Telephone interface circuit for providing over-current and over-voltage protection

In a telephone interface circuit, a first NPN transistor switch-controls a connection between a speech circuit and a pair of subscriber lines. A second PNP transistor controls an on/off state of the first transistor. A positive feedback circuit connects a collector terminal of the first transistor to a base terminal of the second transistor. An internal power source supplies current for driving the second transistor. The first transistor operates in a saturated region when a voltage that is in a range of standard voltages delivered over a pair of subscriber lines for normal operating conditions of a subscriber line device that is not being subjected to an over-voltage or an over-current event is applied in between the pair of the subscriber lines. The first transistor operates in an unsaturated region when an overvoltage exceeding said range of standard voltages is applied in between the pair of the subscriber lines.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an interface circuit which is capable of protecting a telephone set from transient overvoltage such as surge voltage, and from continuous inflow of overcurrent due to a fault contact between a commercial power line and a pair of subscriber lines.

2. Description of the Related Art

Since the subscriber lines being air-suspended have the possibility of getting transient lightning-induced voltage propagation thereto due to a lightning strike, or receiving continuous inflow of overcurrent for a somewhat long period of time due to a fault contact (or short circuit in connection) with the commercial power line, a protection circuit is often provided in the interface between the telephone set and the subscriber line. As counter measures to possible lightning surges, for example, a structure with a varistor element connected between two subscriber lines, a structure with a varistor element connected between the subscriber line and the grounding wire, etc. are known. When a transient surge voltage exceeding a varistor voltage is applied to the subscriber line, the varistor element will function to protect a speech circuit inside the telephone set by shifting to a conduction mode to absorb the surge voltage.

Moreover, as a counter measure to possible heat generation and fires in the telephone set due to a fault contact between the commercial power line and the subscriber line, for example, a structure having a PTC (positive temperature coefficient) thermistor inserted to the interface between the subscriber line and the telephone set is known. When there is a continuous inflow of overcurrent at the PTC thermistor for a some period of time, an input impedance at the interface will increase along with a rise in the element temperature, whereby the inflow of overcurrent into the telephone set can be prevented.

FIG. 3is a circuit diagram showing a conventional telephone interface circuit30. The telephone interface circuit30is to perform interface control between a speech circuit20, which is to process audio signals, and a pair of subscriber lines L1and L2.

The telephone interface circuit30mainly includes a diode bridge40which serves to rectify signals traveling inside the pair of the subscriber lines L1and L2to supply the signals to the speech circuit20, a transistor Tr3which functions as a hook switch for switch-controlling the connection between the pair of the subscriber lines L1and L2and the speech circuit20, a transistor Tr4which functions as a driver for switch-controlling the on/off state of the transistor Tr3, and a zener diode D5which serves to absorb possible overvoltage that could be applied to the subscriber lines L1and L2.

The transistor Tr3is to turn on at an off-the-hook state so as to connect the pair of the subscriber lines L1and L2to the speech circuit20, whereas it turns off at an on-the-hook state so as to disconnect the pair of the subscriber lines L1and L2from the speech circuit20.

An emitter terminal E3of the transistor Tr3is connected to the subscriber line L1.

A collector terminal C3of the transistor Tr3is connected to the speech circuit20through a resistor R7.

A base terminal B3of the transistor Tr3is connected to a collector terminal C4of the transistor Tr4through a resistor R4.

A resistor R3is connected between the emitter terminal E3of the transistor Tr3and the base terminal B3of the transistor Tr3.

A base terminal B4of the transistor Tr4is divided into two lines, one connected to a terminal HC via a resistor R5and the other connected to the subscriber line L2via a resistor R6.

The transistor Tr3is a PNP transistor whereas the transistor Tr4is an NPN transistor.

The terminal HC is connected to a microcomputer (not shown). At the time when off-the-hook operation, on-the-hook operation, dial pulse transmitting operation or the like is to be carried out, this microcomputer serves to control a voltage V4at the terminal HC in order to control a base potential of the transistor Tr4.

For instance, in the off-the-hook state, the microcomputer will control the voltage V4at the terminal HC such that the voltage V4will be at high voltage. Then the base potential of the transistor Tr4will rise as an electric potential of the terminal HC rises, whereby the transistor Tr4will turn on. Then, because a base potential of the transistor Tr3will drop, the transistor Tr3will turn on, and thus the pair of the subscriber lines L1and L2will be connected to the speech circuit20.

In the off-the-hook state, in response to a dial input, the microcomputer will control the voltage V4at the terminal HC. Thereby, the transistor Tr4will transmit a dial pulse signal.

In the on-the-hook state, the microcomputer will control the voltage V4at the terminal HC such that the voltage V4will be at low voltage. Then the base potential of the transistor Tr4will drop, whereby the transistor Tr4will be cut off. Then, because the base potential of the transistor Tr3will rise, the transistor Tr3will be cut off, and thus the pair of the subscriber lines L1and L2will be disconnected from the speech circuit20.

With respect to the above-described telephone interface circuit30, however, transistors with high pressure resistance, which are quite expensive, are required to be used as the transistors Tr3and Tr4to be connected between the pair of the subscriber lines L1and L2, and this leads to increase in manufacturing costs.

Moreover, as the above-described telephone interface circuit30has to have the resistor R4inserted in between the two transistors Tr3and Tr4, the telephone interface circuit30is left with little design flexibility.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a telephone interface circuit which can adopt transistors with low pressure resistance, which are inexpensive, as a first transistor that switch-controls a connection between a pair of subscriber lines and a speech circuit, and a second transistor that drive-controls the first transistor.

Furthermore, another object of the present invention is to provide a telephone interface circuit which does not require a resistor to be inserted in between the first and the second transistors, and which therefore is capable of having more design flexibility.

For the purpose of achieving the above-mentioned objects, a telephone interface circuit according to the present invention comprises: a first transistor which switch-controls a connection between a speech circuit and a pair of subscriber lines; a second transistor which controls an on/off state of the first transistor; a positive feedback circuit which connects a collector terminal of the first transistor to a base terminal of the second transistor; and an internal power source which supplies current for driving the second transistor. A circuit constant is set such that the first transistor is to operate in a saturated region when a voltage in a range of voltage for normal use is applied in between the pair of the subscriber lines, and such that the first transistor is to operate in an unsaturated region when an overvoltage exceeding the range of voltage for normal use is applied in between the pair of the subscriber lines.

Since the second transistor can operate by the current supplied by the internal power source, the second transistor does not need to have current supplied by the pair of the subscriber lines. Therefore, the first and the second transistors should be sufficient as long as they have pressure resistance based on the output voltage of the internal power source, and thus transistors with low pressure resistance which are available at low price can be used as the first and the second transistors.

Moreover, with respect to the telephone interface circuit according to the present invention, since it is not necessary to have a resistor to be inserted in between the first and the second transistors, the telephone interface circuit is allowed to have more design flexibility.

Furthermore, with respect to the telephone interface circuit according to the present invention, as the first transistor operates in the unsaturated region when an overvoltage is being applied to the pair of the subscriber lines, the first transistor, the second transistor and the positive feedback circuit will function as a self-propelled pulse generator. Accordingly, the first transistor and the second transistor will start oscillating while alternating between an on state and an off state, whereby the overcurrent flowing into the first transistor will be able to be cut off intermittently. In addition to that, it is possible to reduce the average value of overcurrent to a considerable extent, whereby the first transistor can be protected from the overcurrent.

The circuit constant is supposed to determine the base current of the second transistor. A boundary between the saturated region and the unsaturated region of the second transistor can be set based on the value of the base current.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1is a diagram illustrating a circuit structure of a telephone interface circuit10according to an embodiment of the present invention.

The telephone interface circuit10is to perform interface control in between a speech circuit20, which is to process audio signals, and a pair of subscriber lines L1and L2. The telephone interface circuit10mainly includes a transistor Tr1which functions as a hook switch for switch-controlling the connection between the pair of the subscriber lines L1and L2and the speech circuit20, and a transistor Tr2which functions as a driver for switch-controlling the on/off state of the transistor Tr1.

The transistor Tr1is to turn on at an off-the-hook state so as to connect the pair of the subscriber lines L1and L2to the speech circuit20, whereas it turns off at an on-the-hook state so as to disconnect the pair of the subscriber lines L1and L2from the speech circuit20.

A collector terminal C1of the transistor Tr1is connected to the subscriber line L2through a resistor R2, while an emitter terminal E1of the transistor Tr1is connected to the speech circuit20and a base terminal B1of the transistor Tr1is connected to a collector terminal C2of the transistor Tr2. Moreover, the emitter terminal E1of the transistor Tr1is grounded.

A base terminal B2of the transistor Tr2is connected to a terminal HC through a resistor R1, while an emitter terminal E2of the transistor Tr2is connected to an internal power source V2. The internal power source V2is a source of power that can be obtained, for instance, by converting an alternate-current power supplied by a commercial power source into a predetermined direct-current power using a power conversion module. The transistor Tr2is to operate relying on the current supplied by the internal power source V2. The collector terminal C1of the transistor Tr1is also connected to the base terminal B2of the transistor Tr2through a capacitor C1.

The transistor Tr1is an NPN transistor whereas the transistor Tr2is a PNP transistor. It is noted that inFIG. 1, a zener diode which serves to absorb possible overvoltage that could be applied to the subscriber lines L1and L2, and a diode bridge which serves to rectify signals traveling inside the subscriber lines L1and L2in order to supply the signals to the speech circuit20are being omitted.

The terminal HC is connected to a microcomputer30. At the time when off-the-hook operation, on-the-hook operation, dial pulse transmitting operation or the like is to be carried out, this microcomputer30serves to conduct negative logic-based control on a voltage V1at the terminal HC in order to control a base potential of the transistor Tr2.

For instance, in the off-the-hook state, the microcomputer30will control the voltage V1at the terminal HC such that the voltage V1will be at low voltage. Then the base potential of the transistor Tr2will drop as an electric potential of the terminal HC drops, whereby the transistor Tr2will turn on. Consequently, a collector current of the transistor Tr2will start flowing. Since the collector current of the transistor Tr2is equivalent to a base current of the transistor Tr1, the transistor Tr1will turn on.

The lowered base potential of the transistor Tr2will be positively fed back to the collector terminal C1of the transistor Tr1through the capacitor C1that functions as a positive feedback circuit. In a case when a line voltage V3in between the pair of the subscriber lines L1and L2stays within a range of voltage for normal use, the oscillation condition will not be met because a circuit constant, which is to determine the value of base current of the transistor Tr1, has been selected such that the transistor Tr1will operate in a saturated region. Therefore, under the off-the-hook state, as long as the line voltage V3in between the pair of the subscriber lines L1and L2is within the range of voltage for normal use, the transistor Tr1will keep itself at the state of being turned on.

On the other hand, in a case when the line voltage V3in between the pair of the subscriber lines L1and L2exceeds the range of voltage for normal use under the off-the-hook state, the oscillation condition will be met because a circuit constant, which is to determine the value of base current of the transistor Tr1, has been selected such that the transistor Tr1will operate in an unsaturated region. As the oscillation condition is met, the transistors Tr1and Tr2as a pair will start oscillating according to the same oscillation principle as that of a multi-vibrator. Consequently, as a loop current flowing at the transistor Tr1will be cut off intermittently, the transistor Tr1will be able to be protected from the overcurrent.

In the off-the-hook state, in response to a dial input, the microcomputer30will control the voltage V1at the terminal HC. Thereby, the transistor Tr2will transmit a dial pulse signal.

In the on-the-hook state, the microcomputer30will control the voltage V1at the terminal HC such that the voltage V1will be at high voltage. Then the base potential of the transistor Tr2will rise, whereby the transistor Tr2will be cut off. Consequently, since the collector current of the transistor Tr2will not flow, the transistor Tr1will be cut off.

Next, a principle on the basis of which the transistors Tr1and Tr2are to oscillate when an overvoltage is being applied in between the pair of the subscriber lines L1and L2will be described.

As can be understood from the fact that a part of the base current of the transistor Tr2is to be positively fed back to the collector terminal C1of the transistor Tr1, as shown inFIG. 1, the transistors Tr1and Tr2as a pair are composing an in-phase amplifier circuit.

As mentioned above, when an overvoltage is applied in between the pair of the subscriber lines L1and L2, transistors Tr1and Tr2as a pair will start oscillating according to the same oscillation principle as that of a multi-vibrator (or a self-propelled pulse generator), because the circuit constant has been selected such that the transistor Tr1will operate in the unsaturated region. As the oscillation starts, the pair of the transistors Tr1and Tr2will alternate between an on state and an off state with a cycle period proportional to a time constant CIR1. For instance, when the transistor Tr2is at an off state at a certain moment, the transistor Tr1is also at an off state. At this moment, since the voltage V1of the HC terminal has been set at low voltage, a first electrode, among first and second electrodes composing the capacitor C1, which is connected to the base terminal B2of the transistor Tr2, will have its electric potential become lower than that of the second electrode, whereby the transistor Tr2will shift to an on state in due course, while the transistor Tr1will also shift to an on state. Due to charging and discharging by the capacitor C1, the pair of the transistor Tr1and Tr2will simultaneously alternate between an on state and an off state. A cycle period of alternation Tm is about “0.69×C1×R1”.

FIG. 2shows a static characteristic of the transistor Tr1.

A horizontal axis in a graph ofFIG. 2indicates a voltage difference between the collector and emitter of the transistor Tr1, that is, the line voltage V3, whereas a vertical axis indicates a collector current IC1of the transistor Tr1. Reference numeral40shows one example of load profile at the time when the line voltage V3is within the range of voltage for normal use, while reference numeral50shows one example of load profile at the time when the line voltage V3exceeds the range of voltage for normal use.

At this point, provided that a base current of the transistor Tr1is indicated by IB1, a collector current of the transistor Tr1is indicted by IC1, a direct current gain of the transistor Tr1is indicate by hFE1, a base current of the transistor Tr2is indicated by IB2, a collector current of the transistor Tr2is indicted by IC2, a direct current gain of the transistor Tr2is indicate by hFE2, and a voltage difference between the base and emitter of the transistor Tr1and a voltage difference between the collector and emitter of the transistor Tr2with respect to the line voltage V3are disregarded, the following expressions can be derived.
IB2=(V1−0.6)/R1  (1)
IB1=IC2=IB2×hFE2(2)
IC1=V3/R2  (3)
IC1=IB1×hFE1(4)

Based on expressions (1) to (4), the following expression can be derived.
V3=(V1−0.6)×hFE1×hFE2×R2/R1  (5)

From expression (5), it can be understood that a value of the voltage V3at the time when the pair of the transistor Tr1and Tr2start oscillating (i.e. an oscillation start voltage) is inversely proportional to a resistance value of the resistor R1, and the oscillation start voltage can be adjusted arbitrarily with the resistance value of the resistor R1.

For example, when the line voltage V3is 10 V (i.e. when the line voltage V3is within the range of voltage for normal use), the transistor Tr1will need a base current IB1of 50 μA in order to enter the saturated region, as indicated by a point A of intersection between a static characteristic curve of the transistor Tr1and the load profile40. According to the present embodiment, the circuit constant, which is to determine the value of the base current IB1of the transistor Tr1, has been selected such that the operating point of the transistor Tr1will enter the saturated region when the line voltage V3is within the range of voltage for normal use. Since the transistor Tr1will not have an amplifying function in the saturated region, the oscillation condition will not be met even when a part of the base current of the transistor Tr2is to be positively fed back to the collector terminal C1of the transistor Tr1through the capacitor C1.

Meanwhile, when the line voltage V3is 50 V (i.e. when the line voltage V3is an overvoltage that exceeds the range of voltage for normal use), the transistor Tr1will need a base current IB1of 200 μA in order to enter the saturated region, as indicated by a point B of intersection between a static characteristic curve of the transistor Tr1and the load profile50. According to the present embodiment, the circuit constant, which is to determine the value of the base current IB1of the transistor Tr1, has been selected such that the operating point of the transistor Tr1will enter the unsaturated region when the line voltage V3is an overvoltage that exceeds the range of voltage for normal use. Since the transistor Tr1will have an amplifying function in the unsaturated region, the oscillation condition will be met, whereby the pair of the transistors Tr1and Tr2will start oscillating.

Since such oscillation will start at the very instant when an overvoltage is applied in between the pair of the subscriber lines L1and L2, an overcurrent passing through the transistor Tr1will be cut off at the instant when the overvoltage is applied in between the pair of the subscriber lines L1and L2. Although the overcurrent will start flowing into the transistor Tr1again when the cycle period of alternation Tm elapses from the time the overcurrent has been cut off, the overcurrent will be cut off again at the very instant when the overcurrent starts flowing. In this way, the overcurrent flowing into the transistor Tr1will be cut off intermittently. By adjusting the value of time constant C1R1to an appropriate value, it is possible to set a ratio of the period, during which the overvoltage is being applied in between the pair of the subscriber lines L1and L2, to the period, during which the overcurrent is flowing into the transistor Tr1during the period when the overvoltage is being applied in between the pair of the subscriber lines L1and L2, to about 10:1, for instance. Accordingly, it is possible to reduce the average value of overcurrent of the transistor Tr1to a considerable extent.

The oscillating behavior by the transistors Tr1and Tr2will continue during the time period when the overvoltage is being applied in between the pair of the subscriber lines L1and L2(i.e. during the time period when the oscillation condition is being met). After that, as the line voltage V3becomes lower to fall into the range of voltage for normal use, the operating point of the transistor Tr1will return to the saturated region again, whereby the oscillating behavior will stop, for the oscillating condition will no longer be met. In this way, since the protection function of the telephone interface circuit10for protecting the transistor Tr1has a self-recovery function, it will become available for normal use at the very moment when the overvoltage is stopped being applied to the pair of the subscriber lines L1and L2.

As for the circuit constant which is to determine the value of the base current IB1of the transistor Tr1, for example, the voltage V1of the terminal HC, the resistance value of the resistor R1, etc. can be considered. However, the circuit constant is not to be limited to these examples.

With respect to the telephone interface circuit10according to the present embodiment of the invention, since the transistor Tr2can operate by the current supplied by the internal power source V2, the transistor Tr2does not need to have current supplied by the pair of the subscriber lines L1and L2. Therefore, the transistor Tr2should be sufficient as long as they have pressure resistance based on the output voltage of the internal power source V2, and thus transistors with low pressure resistance which are available at low price can be used as the transistor Tr2.

Furthermore, with respect to the telephone interface circuit10according to the present embodiment of the invention, since it is not necessary to have a resistor to be inserted in between the two transistors Tr1and Tr2, the telephone interface circuit10is allowed to have more design flexibility.

In case of power outage, the telephone interface circuit10should not be connected to the pair of the subscriber lines L1and L2. In the telephone interface circuit10according to the present embodiment of the invention, since the internal power source V2is to have power supplied by the commercial power source, the output voltage of the internal power source V2will become zero in the case of power outage. Therefore, the transistor Tr2will not turn on in the case of power outage. This means that it is guaranteed that the telephone interface circuit10will not be connected to the pair of the subscriber lines L1and L2in the case of power outage.