JPS5949749B2 - Power supply circuit for subscriber's telephone - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a power supply circuit for subscribers of an exchange, primarily a telephone exchange.

Conventionally, electromagnetic devices such as electromagnetic relays are widely used in exchanges, and power is often supplied to subscriber telephones through the contacts of the relays in the exchange circuit. However, with the computerization of exchanges, switching circuits perform switching operations only on signals, such as call signals, and supply power to subscriber's telephones by providing power supply capability to the subscriber's circuit, and direct current power from this circuit to the telephone. A method is used in which the signal components superimposed on this DC are separated using a transformer or the like and then supplied to a switching circuit. One example is the circuit shown in Figure 1,
FIG. 1 shows an example of a power supply circuit (sometimes referred to as a subscriber circuit) using a transformer. A subscriber's telephone is connected to a terminal 101, and the power supply for the subscriber's telephone is provided by the primary windings 106, 107 of a transformer 108. It is supplied from a power source 102 whose current is limited by current-limiting resistors 103 and 104. In this case winding 106
, 107 are connected by a capacitor 105 with an impedance sufficiently small at the speech signal frequency used. In this way, the terminal 101 is supplied with direct current in a balanced state with respect to the earth, and the speech signal component appears at the secondary terminal 110 through the secondary winding 109 of the transformer 108 and is connected to the switching circuit. In such cases, all-electronic switching circuits are used, such as PCM time-division switching,
Various new technologies are used, such as an electronic contact space division method using a PNPN diode or a PNPN tri-auto (for example, a controlled rectifier), and a time division switching method using a pulse amplitude modulation method. If a transformer is used in the power supply circuit for such subscribers, it is possible to construct a circuit that is easy to handle in terms of line balance, insulation resistance, etc., but in the circuit shown in Figure 1, DC is supplied to the subscribers. Since the core of the transformer 108 is excited by direct current, it is necessary to take measures such as increasing the cross-sectional area of the core of the transformer, introducing an air gap, and increasing the number of windings in order to prevent distortions caused by this. This results in an increase in the size of the transformer, an increase in call loss, etc., and is a factor that hinders the miniaturization and economicalization of devices. The present invention aims to reduce the adverse effects caused by direct current excitation of the iron core in such cases. The present invention basically uses two constant current sources to supply high impedance AC power and direct current power to two balanced lines. If the capacitances are not completely the same, the voltage will be biased toward one power source, and the other power source may become saturated due to excessive current, resulting in low impedance.

In order to prevent this, the present invention intentionally makes a difference in the capacity of the two constant current sources and supplies only the insufficient amount through the transformer. In other words, a transformer has at least two windings with the same number of turns as the primary element connected to the subscriber, and two constant current power supplies with mutually different polarities are connected with the earth as the midpoint, Connect these two power supplies to the subscriber line. Make one of these two power sources smaller than the other. This difference is determined by taking into account changes at the time of manufacture or changes over time. One end of the primary winding is connected in parallel with this smaller power supply, and the other end of this winding is connected to the reference potential of the smaller capacity power supply. In other words, if we set the capacity of the current source supplied from -48V in the negative direction and make the positive power supply supplied from the earth slightly smaller than this, the voltage will be concentrated at both ends of the smaller power supply, and the difference current will flow from this power supply. subtract j
trying to get it out. At this time, if one end of one winding of the transformer is connected to this line and the other end is connected to the reference point of this smaller power supply, here the ground, only the difference in current will be supplied through the transformer. Ru. On the other hand, another line, that is, a constant current source with large capacity, is connected. The other winding constituting the primary winding is connected to the connected line, blocking direct current, and the other end is connected to the line that is not connected to the line of the other winding. By connecting it to the end, it forms a path for passing the call signal to the primary winding. This connection does not have to be direct current, but only needs to be connected with a sufficiently small impedance with respect to the alternating current frequency that is the signal component. In this case, if you are not careful, the voltage drop due to the resistance of the winding will be small, resulting in a decrease in the voltage across the constant current power supply, and in some cases, there is a risk that the call signal will be clipped and distorted. occurs.
To prevent this, increasing the resistance of the winding will cause call loss, so connect something like a battery, such as a zener diode, and a sufficient bypass capacitor in parallel to one end of the winding. By connecting those connected in series, the output voltage of the constant current power supply can be maintained at a constant value or higher, and the above-mentioned distortion can be avoided. However, if this voltage is set too high, the amount consumed by the resistance of the line will be reduced accordingly, so it is desirable to use an appropriate voltage.
The present invention will be explained in detail below with reference to Examples. FIG. 2 shows an embodiment of the present invention, in which a subscriber's telephone is connected to a terminal 201 and a constant current power supply 202 supplies negative current to the power supply. This power supply can actually be divided into a negative power supply of around -48V and a constant current circuit of around 50mA connected to direct current. On the other hand, a positive power source 203 is connected to the other end of the terminal connected to the subscriber.
This positive power supply is connected in series with the ground air in this diagram.
It can be represented by a constant current circuit whose absolute value is 0 mA or less and whose absolute value is smaller than the capacity of the power supply 202. On this side, earth is used, but a constant current circuit may be connected in series to a positive voltage power source, for example, +24. Here, the power supply 203 has a current value of 40 mA compared to the 50 mA of the power supply 202 in consideration of manufacturing distribution. In this way, when current flows through the subscriber's telephone, half of the primary winding 201 of the transformer 208 is connected to this line in order to compensate for the shortage of current. A current of 10 mA flows from the ground surface through the resistor 204. In this example, the resistor 204 is connected to the ground because it is assumed that the power supply 203 is connected to the ground, and if it is connected to +12V, for example,
One end of this is connected to +12V. This resistor 204 may be omitted, but as mentioned earlier, if not much voltage is generated here, the voltage at the connection point with the line of the power supply 203 may drop abnormally, causing signal distortion. be. However, if the voltage is too large, the voltage that can be consumed by the line will decrease accordingly, and the line length will be shortened. Therefore, in ordinary telephones, a resistor of around 300Ω is used to generate a voltage of around 3V. Further, the voltage at the connection point between the resistor 204 and the winding 207 is also useful for checking the operation of the subscriber. On the other hand, the line connected to the power supply 202 has
Block the direct current with a capacitor 205 and connect half of the primary winding 20
6 is connected. This winding 206 has the other end connected to the winding 2
07 to form the primary winding. In this case, winding 2
Although 06 and 207 are directly connected, they may be connected in an alternating current manner through a capacitor as long as the impedance is sufficiently small. In this way, the call signal from the subscriber flows through the windings 206 and 207 and reaches the secondary winding 209.
induces a current in and is coupled from terminal 210 to the switching circuit. Diode 211 acts as a directional element that prevents current from flowing through winding 207 when no call current flows through the subscriber.

On the side shown in Figure 2, the voltage is generated by the resistor 204, or if the current difference between the two power supplies changes due to manufacturing errors, the voltage changes and the allowable consumption voltage for the line, in other words, the allowable line length. This may make the design difficult.

In order to solve this problem, a constant voltage element is introduced in the example shown in FIG. In the embodiment shown in FIG. 3, a constant voltage diode is used as the constant voltage element. In FIG. 3, the subscriber is connected to terminal 301 and the switching circuit is connected to terminal 313. The constant current sources 302 and 303 for the subscriber are assumed to operate in the same manner as 202 and 203 in FIG. Further, the capacitor 304 is connected to the capacitor 205 in FIG. 2 and the winding 309.
, 310 and resistor 308 are respectively 206 and 20 in FIG.
7,204. In this case, the resistor 308 has a constant voltage diode 306 connected to its end 5.
Since resistor 204 generates a constant voltage, it may be smaller than that of resistor 204 or may be omitted. This constant voltage diode 3
06 includes a reverse voltage blocking diode 305 and a bypass capacitor 307 in series. This is for a ringer signal and is not directly related to the present invention. The call signal flows from the secondary winding 311 of the transformer 312 to the switching circuit via the terminal 313, as in FIG. The examples in Figures 2 and 3 use primary voltage generating elements. Although it is provided at the midpoint of the winding, if a sufficiently large bypass capacitor is used, a voltage generating element such as a resistor or a constant voltage diode may be connected between the line and the winding.

FIG. 4 shows an example in which FIGS. 2 and 3 are more specifically described. In this figure, the subscriber has terminals 400 and 4
Connected between 01 and 01. One power source that supplies current to the subscriber is 402, and inside is a reverse voltage blocking diode 41.
The collector characteristics of an NPN transistor 418 having 7 transistors connected in series are used as a constant current source.

The base of the transistor 418 is supplied with current through a resistor 415 from the ground, and is clamped by a power source 414 through a clamping diode 416. On the other hand, the emitter of this transistor 418 is connected to a power source 419 through a resistor 428. As a practical example, the power supply 414 is -48V,
Resistor 415 is 22KΩ, power supply 419 is -60■, resistor 4
If 28 is 240Ω, -50mA constant current power supply 40
2 is obtained. Positive power supply 40 connected to terminal 401 side
3 has a capacity of 40 mA. This power supply 403 uses a PNP transistor 422 connected in series with a reverse blocking diode 423.

The base of this transistor 422 receives a current from a negative power supply 419 through a resistor 420, and a diode 42
1 and it is taramped to earth. On the other hand, the emitter is connected to a power source 425 through a resistor 424, forming a constant current circuit. In this circuit, the resistor 420 is 33KΩ, and the resistor 4
If 24 is 120Ω and power supply 425 is +5V, approximately 40
It becomes a power source of mA. Now, when the subscriber operates and current flows, the 10 mA difference between the two supplies is supplied from the ground through half 410 of the primary winding of transformer 412. One end of the winding 410 on the non-line side is connected to the other half of the winding 409.
and diode 40 through resistor 408.
It is connected to a constant voltage element 406 in which -4 pieces of 5 are connected in series. This constant voltage element 406 is bypassed by a capacitor 407 and produces a voltage of approximately 2.8V if the diode is silicon. Since this voltage is sufficient for normal audio, it can be said that the resistor 408 is not necessary, but a resistor of about 100Ω is used to protect against the inflow of abnormal current.
The part related to the current supplied to the subscriber has been described above, but the call signal flows through the capacitor 404 to the primary winding 409, 4]0, passes through the secondary winding 411 to the terminal 4
13 and is connected to the switching circuit. By using the configuration described above, the current flowing through the transformer is reduced, making it possible to downsize the transformer used in the power supply circuit and to make the power supply circuit more economical.

Although the present invention has been described above with reference to Examples, the scope of the present invention is not limited by this description.

[Brief explanation of the drawing]

FIG. 1 is an example of a conventionally used power supply circuit. FIG. 2 shows an embodiment of the present invention, in which 202 is a negative constant current power supply, 203 is a positive constant current power supply with a smaller capacity than 202,
208 is a transformer, 204 is a resistor, 211 is a diode,
207 is half of the primary winding, and together with 206 constitutes the primary winding. 205 is a DC blocking capacitor. FIG. 3 shows an embodiment of the present invention using a constant voltage diode, in which 302 is a negative constant current power supply, 303 is a positive constant current power supply,
306 is a constant voltage diode, 309 and 310 are windings forming a primary winding, 312 is a transformer, and 308 is a resistor. FIG. 4 shows a specific embodiment of the present invention, in which 402 is a negative constant current source, 403 is a positive constant current source with a smaller capacity than 402, 406 is a constant voltage source using the forward drop of a diode,
404 is a DC blocking capacitor, 409 and 410 are primary windings, 412 is a transformer, and 411 is a secondary winding.

Claims (1)

[Scope of Claims] 1. A transformer having at least two identical windings, that is, a first winding and a second winding, and two constant current power supplies having mutually different polarities; Two constant current power supplies are connected with a suitable potential as a reference point, and the output sides of each are connected to a subscriber's line pair, and the current capacity of one of the two power supplies is made smaller than the other. One end of the first winding is connected to the line connected to the power source, and the other end of the first winding is connected to the reference potential of the smaller power source, and one end of the second winding is connected to the reference potential of the smaller power source. connected to the other of the line pair via a DC blocking capacitor, the other end of the second winding is connected to one end of the first winding on the non-line side, and 1. A power supply circuit for a subscriber's telephone, characterized in that a directional element is provided that does not cause a return path of current. 2. Claim 1, characterized in that one end of the first winding connected to the smaller power source is provided with means for maintaining the output voltage value of the smaller power source above a certain value. Power supply circuit for the subscriber's telephone set forth in Section 1.