JPH0537291Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a constant current circuit suitable for, for example, a trunk of a telephone system such as a button telephone device or a home telephone. Regarding circuits.

[Prior Art] A constant current circuit in a trunk of a conventional telephone system includes a protective resistor 3, a resistor 4, and a constant current transistor 5 between a power supply terminal 1 and a constant current supply terminal 2, as shown in FIG. A current setting resistor 6 and a reverse current blocking diode 5 are connected in sequence, a base current supplying resistor 8 is connected between the left end of the resistor 4 and the base of the transistor 5, and a base current supplying resistor 8 is connected between the base and the right end of the resistor 6. Two bias setting diodes 9, 1 between
0 is connected, and an electrolytic capacitor 11 for power supply noise prevention is connected between the left end of the resistor 4 and the ground.

According to this constant current circuit, diodes 9, 10
Since a constant bias voltage can be applied to the series circuit of the base-emitter junction of the transistor 5 and the resistor 6, a load 12 consisting of a telephone connected between the constant current supply terminal 2 and the ground
A constant current (bias DC current) can be supplied to the

[Problems to be solved by the invention] By the way, the impedance frequency characteristics of the constant current circuit shown in Fig. 10 are as shown in Fig. 11, and
When the load 12 is 100Ω, the impedance value Z seen from the constant current supply terminal 2 is about 500 to 600Ω.
Even if the impedance value of the constant current circuit is the above value, there is no problem when the load 12 is used alone, but if the button telephone device 13 is connected to the load 12 and the constant current circuit as shown in FIG. There is a problem when connecting at the same time.

Next, this problem will be explained with reference to FIG.
The extension call path system of the button telephone device 13 includes an extension trunk 14, key telephone handsets 15 and 16,
The button telephone handsets 15 and 16 are configured to be able to talk to each other and also to a single telephone 18 of the load 12 connected via a non-polar coupling capacitor 17. In addition, each button telephone cordless handset 15, 16 has a built-in switch 1.
9 and 20, and are connected to the extension transformer 14. Also, the standalone telephone 18 is connected to the standalone telephone trunk 2 via the switch 21.
2, and is also connected to extension telephone handsets 15 and 16. When the standalone telephone 18 and the button telephone handset 15 or 16 communicate, the coupling capacitor 1
7, and the load of the call signal sent from the key telephone handset 15 or 16 to the standalone telephone set 18 is both the standalone telephone set 18 and the standalone telephone trunk 22. Note that the extension trunk 14 can also be a load, but it does not pose a particular problem because it is normally formed with high impedance. Since the single telephone trunk 22 includes a constant current circuit shown in FIG. 10, if the impedance of this constant current circuit is low, the speech level will be reduced. Also, the balance of sidetones becomes worse. Therefore, it is ideal that the impedance of the constant current circuit of a single telephone is infinite, and realistically, 6 kΩ or more is desired.

Therefore, an object of the present invention is to provide a constant current circuit with high impedance.

[Means for Solving the Problems] The present invention for solving the above problems and achieving the above objects will be described with reference to the symbols in FIG. a transistor 5 connected in series with the terminal 2; and a protective resistor 3 connected in series with the transistor 5.
a current setting resistor 6 connected to the emitter of the transistor 5 and arranged in series between the power supply terminal 1 and the constant current supply terminal 2; A series circuit of at least two PN junction elements 9, 10 connected in parallel to the base-emitter junction of the base emitter and the current setting resistor 6, and a series circuit of the at least two PN junction elements. This relates to a constant current circuit consisting of a capacitor 23 connected in parallel and a constant current diode 24 connected between the base and collector of the transistor 5.

[Operation] In the constant current circuit of the above invention, the bias current to the collector can be kept constant by connecting the low current diode 24 between the collector and base of the transistor, so that the DC resistance becomes high. Further, the capacitor 23 connected in parallel to the bias PN junction has a function of increasing the impedance seen from the constant current supply terminal 2 side when the transistor 5 is off (when the bias current is extremely small).

[First Embodiment] Next, a constant current circuit in a button telephone system according to a first embodiment of the present invention will be explained based on FIG. However, parts in FIG. 1 that are common to FIGS. 10 and 12 are designated by the same reference numerals, and their explanations will be omitted.

The constant current circuit shown in FIG. 1 is included in the independent telephone trunk 22 shown in FIG. 6, a PNP type transistor 5, and a reverse current blocking diode 7 are connected in series in this order, and two diodes 9 and 10 are connected in the forward direction between the left end of the resistor 6 and the base of the transistor 5. It is constructed by connecting a polar capacitor 23 made of an electrolytic capacitor in parallel to the diodes 9 and 10, and connecting a constant current diode 24 via a resistor 8 between the base and collector. The capacitor 23 is connected with a positive polarity on the power supply terminal 1 side.

Next, specific examples of each part will be shown. Resistor 3 is 47Ω (usable range is 14 to 140Ω), resistor 6 is 12Ω (usable range is 3.6 to 36Ω), capacitor 23 is 3.3μF/
50V (usable range is 1 to 10μF), diode 9,
10 are general-purpose rectifier diode transistors 5, respectively.
is a 1W rated PNP transistor, resistor 8 is 10kΩ (usable range is 0 to 30kΩ), reverse current blocking diode 7 is a general-purpose rectifier diode, constant current diode 2
4 is a constant current diode 501 made by Ishizuka Electronics Co., Ltd., which can flow 500 μA (usable range is 150 μA to 1.5 mA).

[Operation] When the load 12 is connected to the constant current supply terminal 2,
The forward voltage with diodes 9 and 10 is at the connection point 25
A constant bias voltage is applied between the transistor 5 and the base, and a corresponding current flows through the transistor 5. The direct current that is made constant by the transistor 5 and flows into the constant current supply terminal 2 can be adjusted by the value of the resistor 6, and when this value is 12Ω, it is approximately 35 mA.

The impedance value of the constant current circuit in Figure 1 is
As shown in the figure, it is about 6kΩ at 200Hz, and increases as the frequency increases. Such a high impedance value is obtained because the impedance of the constant current diode 24 when viewed from the constant current supply terminal 2 is large.

When the impedance value of the constant current circuit becomes high in this way, when the button telephone handset in the button telephone device 13 and the individual telephone of the load 12 make a call, the loss due to the constant current circuit of the call signal is reduced,
Decrease in call level is reduced.

Furthermore, if the impedance of the constant current circuit becomes large, the power supply nozzle is significantly attenuated before reaching the load 12, so the noise removal capacitor 11 provided in FIG. 10 can be omitted.

In addition, as shown in FIG. 1, the transistor 5 is
If the PNP type is used, even if the bias DC current (constant current) flowing through the load 12 is extremely small, the impedance value seen from the constant current supply terminal 2 can be made high. When the transistor 5 in FIG. 10 is of the NPN type, the impedance seen from the constant current supply terminal 2 side can be made low when the DC bias current of the load 12 is extremely small. Therefore, if the constant current circuit shown in Fig. 1 is used, when the bias DC current (constant current) flowing through the load is extremely small, for example, in operation modes such as extension on-hook dialing, extension speaker listening, etc., where no extension trunk current flows. Since the impedance seen from the constant current supply terminal 2 can be increased, the key telephone device 13 shown in FIG. 12 can be used in the extension trunk 14 without adversely affecting telephone calls.

[Second Embodiment] FIG. 3 shows a constant current circuit according to a second embodiment of the present invention. Since this constant current circuit allows a relatively large bias direct current (constant current) to flow through the load 12, the transistor 5 is connected as shown in FIG.
NPN type is used. In FIG. 3, a resistor 3,
A transistor 5, a resistor 6, and a reverse current blocking diode 7 are connected in this order. Diode 9,1
0 and the capacitor 23 are connected in parallel to the emitter junction and the resistor 6 as in FIG. 1, and the resistor 8 and the constant current diode 24 are connected between the collector and the base. The constant current circuit of FIG. 3 also operates in substantially the same manner as that of FIG. 1, and has a high impedance value, thereby reducing the drop in the level of the speech signal.

[Embodiments] The present invention is not limited to the above-mentioned embodiments, but includes the following possible modifications, for example.

(1) As shown in FIG. 4, if there is no need to limit the current flowing through the constant current diode 24, connect the constant current diode 24 between the base and collector of the transistor 5 without using the resistor 8 shown in FIG. You may.

(2) Even when the transistor 5 is of the NPN type as shown in FIG. 5, it is not necessary to provide a resistor 8 equivalent to the resistor 8 in FIG.

(3) It is also applicable to cases where the power supply terminal 1 is negative (-Vcc) as shown in FIGS. 6, 7, 8, and 9.

(4) The present invention is not limited to constant current power supply circuits for circuits including button telephone devices and standalone telephones, but can also be applied to other constant current circuits that require high impedance.

(5) If the backflow blocking diode 7 is not required, it may be omitted as shown in FIGS. 4, 5, 8, and 9.

[Effects of the Invention] As explained above, according to the present invention, a constant current circuit with high impedance can be obtained with a simple configuration.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing a constant current circuit according to a first embodiment of the present invention, Fig. 2 is an impedance frequency characteristic diagram of the constant current circuit of Fig. 1, and Fig. 3 is a circuit diagram showing a constant current circuit according to a first embodiment of the present invention. Circuit diagram showing an example constant current circuit, No. 4
5, 6, 7, 8, and 9 are circuit diagrams showing constant current circuits of various modifications.
FIG. 0 is a circuit diagram showing a conventional constant current circuit, FIG. 11 is an impedance frequency characteristic diagram of the constant current circuit of FIG. 10, and FIG. 12 is a block diagram showing an extension communication line system in a key telephone device. 1...Power supply terminal, 2...Constant current supply terminal, 3...
...Protection resistor, 5...Transistor, 6...Current setting resistor, 7...Reverse current blocking diode, 9,
10...Diode, 23...Capacitor, 24
... Constant current diode.

Claims (1)

[Claims for Utility Model Registration] [1] A transistor 5 connected in series between a power supply terminal 1 and a constant current supply terminal 2, a protective resistor 3 connected in series to the transistor 5, and the transistor a current setting resistor 6 connected to the emitter of 5 and arranged in series between the power supply terminal 1 and the constant current supply terminal 2;
and a series circuit of at least two PN junction elements 9 and 10 connected in parallel to the base-emitter junction of the transistor 5 and the current setting resistor 6 to provide a constant bias voltage; A constant current circuit consisting of a capacitor 23 connected in parallel to a series circuit of two PN junction elements, and a constant current diode 24 connected between the base and collector of the transistor 5. [2] The constant current circuit according to claim 1, wherein the PN junction element is a diode connected to obtain a forward voltage.