JPH088616B2 - Call current supply circuit - Google Patents

Description

The present invention relates to a circuit for supplying a call current to a telephone in a telephone exchange or a key telephone device.

"Prior Art" In a telephone exchange or a key telephone device, a call current supply circuit for operating a telephone connected to a call line is provided.

The call current supply circuit needs not only a direct current supply function required for operating the telephone, but also an AC element function for preventing leakage of a voice signal.

As a conventional call current supply circuit using an electronic circuit, there is a circuit shown in FIG. 2, for example.

In FIG. 2, 1 and 2 are a pair of terminals connected to a communication line, 3 is a DC power supply, and 4 and 5 are DC power supply circuits, respectively. On the other hand, the DC power supply circuit 4 includes a transistor 6, resistors 7 and 8, a Zener diode 9 and a capacitor 10.
And is connected between the + side of the DC power supply 3 and the terminal 1. The other DC power supply circuit is a transistor 1
1, resistors 12, 13, Zener diode 14 and capacitor 15
And is connected between the minus side (ground) of the DC power supply 3 and the terminal 21. The main current circuit of the transistors 6 and 11 forms a communication current flow path and has a constant current characteristic.

When the terminals 1 and 2 of the call current supply circuit are connected to the call line and a loop is formed, a direct current as a call current can be supplied with the above-mentioned function. Here, a case will be described in which a ground fault (short-circuit with the ground) or contact (short-circuit with the power supply) occurs on the telephone line, that is, with the subscriber terminal on the telephone side. For example, when the terminal 1 is ground-faulted, a ground-fault current flows through the DC power supply 3, the DC power supply circuit 4, the terminal 1, and the path of the ground, increasing the power loss of the transistor 6 and burning it. The same applies to the DC power supply circuit 5, and when the terminals 2 come into contact with each other, the power loss of the transistor 11 increases significantly and the transistor 11 burns out. Therefore, in order to prevent this, a transistor and a resistor with a large allowable power loss are used and a large heat dissipation plate is used, which is uneconomical and hinders downsizing. Further, there is a drawback that the influence on other circuits due to heat generation cannot be avoided. Proposals for solving these problems have already been made, but there have been no satisfactory ones such as power loss due to detection of a ground fault or contact touch, and need of a fusing element.

"Problems to be Solved by the Invention" The present invention has been devised in view of the above-mentioned problems, and has a small power loss for detection of a ground fault or a contact touch, and further, an operation is reliable, and a fusing element, etc. An object of the present invention is to provide a speech current supply circuit that does not require any special component.

[Means for Solving Problems] Therefore, the present invention provides a call current supply circuit configured by providing transistors connected to a main current circuit so as to form a flow path of a call current on both positive and negative sides of a call line. A detection circuit for detecting the difference between the reference potential and the midpoint potential between the communication lines and transmitting the output, and a control circuit for controlling the overpower of the transistor by the detection output of the detection circuit. It was done.

[Embodiment] FIG. 1 shows an embodiment of a call current supply circuit according to the present invention.

Reference numerals 21 and 22 are terminals for connecting to a communication line, 23 is a DC power source, and 24 and 25 are transistors connected in Darlington, respectively, and their main current circuits are connected so as to form a communication current flow path. Resistors 26, 27, 28, 29, 30, 31 and diodes 32,
The series circuit of 33, 34, 35 constitutes a bias path of the transistors 24, 25 in the normal state, and a reference potential described later is set at the point x. Above resistance 26 ~
The 31 and the diodes 32 to 35 are electrically symmetrically connected with the point x as the middle point, and the voltage to the ground at the point x is a DC power supply.
It is half the voltage of 23. Reference numerals 36 and 37 are transistors forming a detection circuit for monitoring the state of the communication line connected to the terminals 21 and 22, each base is connected to an x point, and each emitter is connected to ay point. Point y is the middle point of connection of resistors 38 and 39 connected to terminals 21 and 22.

Since the resistance values of the resistors 38 and 39 are selected to be equal, a midpoint potential between the communication lines connected to the terminals 21 and 22 can be obtained at the point y, and in a normal state, as will be understood from the description below,
It is half the voltage of the DC power supply 23. That is, the point y has the same potential as the reference potential (point x) described above, and the transistors 36 and 37 are both OFF. Reference numerals 40 and 41 are transistors connected so as to be turned on when the transistors 36 and 37 are turned on. The transistors 40 and 41 are connected so that the bias of the transistor 24 described above is changed when the transistor 40 is turned on and the bias of the transistor 25 is changed when the transistor 41 is turned on. Has been done. Although the transistors 42 and 43 are not directly related to the present invention, they are provided to prevent saturation of the transistors 24 and 25 when the loop resistance of the communication line is large, so that the transistors 24 and 25 can be set to a predetermined value. A constant current type operation is performed within the range of the loop resistance of, and a constant resistance type operation is performed when the loop resistance becomes larger than that, which will be described later in detail.

 Next, the operation will be described.

When a call line is connected to terminals 21 and 22, DC power supply 23-resistor 44-transistor 24-terminal 21-call line loop-terminal 22
-Transistor 25-Call current is supplied through the earth path. Then, the transistors 24 and 25 perform a constant current operation determined by the bias path and the resistors 44 and 45 described above.

Next, if the terminal 21 side is grounded, for example, the DC power supply
23-Resistance 44-Transistor 24-Terminal 21-Ground fault causes a ground fault current to flow. However, the ground fault causes the potential at the point y to approach the ground potential, so it is lower than the reference potential at the point x. Become. This causes a base current to flow through the transistor 36, turning on the transistor 36 and turning on the transistor 40.
Therefore, the bias path is changed so that the base potential of the transistor 24 becomes higher, the base current decreases, and the overpower of the transistor 24 is prevented. When the terminal 22 side contacts the power source, current flows through the route of power source-terminal 22-transistor 25-resistor 45-ground air. At this time, the potential at the y point rises, and the potential at the x point is higher than the reference potential. As a result, the base current flows through the transistor 37, the transistor 37 is turned on, and the transistor 41 is also turned on.This changes the bias path of the transistor 25 as described above, and the base current of the transistor 25 decreases. As a result, the overpower of the transistor 25 is prevented. As can be understood from the above description, the circuit for detecting the ground fault and the touch contact is not inserted in the flow path of the call current supply, and the midpoint potential of the call line is detected. There is an advantage that there are few.

Next, the operation of the transistors 42 and 43 will be described. When the rope resistance of the communication line is high, the voltage between the terminals 21 and 22 rises, and the transistors 24 and 25 approach saturation, but this raises the base potential of the transistor 42 and the transistor 43. When the base potential is lowered and turned on, the bias paths of the transistors 24 and 25 are changed to act so as to limit the main currents of the transistors 24 and 25, respectively, so that saturation is prevented. Since the potential at the y point does not change depending on the magnitude of the loop resistance, the transistors 36 and 37 are not turned on.

[Advantages of the Invention] As described above, according to the present invention, there is little power loss for detecting a ground fault or contact, and the operation is reliable. Further, since no special component such as a fusing element is required, the circuit can be integrated and the entire structure can be made compact.

[Brief description of drawings]

FIG. 1 is an embodiment of a call current supply circuit according to the present invention, and FIG. 2 is a circuit showing a conventional call current supply circuit. 1,2 ...... Terminal, 3 ...... DC power supply, 4,5 ...... DC power supply circuit,
6,11,24,25,36,37 …… Transistor, 7,8,12,13,26,27,
28,29,30,31 …… Resistance, 9,14 …… Zener diode, 1
0,15 …… Capacitor, 21,22 …… Connection terminal, 23 …… DC power supply, 32,33,34,35 …… Diode

Claims (1)

[Claims] 1. In a call current supply circuit formed by providing transistors connected to a main current circuit so as to form a flow path of a call current on both positive and negative sides of a call line, between a power source and ground. A voltage dividing circuit configured to be electrically symmetrical and to be electrically symmetrical between the reference potential and the communication line with the midpoint potential of the series circuit serving as the bias path of the pair of transistors as a reference potential. A pair of detection circuits for detecting a difference from the midpoint potential and transmitting the output, and a control circuit for controlling the overpower of the transistor by the detection outputs of the pair of detection circuits are provided. Call current supply circuit