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 
     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. 
     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. 
     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 
     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 (registered trademark) as a protection circuit is necessary. 
     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. 
     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 
         FIG. 1  is a circuit diagram of a telephone interface circuit of this embodiment. 
         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. 
         FIG. 3  is a graph showing change in time of a surge voltage applied across subscriber lines. 
         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. 
         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 
       FIG. 1  shows a circuit configuration for a telephone interface circuit  10  of this embodiment. 
     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 . 
     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 . 
     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 . 
     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 . 
     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 . 
     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 . 
     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. 
     The emitter terminal E 1  of transistor Q 1  is connected to the subscriber line L 1 . 
     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 . 
     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 . 
     A resistor R 14  is connected across emitter terminal E 1  and base terminal B 1  of transistor Q 1 . 
     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 ). 
     Emitter terminal E 2  of the transistor Q 2  is connected to the subscriber line L 2  via the diode bridge  20 . 
     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 . 
     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. 
     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. 
     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 . 
     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. 
     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. 
     Next, a description is given of the operation when an overvoltage is applied to the subscriber lines L 1 , L 2 . 
     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. 
     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. 
     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. 
       FIG. 2  shows the change in time of voltage VBE across the base and emitter of transistor Q 2 . 
     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 . 
     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. 
     Next, a description is given of the results of this embodiment while referring to  FIG. 3  to  FIG. 5 . 
       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. 
       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. 
       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. 
     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.