AU634187B2 - A protection circuit - Google Patents

Description

(1- I- c -IT _rXi OPI DATE 10/07/90 AOJP DATE 09/08/90 APPLN. ID 45226 89 PCT NUMBER PCT/AU89/00491 r INTERNATIONAL APPLICATION PUBLISHED UNDER THE'PATENT COOPERATION TREATY (PCT) (51) International Patent Classification 5 International Publication Number: WO 90/07213 H02H 3/22, 9/04, HO4M 1/74 Al H02H 3/22, 9/04, H04M 1/74 A (43) International Publication Date: 28 June 1990 (28.06.90) .21) International Application Number: PCT/AU89/00491 (74) Agent: O'CONNOR, Patent Department. Standard Telephones and Cables Ptv. Limited, 252-280 Botany (22) International Filing Date: 17 November 1989(17.11.89) Road, Alexandria, NSW 2015 (AU).

Priority data: (81) Designated States: AU. JP, US.

Pi 1884 12 December 1988 (12.12.88) AU Published (71)Applicant (for AU only), STANDARD TEL-EPHONES r international search report, AND CABLES PTY. tIMITED [AU/AU]; 252-280 Bo- T tan', Road, Alexandria, NSW 2015 'S (71)Applcant (for JP only): ALCATEL N.V. [NL'NL]: Strawinsk\laan 341, NL-1077 XX Amsterdam (72) Inventor: and Inventor/Applicant 'for LS on/l). FOX. Ronald. Christo-, pher, Shaw [AU/AU]: Flat 4/89 Gilderthorpe Avenue, Randwick, NSW 2031 (AU).

634187 (54)Title: A PROTECTION CIRCUIT R1 L1 02 TR2 D3

TX

To Control Cct.

TR3 L2 (57) Abstract A protection circuit for protecting circuit elements (TX) connected to line via terminal means (LI and L2) from the effects of a transient abnormal high voltage surge, comprising a semiconductor switch (TRI. TR2), a first circuit comprising a res.,tor (RI) in series with the conductor path of the semiconductor switch and a voltage reference means (DI, D2) in parallel with the resistor and a junction of the semiconductor, and a second circuit comprising the first voltage reference means in series with a second voltage reference means the second circuit being connected in parallel with the resistor. The second circuit further includes a voltage dependent resistance (VCI). When the currert through the resistor exceeds normal line current the voltage reference means (DI. D2. D3) clamp the voltage drop across the resistor and turn off the semiconductor switch.

S WO 90/07213 PCT/AU89/00491 A Protection Circuit Technical Field This invention relates to a protective circuit arrangement for pro-- I tecting circuit elements connected to a line against the damaging effects I of transient abnormal high voltage surges on the line caused by, for example, lightning or fluctuations in adjacent power transmission lines. The circuit arrangement of the present invention is particularly, though not I exclusively, suitable for protecting line associated elements of a telephone subset connected to an exchange line via a solid state line switch, the line switch providing a hook-switch function and loop-current interrupt functions.

SProtection circuits for telephone subsets of the aforementioned kind are known and comprise a voltage limiting circuit and a current limiting circuit. The voltage limiting circuit comprises a voltage dependent nonlinear resistor connected across the subset's line terminals to substantially absorb transient high voltage surges on the line, thus protecting circuit elements in the subset that are associated with the line terminal end of the subset's circuit. In addition to the voltage limiting circuit, a current limiting circuit is also provided. The current limiting circuit comprises an arrangement of a resistor in series with and on the line side of the conductive path of the line switch, and a diode arrangement connected between the line end of the resistor and the line switch's control element. The value of the resistor is chosen so that when a current threshold is exceeded, the voltage drop across the resistor exceeds the forward drop across the diode arrangement thus limiting the current throug.

the conductive path of the line switch.

A disadvantage of the known protection circuit is that a proportion of the current produced by the transient voltage surge passes through the line switch ani therefore the line switch's power rating mst be sufficien 2 accommodate this transient current and the power rating chosen accordingly. This reflects in the cost of the transistors used in the line switch.

It is therefore an object of the present invention to provide an improved protection circuit for protecting electrical equipment connected to a line via a solid state switch from the damaging effects of transient abnormai voltage surges on the line, and thereby permitting the power rating of the transistors used in the line switch to be lowered.

ISummary of Invention According to a first aspect of the present invention there is provided a pro- I tection circuit for protecting circuit elements connected to a line via line terminal means from the effects of a transient abnormal high voltage surge, said circuit i:1 comprising a controllable semiconductor switch a conductive path of which is se- I rially connected between said line terminal means and said circuit elements and I ,5 responsive to an associated con"rol element for selectively connecting said circuit elements to said line a conp-';Aon between said conductive path and said control element defining a semiconductor junction, said protection circuit comprising a first circuit comprising a first resistance means of a predetermined value connected in series between one terminal of the line terminal means and the conductive path of the said semiconductor switch, and a first voltage reference means coupled in parallbl with said first resistance means and said semiconductor junction and a second circuit comprising the first voltage reference means in series with a second voltage reference means, said second circuit being coupled in I parallel with said first resistance means, said second circuit further including a 25 voltage dependent second resistance means having a predetermined threshold value connected between the junction of the first said resistance means and the .i conductive path of the said semiconductor switch and another termisal of said line S' terminal means, the resistance value of said first resistance means being choser.

*i so that with a normal magnitude of line current the sum of the voltage drop,.

across the first resistance means and the said semiconductor junction is less than the reference voltage of the first reference means, and when the magnitude of the line current exceeds the normal magnitude, the said sum of the voltage drops in- I creases accordingly until it reaches the reference voltage of the first reference voltage means whereupon said reference voltage means clamps the voltage 2,1 I across said first resistance means and thereby limits the current therethrough, and upon said voltage dependent second resistance means detectii K U, z4 3 ing its thi-eshold voltage it permits current to increase through said first resistance means until the voltage drop is less than the reference voltage of the first reference means, and when the magnitude of the line current exceeds the normal magnitude, the said sum of the voltage drops increases accordingly until it reaches the reference voltage of the first reference voltage means whereupon said reference voltage means clamps the voltage across said first resistance means and thereby limits the current therethrough, and upon said voltage dependent second resistance means detecting its threshold voltage it permits current to increase through said first resistance means until the voltage drop thereon equals the sum of the reference voltages of the first and second reference voltage means whereupon the voltage across said first resistor means is clamped thereby and the semiconductor switch means rendered non-conducting by the reference voltage of said second voltage reference means.

According to a further aspect of the present invention there is provided a protection circuit wherein said first voltage reference means comprises two serially connected semiconductor diodes.

Brief Description of drawings In order that the invention may be carried into effect, embodiments thereof will now be described in relation to the drav-igs, in which: Figure 1 shows part of the line circuit of a telephone subset incorporating a known protection circuit.

Figure 2 shows part of the !ine circuit of a telephone subset incorporating the protection circuit of the present invontion.

Figure 3 shows a circuit of an AC power switching arrangement incorporating the protection circuit of the present invention.

Best Mode of Carrying out the Invention Referring to Figure 1, the circuit comprises line terminals L1 and L2 for connection to an exchange line (not shown). Across terminals L1 and L2 is connected a voltage protection varistor VC1. A line switch in the form of a complementary pair of transistors comprising an NPN transistor TR1 and a PNP transistor TR2 is serially connected between the line terminals and IJ WO 9 :wow

I

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*I

I

'0/07213 PCT/A L189/00491 4 the subset's transmission circuit TX. The collector/eritter Junction of a line switch control transistor TR3 is connected between the base element o' transistor TR2 and terminal L2. A resistor R2 is connected between the base of transistor TR2 and the collector element of transistor TR3 to provide a small but sufficient base current for transistor TR2. The base element of transistor TR3 is coupled to an output of a micro-processor (not shown) in the transmission circuit. Signals at this output control the functions of the line switch in a known manner. A resistor Rl is serially connected between terminal Ll and the collector/emitter junction of transistor TRi and a serial arrangement of two diodes DJ and D2 are connected between terminal Ll and the base element of transistor TR2. Resistor Rl and diodes Dl and D2 form the current limiting section of the protection circuit. The operation of this known c'Arrent protector circuit has been described above.

Referring to Figure 2, except for the protection circuit, the circuit is identical to that of Figure 1 arid will not be described. The protection circuit shown in Figure 2 comprises resistor RI serially connected between line terminals Ll and the collector/emitter junction of transistor TR1; diodes Dl and D2 serially connected between terminal Li arnd the base element of transistor TR2; a varistor VC1 connected between the junction of resistor Rl and the emitter of transistor TR2, and terminal L2; and a further diode D3 connected across the emitter and base of transistor TR2. The resistance value of resistor RI is chosen so that the voltage drop across it at normal operating line current 150 ma) is less than the forward bias voltage of diode Dl. Typically, the resistance of resistor Rl is approximwtely 3 ohms.

In operation, with normal line current flowing through resistor R1 and the saturated line switch, tbh forward bias voltage across the base/emitter Junction of transistor TR2 is 0,6 volts and the voltage drop across resistor Ri is less than 0.6 volt.. Therefore the voltage drop across diodes Dl iwo' i; ii

I

Vt )0/07213

I

PCT/A U89/0049 1 5 and D2 is the sum of the forward bias voltage across the base/emitter junction of transistor TR2 and the voltage drop across resistor RI, which when' normal line current is flowing is less than the sum of the forward bias voltages of diodes D1 and D2 (1.2 volts).

In the event of a high voltage surge on the line, the line current begins to increase causing the voltage drop across resistor Rl to increase accordingly, the voltage across the base/emitter junction of transistor TR2,-of course, does not change. When the sum of the voltage drops across resistor Rl and the base/emitter junction exceeds the sum of the forward bias voltages of diodes D1 and D2, the latter become forward biased. The forwardly biased diodes cla.mp the voltage across resistor Rl and the base/emitter junction of transistor TR2 and hence prevent an increase in base drive current to transistor TR2, thereby limiting the current through transistor TR1.

At the same time the voltage across varistor VC1 is rising and upon reaching the operating value of varistor VC1, typicaly 180-250 volts, current through resistor R1 increases until the sum of the voltage drops across resistor Rl and base/emitter junction of transistor TR2 exceeds the sum of the forward bias voltages of diodes Dl, D2 and A current path is now established from line terminal LI, diodes Dl, D2, D3, varistor VC1 to line terminal L2. The forward bias voltage of diode D3 reverse bias' transistor TR2 which turns off, thereby turning off transistor TR1 and preventing virtually any current from flowing through the line switch during the remainder of the voltage surge.

Preferably, a polarity guard circuit (not shown) is interposed between the line terminals and the line switch to ensure that the rolarity of the surge voltage is correctly presented to the diodes of the protection circuit.

Referring to Figure 3, the AC power switching circuit comprises an AC input coupled to a load RL via a pair of solid state switching circuits ilWO t i wo 9 i f-I li 1

*I

11 I 'j Rli i 0/07213 :i PCT/AU89/00491 6 each incorporating the protection circuit according to the present invention to provide full-wave protection in the event of an abnormal power surge on a line (not shown) connected to the input. Varistor VC1 is common to both protection circuits. Switch contacts Sl and S2 when activated switch transistors TR3 and TR23 which in turn switch on the line switch transistors TR1, TR2 and TR21, TR22.

In operation, during the first half cycle the current path for normal line-current is via resistor R1, collector/emitter junction of transistor TR1, load RL, diode D24 to line. The elemnts of the protection circuit have the same reference labels as in Figure 2. If a voltage surge occurs it will cause varistor VC1 to conduct and the resulting forward bias voltage of diode D3 will reverse bias transistor TR2 which will turn off and hence turn off transistor TR1 preventinL ,urrent from flowing into the load.

Daring the second half cycle the current path for normal line current is resistor R21, collector/emitter junction of transistor TR21, load RL diode D4 to line. When the voltage across varistor VC1 exceeds its threshold and begins to conduct, the resulting forward bias voltage of diode D23 will reverse bias transistor TR22 which will turn off and turn off transistor TR21. DicJes D5, D6, D25 and D26 divert the surge current away from the load RL.

Whnile 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.

Industrial Applicability The above described invention may be advantageously applied in the manufacture of telephone subset apparatus.

Claims (10)

1. 2. A protecion circuit as claimed in claim 1, wherein said first voltage reference means comprises two serially connected semiconductor di- odes.
2. 3. A protection circuit as claimed in claims 1 or 2, wherein said sec- ond voltage reference means comprises a semiconductor diode.
3. 4. A protection circuit as claimed in any one of claims 1 to 3, wherein said voltage dependent second resistance means comprises a varistor. A protection circuit as claimed in any one of the preceding claims, Swherein said controllable semiconductor switch comprises at least one bipo- lar transistor whose collector/emitter junction forms the said conductive path and whose base element is coupled to control means,
4. 6. A protection circuit as claimed in claim 5, wherein said base ele- ment is coupled to a conductive path of a further transistor whose base el- ement is coupled to control means.
5. 7. A protection circuit as claimed in claim 5 or 6, wherein said semi- conductor switch comprises a complementary configuration of an NPN and a PNP transistor, the collector/emitter Junction of the NPN transistor form- ing said conductive path, the first voltage reference means being connected in parallel with the base/emitter junction of said PNP transistor.
6. 8. A protection circuit as claimed in claim 7, wherein said semicon- ductor switch is a high voltage bipolar device.
7. 9. A protection circuit as claimed in any one of the preceding claims, wherein said line terminal means are operatively connected across the con- stant polarity DC output means of a polarity guard device whose AC input means are for connection to said line. 9 A protection circuit as claimned in any one of the preceding claims, wherein said controllable semiconductor switch is a telephone subset line switch whose control element is coupled to control means associated with a telephone subset .4 circuit.
8. 11. An AC power switch arrangement for connecting an AC power line to a load, said arrangement comprising a pair of line terminals means, a pair of load terminal means, one terminal of the line terminal :neans being coupled to one terminal of said load terminal means via said first resistor means and said conductive path of the semiconductor switch of the potlection circuit as claimed in any one of claims 1 to 9, the other terminal of said line terminal means being coupled to the other terminal of the load terminal means via said first resistor means and said conductive path of the semiconductor switch of another protection Scircuit as claimed in any one of claims 1 to 9.
9. 12. A protection circuit substantially as herein described with reference to Fig- ure 2 of the accompanying drawings,
10. 13. An AC power switch arrangement, substantially as herein described with ii reference to Figure 3 of the accompanying drawings,