JPS5820123B2 - Surge absorber for high frequency signal transmission line - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is applied to a surge absorber that handles high frequency signals equivalent to telephone communications, and is applied to circuits that do not want to insert excessive capacitance between signal lines or between signal lines and top and bottom. This provides a surge absorber with low capacitance while maintaining the conventional surge absorbing ability.

Originally, varistors containing metal oxide as the main component, such as zinc oxide varistors, have an extremely excellent voltage-dependent nonlinear index and a large surge absorption capacity, so they are currently widely used for the purpose of absorbing abnormal voltages. It is used.

FIG. 1 shows the voltage-current characteristics.

1 is a curve showing the voltage-current characteristics of the varistor, 2 and 2
' is the voltage point at which current suddenly begins to flow, and is usually called the varistor voltage.

As can be seen from FIG. 1, if the varistor voltage is set higher than the circuit voltage, there is almost no leakage current under normal conditions, and the varistor operates as an insulator.

When a surge voltage caused by lightning or the like is applied, the varistor exhibits an extremely low resistance value and absorbs the surge voltage by flowing a surge current.

Figure 2 shows a cross section of a conventional disc-shaped varistor.
3 is the varistor body, 4 and 41 are electrodes baked onto the varistor body 3, and 5 and 5' are electrical terminals, which are connected to the electrodes 4 and 4/ through soldering parts 6 and 61, respectively. .

Usually, the dielectric constant εS of the varistor is relatively high, and the apparent εS reaches as high as 2000.

Therefore, the capacitance between terminals 5, 5/ is generally large,
Its value is inversely proportional to the element thickness t and directly proportional to the electrode effective area S.

That is, apart from the capacitance C, varistors used in signal lines etc. for the purpose of absorbing surges are inserted between the lines and between the lines, as shown in FIGS. 3 and 4, respectively. Inserting a varistor increases line-to-ground capacitance or line-to-line capacitance, which attenuates the signal component of the signal line and makes it more susceptible to induction. This is especially true for telephone signal lines, etc. A surge absorber with a smaller capacity was desired.

In addition, in FIGS. 3 and 4, 7.7' is a signal line, 8 is a signal controller, 9 and 91 are varistors applied between the lines and the top and bottom, and 10 is a varistor applied between the lines. .

Up to now, varistors or surge absorbers with small capacitance while maintaining the varistor's surge absorption characteristics have been studied, but no satisfactory solution has been found.

The reason for this is as described below.

a) In order to reduce the capacitance, the thickness t in Fig. 2 can be increased, but since the varistor voltage is proportional to t,
If the length becomes longer, the varistor voltage becomes higher and the limiting voltage also becomes higher, which causes a problem that the surge protection characteristics deteriorate.

b) In order to reduce the capacitance, the area S in Figure 2 can be made smaller, but since the surge resistance is proportional to S,
If the voltage decreases, the surge resistance also decreases, causing the problem that large surge currents cannot be absorbed.

The present invention aims to solve the above problems and provide a surge absorber that maintains the conventional surge absorbing ability and protection characteristics and has low capacitance.

An embodiment of the present invention will be described below with reference to FIG.

In FIG. 5, 11 and 12 are metal oxide varistors whose main component is zinc oxide, for example, and 13 is a gap.

The varistor 12 and the gap 13 are connected in series, and the varistor is connected in parallel with this series circuit.

Here, the varistor 11.1 when the gap 13 is discharged
The parallel circuit of 2 shall have the same surge withstand capacity and varistor voltage as the varistor shown in FIG. The total electrode area of .12 and the electrode area S of FIG. 2 are the same.

As mentioned earlier, at normal voltages the varistor is made of an insulator, that is, a ceramic capacitor, so in the region below the varistor voltage, the varistor can be thought of as a capacitor.

Therefore, the capacitive equivalent circuit of the surge absorber of FIG. 5 is as shown in FIG. 6.

C1 is the capacitance of the varistor 11, C2 is the capacitance of the varistor 12, and Cg is the capacitance of the gap 13.

Therefore, the total capacitance C in FIG. 6 is expressed by the following formula: Here, since C2>Cg (Cg is usually a few pF or less), c=c1+c g, and since C1>Cg, C'' -1C1.

Here, if the capacitance of the varistor in Fig. 2 is co, then
As is clear from the above explanation, since C0<Co, C
Therefore, C8 holds true, and it is possible to significantly reduce the capacitance.

Although the surge absorber shown in FIG. 5 has made it possible to reduce the capacitance at normal circuit voltages, we will now discuss whether the surge absorbing ability can be maintained as before.

When discussing this surge absorbing ability, it becomes clear by assuming the situation when a surge voltage is applied between terminals 14 and 15 in FIG. 5.

7 shows the voltage-current characteristics of the surge absorber of FIG. 5, 16 shows the voltage-current characteristics of the varistor 11, and 17 shows the voltage-current of the combined varistor when the varistors 11 and 12 are connected in parallel. As is clear from the above description, the characteristics are the same as those of the conventional varistor shown in FIG.

Now, when a relatively small surge voltage is applied between the terminals 14 and 15, the surge current flows as shown at 11 in FIG. 5, and only the varistor 11 operates to limit the thermal layer pressure.

At this time, the varistor 12 does not operate because the gap 13 is not discharged.

However, if the limited voltage of the varistor 11 becomes considerably high,
In other words, when a large surge current flows into the varistor 11, the gap 13 starts discharging, resulting in the surge current ■2 in Fig. 5.
The surge begins to flow, and varistors 11 and 12 are connected in parallel to absorb the surge.

That is, the product of the present invention has a voltage-current characteristic in the shape of the arrow in FIG. 7, and it is clear that the product maintains the surge resistance of the conventional varistor.

Note that 18 in FIG. 7 has characteristic 1 due to the discharge in the gap 13.
At the point where characteristic 6 changes to characteristic 17, the discharge start voltage of the gap 13 must be set so that the current value is normally lower than the surge withstand capacity of the varistor 11.

Here, by making the varistor voltage of varistor 12 in FIG. 5 lower than that of varistor 11, a better limiting voltage effect, that is, better protection characteristics can be obtained.

Further, in the above embodiment, a gap is used as a switching element that is opened and closed by voltage, but the same effect can be obtained by using a semiconductor switching element such as a bipolar thyristor.

The surge absorber of the present invention is configured as described above, and is capable of significantly reducing capacitance at steady voltage and maintaining conventional surge withstand characteristics, especially for telephone signal lines. It is effective for application to

[Brief explanation of the drawings] Figure 1 is a diagram showing the voltage-current characteristics of a metal oxide varistor, Figure 2 is a cross-sectional view of a conventional varistor, and Figures 3 and 4 are respective circuits of the same varistor. 5 is a circuit diagram showing an embodiment of the surge absorber according to the present invention, FIG. 6 is a capacitance equivalent circuit diagram of FIG. 5 is a diagram showing the voltage-current characteristics of FIG. 5. FIG. 11... Second metal oxide varistor, 12...
...First metal oxide varistor, 13...
Switching element (gap).

Claims (1)

[Claims] 1. Consists of two metal oxide varistors with excellent voltage-dependent nonlinearity index and a switching element represented by a gap, with the first metal oxide varistor connected in series with the switching element. A surge absorber characterized in that a second metal oxide varistor is connected in parallel to this series circuit. 2. The surge absorber according to claim 1, wherein the varistor voltage of the first metal oxide varistor is set lower than the varistor voltage of the second metal oxide varistor.