JPS59123422A - Ground-fault protecting system for dc current supply circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention The present invention relates to an electronic DC current supply circuit in a telephone exchange, and particularly to a DC current supply circuit with semi-constant current characteristics, which can be used even in the event of a line ground fault. This invention relates to a ground fault protection method for a DC current supply circuit that does not consume excessive power.

Prior art and problems The computerized DC current supply circuit in telephone exchanges is
Because it is small and inexpensive, it has gradually become widely used as an alternative to the conventional direct current supply method using coils connected in parallel to a balanced line.

Conventionally, circuits with constant voltage characteristics and circuits with constant current characteristics have been used as DC current supply circuits using electronic circuits, but in the case of the former, the balanced line is short and the loop resistance is small, but The drawback is that it supplies a larger current than necessary, increasing power consumption.In the latter case, if the current value is set large, overload will occur if the loop resistance is thick. If it is made small, it cannot be used in exchangers that require a relatively large current.

On the other hand, by providing a semi-constant current characteristic, when the loop resistance is small, the current flows more than necessary, reducing power consumption completely, and when the loop resistance is large, saturation of the circuit is completely prevented. A DC current supply circuit which can increase the supply current as much as possible has already been filed by the same applicant in Japanese Patent Application No. 122,660/1982.

FIG. 1 shows the configuration of a conventional DC current supply circuit having semi-constant current characteristics. In the figure, 1°2 are the first and second transistor circuits, 3 is the coupling resistor, and 4 is the
indicates a power supply, and a and b are line terminals. In the transistor circuit 1, 11 is a grounded emitter transistor, 12 to 14 are resistors, 15 is a capacitor, and 16 is a diode. In the transistor circuit 2, 21 is a grounded emitter transistor, 22 to 24 are resistors, 25 is a capacitor, 26 is a diode. For example, power supply 4 is -
48V is used.

In Figure 1, diode 1 is connected from the + side (ground) of the power supply.
6, resistors 13, 3.23 and diode 26 and return to one side of the power supply through
1 is formed. Now considering the case where the resistors 12 and 22 are not provided, the base voltage of each transistor 11 and 21 is uniquely determined by the first bias circuit at -1υ, and therefore the transistor circuits 1 and 2 have constant current characteristics. However, by connecting resistors 12 and 22', transistor 11 is biased by the potential difference between terminals A and ground, and transistor 21 is biased by the potential difference between terminal a and one side of power supply 4. However, the potential difference between terminal A and ground and the potential difference between terminal A and one side of the power supply vary depending on the loop resistance value of the subscriber line. It exhibits semi-constant current characteristics in which the supplied current gradually increases as the loop resistance decreases.

FIG. 2 shows the connections of the DC current supply circuit shown in FIG. 1 in use. In the figure, the same parts as in FIG. 1 are designated by the same numbers, 5 and 6 are line resistances, 7 is a load relay, and 8 is a 2-wire to 4-wire converter.

In FIG. 2, a loop resistance is formed by the line resistances 5, 6 and the resistance of the load relay 7, and the DC current supply circuit supplies DC current IL to this through the terminal a#bi. 2
The line-4-wire converter 8 is, for example, a telephone exchange, and includes a 2-wire side including all the load relays 7, a 4-wire sending side (4WS), and a receiving side (4WR)'! 5 Connect.

Figure 3 shows all the loop resistance-loop current characteristics of the DC current supply circuit in the connection shown in Figure 2, and shows that as the loop resistance increases, the loop current gradually decreases to a semi-constant current characteristic. ing.

In Fig. 2, if a ground fault occurs at one end of the line connected to terminal a, as shown by the person, the power supply voltage will be applied across the transistor circuit 2 and the line resistance 5, causing a current flow. increase In FIG. 4, the solid line shows the line resistance-earth fault current characteristic at the time of a ground fault accident, and when the line resistance is small, the ground fault current becomes very large. Therefore, the power consumption in the transistor circuit 2 increases, causing a temperature rise, and in the worst case, the transistor 21 is destroyed.

As a method to reduce power consumption in the event of a ground fault in such a DC current supply circuit and to prevent damage to the element, for example, when a voltage exceeding the normal level is generated in one transistor circuit, this is completely detected. death.

One possible method is to cut off the current. However, during normal operation, there is a high possibility that a surge voltage may be applied from the line side, and once the circuit is cut off due to such a cause, the balance between the residual voltages in both transistor circuits will be lost, and the transistor circuit on the cut-off side will Since the full power supply voltage is applied to the circuit, it becomes impossible to recover from the cut-off state.

Purpose of the Invention The present invention attempts to solve all of the problems of the prior art, and its purpose is to reduce the amount of direct current in the event of a ground fault in an electronic direct current supply circuit having semi-constant current characteristics. It is an object of the present invention to provide a circuit type capable of suppressing power consumption in a transistor circuit having an Ft in a supply circuit.

Embodiment of the Invention FIG. 5 shows the structure of an embodiment of the ground fault protection system for a DC current supply circuit according to the present invention. In this figure, the same parts as in FIG. 1 are designated by the same numbers, and 9 is a protection circuit. In the protection circuit 9, 91 is a grounded emitter transistor, 92 is a resistor, 93 is a Zener diode, 94 and 95 are resistors, and 96 is a diode.

In Figure 5, the configuration of the circuit with all protection circuits 9 removed is the first one.
Same as in the figure. In the protection circuit 9, the Zener diode 93 is selected to have a total operating voltage of 27.5V when the power supply voltage is -48V, for example.

On the other hand 1. Forward voltage of diode 96 and transistor 9
The base emitter voltage VBE of 1 is about 0.5V, so the voltage between terminal a and one side of power supply 4 is 28V.
When V'i is exceeded, a Zener current flows.

During normal operation, both transistor circuits 1 and 2 operate in a balanced state, and their respective residual voltages VLI
, VL2 are equal, and even if the loop resistance is zero, each
4V and does not reach 28V. Therefore, no Zener current flows, transistor 91 is cut off, and protection circuit 9 does not operate at all. Therefore, the operation of the circuit in this state is no different from that in FIG.

During normal operation, the respective residual voltages of the current source circuits 1.2 are balanced. Therefore, the maximum power consumption of both circuits is as follows, assuming that the loop resistance is zero in FIG.

48 (V) ÷ 2 x 31 (mA) = 744 mW
Now, a ground fault has occurred on the A line side and the transistor circuit 2
The residual voltage VL2. When exceeds 28 Vi, current flows through the Zener diode 93, biasing the transistor 91 and making it conductive. Current amplification factor h of transistor 91
Assuming that fe is sufficiently large, collector current IQI is given by the following equation.

That is, the collector current I91 of the transistor 91 is
28 V Flows in proportion to the total excess fL voltage. Here, R92, Rt4. Res are resistances 92 and 94, respectively.
, 95 resistance values.

On the other hand, if the current amplification factor "fe" of the transistor 21 is sufficiently large, the base current will hardly flow, and therefore the resistor 2
The electric current 28 flowing through 3 is given by the following equation.

I2g-I8 +I2゜-I e 1(2) where I8
is the current flowing through the resistor 3, and since the value of the resistor 3 is selected to be sufficiently larger than that of the resistor 23, the current III
can be considered a constant current.

Therefore, if the rate of change of the current factor 1 is set larger than the rate of change of the current factor 22 with respect to the change in voltage exceeding 28 V'i, the current I211 can be reduced from the relationship in equation (2). can.

On the other hand, the collector current I21 of the transistor 21 can be considered to flow in proportion to the electromotive force of the resistor 23 based on the current 128. Therefore, if the current I2g is made to decrease when the voltage exceeding 28 V i increases, The collector current 21 of the transistor 21 decreases as the voltage exceeds 28 V, and a negative resistance characteristic is exhibited between the terminal a and one side of the power source 4. Therefore, according to the circuit shown in FIG. 5, if a ground fault occurs, the residual voltage VL2 of the current source circuit 2
When exceeds a certain value that is larger than the normal value, the negative resistance characteristic allows the rate of increase in ground fault current to be smaller than in the case without the protection circuit 9.

Note that increasing the rate of change of current I91 with respect to the rate of change of current 122 with respect to a change in voltage exceeding 28V,
This can be easily achieved as follows. That is, by selecting the value of the resistor 22 to be sufficiently large compared to the resistor 23, the electric current 2□ is approximately determined by the following equation.

Therefore, the current I22 increases in proportion to the residual voltage L2. On the other hand, the electric current, l, is determined by the relationship in equation (1) above, and its magnitude changes depending on the selection of each constant value in equation (1). Therefore, by choosing these constants, the current 11! for voltage changes over 28 Vf! +
The rate of change of the current 1111 can be made larger than the rate of change of +.

FIG. 6 is a diagram showing the correlation between residual voltage and ground fault current in the embodiment of the present invention shown in FIG. In the figure, the solid line shows the case without a protection circuit, and the broken line shows the case with a protection circuit, and both show the case where the line resistance is zero. As is clear from the figure, in the absence of a protection circuit, the ground fault current increases with the residual voltage value, but in the case of a protection circuit, the residual voltage ranges from 28V (point A) to 48V (8V).
Between points), the ground fault current decreases as the residual voltage increases, indicating negative resistance characteristics.

Further, FIG. 7 shows the relationship shown in FIG. 6 as a relationship between residual voltage and power consumption. In the same figure, the solid line shows the case without a protection circuit, and the broken line shows the case with a protection circuit. As is clear from the figure, when there is no protection circuit, the power consumption reaches a maximum value of 2.3 W when the residual voltage is 48 V, which is significantly higher than that during normal operation as described above. On the other hand, when a protection circuit is provided, the maximum power is 1.25 W, which is half that of the case without the protection circuit, and does not increase much compared to the case of normal operation.

In the method of the present invention, the negative resistance characteristic can be arbitrarily set by selecting the constants in the protection circuit 9 as described above, and therefore, in addition to providing the drooping characteristic as shown in FIG. For example, it is possible to provide constant current characteristics above a certain residual voltage value.

Effects of the Invention' As explained above, the ground fault protection method of the DC current supply circuit of the present invention is to provide a grounded emitter transistor circuit in series in each current supply path to a balanced line, and to apply a constant bias to the pace of each transistor. In a DC current supply circuit that is provided with a first bias path that provides a bias proportional to the voltage drop of one transistor circuit, and a second bias path that provides a bias proportional to the voltage drop of each transistor circuit at each base, the voltage drop of one transistor circuit is We have installed a protection circuit that shunts the base current of the transistor in the transistor circuit according to the excess voltage when a certain voltage is exceeded, so that when a voltage exceeding the normal voltage is applied to one transistor circuit due to a ground fault, etc. The transistor circuit exhibits negative resistance characteristics to reduce power consumption of the trunk circuit and provide protection.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the configuration of a conventional DC current supply circuit having semi-constant current characteristics, Fig. 2 is a diagram showing the connection of the DC current supply circuit shown in Fig. 1 in use, and Fig. 3 is a diagram showing the configuration of a conventional DC current supply circuit having semi-constant current characteristics. Second
Figure 4 is a diagram showing the loop resistance-loop current characteristics of the DC current supply circuit in the usage state shown in the figure, Figure 4 is a diagram showing the line resistance-line current characteristics at the time of a ground fault in the DC current supply circuit, and Figure 5 is FIG. 6 is a diagram showing the configuration of an embodiment of the ground fault protection method for the DC current supply circuit of the present invention, and FIG. 6 shows the correlation between the residual voltage and the ground fault current in the embodiment of the present invention shown in FIG. The diagram shown in FIG. 7 is a diagram similarly showing the relationship between residual voltage and power consumption. 1.2...Current source circuit, 3...Coupling resistance, 4...
Mold temperature, 5.6... Line resistance, 7... Load relay, 8
... 2-wire to 4-wire conversion section, 9... Protection circuit, 11...
- Emitter grounded transistor, 12-14... Resistor, 15... Oscillation prevention capacitor, 16... Diode, 21... Emitter grounded transistor, 22-
24...Resistor, 25...Capacitor for oscillation prevention, 2
6... Diode, 91... Emitter grounded transistor, 92... Resistor, 93... Zener diode, 94, 95... Resistor, 96... Diode. Patent Applicant Fujitsu Limited Patent Attorney Hisashi Tamamushi (3 others) Fig. 1 Fig. 2 Fig. 3 0F-11 Roof resistance (K, Ω,) Fig. 4 1-III! Line resistance at the time of circuit (standing) Fig. 5 Fig. 6 Fig. 7 Fig. 11 + Residual voltage (V)

Claims (1)

[Claims] In the current supply path for the balanced line, a grounded emitter transistor circuit is provided in full series, and a first bias path is provided to give a constant bias to the base of each transistor circuit, and a voltage drop of each transistor circuit is provided to each base. In a DC current supply circuit provided with a second bias path that provides a bias proportional to A ground fault protection method for DC current supply circuits that is characterized by the provision of a protection circuit.