JPH0644767B2 - Overcurrent protection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] In a data interface device that operates based on electric power supplied from an exchange, when an overcurrent is supplied from a driver due to a ground fault of a cable for transmitting data, the data interface Since the output voltage drops due to the small capacity of the power supply circuit of the device, this output voltage drop is detected, the output impedance of the driver is increased to prevent overcurrent, and the control circuit that controls the driver is reset. Then, the data transmission is stopped.

[Industrial application field]

The present invention relates to an overcurrent prevention circuit which detects that an output voltage of a power supply circuit of a data interface device is lowered by an overcurrent supply and prevents the overcurrent supply.

In the data interface device, an overcurrent may flow in the driver due to a ground fault or a short circuit of the cable, which may damage the circuit element. Therefore, it is necessary to prevent overcurrent.

[Conventional technology]

The data transmission system is, for example, as shown in FIG.
The terminal devices 21 and 25 are the data interface device 2
2 and 24 are connected to the exchange 23, and the exchange 23 and the data interface devices 22, 24 are connected.
Are connected via subscriber lines 27, 28, and between the data interface devices 22, 24 and the terminal devices 21, 25 are, for example, a cable 2 based on the RS-232C standard.
Data transmission is performed between the terminal devices 21 and 25 which are connected to each other via the exchange 23.

The data interface devices 22 and 24 normally use a configuration in which electric power is supplied from a commercial power source (100 V AC) via an AC adapter. However, there is a high possibility that noise will be mixed in from the AC adapter and a data error will occur, and it is necessary to connect several stages of filters, common mode choke coils, etc. for removing the noise, so the AC adapter is expensive. Become. Therefore, by using the electric power supplied from the exchange 23, the data interface device 2
It is conceivable that a low-priced structure can be obtained by operating the circuits 2 and 24.

FIG. 4 is a block diagram of a conventional example of a data interface device, in which a DC voltage supplied from an exchange is supplied to a power supply circuit 3
In addition to 1, the voltage required for each part by this power supply circuit 31, for example, V ₁ = + 5V, V ₂ = + 8V, V ₃ = −8
To form V. The control circuit 32 is the power supply circuit 3
It operates by the output voltage V _{1 of 1} and controls the driver 33 according to the transmission data. The driver 33 has a switch 34 composed of a transistor or the like for switching and outputting the output voltages V ₂ and V ₃ of the power supply circuit 31 in accordance with transmission data, and a resistor R _{1 for} setting an output impedance of a predetermined value.
Is to have.

The 35 and 36 is a constant voltage circuit, the resistor _R 4, is connected with _{R 5} in parallel, is a voltage drop due to the resistance _R 4, _{R 5} which operates to constant reduction. That is, a constant current characteristic can be obtained. Further, 37 is a partner device such as a terminal device, R ₃ is an internal impedance, and 38 is a cable.

From the driver 33, the output voltages V ₂ and V ₃ of the power supply circuit 31 of + 8V or −8V are sent out via the resistors R ₄ and R ₅ , the switch 34, and the resistor R _{1 in accordance} with the transmission data. In the partner device 37, for example, in the case of + V, “1”,
In the case of -V, it will be identified as "0" data.

Further, when the cable 38 has a ground fault or a short circuit, or the resistance R ₃ which is the internal impedance of the counterpart device 37 is equivalently short-circuited, an overcurrent is supplied from the driver 33, but the resistance of the resistors R ₄ and R ₅ is increased. By operating the constant voltage circuits 35 and 36 so that the voltage drop becomes constant, it is possible to prevent an overcurrent from being supplied by supplying a constant current or more.

The power supply circuit 31 has a self-exciting DC / DC converter configuration as shown in FIG. 5, for example.
The DC voltage supplied from the exchange is applied to the + terminal and the-terminal, and the voltages V ₁ (+ 5V), V ₂ (+ 8V), V _{3 are applied.}
It outputs (-8V). Further, Q ₁ is a switching transistor, R ₆ and R ₇ are resistors, T is a transformer, n _{1 to} n ₅ are windings, D _{1 to} D ₃ are rectifying diodes, and C _{1 to} C ₃ are capacitors. .

DC voltage supplied from the exchange is +, - when added to the terminal shown, the base current of the transistor Q ₁ is supplied through the resistor R _6, whereby the collector current of the transistor Q ₁ is the winding n ₁ Flowing. This collector current induces a voltage in the windings n _{2 to} n ₅ , but the windings n ₃ to n ₅
The voltage induced in n ₅ has a reverse polarity with respect to the diodes D _{1 to} D ₃ , and no current flows. Voltage induced in Matamaki line n ₂ becomes a polarity in a direction increasing the base current of the transistor Q _1, the collector current of the transistor Q ₁ which flows through the winding n ₁ of the transformer T is increased.

Resistors _R 6, _{R 7} is for adjusting the base current of the transistor _{Q 1,} the collector current of the transistor _{Q 1} is the resistance R
Saturates at a value corresponding to the set values of ₆ and R ₇ . That is, the increase in collector current stops at a certain value. When the collector current stops increasing, the polarities of the voltages induced in the windings n _{2 to} n ₅ are opposite to those described above, and the voltage induced in the winding n ₂ causes the base current to decrease and the collector current to Correspondingly reduced. Further, the capacitors C ₁ to C ₆ are connected via the voltage diodes D _{1 to} D ₆ induced in the windings n _{3 to} n _5.
It is output as well as being charged to C ₃ .

It is possible to obtain the desired output voltages V _{1 to} V ₃ by repeating the above-described operation, and the output voltages V _{1 to} V ₃ are
It is determined by selecting the turns ratio of _{3 to} n ₅ .

Also, in the RS-232C standard, the termination load resistance is 3KΩ.
The voltage at that time is regulated to 5 to 15 V in absolute value. Further, "1" of the transmission data is represented by a negative voltage of -3V or less, and "0" is represented by a positive voltage of + 3V or more. The output impedance is specified to be 300Ω at the minimum. Therefore, in the conventional example shown in FIG.
₁ is connected.

In order to reduce the power consumption of the data interface devices 22 and 24, the voltage output from the driver 33 may be lowered as much as possible. In that case, SR-23
In order to satisfy the 2C standard, for example, the termination load resistance R ₃ is ₃ KΩ, the resistance R ₁ is 300 Ω, and the minimum voltage of 5 V is applied to the termination load resistance R ₃ , the power supply voltage is
It should be set to 5.5V. However, the power supply circuit 31
In order to apply an output voltage from the driver 33 to the driver 33, the resistors R ₄ and R ₅ are connected, and there is a voltage drop due to that,
Further, in consideration of the fluctuation of the power supply voltage, the internal impedance of the driver 33, the margin, etc., it is necessary to set the voltage higher than 5.5V by several V. For example, as described above, the output voltages V ₂ , V of the power supply circuit 31 are set. ₃ will be set to + 8V and -8V.

[Problems to be solved by the invention]

When the power consumption of the data interface devices 22 and 24 is supplied from the commercial power source, there are drawbacks that noise is mixed into the data and the AC adapter is expensive as described above. Therefore, the electric power supplied from the exchange 23 is used, but the subscriber lines 27 and 28 are required to have a long distance and cannot consume a large amount of electric power. It needs to be converted to electricity.

However, the conventional data interface devices 22, 24
Is a resistance R _{4 for} preventing overcurrent, as shown in FIG.
Since R ₅ and the resistor R ₁ that determines the output impedance are connected in series, the output voltages V ₂ , V of the power supply circuit 31
₃ must be at least higher than the specified voltage by a voltage drop due to the resistors R ₄ and R ₅ . Therefore, it is not easy to reduce the power consumption.

Further, when a signal not related to direct data transmission such as a ringing signal (RI signal) is short-circuited at the input terminal of the partner device, an overcurrent will be supplied, but the constant voltage circuit 3
5, 36 suppresses the overcurrent, and the control circuit 32
Will send data without noticing an abnormal condition.
In that case, at least the other device has a receiver short circuit, which results in a low-reliability data transmission system.

It is an object of the present invention to reduce power consumption by lowering the output voltage of a power supply circuit, and to reset the control circuit to stop data transmission in a fault state in which an overcurrent flows.

[Means for solving problems]

The overcurrent prevention circuit of the present invention reduces the output voltage at the time of overcurrent by making the capacity of the power supply circuit relatively small,
The decrease in the output voltage is detected to prevent overcurrent. This will be described below with reference to FIG. A data interface device including a power supply circuit 1 which is supplied with a DC voltage from an exchange and supplies an output voltage to each unit, a control circuit 2 which controls transmission of data, and a driver 3 which is controlled by the control circuit 2. The driver 3 has a configuration of switching the output impedance, and the control circuit 2 has a configuration of controlling the driver 3 according to the transmission data to send the data to the line and resetting the driver 3 to stop the control. Further, since the output voltage of the power supply circuit 1 drops when the output current of the driver 3 exceeds a predetermined value, the output voltage drop is detected, the control circuit 2 is reset to the initial state, and the output impedance of the driver 3 is adjusted. Switching control is performed so as to increase the value.

[Action]

When an overcurrent is supplied from the driver 3, the output voltage drops because the power supply exceeds the allowable power supply of the power supply circuit 1. The decrease in the output voltage is detected by the voltage detection circuit 4, and the output impedance of the driver 3 is switched so as to be increased. The output impedance of the power supply circuit 1 is reduced because the output impedance can be kept low at all times. be able to. Further, when the output voltage of the power supply circuit 1 drops, the control circuit 2 is reset to stop the operation of controlling the driver 3 to send data and prevent invalid data sending.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 2 is a block diagram of an embodiment of the present invention. 11 is a power supply circuit, 12 is a control circuit, 13 is a driver, 14 is a voltage detection circuit, 15 is a partner device such as a terminal device, 16 is a cable, and 17 is Light emitting diodes, SW ₁ and SW ₂ are switches composed of transistors and the like, and R _{1 to} R ₃ are resistors.

The power supply circuit 11 has, for example, the configuration shown in FIG. 5, is supplied with a DC voltage from the exchange, and has V ₁ (= + 5V), V ₂
(= + _8V), and outputs a voltage V 3 (= -8V), supplies the output voltages _{V 1} to the control circuit 12. This control circuit 1
When the transmission data from the exchange or the like is input, the switch ₂ controls the switch SW _{1 of the} driver 13 to change the positive and negative output voltages V ₂ and V ₃ of the power supply circuit 11 to “1” of the transmission data.
It is switched according to "0" and is sent to the line such as the cable 16, and when a reset signal from the voltage detection circuit 14 is applied, the initial state is restored and the driver 13 is returned.
The control circuit 12 can be easily realized by the control function of the logic circuit or the microprocessor that performs the control operation as described above.

Also, the driver 13 has a configuration in which the output impedance is switched by the switch SW ₂ , and the positive and negative output voltages V ₂ and V ₃ of the power supply circuit 11 are supplied to the driver 13 without passing through a resistor, and the switch SW is normally provided. ₂ is ON, the positive and negative output voltages V ₂ and V ₃ are controlled by the switch SW ₁ controlled by the control circuit 12 according to the transmission data.
Will be switched and output.

Further, the resistor R _{2 of the} driver 13 is normally short-circuited by the switch SW ₂ , and therefore the output voltage of the driver 13 is the voltage drop of the resistor R ₁ from the output voltages V ₂ and V ₃ of the power supply circuit 11. The resistance R ₁ can be set to, for example, 300Ω, and since the resistors R ₄ and R _{5 in} FIG. ₄ are not connected, the output voltages V ₂ and V ₃ of the power supply circuit 11 are It can be lowered as compared with the case shown in FIG. That is, low power consumption can be achieved.

The voltage detection circuit 14 shows the case where the output voltage V ₁ of the power supply circuit 11 supplied to the control circuit 12 is monitored.
Of course, it is also possible to adopt a configuration in which other output voltages V ₂ and V ₃ are monitored.

When an overcurrent is supplied from the driver 13 due to a short circuit of the cable 16 or the like, in the power supply circuit 11, the overcurrent is output from either terminal of the output voltages V ₂ and V _{3 in accordance} with the switching state of the switch SW ₁ of the driver 13. Will be supplied. The power supply circuit 11 is designed to have maximum efficiency when supplying power to the rated load, and since the power supplied from the exchange is constant, when overcurrent is supplied, The output voltages V ₂ and V ₃ of the power supply circuit 11 decrease. At the same time, other output voltages in the configuration shown in FIG. 5 are also reduced by an amount corresponding to the turn ratio of the windings n ₃ , n ₄ and n ₅ of the transformer T. The decrease in the output voltage V ₁ is detected by the voltage detection circuit 14 to detect the overcurrent state.

When the voltage detection circuit 14 detects the decrease in the output voltage of the power supply circuit 11 as described above, the reset signal is applied to the control circuit 12 and the output impedance control signal is applied to the driver 13. The control circuit 12 is reset so as to be in the initial state, and all the output terminals of the control circuit 12 are set to the high level by this. Therefore, when the current is normally controlled to flow through the light emitting diode, the light emitting diode 17 The current flowing due to the output voltage V ₁ stops flowing at the time of reset, and the light emitting diode 17 does not emit light. Therefore, the maintenance person can recognize that it is in the overcurrent state and start the fault search.

In the drive 13, the switch SW ₂ is turned off by the output impedance control signal, so that the resistance R ₁
The resistor R ₂ is connected in series with the output impedance and the output impedance increases. Therefore, the overcurrent is suppressed. In this case, by suppressing the overcurrent, the output voltages V _{1 to} V ₃ of the power supply circuit 11 return to the rated values, and when the voltage detection circuit 14 detects it, the switch SW _{2 of the} driver 13 is turned on. Therefore, when the short circuit of the cable 06 is not recovered, the overcurrent flows again, and the above operation is repeated. Switch S
The average output impedance is lowered by repeatedly turning on and off W ₂ , but by increasing the resistance R ₂ as compared with the case of only the resistance R ₁ ,
Overcurrent can be suppressed sufficiently.

〔The invention's effect〕

As described above, according to the present invention, the power supply circuit 1 that is supplied with the DC voltage from the exchange and supplies the predetermined output voltage to each unit.
A control circuit 2 for transmitting data, a driver 3 controlled by the control circuit 2, and a voltage detection circuit 4 for detecting a decrease in the output voltage of the power supply circuit 1. The voltage detection circuit 4 supplies a power supply circuit. Detecting the output voltage drop of 1,
The control circuit 2 is reset and switched so that the output impedance of the driver 3 is increased.
Since it is possible to reduce the voltage output via the driver, it is possible to reduce the power consumption, and it is possible to suppress the overcurrent by increasing the output impedance of the driver 3. Also, by resetting the control circuit 2, data transmission can be stopped,
It is not necessary to perform invalid data transmission, and the user can easily recognize the failure occurrence.

[Brief description of drawings]

FIG. 1 is a block diagram of the principle of the present invention, FIG. 2 is a block diagram of an embodiment of the present invention, FIG. 3 is a block diagram of a data transmission system, FIG. 4 is a block diagram of a conventional example, and FIG. It is a principal part circuit diagram of a circuit. 1 is a power supply circuit, 2 is a control circuit, 3 is a driver, 4 is a voltage detection circuit, 11 is a power supply circuit, V ₁ , V ₂ and V ₃ are output voltages, 12 is a control circuit, 13 is a driver, and 14 is a voltage detection circuit. Circuit, 15 is a partner device such as a terminal device, 16 is a cable,
Reference numeral 17 is a light emitting diode, SW ₁ and SW ₂ are switches composed of transistors and the like, and R ₁ , R ₂ and R ₃ are resistors.

Claims (1)

[Claims] 1. A power supply circuit (1) which is supplied with a DC voltage from an exchange and supplies a predetermined output voltage to each part, and a control circuit (2) which is operated by being supplied with power from the power supply circuit (1).
And a driver (3) controlled by the control circuit (2), the driver (3) has a configuration for switching output impedance, and the control circuit (2) is The driver (3) is controlled in accordance with the transmission data to send the data to the line, and the control of the driver (3) is stopped by a reset, and the output current of the driver (3) is a predetermined value or more. When the output voltage of the power supply circuit (1) drops, the control circuit (2) is reset to the initial state and the output impedance of the driver (3) is increased to control the switching. An overcurrent prevention circuit characterized in that a voltage detection circuit (4) is provided.