JPH0116091B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention protects a system, such as a communication device, that has an internal resistance and a DC power supply circuit from induced lightning surges that occur in communication lines, etc. Regarding surge protection circuits.

[Prior Art] Conventionally, in communication lines, etc., discharge tubes, varistors, Zener diodes, etc. have been used to protect equipment from lightning surges. Increasingly, there is a demand for higher performance such as faster protection speed, smaller size, and more stable and uniform protection voltage. Here, the discharge tube has problems such as variations in the discharge starting voltage and a time delay until the discharge starts. Additionally, Zener diodes are excellent in high speed, but because they clamp surges at a constant voltage, this type of protection element, which has a large voltage/current product, requires an element with performance that can handle large amounts of power, making it expensive. There is a drawback.

On the other hand, a protection circuit using a thyristor to protect against abnormal output of a voltage regulator is disclosed in Japanese Patent Laid-Open No. 48-94844 and US Pat. No. 3,551,745.

[Problems to be Solved by the Invention] Generally, once a thyristor is fired, it remains in the fired state as long as a direct current greater than the holding current can flow through the thyristor. Therefore, when a DC power supply circuit is connected to a protection circuit using a thyristor,
A thyristor that is once ignited due to a surge or the like remains conductive due to the direct current from the power supply circuit, and there is a problem in that it cannot recover by itself even after the surge has ended. Incidentally, in response to this problem, the above-mentioned US patent explains that the SCR (thyristor) can be reset by turning off the power to the regulator, and the current situation is to manually reset it.

However, in communication devices, it is strongly desired that the thyristor automatically return to its initial state when the external surge subsides, that is, a maintenance-free arrester.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a surge protection circuit that automatically restores a thyristor to a cut-off state after a surge has ended.

[Means for Solving the Problems] To achieve the above object, the present invention provides a system having an internal resistor and a DC power supply circuit connected in series between a pair of terminals, in which an anode is connected to the positive terminal of the system. , a thyristor having a cathode connected to its negative terminal; a constant voltage element connected between the gate and anode of the thyristor and exhibiting a constant voltage drop with respect to a current flowing from the anode to the cathode of the thyristor; A surge protection circuit was constituted by a resistor that short-circuited between the gate and the cathode, and whose effective resistance value was determined so that the holding current of the thyristor was effectively larger than the short-circuit DC current in the system.

[Function] Once a thyristor is fired, it remains fired as long as a direct current greater than the holding current can flow through it. In the present invention, the resistance that short-circuits between the gate and cathode of the thyristor, that is, its effective resistance value, is a resistance determined such that the holding current of the thyristor is effectively larger than the short-circuit DC current in the system in which it is used. Therefore, when the external surge subsides and the short-circuit DC current in the system becomes smaller than the holding current of the thyristor,
The thyristor is no longer able to maintain conduction and automatically returns to the cut-off state.

[Example] The present invention will be explained in detail below using the drawings.

FIG. 1 is a circuit diagram showing a first embodiment of the present invention.

In FIG. 1, a system 6 to be protected by the surge protection circuit of the present invention has a DC power supply circuit 7 and an internal resistor 8, and a thyristor 1 constituting the surge protection circuit is connected to the positive terminal of the system 6. The cathode of the thyristor 1 is connected to the negative terminal of the anode of the thyristor 1 .

A resistor 2 is connected between the gate G and cathode K of the thyristor 1, a Zener diode 3 is further connected as a constant voltage element between the gate G and the anode A, and a terminal is connected between the anode A and the cathode K of the thyristor 1. 4 and 5 constitute a surge protection circuit.

The resistor 2 is selected to have a low resistance of, for example, several ohms for reasons described later. The Zener diode 3 is connected with a polarity such that when the potential of the terminal 4 exceeds a certain positive voltage when the terminal 5 is set as a reference potential, current flows from the terminal 4 to the resistor 2 through the Zener diode 3. ing.

With this configuration, when a surge voltage is applied between the terminals 4 and 5, the Zener diode 3 operates immediately when the voltage exceeds a certain level, and acts to prevent the potential difference between the terminals 4 and 5 from increasing any further. .
At this time, if the internal impedance of the surge source is low, a large surge current flows through the Zener diode 3. As a result, the voltage drop across resistor 2 reaches a value sufficient to fire thyristor 1 (usually about 0.7 volts or more), causing thyristor 1 to fire, and terminals 4 and 5 are short-circuited by thyristor 1, resulting in a low voltage. (usually a few volts). Once ignited, thyristor 1 can remain conductive as long as the flowing current does not decrease, so the surge is maintained at a low voltage. At this time, there is no power consumption in Zener diode 3, and overall power consumption is small. . Of course, if the surge is a large voltage but the internal impedance of the surge source is large so that a very large current does not flow, the Zener diode 3 alone can clamp the terminals 4 and 5 to a constant voltage.

Here, due to the use of the thyristor 1, there are matters to be noted. That is, in addition to the DC power supply circuit 7 and the internal resistor 8, the system 6 shown by the broken line in FIG. 1 includes components such as an IC, a transistor, and a capacitor (not shown). Such a configuration actually exists in telephone exchanges and the like. If a lightning surge etc. is applied from outside to these configurations, thyristor 1 is ignited and protects system 6 from the lightning surge, then even after the lightning surge disappears, current will be supplied from system 6's DC power supply circuit 7 to thyristor 1. This will continue to be the case. Since this will impede the function of the system 6, it is necessary to be able to restore the thyristor 1 to the cut-off state again after the surge has disappeared.

Generally, when the current flowing through a conducting thyristor is gradually reduced, the thyristor suddenly shuts off at a certain current value. This is called the holding current of the thyristor, and this holding current is closely related to the magnitude of the short-circuit resistance between the gate and cathode of the thyristor, and as the short-circuit resistance is reduced, the holding current increases. Therefore, if the holding current of the thyristor is set to be larger than the current flowing from the system 6 (determined by the DC power supply circuit 7 and internal resistance 8) when terminals 4 and 5 in Fig. 1 are short-circuited, it is possible to There is no need to worry; once the surge disappears, the surge protection circuit can return to its cut-off state. For this reason, as mentioned above, the resistance value of the resistor 2 is selected to be quite low. Generally, the line-to-line short circuit current in the system is around 100 to 200 mA at most, and the thyristors are
It is sufficient to design it so that the holding current is 100mA.

Let's summarize the relationship between the thyristor's holding current and the short-circuit resistance between the gate and cathode.

Generally, the thyristor is composed of two
It can be written as two transistors. In Figure 6, Ia is the anode current, αp, αn are the current transmission rates,
Rg is the short circuit resistance and Ig is the gate current.

Here, the conditions for ignition can be expressed by the following equations (1) and (2).

αp＋αn≧1 ……(1) Vg0.7v ……(2) Also, Vg can be expressed as Vg≦Rg・Ig ……(3). Therefore, at the time of ignition, even if the short circuit resistance Rg in equation (3) is small, the current Ig due to the lightning surge is sufficiently large, so Vg (0.7v) can be easily obtained and the thyristor will ignite.

On the other hand, the retention condition is generally that the gate current is
When Ig is stopped, it can be expressed as Vg≦Rg・Ia・αp…(4). Therefore, under the holding condition of equation (4), reducing the short-circuit resistance Rg means that the anode current Ia required for holding increases, and the short-circuit resistance Rg
The value of will greatly contribute to the retention conditions.

FIG. 2 is a circuit diagram showing a second embodiment of the present invention. In Figure 2, Zener diode 3 is
It is connected to the base of the NPN transistor 10, its emitter is connected to the gate G of the thyristor 1, and its collector is connected to the terminal 4. With this configuration, the current from Zener diode 3 is
Since the current is amplified by the NPN transistor 10 and added to the resistor 2 and thyristor 1, a Zener diode with a small capacity can be used. The current flowing into
This has the advantage that it is possible to fire the thyristor 1 at a higher speed than in the illustrated embodiment.
The rest is the same as the above description.

FIG. 3 is a circuit diagram showing a third embodiment of the present invention, in which a diode 1 is added to the circuit shown in FIG.
1 has been added as shown. In the embodiments shown in FIGS. 1 and 2, whenever the potential at terminal 5 becomes more positive than at terminal 4, current flows backward between terminals 4 and 5 through the Zener diode 3 or the emitter-base of the transistor 10. There is a drawback. This is because, in general, the base-emitter of an NPN transistor can only withstand a voltage of about 10 volts at most, and current will continue to flow in response to higher reverse voltages. Therefore, the third
In the figure, a diode 11 is provided to prevent backflow. Note that since the thyristor 1 originally blocks current also in the reverse direction, no special considerations are necessary.

FIG. 4 shows a fourth embodiment of the present invention, in which the circuits of the embodiment shown in FIG. 3 are connected in antiparallel to each other to provide protection against surges in both positive and negative directions. That is, in the embodiments of FIGS. 1 and 2, the system 6
Although we have considered only the case where the current flows from the to the communication line etc. in a fixed direction, some systems may be equipped with an internal switch for reversing the current to supply current in the opposite direction. Therefore, in this embodiment, a surge protection circuit is constructed of a thyristor 1, a resistor 2, a Zener diode 3, a transistor 10, and a diode 11, and a surge protection circuit is composed of a thyristor 1', a resistor 2', a Zener diode 3', a transistor 10', and a diode 11'. A surge protection circuit is connected in antiparallel to protect against surges in both positive and negative directions between terminals 4 and 5. Furthermore, by connecting resistors 12, 12' between the bases and emitters of the NPN transistors 10, 10', the effective collector-emitter breakdown voltage of the transistors 10, 10' can be increased, allowing stable operation. Effects can be obtained. In the drawings, a dash mark (') attached to a symbol represents the same component as a symbol without that mark. Operations and functions can be understood from the above description.

FIG. 5 shows a fifth embodiment of the present invention, which, like FIG. 4, is intended to protect against surges in both positive and negative directions applied to the terminals 4 and 5.
Zener diodes 3, 3' connected in series in opposite directions are connected to the respective gates of two thyristors 1, 1' connected in antiparallel. With this configuration, the surge applied to terminals 4 and 5 first flows through resistor 2' - Zener diode 3 - Zener diode 3 - resistor 2 (or in the opposite direction) and is clamped to a constant voltage, and then the surge current If it is large, thyristor 1 or 1' is fired to protect it.

It goes without saying that the two Zener diodes 3 and 3' may be replaced with one element having constant voltage properties in both positive and negative directions.

[Effects of the Invention] As explained in detail above, according to the present invention,
A resistor that short-circuits between the gate and cathode of a thyristor, the effective resistance of which is determined such that the holding current of the thyristor is effectively larger than the short-circuit direct current in the system in which it is used. Therefore, when the external surge subsides and the short-circuit DC current in the system becomes smaller than the holding current of the thyristor, the thyristor can no longer maintain the conductive state and automatically returns to the cut-off state. In other words, a maintenance-free arrester can be realized.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a first embodiment of the present invention, and FIGS. 2 to 5 are a circuit diagram showing a second embodiment of the present invention.
The circuit diagrams of the third, fourth, and fifth embodiments, and FIG. 6 are circuit diagrams for explaining ignition and holding of the thyristor. 1, 1'... Thyristor, 2, 2'... Resistor,
3, 3'... Constant voltage element, 4, 5... Terminal, 6
...System, 7...DC power supply circuit, 8...Internal resistance, 10,10'...NPN transistor, 1
1, 11'...Diode, 12, 12'...Resistor.

Claims (1)

[Claims] 1 In a system having an internal resistance and a DC power supply circuit connected in series between a pair of terminals, a thyristor having an anode connected to the positive terminal and a cathode to the negative terminal of the system, and the gate anode of this thyristor A constant voltage element is connected between the thyristor and exhibits a constant voltage drop with respect to the current flowing from the anode to the cathode of the thyristor, and a resistor short-circuits between the gate and cathode of the thyristor, the effective resistance of which is the same as that of the thyristor. and a resistor determined such that the holding current is effectively greater than the short-circuit DC current in the system.