JPH07108001B2 - Ground fault detection circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a ground fault detection circuit for a long-distance subscriber circuit in a small-capacity digital electronic exchange (such as a private branch exchange), and in particular to a DC/DC
This relates to a ground fault detection circuit that has the function of protecting long-distance subscriber circuits by stopping the converter.

The functions provided in the subscriber circuits used in digital electronic switching systems include power supply (B), overvoltage protection (O), ringing (R), monitoring (S), analog (voice)/digital mutual conversion (C), two-wire/four-wire conversion (H), and test (T), which are so-called BORSCHT functions. This invention relates to a circuit that realizes earth fault detection, which is one of the monitoring functions (S).

Ground faults occur when uninsulated telephone lines come into contact with metal or the earth that has been accidentally grounded during construction. This is because when a ground fault occurs, the current in the power supply circuit increases by 2 times the normal
This is because the power consumption is doubled and four times that of normal batteries, which can lead to dangerous conditions such as overheating.

On the other hand, large-capacity digital electronic exchanges (office exchanges, etc.) generally use a -48V power supply to power subscriber circuits, but small-capacity digital electronic exchanges (PBXs, etc.) used to configure small-scale systems may use a -24V power supply to power them because the telephone lines connecting the exchange and telephones can be short. However, if the telephone lines are long, a -24V power supply cannot supply sufficient call current, so a higher voltage, -48
To achieve this, a voltage such as V is required.
A converter is used in the power supply circuit of the subscriber circuit, and the -24V voltage is boosted to -48V. For long-distance subscriber circuits, the circuit conditions are stricter, so a power supply circuit using resistors and a transformer is advantageous. The advantage of a power supply using resistors is that it does not require consideration of the dynamic range required for power supply circuits using transistor circuits, and that current can flow in both directions without distortion. In other words, the constant current protection circuit method often used in power supply circuits using transistor circuits is difficult to use in power supply circuits for long-distance subscriber circuits.

BACKGROUND ART A circuit diagram of a ground fault detection circuit for a subscriber circuit using a conventional DC/DC converter in the power supply circuit is shown in Figure 1. In Figure 1, 1 is the DC/DC converter, 2 is a comparator, 3 is a power supply transformer, and 4 is a subscriber telephone set.

DC/DC converter 1 is a DC transformer circuit for boosting -24V voltage (system power supply for the exchange) to -48V voltage, and comparator 2 is a circuit that compares it with a reference voltage to detect a ground fault and send a monitoring signal to the exchange's control unit. When a ground fault occurs in the subscriber line, if the line resistance is 0, the power supply current I will be twice as large as normal. Therefore, DC/DC
When the voltage of converter 1 is fixed at -48V, the power supply current value increases. Also, the earth fault detection voltage _VE becomes ground, comparator 2 is inverted, and a signal "1" is sent to the exchange control unit.
Even if a ground fault occurs, the -48V power supply continues to supply -48V to the DC/DC converter 1, so the power supply resistors R _T and R _R need to be elements that can withstand four times the power, and the DC/DC converter 1 must be configured to withstand twice the current. As a result, not only does this lead to an increase in price, but the size of the circuit components also increases, placing greater restrictions on component mounting.

SUMMARY OF THE INVENTION Therefore, a primary object of the present invention is to provide a ground fault detection circuit that solves the above problems.

A more specific object of the present invention is to provide a ground fault detection circuit which, when a DC/DC converter is used to supply a -48V voltage to the power supply circuit of a long-distance subscriber circuit, switches the -48V voltage to a -24V voltage when a ground fault is detected, thereby maintaining the ground fault detection state and reducing the current capacity when a ground fault occurs, thereby achieving smaller, lighter, and more economical circuit components.

Disclosure of the Invention The present invention relates to a small-capacity digital electronic exchange (private branch exchange, etc.).
This is a ground fault detection circuit for a long distance subscriber circuit in a telephone network. This ground fault detection circuit has the following elements:

The ground fault detection circuit has a DC/DC converter that converts low potential to high potential as a power supply for power supply, and a comparator that detects the occurrence of a ground fault. This comparator compares a reference voltage obtained by dividing the high voltage converted by the DC/DC converter with a ground fault detection voltage, and upon detecting the occurrence of a ground fault, sends a ground fault monitoring signal to the exchange control unit and also sends a stop signal to the DC/DC converter to stop the DC/DC converter.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a conventional ground fault detection circuit: Figure 2 shows the basic configuration of the present invention: Figure 3 shows the line resistance/power supply current characteristics of the present invention: Figure 4 shows an embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION First, the basic configuration of the present invention will be described with reference to FIG.

In FIG. 2, 1 is a DC/DC converter, 2 is a comparator,
Reference numeral 3 denotes a power supply transformer, and 4 denotes a subscriber telephone. The reference voltage _Vth of the comparator 2 is generated by dividing the -48V output side of the DC/DC converter 1 with resistors _R2 and _R3 and is supplied to the positive terminal of the comparator 2. The earth fault detection voltage _VE is generated by dividing the -48V output side of the DC/DC converter 1 with resistors R3 and R4.
The output side of -48V is connected to the power supply resistor _R and the voltage dividing resistors _R0 and R1 _.
The voltage is divided by and supplied to the negative terminal of the comparator 2. If there is a ground fault in the Tip or Tng line of the subscriber line, the power supply resistance R _R
and the voltage dividing resistor _R0 , the negative terminal of the comparator 2 becomes grounded, the comparator 2 is inverted and sends out a ground fault monitoring signal to the exchange control unit.

As an example, FIG. 3 shows a characteristic diagram of the relationship between the line resistance R of the subscriber circuit connected to the subscriber telephone set 4 and the power supply current I.

In FIG. 3, the solid curves (a) and (b) show the characteristics of the present circuit, and the dotted curves (c) and (d) show the characteristics of the conventional circuit.

First, when a ground fault occurs, the line resistance becomes 0 and the line resistance R is connected to the ground, so the power supply current I is 2 times that of normal.
Therefore, when the voltage of DC/DC converter 1 is -48V, the power supply current value increases as shown by the dotted line. If a ground fault occurs, and the voltage of the DC/DC converter is set to -24V, the power supply current when the line resistance is 0 is halved as shown by the solid line.

The present invention uses this principle to normally use a -48V voltage and is applied to long-distance subscribers with high line resistance, and when a ground fault occurs, the output voltage of the DC/DC converter is switched from -48V to -24V by inverting comparator 2. Even when a ground fault occurs, the power supply current for -24V is half that of -48V, and the ground fault monitoring voltage _VE and reference voltage _Vth sent to comparator 2 are also relatively halved, so the detection circuit for comparator 2 does not change and the monitoring signal to the exchange control unit remains sent. Therefore, even when a ground fault occurs, the power supply of the DC/DC converter is switched from -48V to -24V
By simply returning the comparator 2 to the normal state, the comparator 2 can maintain the abnormal voltage detection circuit, and long-distance subscribers can be accommodated without using circuit elements that can withstand special power.

In FIG. 3, the threshold current at -48V voltage when the DC/DC converter is operating, that is, the current value of the power supply circuit when the comparator is inverted, is _IA , and the threshold current at -2
If the threshold current at 4V is I _B , then I _A is I
At a current value about twice that of _B , in the case of a -48V voltage, the feed current increases as the line resistance decreases, and when the feed current reaches threshold value _IA , comparator 2 inverts and the voltage becomes -24V, and in the case of a -24V voltage, the feed current decreases as the line resistance increases, and when the feed current falls below threshold value _IB , comparator 2 returns to its original state and the voltage becomes -48V. Therefore, the power supply voltage can be adjusted according to the magnitude of the line resistance.

A preferred embodiment of the present invention will now be described.

4 is a diagram showing the configuration of a ground fault detection circuit according to the present invention, in which 1 is a DC/DC converter, 2 is a comparator, 3 is a power supply transformer, and 4 is a subscriber telephone.

The power supply resistors R _T and R _R are 330Ω, the voltage dividing resistors R ₀ and R ₁ for earth fault detection are 33KΩ and 11KΩ, and the reference voltage dividing resistors R ₂ and R
If the resistance of the resistor ₃ is 68KΩ and 10KΩ, the reference voltage _Vth is 6V when the power supply is -48V, and 3V when the power supply is -24V. Therefore, when the line resistance is large, the voltage for detecting earth faults _VE is 12V.
When a ground fault occurs, the ground fault detection voltage _VE becomes low, the comparator 2 is inverted, a signal of "1" is sent to the exchange control unit, an alarm is sent, and the on/off signal to the switching regulator _Q of the DC/DC converter 1 is masked and controlled.
The converter 1 is configured with a switching regulator element Q, an on/off signal receiver that controls the switching regulator element Q, a coil for boosting the voltage, and a capacitor and diode that form a rectifier circuit. This type of configuration is generally called a non-isolated DC/DC converter. The switching regulator element Q
When an on/off signal is applied to the gate of
By switching V with a duty of 50%, the output voltage is converted to -48 V. On the other hand, when the ground fault occurrence monitoring signal "1" arrives from comparator 2, the on/off signal is masked by the gate element, and no control signal is sent to switching regulator element Q. As a result, switching regulator element Q is turned off.

As a result, the configuration is simply a diode and coil connected in series, so the output is -24V and -24V is supplied to the power supply circuit. Therefore, the reference voltage _Vth of comparator 2 also becomes 3, but the ground fault detection voltage _VE is also halved, so comparator 2 remains inverted and the operating state of the exchange control unit is maintained. Because -24V is supplied to the power supply circuit, even if current continues to flow through the power supply resistor R _R due to a ground fault, it will only be about 60mA, and the alarm state can be maintained until maintenance personnel take action.

INDUSTRIAL APPLICABILITY According to the present invention, a DC/DC converter is used in the power supply circuit of a long-distance subscriber circuit.
When a C converter is used to supply -48V, the ground fault detection state can be maintained by switching the -48V voltage to -24V when a ground fault is detected.
By reducing the current capacity during a ground fault, circuit components can be made smaller, lighter, and more economical.

The present invention is suitable for constructing a long-distance subscriber circuit.

Claims (6)

[Claims] [Claim 1] A ground fault detection circuit for a long-distance subscriber circuit in a small-capacity digital electronic exchange, said ground fault detection circuit comprising: a DC/DC converter (1) for converting a low voltage into a high voltage as a power supply; a reference voltage obtained by dividing the converted high voltage by a resistor;
A comparator (2) is provided which compares the voltage obtained by dividing the earth fault detection voltage on the line side of a power supply resistor connected between the power supply and the line and sends a ground fault monitoring signal to the exchange control unit when it detects the occurrence of a ground fault, and the operation of the comparator (2) sends a ground fault monitoring signal to the exchange control unit and also sends a DC/DC converter stop signal to the DC/DC converter (1). 2. The ground fault detection circuit according to claim 1,
The DC/DC converter (1) uses -24V as a low voltage and converts the output voltage (power supply voltage) to a high voltage of -24V to -48V. 3. The ground fault detection circuit according to claim 1,
When the DC/DC converter (1) receives the stop signal, it converts the power supply voltage from the high voltage to the low voltage. 4. The ground fault detection circuit according to claim 1,
The DC/DC converter (1) includes a switching element (Q)
The inverter comprises an ON/OFF signal receiving unit that controls the switching element, a coil for boosting the voltage, and a capacitor and diode that form a rectifier circuit. 5. The ground fault detection circuit according to claim 4,
The DC/DC converter (1) includes a circuit configuration that masks and controls an on/off signal for a switching element (Q) when a stop signal is received. 6. The ground fault detection circuit according to claim 1,
The power supply circuit that supplies voltage to the subscriber has a power supply resistance (R _r ,
R _R ) and a power supply transformer (3).