JPS63290092A - Current clamping circuit - Google Patents

Abstract

PURPOSE:To prevent S/N characteristic from deteriorating by making the current clamping characteristic of a loop forming circuit have a hysteresis. CONSTITUTION:In attaining the hysteresis characteristic, a light-emitting diode 8 forming a photocoupler detects a current when it begins to flow through a current clamping circuit itself, a phototransistor 9 electrically switches a part of the current clamping circuit to change the clamping characteristic. At the time of this change, it is necessary to provide that a more current flows through the diode 8 after the change. Provided with this, the phototransistor 9 will be fed back, and is made not to come released soon when clamped. In such a way, the deterioration of the S/N characteristic can be prevented.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a loop forming circuit for a balanced transmission circuit that utilizes both DC signals and AC signals such as a telephone line, and particularly relates to a loop forming circuit for an analog trunk of a switchboard. The present invention relates to a current clamp circuit suitable for the circuit.

(Prior Art) A typical example of a conventional loop forming circuit having a current clamping function is shown in FIG.

A known example is shown in FIG. 2 described in Japanese Patent Application Laid-Open No. 60-246197 "Talking Current Loop Circuit," but this example does not include a flang circuit using a Zener diode. However, it is common to clamp the current in such loop-forming circuits with Zener diodes.

[Problem that the invention seeks to solve]

FIG. 6 shows an example in which the loop forming circuit is used in an analog trunk of a telephone exchange. The loop forming circuit 10
Corresponds to the circuit shown in the figure. The direct current is passed through a high-impedance relay winding 13 to a line resistance 11, a loop forming circuit 10, a line resistance 12, a relay winding 15, and a power source 14.
It flows along the route of. On the other hand, AC signals are transmitted through terminals 19 and 2.
0 and terminals 21 and 22. capacitor 15
, 16, 17, and 18 are for DC blocking.

FIG. 4 shows the current characteristics of the circuit with the above configuration. Line resistance R
When the current is small, the current is clamped to prevent excessive current from flowing through the relay winding 15 and the like. However, DC 1! If an alternating current signal is superimposed when the current is in a state near point P where clamping begins, the alternating current signal current causes repeated transitions between a clamped state and an unclamped state. This means that the Zener diode 6 shown in FIG. 2 repeats a conductive state and a non-conductive state. Under such conditions, the circuit shown in Figure 2 generates noise, and when used in a system such as that shown in Figure 3, the circuit shown in Figure 2 is
This degrades the S/N characteristics of AC signals transmitted between the two.

Such a problem is generally prevented by creating a current clamp characteristic with hysteresis as shown in FIG. 5, but this is difficult to achieve with a simple circuit configuration.

An object of the present invention is to provide a hysteresis characteristic to the current clamping characteristic of a loop forming circuit with a simple circuit configuration, so as to prevent the transition between a clamped state and a non-clamped state from being caused by a minute alternating current signal. The objective is to prevent deterioration of the S/N characteristics.

Means for Solving Problem c] The above object can be achieved by making the current clamping characteristic of the loop forming circuit into a characteristic having hysteresis as shown in FIG. 5, as described above. To simplify the circuit configuration, a light emitting diode and a phototransistor are combined.
A so-called photo coupler was used.

[Effect]

To achieve hysteresis characteristics, when current begins to flow in the current clamp circuit itself, a light emitting diode that forms a photocoupler detects it, and a phototransistor electrically switches a part of the current clamp circuit to achieve the clamp characteristics. All you have to do is change it. When changing, it is necessary to allow even more current to flow through the light emitting diode after the change. This applies positive feedback to the phototransistor, and once it is clamped, it will not return to its original state immediately.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. Most of the direct current flows from the terminal B through the collector and emitter of the Darlington-connected transistor 1, passes through the resistor 2, and flows to the terminal B. A portion of the direct current is passed through a high resistance resistor 4
, 5, and becomes the base current of transistor 1. The capacitor 3 is used to bypass the alternating current signal so that it does not enter the base of the transistor 1, and as a result, there is a high impedance between terminals AB in terms of alternating current.

The configuration up to this point is the same in the conventional circuit shown in FIG. When the direct current flowing between the terminals AB increases, the voltage drop across the resistor 2 through which most of the current flows increases, and the voltage at the 0 point becomes higher than that at the terminal A.

Normally, the Zener diodes 6 and 7 are in a non-conducting state, but when the voltage with respect to the terminal A at the 0 point becomes larger than the sum of the forward voltages of the Zener diodes 6 and 7 and the light emitting diode 8, the Zener diodes 6 and 7 conducts. When the Zener diode conducts, the voltage to terminal A at point 0 is determined by the Zener diode, so the voltage drop due to resistor 2 is limited to a constant value, and I! Yodo clamp is applied.

In the present invention, the light emitting diode 8 and the phototransistor 9
The feature is that a photocoupler consisting of the following constitutes a part of the clamp circuit. Zener diode 6 as mentioned above
When the light emitting diode 8 and 7 become conductive, the phototransistor 9 changes from the off state to the on state. Then, both ends of the Zener diode 7 are almost short-circuited by the phototransistor 9. Therefore, 0
The voltage to terminal A at point is almost the Zener diode 6
It will be determined by As a result, the voltage drop across the resistor 2 can be further suppressed, so that the current clamp is applied at a low current value. Since the Zener diode 7 is short-circuited by the phototransistor 9, the direction in which current flows further changes in the light emitting diode 8, so that positive feedback is applied. Therefore, once the DC current enters the clamp region, even a minute AC signal current will not return to the state before clamping.

However, when the voltage drop across the resistor 2 becomes sufficiently small, the Zener diode 6 becomes non-conductive and no current flows through the light emitting diode 8.

Then, the phototransistor 9 also turns off, returns to the non-clamped state, and in order to shift to the clamped state again,
The Zener voltage of Zener diodes 6 and 7 must now be exceeded.

As described above, according to the present invention, the current clamp circuit of the loop forming circuit does not repeat state transitions near the boundary between the clamped state and the non-clamped state, thereby preventing deterioration of the S/N characteristics of the entire system. .

Moreover, the only elements necessary for this purpose are a pair of photocouplers and a Zener diode.

〔Effect of the invention〕

According to the present invention, in a loop forming circuit having current clamping characteristics, transition between a clamped state and a non-clamped state is repeated in the vicinity of the clamping start region, thereby preventing deterioration of S/N characteristics.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a loop forming circuit according to an embodiment of the present invention, FIG. 2 is a circuit diagram of a conventional loop forming circuit, FIG. 5 is a functional circuit diagram showing an example of the use of the loop forming circuit, and FIG. FIG. 4 is an explanatory diagram of current clamp characteristics, and FIG. 5 is an explanatory diagram of current clamp characteristics having hysteresis characteristics. 1...Transistor, 2...Resistor, 5...
... Capacitor, 4,5... Resistor, 6.
7... Zener diode, 8... Light emitting diode, 9... Phototransistor, 10... Loop forming circuit, 11.12... Line resistance,

Claims (1)

[Claims] 1. In a loop forming circuit in which direct current is passed through a transistor and high impedance is applied to alternating current, the base terminal of the transistor or the terminal other than the base terminal side of the resistor connected to the base terminal and the terminal of the transistor A light emitting diode forming a photocoupler with a first Zener diode and a second Zener diode is connected in series between the emitter terminal or the terminal of the resistor connected to the emitter terminal that is not on the emitter terminal side. or second
The emitter of the phototransistor forming the above photocoupler to the anode of one of the Zener diodes,
A current clamp circuit characterized in that the collector of the phototransistor is connected to the cathode.