JPH08155333A - Air cleaner - Google Patents

Abstract

PURPOSE: To keep an electrifying filter in an electrified state and to maintain the performance for removing dust by impressing high voltage between a first conductive filter and a second conductive filter. CONSTITUTION: This device is provided with a first activated carbon filter 3 as the first conductive filter, a polyethylene filter 4 as the electrifying filter and a second activated carbon filter 5 as the second conductive filter. The high voltage power source 6 impresses high voltage between the first activated carbon filter 3 and the second activated carbon filter 5, and positive electric charge is electrified on the first activated carbon filter 3 and negative electric charge is electrified on the second activated carbon filter 5. Besides, high voltage is impressed on the polyethylene filter 4 via the first and second activated carbon filters 3, 5 and the electrified state of the polyethylene filter 4 is held. Thus, the dust collection efficiency can highly be maintained for a long period.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air cleaner, and more particularly, to efficiently remove dust floating in the air of automobiles, ships, aircrafts, indoors of general buildings, factories, etc. Air cleaner

[0002]

2. Description of the Related Art Some conventional air cleaners use a charging filter to remove dust floating in the air.

This charging filter is one in which the surface of an insulator is charged with positive or negative charges, and it functions as a filter and a physical filter that uses Coulomb attractive force due to the charging of dust itself, and collects dust. To do.

By the way, since an air cleaner using this charging filter is not so high in efficiency, an air cleaner in which an ionizer is provided to improve the dust collection efficiency is well known.

An air cleaner using an ionizer is, for example, an ionizer that charges dust to a positive side or a negative side by performing corona discharge from an upstream side of an air flow path to a downstream side, and an ionizer from an ionizer discharge electrode. A collector having a float electrode to which a bias voltage is applied by flowing an electric current, a first plate electrode connected to the float electrode, and a second plate electrode charged with a charge different from that charged by the ionizer, a deodorizing filter (or a deodorizing filter) And the ozone decomposition filter) are arranged in order.

When dust-containing air is introduced into the air cleaner, the ionizer charges the dust to the positive or negative side by performing corona discharge.

In parallel with this, a bias voltage is applied to the float electrode by the flow of an ion current from the discharge electrode of the ionizer, and the first plate electrode of the collector has the same charge as the dust charged by the ionizer. Be charged to. In addition, the second plate electrode of the collector is charged with a charge different from that charged by the ionizer. As a result, the collector collects dust in the air introduced by the Coulomb attractive force, and further is deodorized by the deodorizing filter and discharged as clean air. Alternatively, the ozone generated by the ionizer is decomposed by the deodorizing and ozone decomposing filter, and is also deodorized and discharged as clean air.

[0008]

The air cleaner using the above charging filter has a problem that the charging ability of the charging filter is reduced in a short period of time, and the collection ability is reduced in a short period of time. It was In particular, when used in a high humidity environment or when a conductive substance such as carbon is collected, the charging ability is lowered, and the tendency that the collecting ability is apt to be lowered tends to be remarkable.

Further, in the air cleaner using the above-mentioned ionizer, since the number of parts is increased and it is necessary to arrange them along the air flow path, the size in the air flow direction is increased and the cost is also increased. There was a problem of increase. Temporarily, in order to reduce the size, there is a method of removing the deodorizing filter (or the deodorizing and ozone decomposing filter), but such a configuration causes a new problem that the air cleaning effect cannot be sufficiently obtained.

SUMMARY OF THE INVENTION An object of the present invention is to provide an air cleaner which can maintain the dust collecting capability and can be downsized.

[0011]

In order to solve the above problems, the invention according to claim 1 provides a first conductive filter having conductivity, and a charging filter provided on the downstream side of the first conductive filter. And a second conductive filter having conductivity, which is provided on the downstream side of the charging filter, and high voltage applying means for applying a high voltage between the first conductive filter and the second conductive filter. Configure.

According to a second aspect of the present invention, in the first aspect of the present invention, a discharge electrode and a counter electrode are provided on the upstream side of the first conductive filter, and dust is positive or negative by corona discharge. An ionizer that is charged by the electric charge is provided.

According to a third aspect of the present invention, an ionizer having a discharge electrode and a counter electrode for charging dust with a positive or negative electric charge by performing corona discharge is provided, and the ionizer is provided on the downstream side of the ionizer. With
A first electrically conductive filter that is electrically floated and has a conductivity that generates a bias voltage by the ion current from the ionizer; a charging filter provided on the downstream side of the first electrically conductive filter; And a second conductive filter provided on the downstream side and having conductivity.

According to a fourth aspect of the present invention, there is provided an ionizer having a discharge electrode and a counter electrode, which charges the dust with a positive or negative charge by performing corona discharge, and is provided on the downstream side of the ionizer. With
A conductive first conductive filter connected to the counter electrode, a charging filter provided on the downstream side of the first conductive filter, and a second grounded filter provided on the downstream side of the charging filter. It comprises a conductive filter and a voltage drop means having one end connected to an intermediate connection point between the counter electrode and the first conductive filter and the other end connected to the ground.

According to a fifth aspect of the invention, in the fourth aspect of the invention, the voltage drop means is one Zener diode or a plurality of Zener diodes connected in series.

[0016]

According to the invention of claim 1, the high voltage applying means applies a high voltage between the first conductive filter and the second conductive filter.

As a result, a high voltage is always applied to the charging filter, so that the charging filter can maintain the charged state and maintain the dust removing performance.

According to the second aspect of the present invention, in addition to the effect of the first aspect of the invention, a high voltage is applied between the discharge electrode and the counter electrode to cause the ionizer to perform corona discharge, thereby removing dust. It can be charged with either positive or negative charges, and electrically neutral dust can be removed.

According to the third aspect of the present invention, a high voltage is applied between the discharge electrode and the counter electrode of the ionizer to cause the ionizer to perform corona discharge, so that dust is charged with either positive or negative charges. Let

Along with this, a bias voltage is generated by the ion current from the ionizer, and a bias voltage is generated at the float electrode.

As a result, a high voltage corresponding to the bias voltage is applied between the float electrode and the conductive filter.

As a result, a high voltage is always applied to the charging filter, the charging filter can maintain the charged state, and the dust removing performance can be maintained.

According to the fourth aspect of the present invention, a high voltage is applied between the discharge electrode and the counter electrode of the ionizer to cause the ionizer to perform corona discharge, thereby charging the dust with either positive or negative charges. Let

In parallel with this, the voltage drop means steps down the high voltage applied to the ionizer and applies a predetermined voltage between the first conductive filter and the second conductive filter.

As a result, a predetermined voltage is always applied to the charging filter, the charging filter can maintain the charged state, and the dust removing performance can be maintained.

According to the invention of claim 5, in addition to the operation of the invention of claim 4, the voltage drop means is constituted by one Zener diode or a plurality of Zener diodes connected in series. Therefore, a stable voltage can be easily generated and applied.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will now be described with reference to the drawings. First Embodiment FIG. 1 shows a schematic cross-sectional view of a main part of an air cleaner according to a first embodiment corresponding to the invention described in claim 1.

The air cleaner 1 has an air supply fan 2 for introducing air, which is a cleaning target including dust, a first activated carbon filter 3 as a first conductive filter, and a cross section for increasing the surface area. The shape is a zigzag shape, and the polyethylene filter 4 as a charging filter, the second activated carbon filter 5 as a second conductive filter, the charged state of the polyethylene filter 4 are held, and the first activated carbon filter 3 and the second activated carbon are maintained. A high voltage power supply 6 for applying a high voltage (for example, 2 kV) between the first activated carbon filter 3 and the second activated carbon filter 5 in order to improve the dust collection efficiency by setting the filter 5 in a charged state, and on the upstream side of the first activated carbon filter 3. It is provided to remove large dust in advance and extend the life of the first activated carbon filter 3, the second activated carbon filter 5 and the polyethylene filter 4. And it is configured to include a pre-filter 7 for the.

Next, the operation will be described.

First, the air supply fan 2 has dust (dust).
Introducing air to be cleaned including air into the air cleaner 1,
The pre-filter 7 removes large dust from the introduced air. In parallel with this, the high-voltage power supply 6 applies a high voltage between the first activated carbon filter 3 and the second activated carbon filter 5 to charge the first activated carbon filter 3 with a positive charge,
The activated carbon filter 5 is charged with a negative charge.

As a result, the first activated carbon filter 3 functions as a physical filter, and at the same time, dust charged with negative charges is attached and removed by Coulomb attractive force to remove odor. Similarly, the second activated carbon filter 5 functions as a physical filter, and also removes odor by attaching and removing dust charged with positive charges by Coulomb attraction.

Further, a high voltage is applied to the polyethylene filter 4 through the first activated carbon filter 3 and the second activated carbon filter 5, the charged state of the polyethylene filter 4 is maintained, and the dust collection efficiency is kept high for a long period of time. The Rukoto.

As described above, according to the first embodiment, since the charged state of the polyethylene filter 4 as the charging filter is maintained, the dust collecting efficiency can be kept high for a long period of time.

The second activated carbon filter 5 functions not only as a deodorizing filter but also as an ozone decomposing filter. The reaction in this case is represented as follows.

2O ₃ + 2C → 2CO ₂ + O ₂ Second Embodiment Next, FIG. 2 shows a schematic sectional view of a main part of an air cleaner according to a second embodiment of the invention described in claim 2.

The air cleaner 10 includes an air supply fan 11 for introducing air to be cleaned containing dust, a linear discharge electrode 12 connected to the high potential side, and a low potential side (ground side). Has a plate-shaped counter electrode 13 connected to
An ionizer 14 that charges the dust with a positive charge by performing corona discharge between the electrodes 12 and 13, and a first electrode that functions as a float electrode in an electrically floating state.
The activated carbon filter 15, the cross-sectional shape is zigzag in order to increase the surface area, the polyethylene filter 16 as a charging filter, and the second as a conductive filter.
An activated carbon filter 17, a high voltage power source 18 for applying a high voltage between the discharge electrode 12 and the counter electrode 13 in order to maintain the charged state of the polyethylene filter 16 and perform corona discharge, and an upstream side of the first activated carbon filter 15. And a pre-filter 19 for removing large dust in advance and extending the life of the first activated carbon filter 15, the second activated carbon filter 17, and the polyethylene filter 16.

In this case, the installation distance between the counter electrode 13 and the first activated carbon filter 15 must be set to a sufficient distance so as not to cause arc discharge.

Next, the operation will be described.

First, the air supply fan 11 introduces air containing dust, which is an object to be cleaned, into the air cleaner 1, and the pre-filter 19 removes large dust from the introduced air. In parallel with this, the high-voltage power supply 18 is connected to the discharge electrode 12-
A high voltage (for example, about 6 kV) is applied between the counter electrodes 13. As a result, the ionizer 14 performs corona discharge, and the dust passing through the ionizer 14 is charged with a positive charge.

At this time, the discharge electrode 12 of the ionizer 14
Ionic current flows from the first activated carbon filter 15 functioning as a float electrode to the first activated carbon filter 1
A high voltage (for example, 2 kV) as an automatic bias voltage is applied between the fifth activated carbon filter 17 and the second activated carbon filter 17.

As a result, the first activated carbon filter 15 functions as a physical filter and also removes odor by adhering and removing dust charged with negative charges by Coulomb attraction. Similarly, the second activated carbon filter 17 is
In addition to functioning as a physical filter, dust with a positive charge is attached and removed by Coulomb attraction,
Remove the odor.

Further, a high voltage is applied to the polyethylene filter 16 via the first activated carbon filter 15 and the second activated carbon filter 17, the charged state of the polyethylene filter 16 is maintained, and the dust collection efficiency is kept high for a long period of time. The Rukoto.

As described above, according to the second embodiment, since the charged state of the polyethylene filter 4 as the charging filter is maintained, the dust collecting efficiency can be kept high for a long period of time. Furthermore, since the electrically neutral dust is also forcibly charged by the ionizer 14, the dust collection efficiency can be further improved as compared with the first embodiment.

Even in this case, as in the case of the second activated carbon filter 5 of the first embodiment, the second activated carbon filter 1 is also used.
7 functions not only as a deodorizing filter but also as an ozone decomposing filter. Third Embodiment Next, FIG. 3 shows a schematic cross-sectional view of a main part of an air cleaner of a third embodiment corresponding to the invention described in claims 3 and 5.

The air cleaner 20 includes an air supply fan 21 for introducing air to be cleaned including dust, a linear discharge electrode 22 connected to the high potential side, and a low potential side (ground side). Has a plate-shaped counter electrode 23 connected to
An ionizer 24 that charges the dust with a positive charge by performing corona discharge between the electrodes 22 and 23, and a first conductive filter that is connected to the counter electrode and functions as a first conductive filter
The activated carbon filter 25, a zigzag cross-sectional shape for increasing the surface area, a polyethylene filter 26 as a charging filter, and a second activated carbon filter 27 that functions as a second conductive filter and is grounded.
A high-voltage power supply 28 that applies a high voltage (for example, 7 kV) between the discharge electrode 22 and the counter electrode 23 in order to maintain the charged state of the polyethylene filter 26 and perform corona discharge, and the first activated carbon filter 25. A pre-filter 29 provided on the upstream side for removing large dust in advance and extending the life of the first activated carbon filter 25, the second activated carbon filter 27 and the polyethylene filter 26, the counter electrode 23 and the first activated carbon filter 25-grounded. A zener diode 30 (for example, a voltage drop amount of 2 kV) that is connected in the opposite direction between them and functions as a voltage drop means is configured.

Next, the operation will be described.

First, the air supply fan 21 introduces air, which is a cleaning target, containing dust into the air cleaner 1, and the pre-filter 29 removes large dust from the introduced air. In parallel with this, the high voltage power supply 28 is connected to the discharge electrode 22-
High voltage between the counter electrodes 23 (in the above example, about 5 kV
[= 7-2 kV]) is applied. As a result, the ionizer 24 performs corona discharge, and the dust passing through the ionizer 24 is charged with a positive charge.

At this time, a voltage corresponding to the amount of voltage drop (2 kV in the above example) is applied to the first activated carbon filter 25 by the Zener diode 30.
A high voltage (2 kV in the above example) is applied between the activated carbon filter 25 and the second activated carbon filter 27.

As a result, the first activated carbon filter 25 functions as a physical filter and also removes odor by adhering and removing dust charged with negative charges by Coulomb attraction. Similarly, the second activated carbon filter 27 is
In addition to functioning as a physical filter, dust with a positive charge is attached and removed by Coulomb attraction,
Remove the odor.

Further, a high voltage is applied to the polyethylene filter 26 via the first activated carbon filter 25 and the second activated carbon filter 27, and the charged state of the polyethylene filter 26 is maintained, so that the dust collection efficiency is kept high for a long period of time. The Rukoto.

As described above, according to the third embodiment, since the charged state of the polyethylene filter 26 as the charging filter is maintained, the dust collection efficiency can be kept high for a long period of time. Furthermore, since the electrically neutral dust is also forcibly charged by the ionizer 24, the dust collection efficiency can be further improved as compared with the first embodiment.

Even in this case, as with the second activated carbon filter 5 of the first embodiment, the second activated carbon filter 2 is also used.
7 functions not only as a deodorizing filter but also as an ozone decomposing filter.

Further, although only one Zener diode 30 is shown in FIG. 3, a plurality of Zener diodes may be connected in series so as to have the same voltage drop amount.

Further, the voltage effect means is not limited to the Zener diode, and other means such as voltage division by a resistor can be used. Fourth Embodiment Next, FIG. 4 shows a schematic sectional view of a main part of an air cleaner according to a fourth embodiment of the invention as claimed in claim 4.

The air cleaner 40 is provided with an air supply fan 41 for introducing air, which is a cleaning target including dust, and a linear discharge electrode 42 connected to a high potential side of a first high-voltage power supply 48 described later. And a plate-shaped counter electrode 43 connected to a low potential side (ground side) of a first high-voltage power supply 48, which will be described later, and a positive charge is applied to dust by corona discharge between the electrodes 42 and 43. The ionizer 44 to be activated, a first activated carbon filter 45 connected to the high potential side of a second high-voltage power supply 50 described later, and functioning as a first conductive filter, and a zigzag cross-sectional shape for increasing the surface area, and charged. A polyethylene filter 46 as a filter, and a second
While functioning as a conductive filter, the charged state of the second activated carbon filter 47 and the polyethylene filter 46, which are grounded, is maintained and a high voltage (for example, between the discharge electrode 42 and the counter electrode 43) for performing corona discharge.
The first high-voltage power supply 48 for applying 6 kV) and the first activated carbon filter 45 are provided upstream of the first activated carbon filter 45 to remove large dust in advance, and the first activated carbon filter 45 and the second activated carbon filter 4 are provided.
7 and the pre-filter 49 for extending the life of the polyethylene filter 46, and the first activated carbon filter 45 and the ground are connected, and a high voltage (for example, 2 kV) is applied between the first activated carbon filter 45 and the second activated carbon filter 47. Second
A high voltage power supply 50 is provided.

Next, the operation will be described.

First, the air supply fan 41 introduces air containing dust, which is a cleaning object, into the air cleaner 1, and the pre-filter 49 removes large dust from the introduced air. In parallel with this, the first high-voltage power supply 48 controls the discharge electrode 4
A high voltage between the two and the counter electrodes 43 (in the above example, about 6 k
V) is applied. As a result, the ionizer 44 performs corona discharge, and the dust passing through the inside of the ionizer 44 is charged with a positive charge.

At this time, a high voltage (2 kV in the above example) is applied between the first activated carbon filter 45 and the second activated carbon filter 47 by the second high voltage power supply 50, and the first activated carbon filter 45 is thus applied. -Second activated carbon filter 47
A high voltage (2 kV in the above example) is applied between them.

As a result, the first activated carbon filter 45 not only functions as a physical filter, but also removes odors by adhering and removing dust charged with negative charges by Coulomb attraction. Similarly, the second activated carbon filter 47 is
In addition to functioning as a physical filter, dust with a positive charge is attached and removed by Coulomb attraction,
Remove the odor.

Further, a high voltage is applied to the polyethylene filter 46 through the first activated carbon filter 45 and the second activated carbon filter 47, and the charged state of the polyethylene filter 46 is maintained, so that the dust collection efficiency is kept high for a long period of time. The Rukoto.

As described above, according to the third embodiment, since the charged state of the polyethylene filter 46 as the charging filter is maintained, the dust collection efficiency can be kept high for a long period of time. Further, since the electrically neutral dust is also forcibly charged by the ionizer 44, the dust collection efficiency can be further improved as compared with the first embodiment.

As described above, since the first high-voltage power supply 48 which is the high-voltage power supply for the ionizer 44 and the high-voltage power supply for the first and second activated carbon filters 45 and 47 are separately provided, the third embodiment. The power supply voltage can be lowered as compared with the case of the example. Moreover, since the other power source is not affected by the one power source, a more stable air cleaning operation can be performed.

Even in this case, as in the case of the second activated carbon filter 5 of the first embodiment, the second activated carbon filter 4 is also used.
7 functions not only as a deodorizing filter but also as an ozone decomposing filter.

According to each of the above embodiments, 1) the life of the charging filter and the activated carbon filter can be extended, so that the running cost can be substantially reduced.

2) Since the charged state of the charging filter can be maintained, the dust collection efficiency (dust collection rate) can be improved.

3) Since the portion for collecting dust and deodorizing is constituted by a filter such as an activated carbon filter and a polyethylene filter, maintenance is easy and cost can be reduced.

4) Since the dust collection efficiency (dust collection rate) can be maintained high, if the collection efficiency is constant, the apparatus can be downsized.

5) The structure is simple and maintenance is easy. Such an effect can be obtained.

In each of the above embodiments, the polyethylene filter was used as the charging filter, but it is also possible to use a filter made of another insulating material.
Further, although the activated carbon filter is used as the conductive filter or the float electrode, it is also possible to use a filter made of another conductive material such as a metal filter.

Further, although the air supply fan is provided on the upstream side of the air flow path, it may be provided on the downstream side.

[0071]

According to the invention of claim 1, the high voltage applying means applies a high voltage between the first conductive filter and the second conductive filter, so that the high voltage is always applied to the charging filter. Therefore, the charging filter can maintain the charged state, maintain the dust removal performance, extend the life of the charging filter, and thus extend the life of the air cleaner. It will be possible.

According to the invention of claim 2, in addition to the effect of the invention of claim 1, since a high voltage is applied between the discharge electrode and the counter electrode, the ionizer discharges dust by corona discharge. Can be charged with either positive or negative charges, and electrically neutral dust can be removed.

According to the third aspect of the present invention, a high voltage is applied between the discharge electrode and the counter electrode of the ionizer to cause the ionizer to perform corona discharge, so that the dust is charged with either positive or negative charges. As a result, electrically neutral dust can be removed, and since a high voltage according to the bias voltage is applied between the float electrode and the conductive filter, the charging filter maintains the charged state. It is possible to maintain the dust removal performance.

According to the fourth aspect of the present invention, a high voltage is applied between the discharge electrode and the counter electrode of the ionizer to cause the ionizer to perform corona discharge, whereby the dust is charged with either positive or negative charges. .

According to the invention of claim 5, in addition to the effect of the invention of claim 4, the voltage drop means is constituted by one Zener diode or a plurality of Zener diodes connected in series. Therefore, a stable voltage can be easily generated and applied with a simple configuration, and the dust removal performance and reliability of the air cleaner are improved.

In parallel with this, the voltage drop means steps down the high voltage applied to the ionizer, applies a predetermined voltage between the first conductive filter and the second conductive filter, and removes dust from positive or negative. Since it is charged by any one of the electric charges, it is possible to remove electrically neutral dust, and a predetermined voltage is applied between the first conductive filter and the charging filter. Can maintain the charged state and maintain dust removal performance.

Summarizing the above, 1) Since the life of the filter can be extended, the substantial running cost can be reduced.

2) Since the charged state of the charging filter can be maintained, the dust collection efficiency (dust collection rate) can be improved.

3) Since the part for collecting dust and deodorizing is composed of a filter, maintenance is easy and cost can be reduced.

4) Since the dust collection efficiency (dust collection rate) is high, the device can be downsized if the collection efficiency is constant.

5) The structure is simple and maintenance is easy. Such an effect can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic configuration sectional view of an air cleaner according to a first embodiment.

FIG. 2 is a schematic structural cross-sectional view of an air cleaner according to a second embodiment.

FIG. 3 is a schematic structural cross-sectional view of an air cleaner according to a third embodiment.

FIG. 4 is a schematic configuration sectional view of an air cleaner according to a fourth embodiment.

[Explanation of symbols]

 1, 10, 20, 40 Air washer 2, 11, 21, 41 Air supply fan 3, 15, 25, 45 First activated carbon filter 4, 16, 26, 46 Polyethylene filter 5, 17, 27, 47 Second Activated carbon filter 6, 18, 28 High voltage power supply 7, 19, 29, 49 Pre-filter 12, 22, 42 Discharge electrode 13, 23, 43 Counter electrode 14, 24, 44 Ionizer 30 Zener diode 48 First high voltage power supply 50 Second high voltage Power supply

Claims (5)

[Claims] 1. A first conductive filter having conductivity, a charging filter provided on the downstream side of the first conductive filter, and a second conductive filter provided on the downstream side of the charging filter.
An air cleaner comprising: a conductive filter; and a high voltage applying unit that applies a high voltage between the first conductive filter and the second conductive filter. 2. The air cleaner according to claim 1, further comprising a discharge electrode and a counter electrode on an upstream side of the first conductive filter, the dust being charged with positive or negative charges by performing corona discharge. An air cleaner equipped with an ionizer that is electrically charged by. 3. An ionizer having a discharge electrode and a counter electrode for charging dust by positive or negative charges by performing corona discharge, and an ionizer provided downstream of the ionizer and electrically floating. A first conductive layer that is brought into a state and that generates a bias voltage by the ion current from the ionizer.
A conductive filter, a charging filter provided on the downstream side of the first conductive filter, and a second conductive filter provided on the downstream side of the charging filter.
An air cleaner comprising: a conductive filter. 4. An ionizer having a discharge electrode and a counter electrode for charging dust by positive or negative charges by performing corona discharge; and an ionizer provided downstream of the ionizer and provided on the counter electrode. A connected first conductive filter having conductivity, a charging filter provided on the downstream side of the first conductive filter, and a second grounded filter provided on the downstream side of the charging filter.
An air cleaner comprising: a conductive filter; and a voltage drop unit having one end connected to an intermediate connection point between the counter electrode and the first conductive filter and the other end connected to ground. 5. The air cleaner according to claim 4, wherein the voltage drop means is one Zener diode or a plurality of Zener diodes connected in series.