Abstract:
A telephone interface circuit comprises a first transistor for controlling opening and closing between a speech circuit and subscriber lines, a second transistor for controlling the first transistor to turn on and off, a positive feedback circuit connecting a collector terminal of the first transistor and a base terminal of the second transistor, an overcurrent detection circuit detecting overcurrent applied to the subscriber lines, and a breaker circuit for turning off the first transistor by lowering the base potential of the second transistor to a low potential when overcurrent is detected at the overcurrent detection circuit. Here, the base terminal of the first transistor and a collector terminal of the second transistor are connected. Further, when off-hook, the base potential of the second transistor is controlled in such a manner as to become a high potential by a microcomputer.

Description:
BACKGROUND 
       [0001]    The present invention relates to an interface circuit for protecting a telephone from overcurrents flowing in continuously due to transient overvoltages such as surge voltages or mixing etc. of mains power lines and subscriber lines. 
         [0002]    A protection circuit is provided at an interface of a telephone and subscriber lines because of the possibility of a transient indirect lightening stroke accompanying a lightening strike being propagated to subscriber lines hanging in space or the possibility of overcurrents continuously flowing over a long period of time to a certain extent due to mixing with a mains power line. Configurations such as, for example, connecting a varistor element between two subscriber lines or connecting a varistor element between a subscriber line and earth are well-known as lightening surge countermeasures. When a transient surge voltage exceeding the varistor voltage is applied to a subscriber line, the surge voltage is absorbed as a result of the varistor element making a transition to conducting mode, and a speech circuit within the telephone is protected. 
         [0003]    Further, configurations where, for example, a PTC thermistor (Positive Temperature Coefficient Thermistor) is interposed at an interface between a subscriber line and a telephone are also well known as a countermeasure for heating and combustion of a telephone due to mixing of subscriber lines and mains power lines. When an overcurrent flows into a PCT thermistor continuously over a certain period of time, input impedance of the interface increases in accompaniment with rise in element temperature and flowing in of overcurrents to within the telephone can be suppressed. 
       SUMMARY 
       [0004]    However, in a telephone interface circuit using a semiconductor element for opening and closing a connection between a speech circuit and subscriber lines, strict adherence to ratings for current and voltage are necessary in order to avoid a secondary breakdown phenomenon peculiar to the semiconductor element, and use of an expensive Sidac® as a protection circuit is necessary. 
         [0005]    The present invention therefore sets out to resolve the problem of providing a low-price telephone interface circuit capable of maintaining reliability of an interface circuit for protecting a telephone from the flowing in of overcurrents. 
         [0006]    In order to resolve the aforementioned problems, a telephone interface circuit of the present invention comprises a first transistor for controlling opening and closing between a speech circuit and subscriber lines, a second transistor for controlling the first transistor to turn on and off, a positive feedback circuit connecting a collector terminal of the first transistor and a base terminal of the second transistor, an overcurrent detection circuit detecting overcurrent flowing in to the subscriber lines, and a breaker circuit for turning off the first transistor by lowering the base potential of the second transistor to a low potential when overcurrent is detected at the overcurrent detection circuit. Here, the base terminal of the first transistor and a collector terminal of the second transistor are connected. Further, when off-hook, the base potential of the second transistor is controlled in such a manner as to become a high potential by a microcomputer. 
     
     
       DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a circuit diagram of a telephone interface circuit of this embodiment. 
           [0008]      FIG. 2  is a graph showing change over time of a voltage across a base and an emitter of a transistor when an overcurrent is continuously applied to a subscriber line. 
           [0009]      FIG. 3  is a graph showing change in time of a surge voltage applied across subscriber lines. 
           [0010]      FIG. 4  is a graph showing change in time of a current passing through a transistor when a surge voltage is applied across subscriber lines in a telephone interface of the related art. 
           [0011]      FIG. 5  is a graph showing change in time of a current passing through a transistor when a surge voltage is applied across subscriber lines in a telephone interface of this embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]      FIG. 1  shows a circuit configuration for a telephone interface circuit  10  of this embodiment. 
         [0013]    The telephone interface circuit  10  controls interfacing between a speech circuit  30  and subscriber lines L 1 , L 2 . The telephone interface circuit  10  is mainly comprised of a varistor element V 1 , Zener diode  40 , diode bridge  20 , transistors Q 1 , Q 2 , positive feedback circuit  50 , overcurrent detection circuit  40 , and breaker circuit  70 . 
         [0014]    The varistor element V 1  is arranged at a front stage of the diode bridge  20 , and absorbs overvoltage (for example, high voltages of 270V or more) between the subscriber lines L 1  and L 2 . 
         [0015]    The Zener diode  40  is arranged at a front stage of the speech circuit  30 , and absorbs overvoltage (for example, high voltages of 9V or more) between the subscriber lines L 1  and L 2 . 
         [0016]    The diode bridge  20  regulates the signal flowing through the subscriber lines L 1 , L 2  for supply to the speech circuit  30 . The diode bridge  20  is configured from four diodes D 1  to D 4 . 
         [0017]    The overcurrent detection circuit  40  is a circuit for detecting overvoltages between the subscriber lines L 1 , L 2 . The overcurrent detection circuit  40  contains a resistor R 23 . 
         [0018]    The breaker circuit  70  is a circuit for turning the transistor Q 2  off by applying a reverse bias voltage across base terminal B 2  and emitter terminal E 2  of transistor Q 2  when an overvoltage is applied between the subscriber lines L 1 , L 2 . Breaker circuit  70  contains a Zener diode D 20 . 
         [0019]    When the telephone is off the hook, the transistor Q 1  is turned on so as to connect the subscriber lines L 1 , L 2  and speech circuit  30 , while when the telephone is on the hook, the transistor Q 1  is turned off so that the subscriber lines L 1  and L 2  and the speech circuit  30  are disconnected. 
         [0020]    The emitter terminal E 1  of transistor Q 1  is connected to the subscriber line L 1 . 
         [0021]    The base terminal B 1  of the transistor Q 1  is connected to collector terminal C 2  of transistor Q 2  via a resistor R 12 . 
         [0022]    The collector terminal C 1  of the transistor Q 1  branches, with one branch connected to the speech circuit  30  and the other branch being connected to the positive feedback circuit  50 . The positive feedback circuit  50  has a capacitor C 22 . 
         [0023]    A resistor R 14  is connected across emitter terminal E 1  and base terminal B 1  of transistor Q 1 . 
         [0024]    Base terminal B 2  of transistor Q 2  branches into three, with one branch being connected to the positive feedback circuit  50 , another branch being connected to the breaker circuit  70 , and the remaining branch being connected to a microcomputer (not shown) via an RC circuit (a circuit containing a resistor R 24  and a capacitor C 21 ). 
         [0025]    Emitter terminal E 2  of the transistor Q 2  is connected to the subscriber line L 2  via the diode bridge  20 . 
         [0026]    As a result of the above circuit configuration, the transistors Q 1  and Q 2  and the positive feedback circuit  50  function as a Schmitt trigger  60 . Namely, the base terminal B 2  of transistor Q 2  functions as a gate terminal G of the Schmitt trigger  60 . 
         [0027]    Transistor Q 1  is a switching element comprised of a PNP transistor and transistor Q 2  is a switching element comprised of an NPN transistor. 
         [0028]    A terminal HC is connected to a microcomputer (not shown). The microcomputer (not shown) controls base potential of the transistor Q 2  by controlling the potential of terminal HC at the time of an off-hook operation, on-hook operation, or dial pulse transmission operation, etc. 
         [0029]    For example, when off-hook, the potential of the terminal HC is controlled to be a high potential as a result of control by the microcomputer (not shown). In doing so, as a result of the rise in potential of the terminal HC, the base potential of the transistor Q 2  rises, and the transistor Q 2  turns on. As a result, the base potential of transistor Q 1  rises, and the transistor Q 1  therefore turns on. The rise in the collector potential of the transistor Q 1  is then positively fed-back to the base terminal B 2  of transistor Q 2  via the positive feedback circuit  50 . At this time, the capacitor C 22  has a function for shortening the turn on time of the transistor Q 2 . 
         [0030]    When a dial input takes place in an off-hook state, the microcomputer (not shown) controls the potential of the terminal HC so as to correspond to the dial input. As a result, the transistor Q 1  sends a dial pulse signal. 
         [0031]    On the other hand, when on-hook, the potential of the terminal HC is controlled to be a low potential as a result of control by the microcomputer (not shown). As a result, the base potential of transistor Q 2  falls, and the transistor Q 2  therefore turns off. In doing so, the base potential of transistor Q 1  falls, and the transistor Q 1  therefore turns off. 
         [0032]    Next, a description is given of the operation when an overvoltage is applied to the subscriber lines L 1 , L 2 . 
         [0033]    When an overvoltage is applied to the subscriber lines L 1 , L 2 , a large voltage drop occurs at the overcurrent detection circuit  40 . When this voltage drop exceeds the Zener voltage, the Zener diode D 20  enters a breakdown state. The breaker circuit  70  then causes the base potential of the transistor Q 2  to fall. At this time, a reverse bias voltage is applied across the base terminal B 2  and emitter terminal E 2  of the transistor Q 2 . The transistor Q 2  therefore turns off the instant (within two microseconds) the overvoltage is applied across the subscriber lines L 1 , L 2 . In doing so, the base potential of transistor Q 1  falls, and the transistor Q 1  therefore also turns off. 
         [0034]    The magnitude of the overcurrent necessary for the breaker circuit  70  to operate depends on the resistance of resistor R 23 , Zener voltage of Zener diode D 20 , and reverse bias voltage across the base and emitter in order to turn the transistor Q 2  off, etc. 
         [0035]    When an overvoltage is applied across the subscriber lines L 1 , L 2  when off-hook, the breaker circuit  70  operates as described above, and the transistor Q 1  is made to turn off. However, when off-hook, the terminal HC is controlled to be a high potential by the microcomputer (not shown), and the base potential of the transistor Q 2  rises immediately. When the voltage across the base and emitter of the transistor Q 2  exceeds the threshold voltage, the transistor Q 2  is turned on again. As a result, the base potential of transistor Q 1  rises, and the transistor Q 1  is therefore also turned on again. In this way, the transistor Q 1  has self-returning function. When the transistor Q 1  turns on again due to this self-returning function, in the event that an overvoltage is applied across the subscriber lines L 1 , L 2  as before, the breaker circuit  70  operates as described above, and the transistor Q 1  is turned off. In this way, in the event that overvoltages are successively applied across the subscriber lines L 1 , L 2 , the transistor Q 1  repeatedly alternates between a state of being turned on and a state of being turned off. 
         [0036]      FIG. 2  shows the change in time of voltage VBE across the base and emitter of transistor Q 2 . 
         [0037]    At time t 1 , when an overvoltage is applied across the subscriber lines L 1 , L 2 , the breaker circuit  70  operates and the transistor Q 2  is made to turn off. However, in an off-hook state, the potential of terminal HC is controlled to a high potential. The voltage VBE across the base and emitter of transistor Q 2  therefore immediately rises, and the voltage VBE reaches the threshold voltage VT at the time t 2 . In doing so, the transistor Q 2  is turned on again. As an overvoltage is then applied continuously across the subscriber lines L 1 , L 2 , the breaker circuit  70  operates the instant the transistor Q 2  is turned on, and the transistor Q 2  is turned off. After this, the voltage VBE across the base and emitter of transistor Q 2  rises immediately, and the voltage VBE reaches the threshold voltage VT at time t 3 . In doing so, the transistor Q 2  is turned on again. As an overvoltage is then applied continuously across the subscriber lines L 1 , L 2 , the breaker circuit  70  operates the instant the transistor Q 2  is turned on, and the transistor Q 2  is turned off. The same operation is then repeated at time t 4 . 
         [0038]    A period T where the transistor returns to being on from being turned off due to its self-returning function is determined by the size of the overvoltage applied across the subscriber lines L 1 , L 2  and the time constant of the RC circuit (circuit containing resistor R 24  and capacitor C 21 ) connected to the base terminal B 2  of the transistor Q 2 . The period the transistor Q 1  is disconnected for is longer for a larger overvoltage applied across the subscriber lines L 1 , L 2  and thermal fracturing due to collector loss of transistor Q 1  can be suppressed. 
         [0039]    Next, a description is given of the results of this embodiment while referring to  FIG. 3  to  FIG. 5 . 
         [0040]      FIG. 3  shows a waveform for a surge voltage applied across subscriber lines L 1 , L 2 . In the same drawing, the horizontal axis shows time, and the vertical axis shows voltage. 
         [0041]      FIG. 4  shows a waveform for current passing through transistor Q 1  when the surge voltage shown in  FIG. 3  is applied across subscriber lines L 1 , L 2  at the telephone interface of the related art. In the same drawing, the horizontal axis shows time, and the vertical axis shows current. 
         [0042]      FIG. 5  shows a waveform for current passing through transistor Q 1  when the surge voltage shown in  FIG. 3  is applied across subscriber lines L 1 , L 2  at the telephone interface  10  of this embodiment. In the same drawing, the horizontal axis shows time, and the vertical axis shows current. In this drawing, the transistor Q 1  repeatedly alternates between being turned on and being turned off, with it being shown that the period the transistor Q 1  is disconnected for is longer for a larger overvoltage. 
         [0043]    As described above, according to the telephone interface circuit  10  of this embodiment, it is possible to turn off the transistor Q 1  the instant an overvoltage is applied across the subscriber lines L 1 , L 2 , and it is possible for the transistor Q 1  to be restored by a self-returning function. In particular, it is possible for the period the transistor Q 1  is disconnected for to be longer for a larger overvoltage applied across the subscriber lines L 1 , L 2  and for thermal fracturing due to collector loss of transistor Q 1  to be suppressed.