JPS62230141A - Overcurrent preventing circuit - Google Patents

Abstract

PURPOSE:To reduce power consumption for an interface device and to prevent overcurrent by detecting an output voltage decrease due to the overcurrent so as to reset a control circuit and to increase an output impedance of a driver. CONSTITUTION:If an output current of the driver 13 controlled by a control circuit 12 applying data transmission control reaches a prescribed value or over by short-circuit or the like of a cable 16, the output voltage of a power circuit 11 is lowered. At the time of a voltage detection circuit 14 detects the redunction in the output voltage of the circuit 11, the circuit 14 gives a reset signal to the circuit 12 to bring the circuit into the initial state, an output impedance control signal is fed to the driver 13 to turn off a switch SW2 thereby increasing the output impedance.

Description

[Detailed Description of the Invention] [Summary] In a data interface device that operates based on power supplied from an exchange, when an overcurrent is supplied from a driver due to a ground fault in a cable that sends data, etc., the data interface device Since the output voltage drops due to the small capacity of the device's power supply circuit, this drop in output voltage is detected and the output impedance of the driver is increased to prevent overcurrent, and the control circuit that controls the driver is reset. Then, data transmission is stopped.

[Industrial application field]

The present invention relates to an overcurrent prevention circuit that detects a decrease in the output voltage of a power supply circuit of a data interface device due to overcurrent supply and prevents the overcurrent supply.

In a data interface device, an overcurrent may flow through a driver due to a ground fault or short circuit in a cable, thereby damaging circuit elements. Therefore, it is necessary to prevent overcurrent.

[Conventional technology]

The data transmission system is, for example, as shown in FIG.
The terminal device 21.25 is a data interface device 22.
．． The switch 23 and the data interface device 22.24 are connected via the subscriber line 27.28, and the data interface device 22.24 and the terminal device 21.25, for example, cable 2 according to the R3-232C standard.
6.29, and data transmission is performed between the terminal devices 21 and 25 connected via the exchange 23.

The data interface device 22.24 normally uses a configuration in which power is supplied from a commercial power source (AClooV) via an AC adapter. However, there is a high possibility that data errors may occur due to noise entering from the AC adapter, and it is necessary to connect several stages of filters, common mode choke coils, etc. to remove the noise, so AC adapters are expensive. Become. Therefore, using the power supplied from the exchange 23, the data interface device 2
It is considered that an inexpensive configuration can be achieved by operating 2.24.

FIG. 4 is a block diagram of a conventional example of a data interface device, in which DC voltage supplied from an exchange is connected to a power supply circuit 3.
1, the voltage required for each part by this power supply circuit 31, for example, V, = +5V, V2 = +8V, V3 = -8
It forms a V. The control circuit 32 is the power supply circuit 3
It operates with an output voltage Vl of 1, and controls the driver 33 in response to transmission data. The driver 33 outputs the output voltage v2. of the power supply circuit 31. It has a switch 34 consisting of a transistor or the like that switches and outputs V3 in accordance with the transmitted data, and a resistor R1 for setting the output impedance to a predetermined value.
It has the following.

Further, reference numerals 35 and 36 are constant voltage circuits, which are connected in parallel with the resistors R1 and R5, and operate to constantize the voltage drop caused by the resistors R, , R5.

That is, constant current characteristics can be obtained. Further, 37 is a counterpart device such as a terminal device, R3 indicates internal impedance, and 38 is a cable.

From the driver 33, the output voltage 2, v3 of the power supply circuit 31 of +8V or -8V is sent out via the resistors R, , R5, the switch 34, and the resistor R, corresponding to the transmission data, and is sent to the other party. In the device 37, for example, in the case of +V, "1",
In the case of −■, it is identified as “O” data.

In addition, when there is a ground fault or short circuit in the cable 38 or an equivalent short circuit in the resistor R3, which is the internal impedance of the counterpart device 37, an overcurrent will be supplied from the driver 33, but the voltage across the resistors R4 and R5 will be By operating the constant voltage circuits 35 and 36 so that the drop is constant, overcurrent can be prevented by not supplying more than a certain current.

Further, the power supply circuit 31 has a configuration of a self-excited DC-DC converter, as shown in FIG. 5, for example.
The DC voltage supplied from the exchange is applied to the middle terminal and one terminal, and the voltages V+ (+5V), V2 (+8V),
It outputs V3 (-8V). Also, Q is a switching transistor, Rh and R'r are resistors,
T is a transformer, nl-n5 is a winding, D, ~D3 are rectifying diodes, and CI ``'' C3 is a capacitor.

When a DC voltage supplied from the exchange is applied to the terminals indicated by + and -, the base current of the transistor Q1 is supplied via the resistor R, so that the collector current of the transistor Q flows to the winding nl. This collector current induces a voltage in tJAnz~n5, but winding n3
The voltage induced in ~n5 has opposite polarity to the diodes D and ~D3, and no current flows. Further, the voltage induced in the winding n2 has a polarity that increases the base current of the transistor Q, and the collector current of the transistor Q flowing through the winding n1 of the transformer T increases.

The resistors Rh and R7 are for adjusting the base current of the transistor Q, and the collector current of the transistor Q is saturated at a value corresponding to the setting values of the resistors Rh and Rq. That is,
The collector current stops increasing at this value. When the collector current stops increasing, the winding n Z ”” n 5
The polarity of the voltage induced in the winding n2 is opposite to that in the previous case, and the base current decreases due to the voltage induced in the winding n2.
The collector current decreases correspondingly. Also winding n3~
The voltage induced in n5 is charged to capacitors 01 to C3 via diodes D and D3, and is outputted.

Desired output voltages ■1 to ■3 can be obtained by repeating the above-mentioned operations, and the output voltages ■- to v3 are determined by the winding n.
This is determined by selecting the turns ratio from 3 to n5.

Further, in the R3-232C standard, the terminal load resistance is set to 3Ω to 7Ω, and the voltage at that time is specified to be 5 to 15V in absolute value. Further, "1" of the transmission data is represented by a negative voltage of -3V or less, and "O" is represented by a positive voltage of +3V or more. Further, the output impedance is specified to be 300Ω at the minimum. Therefore, in the conventional example shown in FIG.
, is connected.

In order to reduce the power consumption of the data interface devices 22, 24, it is conceivable to lower the voltage output from the driver 33 as much as possible.

In that case, in order to satisfy the R3-232G standard,
For example, the terminal load resistance R3 is set to 3Ω, and the resistance R1 is set to 300Ω.
Ω, and in order to apply the lowest voltage of 5V to the terminal load resistor R3, it is sufficient to set the power supply voltage to 5.5μ. However, in order to apply the output voltage from the power supply circuit 31 to the driver 33, resistors R, R5 are connected, which causes a voltage drop, and also takes into account fluctuations in the power supply voltage, internal impedance of the driver 33, margin, etc. Then, 5.5
It is necessary to set the voltage to several V higher than V2.V, for example, as described above, the output voltage v2. v3 will be set to +8V and -8V.

[Problem that the invention seeks to solve]

When the power consumption of the data interface devices 22, 24 is supplied from a commercial power supply, as described above, there are disadvantages in that noise is mixed into the data and the AC adapter is expensive. Therefore, the power supplied from the exchange 23 is used, but since the subscriber lines 27 and 28 are required to be long distances and cannot consume large amounts of power, the interface devices 22 and 24 are designed to have low consumption. It is necessary to electrify.

However, the conventional data interface equipment R22, 24 has resistors R4, R24 for overcurrent prevention, as shown in FIG.
s and the resistor R, which determines the output impedance, are connected in series, so that the output voltage V2. v3
needs to be higher than the specified voltage by at least the voltage drop caused by the resistors R4, R. Therefore, it has not been easy to reduce power consumption.

In addition, when the calling signal (RI double signal) is a signal not related to direct data transmission, if there is a short circuit at the input terminal of the other device, 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 the abnormal condition.

In that case, at least the other device has a failure due to a receiver short circuit, resulting in a data transmission system with low reliability.

An object of the present invention is to reduce power consumption by lowering the output voltage of a power supply circuit, and to reset a control circuit to stop data transmission in a fault condition where 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.
It detects the drop in the output voltage and prevents overcurrent. This will be explained below with reference to FIG. A data interface device includes a power supply circuit 1 that is supplied with DC voltage from an exchange and supplies output voltage to various parts, a control circuit 2 that controls data transmission, and a driver 3 that is controlled by this control circuit 2. When the output current of the driver 3 exceeds a predetermined value due to a ground fault or short circuit in the cable, the output voltage of the power supply circuit 1 decreases, so the voltage drop is detected and a reset signal is applied to the control circuit 2. to reset to the initial state, and also apply an output impedance control signal to the driver 3 to increase its output impedance. It is for wholesale.

[Effect]

When an overcurrent is supplied from the driver 3, the output voltage decreases because the power exceeds the allowable supply power of the power supply circuit 1. The drop in the output voltage is detected by the voltage detection circuit 4, and the output impedance of the driver 3 is switched to be high.Since the output impedance can be kept low at all times, the output voltage of the power supply circuit 1 is reduced. can do. Also, when the output voltage of the power supply circuit 1 decreases, the control circuit 2
By resetting , the driver 3 is controlled to stop the operation of transmitting data, and to prevent invalid data transmission.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 2 is a block diagram of an embodiment of the present invention, in which 11 is a power supply circuit, 12 is a control circuit, 13 is a driver, 14 is a voltage detection circuit, 15 is a counterpart device such as a terminal device, 16 is a cable, and 17 is a The light emitting diodes, sw, , sw2 are switches made of transistors, etc., and R1'''R3 is a resistor.

The power supply circuit 11 has, for example, the configuration shown in FIG.
z (-+8V) and V3 (=8V), and the control circuit 12 receives the output voltage V.

supply. Further, the output voltage V 2 + V 3 is directly supplied to the driver 13 without going through a resistor or the like,
The positive and negative output voltages v2. V3 will be switched and output.

Further, the resistor R2 of the driver 13 is normally short-circuited by the switch sw2, so that the output voltage of the driver 13 is equal to the output voltage v2. It is the value obtained by subtracting the voltage drop across the resistor R from v3, and the resistor R1 can be set to, for example, 300Ω, and since the resistors R4 and R5 in FIG. 4 are not connected, the output voltage of the power supply circuit 11 is 2. (3) can be lowered compared to the case shown in FIG. That is, it is possible to reduce power consumption.

Further, although the case is shown in which the voltage detection circuit 14 monitors the output voltage V of the power supply circuit 11 that is supplied to the control circuit 12,
Other output voltage v2. Of course, it is also possible to configure the system to monitor V3.

When an overcurrent is supplied from the driver 13 due to a short circuit in the cable 16, etc., the power supply circuit 11 supplies the overcurrent from either terminal of the output voltage Vz or V3 according to the switching state of the switch swl of the driver 13. That will happen. The power supply circuit 11 is designed to have the highest efficiency when supplying power to the rated load, and since the power supplied from the exchange is constant, when an overcurrent is supplied, The output voltage V 2 *V 3 of the power supply circuit 11 decreases. At the same time, the other output voltage V, in the case of the configuration shown in FIG.
*Decreased by an amount corresponding to the turns ratio of n5.

This drop in the output voltage V is detected by the voltage detection circuit 14,
An overcurrent condition will be detected.

When the voltage detection circuit 14 detects a drop in the output voltage of the power supply circuit 11 as described above, it applies a reset signal to the control circuit 12 and applies an output impedance control signal to the driver 13. The control circuit 12 is reset to the initial state, and all output terminals of the control circuit 12 are at a high level. Therefore, if the control circuit 12 is controlled so that current flows through the light emitting diode, the light emitting diode 17 The current that was flowing due to the output voltage vI stops flowing at the time of reset, and the light emitting diode 17 enters a state in which it does not emit light. Therefore, the maintenance person can recognize that there is an overcurrent condition and start searching for a fault.

In the driver 13, since the switch SW2 is turned off by the output impedance control signal, the resistors R,
Since the resistor R2 is connected in series with the output impedance, the output impedance becomes high. Therefore, overcurrent is suppressed. In this case, by suppressing the overcurrent, the output voltages ■, ~v3 of the power supply circuit 11 return to their rated values, and when the voltage detection circuit 14 detects this, the switch SW2 of the driver 13 is controlled to turn on. Therefore, if the short circuit in the cable 16 is not recovered, the overcurrent will flow again and the above-described operation will be repeated. switch S
By repeatedly turning on and off W2, the average output impedance becomes lower, but by making the resistor R2 larger than when only the resistor R is used,
Overcurrent can be sufficiently suppressed.

〔Effect of the invention〕

As explained above, the present invention provides a power supply circuit 1 that is supplied with DC voltage from an exchange and supplies a predetermined output voltage to each part.
, 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 drop in the output voltage of the power supply circuit 1. Detecting a drop in the output voltage of 1,
This resets the control circuit 2 and switches the output impedance of the driver 3 to high.
Since it is possible to lower the voltage output through the driver 3, power consumption can be reduced, and overcurrent can be suppressed by increasing the output impedance of the driver 3.

Furthermore, data transmission can be stopped by resetting the control circuit 2, so that invalid data transmission is not performed, and the user can easily recognize the occurrence of a failure.

[Brief explanation of drawings]

Figure 1 is a block diagram of the principle of the present invention, Figure 2 is a block diagram of an embodiment of the present invention, Figure 3 is a block diagram of a data transmission system, Figure 4 is a block diagram of a conventional example, and Figure 5 is a power supply. It is a circuit diagram of the main part 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, , v2, v3 are output voltages, 12 is a control circuit, 13 is a driver, 14 is a voltage detection circuit, 15 is a counterpart device such as a terminal device, 16 is a cable,
17 is a light emitting diode, sw, sw2 is a switch made of a transistor, etc., and RI + R2 + R3 is a resistor.

Claims (1)

[Claims] A power supply circuit (1), a control circuit (2) that operates by being supplied with power from the power supply circuit (1), and a driver (3) that is controlled by the control circuit (2). The data interface device includes a data interface device that detects a drop in the output voltage of the power supply circuit (1) due to the output current of the driver (3) exceeding a predetermined value, and returns the control circuit (2) to an initial state. a voltage detection circuit (
4) An overcurrent prevention circuit characterized by providing the following.