Voltaqs clamping circui~ry 11 is effective ~or protectlng the ~lephone equipm~nt connected to the OU~P~T o~ circuit 10 ~ro~ hazardous voltage~, ~or exa~-ple, due to lightning and power surges. The gas ~is-charg¢ tube 12 cla~ps transient voltages that appear bot~ acro~s thQ TIP and RING conduc~or~ with respect to the ground line ~ and provides ~ ~igh i~ped~nc~ to th~
ground line G when a hig~ voltag~ tr~n~ien~ ~ B no~ pra~

; : ~

_5~ 2 sent. rrhe transient voltage suppressors 18 and 20 con-duct whenever the voltage potential applied to the TIP
and RING lines, respectively, exceeds a predetermined breakdown voltage po~ential selected above the typical 170 volts AC ring signal, such as, for exampl~ 200 volts. Resistors 1~ and 16 limit current to the tran-sient voltage suppressors 18 and 20.
A three electrode gss discharge tube device such as, a Milli-Triac par~ number MLT0090, rated ~or a breakdown voltage of 600 volts, manufactured by Reynolds Industries may be employed Por the ga~ dis~
charge tube 12. Transient voltage ~uppressors of a type manu~actured and sold by ~eneral Semiconductor In-dustries, Inc., a Square D Company under a regis~ered trademark TransZorb, such as part number lN6303A or 1.5KE250C rated for a reverse standof~ voltage without conduction at 202 volts and ~or a br~akdown voltage in a range between 225-275 volts, advantageou~ly may be e~ployed ~or suppressors 18 and 20.
Overvolta~e protection circuit 10 includes second voltage clamping circuitry designated generally by the reference numeral 22 for clamping voltage sig-nal~ on the TIP and RING conductors having a prede~ined thre~hold r~te o~ change and at a ~econd predatermined voltage potential ~bove the voice ~nd data sign~l level and ~ub~tanti~lly le88 than the typic~l ring 170 volt6 AC ring ~ignal. Vol~aqe clamping circuitry 2~ is ef-~ective for filterin~ noise and trans.ient voltage sig-nals occurring below the selected operational voltage o~ ~he circuitry 11 and above a prede~ined thre~hold rate of change with respect to time above the frequency of the ring signal. Voltage clamping circuitry 22 has a voltage amplitude and frequency response for effec-tiv~ly protecting against transient and noise signals sub~tan.ially without interfering with the conventionnl teleph~nc signals. Vol~aga cla~ping circuitry 22 pro--6- ~3~ 2 . . .
vides common mode voltage si~nal rejection by substan-tially eli~inating common mode current flow from the telephone lines so that common mode induced voltages do not produce audible sound in the subscriber's station.
Voltage clamping circuitry 2~ includes a diode bridge arrangement 23 of diodes 24, 26, 28 and 30 conn~cted between the resistors 14 ~nd 16 se.ries con-nected with the TIP and RING conductors. A resistor 32 is connected across t~e center terminals + and - o~
diode bridge 23. A capacitor 34 is connected to the junction of the diode bridge - center terminal and re-sistor 32 and is connected in series with a unipolar voltage suppressor 36 to ground line G. A biasing re-sistor 38 is connected in parallel with the voltage suppressor 36. A capacitor 40 is connected to the junction of the diode bridge ~ c~n~er ter~inal and re-~is~or 32 and is connected in seri~ with ~ unipolar voltage suppres~or 42 to ground line ~. A bia~ing re-sistor 44 i5 connected in parallel wit~ the voltage suppre~sor ~2.
Various commercially available devices can b~
used for diodes 2~, 26, 28 and 30, such as, ~or ~xam-ple, a device MDA20~G ~anufactured and sold by Motorola Inc. Voltage suppre~or~ 36 an~ 42 are ~alec~ed to ~5 hav4 a breakdown vol~ag~ ratlng above ~he voic~ and d~ta ~ignals carried by the TIP and RING conduc~ors, such as, for exampla, in a range between 5 and 10 volts. For example, a device ~ype lN626g sold by vari-ouæ manu~cturers or Gene~al Semiconductor Industries, ~nc. part number l.5KE8.2 an~ having a breakdown volt-age rating of 8.2 volts can be used ~or unipolar volt-age suppr~ssors 36 and 42. T~e e~fectiv~ fr~quency re-sponse of the overvoltag~ cl~mping circuitry 22 ~or vo~tage signals ~bove the ra~ed breakdGwn vol~age i~
determined by the co~ponent v~lue~ o~ th~ r~ or~ 14 ~nd 16 and c~p~oitor~ 34 and 40. C~p~citors 3~ and 40 ~7~ ~3~27~
may be provided in a range between 0.1 and 1 microfarad with resistors 14 and 16 selected in a ran~e between 10 ~nd 30 ohms. For example, 0.68 microfarad capacitors 34 and 40 can be used with 15 ohm resistors 14 and 16.
A high value such as, ~or example, 10 megohms; is used for resistors 38 and 44 providing a discharge path to ground for capacitors 34 and 40. Similarly a high impedance value such ast for example, 2.2 megoh~s, is used for resistor 32 providing a discharge path ~or ca-pacitors 34 and 40.
In operation, positive transient voltage sig-nals on the TIP and RING conductors that exceed both the threshold frequency and the second predetermined voltage potential are coupled via diodes 28 and 30, re-spectively, capacitor flO and suppressor 42 to ground.
Similarly, negative transient voltage sign~ls on the T~P and RING conductors are coupled via diodes 24 and 26, respectively, capacitor 34 and suppressor 36 to ground. ~he frequency responsa o~ circui~ 22 allow the typical 170 volt, low frequency AC ring signal to pass to the subscriber station substanti~lly unaf-~ected. Voice and data signals likewise are substan-tially unaffected by the overvoltage clamping circui~
22 having ~n Q~fective i~pedance of ~he ~erles i~pedance Or t~e diode~ 2~, 26, 28 ~nd 30, ~p~citor~
34 and ~0 and the respective parallel combin~tion of the sUppressor 36 and resistor 3~ and the suppressor 42 ~nd rosistor ~4.
Common mo~e voltage slynal rejection is pro-3Q vided by the substantially instantaneous charging of the capacitors 34 and 40 to particular levels deter-min~d by the amplitude of the co~mon mo~e voltaqe sig-nal~ on t~e lines so that the suppressors 36 and ~2 are disposed in a non-conductive state~ As a re~ult, com-mon ~ode current flow b4tween tho telephone lines i~

~3~27~2 avoided so ~hat common mode induced voltages do not produce audible sound in the su~scriber's sta~ion, As an alternative arrangement, a third unipo-lar voltage suppressor 46 shown in dotted line can be connected between the anode of suppressor 42 and the cathode of suppressor 36 providing volt~ge cl~ping therebetween. An iden~ical device type lN6269 having a breakdown voltage rating of 8.2 volts as used ~or unipolar voltage suppressors 36 and 42 can be employed for the unipolar voltage suppressor 46.
FIGS. 2-8 illustrate alternative embodiments of overvoltage protection circuits according to the in-vention generally designated by the reference numerals 50, 52, 54, 56, 5~, 60 and 62. The same reference nu-merals are used for similar circuitry and co~ponents of FIG. 1. Each of ths overvol~age protectlon circuits 50, 52, 54 56~ 58, 60 and 62 includs~ the firs~ over-vol~age protection circuitry 11 and an altern~tive sec-ond voltage clamping circuitry ~hat replace~ and per-forms similar functions of the voltage clamping cir-cuitry 22.
Referring now to FI~. 2, the overvoltaqe pro-tection circuit S0 includes an alternative s~cond volt-a~e clamping circultry g~nerally desiqn~ted by ~he re~-erenc~ numer~l 6~. Volt~g~ ~lamping circuitry ~4 in-cludes a ~ioda bridge ~6 coupled vi~ ~ p~ir o~ cap~ci-tors 68 and 70 ~etween the r~sis~ors 14 and 16 serias connected wit~ the TIP and RING conductors. Diode bridge 66 includes diodes 7~, 74, 76 and 78 and can be 3Q provid~d with the ~ame device typa as used ~or diodes 24, 26, 28 and 30. The effec~iv~ ~r~quency respon~e of the overvoltage clamping circuitry 64 for voltaqe sig-nals above the rated breakdown voltage is determined by the component values of ~a resis~ors 1~ ~nd 16 and ca-pacitor~ ~8 and 70. C~pacitor~ 6~ ~nd 70 hav~ a 8i~i-lar value ~ u~od for capacitor 34 ~nd 40 o~ FI~. 1.

g :IL3~752 A unipolar voltage suppressor B0 is connec~ed across the center terminals + and - of diode bridge 66.
A bidirectional voltage suppressor 82 is connected in series between the center terminal + of diode bridge 66 and ground G. An identical device type lN6269 havin~ a breakdown voltage rating of ~.2 volt~ as u~ed for unipolar voltage suppressors 36 and 42 in FIG. 1 can be employed for the unipolar voltage suppressor 80. A
similar device as used for suppressors le and 20 havinq a reverse breakdown voltage rating above the peak com-mon mode voltage signal, such as in a range between 20 volts and 68 volts can be used for bidirectional volt-age suppressor ~2. For example, ~ device type lN6282 sold by various manufacturers or General Semiconductor Industries, Inc. part number 1.5KE39C and having a breakdown voltage rating of 40 volts can be used ~or bidirectional voltage suppressor 82.
In operation, positive transi~nt voltage sig-nals abov~ the threshold frequency and the second pre-deter~ined voltage potential on th~ TIP ~nd RING con-ductors are coupled via capacitors 68 and 70, diodes 72 ~nd 74 and suppressor 82 to ground. Similarly, nega-tive transient voltage signals above ~he threshold s~c-ond prede~rmined vol~age potential on ~he ~IP and RING
conductors are coupled via capacitor6 ~ ~nd 70, diode~
76 and 78 and ~uppres~ors 80 and ~2 to ground.
Vol~age cl~mpinq circuitry 64 provide~ co~mon mode voltaga fiignal r~jection by the operation o~ the bidirectional suppressor 8~. Suppressor 82 is disposed in a non-condùc~ive state re~ponslve to comman mode voltage sign~ls on the lines so that common ~ode cur-rent flow is eliminated from the telephone lines an~
com~on mode induced vol~ages dc not produce audible sound in t~e subscrib~r's station.
Rs~erring now to FIG. 3, the overvoltag~ pro tec~ion circuit 52 inclu~s an alternative wcond volt--lo ~3~ 2 age clamping circuitry generally designated by the ref-erence numeral 86. Voltage clamping circuitry 86 in-cludes a star arrangement of a pair of unipolar tran-sient voltage suppressors 88 and 90 and a bidirectional voltage suppressor 92. ~he anode o~ suppressor 88 is connected to the OUTPUT end of the TIP conductor by a capacitor 94. Similarly, a capaci~or 96 connects the suppressor 90 to the RING conductor.
An identical device type lN6269 having a breakdown voltage rating of 8.2 volts as used for unipolar voltage suppressors 36 and 42 in FIG. 1 can be employed for the unipolar voltage suppressors 88 and 90. A bidirectional suppressor device having a reverse breakdown voltage rating above the peak co~mon mode voltage signal, such as in a range between 20 and 68 volts is used for bidirectional volta~e suppressor 32.
An identical device typ~ lN6282 having a breakdown voltage rating o~ 40 volts as used ~or bidirectional voltage suppressor 82 can bs e~ployed or the bidirec-tional voltaqe suppressors 92.
The ef~ective frequency response of the over-voltage clamping circui~ry 8~ for voltage signals above the rated brea~down voltage of suppressors 88, 90 and 92 i8 deter~ined by the co~ponent values o~ tha resis-tors 14 an~ 16 and cap~citors g4 ~nd 96~ Capacitor~ 94 and 96 have ~ si~ r v~lue as used Por capacitor 34 and 40 of FIG. 1.
Vol~age clamping circuitry 86 provldes common mo~e voltag~ signal re~ection by the opera~ion o~ t~e bidirsctional suppressor 92. Suppres60r 92 is disposed in a non-conductive state responsive to com~on mode voltage signals on the lines so that common mode cur-rent flow is eliminated from the telephone lines and co~on ~ode induced volt~ges ~o not produce audible ~ound in the subficriber's st~tion.

1 1-- IL 3 r~ 2 7 ~ ~

Referring now to FIG. 4, the overvoltaqe pro-tection circuit 54 includes an alternative second volt-age clamping c.ircuitry generally designa~ed by the ref-erence numeral 106. Voltage clamping circuitry 106 in-cludes a star arrang~ent of unipolar transient voltagesuppres~ors 108, 110 and 112 coupled via a pair o~ ca-pacitors 114 and 116 between the resistors 14 and 16 series connected with the TIP and RING conductors. A
diode bridge arrangement 11~ of diodes 120, 1~2, 124 and 126 is connected between the cathode of suppressor 112 and ground G. ~ unipolar voltage suppressor 128, a capacitor 130 and a resistor 132 are connected in par-allel across the center terminals + and - of diode bridge 118.
An identical device type lN6269 havin~ a breakdown voltage rating of 8.2 V01~8 as used ~or unipolar vol~age suppres~ors 3~ and 42 can be employed for unipolar vol~aga ~uppressors 108, 110 and 112, Unipolar vol~age suppressor 128 li~its th~ voltage charging lev~l acro6s t~e capacitor 130 and is selected to have a breakdown voltage rating in a range between 20 volts and ~8 volts. A similar unipolar device type lN6282 having a breakdown vol~age rating o~ 40 volts as used for bidirac~ional voltags suppres60r ~2 can be em-ployed ~or t~ unipol~r voltsge suppres~ors 128.
~ he e~Pective ~reque~cy respon~e o~ the over-voltag~ cla~ping clrcuitry 106 ~or voltag~ signals above the rated breakdown voltage o~ suppressor 10~ and 110 ls d~t~rmined by the co~ponent values o~ the resis-3~ tors 14 ~nd 16 and capacitors 114 an~ 116. Capacitors 114 and 116 have a similar value as used for capacitor 34 and 40 of FIG. 1~
In the alternative second voltage cla~pingcircuitry 106, th~ voltage suppressors 108 and 110 pro-vide a high i~ped~nce below the ~econd cl~mping ~olt~q~
or ~.2 volts. Vol~age C1A~Ping circuitry 106 provi~e~ ~

-12- ~3~2~

common mode voltage signal rejection by the operation of the parallel connected capacitor 130 and suppressor 128 via the diode bridge 118.
Referring now to FIG. 5, the overvoltage pro-t~ction circuit 56 includes an alterna~iv~ second volt-age cla~ping circuitry generally designated by th~ r~-erence numeral 134. Voltage clamping circuitry 134 in-cludes a bidirectional voltage suppressor 136 coupled via a pair of capacitors 138 and 140 between the resis-tors 14 and 16 series connected with th2 TIP and RING
conductors. A second bidirectional voltage ~uppressor 142 is connected between tlle junction of the ~he bi~i-rectional voltage suppressor 136 and the capacitors 138 and ground G. Alternatively, the second bidirectional voltage suppressor 142 can connected between tha junc-tion of the the bidirectional voltaqe suppressor 136 an~ the capacitors 140 and ground G. Voltage ~lamping circuitry 134 provide~ common mode voltag~ signal re-jection by the operation o~ the bidirec~ional suppres-sor 142. ;::
A similar bidirectional suppressor device type lN6269 having a breakdown voltage ra~ing o~ ~.2 volts as used for unipolar voltag2 suppressor~ 36 and 42 c~n be ~ployed for the bidlr~c~ional ~olta~e ~up-~5 pro~sors 136~ T~e bidirec~ional suppre~sor dovice 138 ha~ a reverse bre~kdown voltage r~ting above t~e peak com~on mode voltage signal, such a5 in a range between 20 and fi8 vol~s. An id~ntical bidirec~ional device type lN6282 ~ing ~ breakdown voltag~ rating of 40!
volts ~s used for bidirectional voltage suppressor 82 can be e~ployed for the bidirec~ional voltago suppres-sor 142.
~e~erring now ~o FIG. 6, the overvoltage pro-tection circuit 58 inclu~e~ an ~lternative ~econd volt- :
ag~ clamping circuitry generally de~ign~sd by ths re~-erence nu~er~ 4. Voltags cl~ping circul~ry 144 in--13~ 2~2 cludes a parallel combination of a first unipolar volt age suppressor 146 connected in series with an oppo-sitely poled diode 1~ and a second unipolar voltage suppressor 150 connected in series with an oppositely poled diod~ 152. The parallel combination of suppres-sors 146, 150 and diodes 148, 152 is coupled via a pair of capacitors 15~ and 156 between the resistors 14 and 16 series connected with the TIP and ~ING conductors.
The suppressors 146, 150 and diodes 148, 152 provides equivalent functions as th~ bidirectional voltage sup-pressor 136 of FIG. 5 with an advantage of providing a lower capacitance than suppressor 136. A second low capacitance bidirectional common mode rejection sup-pressor function of the bidirectional suppre~sor 142 of FIG. 5 is provided by a parallel combination of a firs~
unipolar voltage suppre~sor 158 connscted in series with an oppo~itely poled diode 160 and a ~econd unipo-lar voltage ~uppressor 162 connected in series with ~n oppositely poled diode 164.
An identical device type 1~6269 having a .
breakdown voltage rating of 8.2 volts as used for unipolar voltage suppres~ors 36 and 42 o~ FIG. 1 can be employed for unipol~r voltage suppressors 146 and 150.
An identical unipolar device type lN6282 having a breakdown voltage r~ting o~ 40 volts as used fer ~he unipolar voltage ~uppres~or 12~ of PIG. 4 c~n be e~-ployed ~or the unipolar voltage suppre~sers 158 and 1~2. Diodes 14~, 152, 160 and 1~4 and can be provided with ~n identical device type as used rOr diode~ for~-ing bridq~ 23 of FIG. 1. ! .
Referring new to FIG. 7, the overvoltage pre-tection circuit 60 includes an altern~ive secon~ volt~
age clamping circuitry generally designa~ed by the ref-erencQ nu~eral 166. Voltage cla~ping circui~ry 166 in~
cludes ~ diod~ bridg~ 168 coupled ~i a pair of c~p~cl-tor~ 170 ~nd 172 between t~e re~istors 1~ ~nd l~ ~arie6 -14- 1~32~

connected with the TIP and RING conductors. Diode bridge 168 includes diodes 174, 176, 178 and 180 and can be provided with an identical device type as used for diodes forming bridge 23 of FIG. 1. A unipolar voltage suppressor 182 is connected across ths center terminals + and - of diode bridge 168. A second diode bridge 184 with a second unipolar voltage suppressor 186 connected across the center terminals + and - of diode bridge is connected in series between the center terminal + of diode bridge 168 and ground G. Diode bridge 184 includes diode~ 188, 190, 192 and 194 and can be provided with an identical device type as used .
for diodes forming bridge 23 of FIG. 1.
An identical device type lN6269 having a breakdown voltage rating oP 8.2 volts as us~d for unipolar voltage suppressor 80 of FIG. 2 can be em-ployed Yor unipolar voltag~ suppressox la2. An identi-cal unipolar device type lN6282 having a bre2kdown voltage rating of 40 voltæ as used for ~he unipolar voltage suppressor 128 of FIG. 4 can be e~ployed for the unipolar voltage suppressor 186.
Re~arrin~ now to FIG. 8, the overvoltaqe pro-~ection circuit 62 includes an alternative ~econd volt-aga clampin~ circuitry ganer~lly ~e~i~na~ed by the ra~-erance nu~eral 1~6. Volt~ge clamping circuitry 196 in-cludes a diode bridge 19~ coupled via a pair o~ capaci-tors ~00 and 202 batween tha resis~or~ 14 and 1~ series conn~ct~d with t~e TIP and RING conductor~. Diode bridqe 198 inc~ludes diodes 204, 206, 208 and 210 and can be provided with an identical device type as used for diodes forming bridge 23 of FIG. 1. Instead of ~e unipolar voltage suppr~ssor 182 of FIG. 7, a plurality of diodee 212, 214, 216, 218, 220 and 222 are connected in series across the center ter~inals + and - of diode bridge l9B. A bidirection~l volt~g~ ~uppr~$eor 224 i~

~L ~ 3 2 7 r5~ 2 connected between the junction of the series connected diode~ 216 and 218 and ground G.
~ he bidirectional suppressor device 224 has a reverse breakdown voltage rating above the peak common mode voltage signal, such as in a range between 20 and 68 volts. An identical bidirectional device type lN6282 having a breakdown voltage rating of 40 volts as used for bidirectional voltage suppressor 82 of FIG. 2 can be employed for the bidirectional voltage suppres-sor 224.
In the voltage clamping circuit 11 of FIGS.
1-8, the gas discharg~ tube 12 can be omitted. In ad-dition, the voltage clamping circuit 11 can be provided without the bidirectional transient voltage suppressors 18 and 20. It should be understood that the gas dis-charge tube 12 and suppressors 18 and 20 can be elimi-nated and the protection circuits 10, 50, 52, 54, 56, 58, 60 and 62 remain ef~ective for suppressing noise and transient voltage signals.
Although the present invention has been de-scribed in connection with details of the preferred em-bodiments, many alterations and modifications may be made without departing from the invention. Accord- -ingly, it i8 intended that all such alterations and modification~ b~ considered as within the spirit and scope o~ the invention as de~ined in the appended claims.
The embodiments of the invention in which an exclusive property or privilege is claimed are defined 30 a~ ~ollows: '

Claims (13)

1. An overvoltage protection circuit used with a pair of telephone lines comprising:
first voltage clamping means for clamping voltage signals on said lines at a first predetermined voltage potential, second voltage clamping means for clamping voltage signals on said lines at a second predetermined voltage potential, said second voltage clamping means including filter means for filtering noise and transient signals from said voltage signals only when said voltage signals exceed said second predetermined voltage potential, said second voltage clamping means including a diode bridge coupled to said lines, a pair of transient voltage suppressor devices, said transient voltage suppressor devices being coupled between a first and second center connection of said diode bridge and ground, and said filter means including resistance means connected in series between said lines and said diode bridge and capacitance means connected between said transient voltage suppressor devices and said diode bridge center connections.

2. An overvoltage protection circuit as recited in claim 1 wherein said transient voltage suppressor devices comprise unidirectional semiconductor devices.

3. An overvoltage protection circuit as recited in claim 1 wherein said second voltage clamping means further include high impedance means connected in parallel with said transient voltage suppressor devices.

4. An overvoltage protection circuit used with a pair of telephone lines comprising:
first voltage clamping means for clamping voltage signals on said lines at a first predetermined voltage potential, second voltage clamping means for clamping voltage signals on said lines at a second predetermined voltage potential, said second voltage clamping means including filter means for filtering noise and transient signals from said voltage signals only when said voltage signals exceed said second predetermined voltage potential, said second voltage clamping means including a diode bridge coupled to said lines, a first transient voltage suppressor device coupled between a first and second center connection of said diode bridge, a second voltage suppressor device connected between the second center connection of said diode bridge and ground, and said filter means including resistance means connected in series with said lines and capacitance means connected between a respective one of said resistance means and said diode bridge.

5. An overvoltage protection circuit as recited in claim 4 wherein said second transient voltage suppressor device has a reverse breakdown voltage above a peak value of common mode voltage signals on said lines.

6. An overvoltage protection circuit as recited in claim 4 wherein said first transient voltage suppressor device is a unipolar transient voltage suppressor device formed by a plurality of series connected diodes and said second transient voltage suppressor device is a bidirectional transient voltage suppressor device.

7. An overvoltage protection circuit used with a pair of telephone lines comprising:
first voltage clamping means for clamping voltage signals on said lines at a first predetermined voltage potential, second voltage clamping means for clamping voltage signals on said lines at a second predetermined voltage potential, said second voltage clamping means including filter means for filtering noise and transient signals from said voltage signals only when said voltage signals exceed said second predetermined voltage potential, said second voltage clamping means including first and second transient voltage suppressor devices coupled between said lines and a third transient voltage suppressor device connected to a junction of said first and second transient voltage suppressor devices, a diode bridge connected between said third transient voltage suppressor device and ground, a fourth transient voltage suppressor device connected across first and second diode bridge center connections; and said filter means including resistance means connected in series with said lines and capacitance means connected between said resistance means and said first and second transient voltage suppressor devices.

8. An overvoltage protection circuit as recited in claim 7 wherein said second voltage clamping means further include a capacitor connected in parallel across said fourth transient voltage suppressor device.

9. An overvoltage protection circuit as recited in claim 7 wherein said second voltage clamping means further include high impedance means connected in parallel across said fourth transient voltage suppressor device.

10. An overvoltage protection circuit used with a pair of telephone lines comprising:
first voltage clamping means for clamping voltage signals on said lines at a first predetermined voltage potential, second voltage clamping means for clamping voltage signals on said lines at a second predetermined voltage potential, said second voltage clamping means including filter means for filtering noise and transient signals from said voltage signals only when said voltage signals exceed said second predetermined voltage potential, said second voltage clamping means including a first bidirectional transient voltage suppressor device coupled between said lines and a second bidirectional transient voltage suppressor device connected to said first bidirectional transient voltage suppressor device and ground, and filter means including resistance means connected in series with said lines and capacitance means connected between said resistance means and said first transient voltage suppressor device.

11. An overvoltage protection circuit as recited in claim 10 wherein said first and second bidirectional voltage suppressor devices include a pair of unipolar voltage suppressors, each of said unipolar voltage suppressors connected in series with an oppositely poled diode and said series connected unipolar voltage suppressors and diodes connected oppositely poled in parallel.

12. An overvoltage protection circuit used with a pair of telephone lines comprising:
first voltage clamping means for clamping voltage signals on said lines at a first predetermined voltage potential, second voltage clamping means for clamping voltage signals on said lines at a second predetermined voltage potential, said second voltage clamping means including filter means for filtering noise and transient signals from said voltage signals only when said voltage signals exceed said second predetermined voltage potential, said second voltage clamping means substantially blocking current flow between said lines responsive to common mode voltage signals on said lines, said second voltage clamping means including a first diode bridge coupled to said lines, a first transient voltage suppressor connected between a first and second center connection of said first diode bridge and a second diode bridge coupled between the second center connection of said first diode bridge and ground and a second transient voltage suppressor connected between and first and second center connection of said second diode bridge, and said filter means include resistance means connected in series with said lines and capacitance means connected between said resistance means and said first diode bridge.

13. An overvoltage protection circuit as recited in claim 12 wherein said second transient voltage suppressor device has a reverse breakdown voltage above a peak value of common mode voltage signals on said lines.