Anti Resonant Electrodes For A Piezoelectric Transformer

Abstract

In a piezoelectric transformer of the class comprising a ferroelectric piezoelectric vibrating element including a driving region provided with driving electrodes on the opposite surfaces thereof and an output region provided with an output electrode, there are provided terminals for the driving electrodes, each of the terminals comprising a relatively wide and thin metal sheet, and connecting means for urging the terminals against the driving electrodes into firm electrical contact at the node of the mechanical vibration of the fundamental vibration mode of the vibration element.

Parent Case Text

CROSS REFERENCE TO RELATED APPLICATION

This application is a Continuation-in-Part of Ser. No. 148,413, filed on June 1, 1971 now abandoned.

Description

BACKGROUND OF THE INVENTION

This invention relates to a piezoelectric transducer or transformer, and more particularly to a piezoelectric transducer including improved driving means.

Ferroelectric ceramic materials are generally used as voltage stepping-up or boosting vibrating elements of piezoelectric transformers. Such piezoelectric transformers have recently been used commercially. A boosting vibrating element comprises a piece of rectangular piezoelectric ceramic, one half of the length of the element being polarized in the direction of thickness to constitute a driving side whereas the other half being polarized in the longitudinal direction to constitute an output side. On the upper and lower surfaces of the driving side are applied driving electrodes which are soldered to lead wires and on the end surface of the output side is applied an output electrode soldered to an output lead wire. Upon application of an electric field across the driving electrodes the boosting vibrating element undergoes a mechanical vibration at its natural frequency of vibration to produce a stepped-up voltage at the output electrode in a manner well known in the art. It will thus be readily understood that in order to cause the vibrating element of the piezoelectric transformer to vibrate at a high efficiency at its natural frequency, it is necessary to support the vibrating element at its node of vibration and that the input lead wires should be soldered to the driving electrodes also at the node. However, even when the input lead wires are soldered to the driving electrodes at the node of vibration it is still necessary to prevent the soldered joints from being peeled off and the lead wires from being broken by fatigue, because even at the nodal point the vibrating element vibrates more or less and because the soldered joints are not true points but actually have certain areas.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an improved piezoelectric transformer which can obviate the difficulties mentioned above.

According to this invention, instead of soldering the input lead wires directly to the driving electrodes, the input lead wires are soldered to the outer ends of relatively wide and thin metal sheets and the inner ends of the thin metal sheets are urged against the driving electrodes into intimate electrical contact therewith at the node of the mechanical vibration of the vibrating element of the piezoelectric transducer by connecting or clamping means of resilient insulator material. Advantageously, the thin metal sheets are provided with a vibration absorbing pad on one or both sides. The thin metal sheets effectively prevent the vibration of the vibrating element from being transmitted to the input lead, thus preventing their breakage due to fatigue and peeling off of the soldered joints between the terminal leads and the thin metal sheets. The thin metal sheets also dissipate the heat generated in the vibrating element. The connecting means can also be used to hold the piezoelectric transformer.

BRIEF DESCRIPTION OF THE DRAWINGS

Further object and advantages of the invention can be better understood from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 shows a longitudinal sectional view of a piezoelectric transformer constructed according to the teaching of the invention;

FIG. 2 shows a perspective view of means for connecting driving electrodes and electrode terminals utilized in this invention;

FIG. 3 shows a perspective view of the connecting means and the electrode terminals connected thereto;

FIGS. 4 and 5 show front elevations of modified connecting means utilized in this invention;

FIG. 6 is a longitudinal sectional view of further modified embodiment of this invention;

FIG. 7 shows an exploded perspective view showing the relationship between the connecting means and the electrode terminals utilized in the embodiment shown in FIG. 7;

FIG. 8 is a view like FIG. 6 of a further modification; and

FIG. 9 is a perspective view of a modification shown in FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The piezoelectric transformer shown in FIG. 1 comprises a vibrating element 1 of a ferroelectric ceramic, lead titanatezirconate, for example, and is polarized as above described. The left hand half which is polarized in the direction of thickness of the element is used as a driving region 2 whereas the right hand half which is polarized in the longitudinal direction is used as an output region 3. A pair of driving electrodes 4 and 5 are applied on the upper and lower surfaces of the driving region 2 and an output electrode 6 is applied on the end surface of the output region 3. The output electrode 6 is connected to an output lead wire 8 via an electro conductive rubber cap 7. Thin metal foils 9 and 10 of copper and aluminum are electrically connected to driving electrodes 4 and 5, respectively in the following manner. As shown in FIG. 2, the connecting means 11 for electrically connecting metal foils to driving electrodes takes the form of a rectangular frame of resilient insulator material such as insulating rubber, for example, having a rectangular opening 12 equal to or a little larger than the cross-section of the vibrating element 1. The inner ends of metal foils 9 and 10, each having substantially equal width as that of the vibrating element 1, are wrapped about transverse legs 13 and 14 of connecting means 11 as shown in FIG. 3. Under these conditions, the vibrating element is forced into opening 12 against the resiliency of the connecting means. Then, the metal foils will be firmly urged against driving electrodes by the resiliency of the connecting means thus improving the electrical connection therebetween. The metal foils secured in this manner serve as the terminals for the driving electrodes and lead wires 15 and 16 are soldered to the outer ends of these foils for supplying the driving power having a frequency equal to the natural frequency of vibration of the vibrating element. The position of connection between the metal foils and the driving electrodes is selected to coincide with the node of the mechanical vibration of the vibrating element so that the connection or joint will not add any lead to the mechanical vibration thus improving the efficiency of vibration. Assuming that .lambda. represents the fundamental oscillation mode of the vibrating element, the joints between the metal foils and the electrodes may be located at a point 1/4 l to the left end of the vibrating element as viewed in FIG. 1 where l represents the longitudinal length of the element. On the other hand, where the fundamental mode of the vibrating element equals .lambda./2 such joints may be located at the center along the length of the vibrating element.

Where an AC driving power is supplied across lead wires 15 and 16, the vibrating element of the piezoelectric transformer vibrates with a large amplitude at the fundamental vibration mode but since metal foils 9 and 10 are firmly clamped by the transverse legs 13 and 14 of the connecting means their position will never be displaced by the vibration. Small mechanical vibrations that may be transmitted to the metal foils from the vibrating element at the node are efficiently absorbed by the resiliency of the connecting means and the metal foils so that lead wires will not be subjected to such vibrations. In the prior art construction, the lead wires are soldered directly to the driving electrodes so that a tendency of peeling-off of the solder or breakage of the lead wires exists. With the invention, these problems can be greatly reduced. In addition, since the width of the metal foils is relatively wide, the heat generated by the mechanical vibration of the vibrating element can be efficiently dissipated through the metal foils.

FIG. 4 shows a modified connecting means 17 having a gap 18 in one vertical leg. In another modification of the connecting means 11 shown in FIG. 5, the upper and lower edges of the opening 12 are serrated as at 19 to improve electrical contact.

In still another modification shown in FIGS. 6 and 7, instead of metal foils utilized in the foregoing embodiments, relatively thick metal sheets 20 and 21 are used. These sheets may be copper or aluminum sheets having a thickness of about 10 to 100 microns. One end of these metal sheets is grooved to receive resilient connecting member 11 so as to be urged against driving electrodes 4 and 5 to form low resistance contacts, Input lead wires 15 and 16 are soldered to the opposite ends of the metal sheets 20 and 21. As shown in FIG. 7, one end of each metal sheet 20 and 21 is bent to form grooves 22 and 23, each having a width equal to that of the connecting means 11, so as to receive transverse legs thereof thus defining opening 12 to receive the driving region of the vibrating element. Again, the contact positions of the metal sheets and the driving electrodes coincide with the node of vibration of the vibrating element. In this embodiment, since terminals for the driving electrodes comprise thin metal sheets thicker than the foils, the metal sheets tend to resonate with the vibrating element. This tendency can be avoided by applying vibration absorbing resilient pads 22 and 23 of rubber and the like on one or both sides of metal sheets 20 and 21 in areas not contacting with the driving electrodes, as shown in FIGS. 6 and 7.

Similarly, in the embodiment shown in FIG. 1, the vibration absorbing characteristic of the electrode terminals can be improved by forming vibration absorbing layers on one or both sides of the metal foils, such layers being conveniently formed by the appplication of a solution of silicone rubber.

As above described, in this invention, it is essential to cause the thin metal terminals to engage the driving electrodes at the node of the vibration of the vibrating element, and this nodal point should coincide with the mounting position of the holder supporting the vibrating element of the piezoelectric transducer. Accordingly, by combining the connecting means of the electrode terminals with the holder for the vibrating means, the construction of the piezoelectric transducer can be greatly simplified.

FIG. 8 is a view similar to FIG. 6 for a modification which provides improved electrical and physical operating characteristics. All elements having the same reference characters are constructed and performed in a manner similar to that previously described, but in this embodiment the metal sheets 20 and 21 make contact with the driving electrodes 4 and 5 respectively by means of an interposed pad of conductive rubber 100 and 101. The view of FIG. 8 shows the components in expanded position for clarity of illustration, but in actuality and as shown in FIG. 9, the resilient rectangular frame 11 urges the metal plates 20 and 21 into contact with the conductive rubber sheets 100 and 101 which in turn are urged into contact with the drive electrodes 4 and 5, respectively. In this manner, electrical contact is established to the driving electrodes 4 and 5 and the resiliency of the conductive rubber sheets 100 and 101 provides mechanical damping to eliminate the vibration and noise attendant upon operation when the motion of the piezoelectric element 3 is coupled to the metal sheets 20 and 21. The resiliency of the conductive rubber inserts 100 and 101 minimizes this coupling and hence eliminates the objectionable noise. At the same time improved electrical contact is made with the electrodes 4 and 5. Generally, the electrical resistance of the electroconductive rubber should be less than ten per cent of the input impedance of the piezoelectric transformer element at the drive electrodes 4 and 5 which is generally in the order of 1,000 ohms and a resistance value for the conductive rubber in the range from 20 to 60 ohms is preferable. The corresponding thickness of the conductive rubber could be approximately 0.2-1.0mm. A suitable thickness for the metal plates 20 and 21 would be in the range of 10-400 microns. The construction shown in FIGS. 8 and 9 accordingly provides a compact and efficient piezoelectric transformer element providing good electrical contact which introduces a minimum of interference with the vibratory motion of the piezoelectric element while making good electrical contact thereto and at the same time providing damping for the connections to the driving electrodes for minimizing vibration and noise which result when such elements are in contact with the vibrating element.

While the invention has been shown and described in terms of certain preferred embodiments thereof it should be understood that many changes and modifications will occur to one skilled in the art within the true spirit and scope of the invention as defined in the appended claims.

Claims

I claim:

1. A piezoelectric transformer comprising a ferroelectric piezoelectric vibrating element including a driving region provided with driving electrodes on the opposite surfaces thereof and an output region provided with an output electrode; terminals for said driving electrodes, each of said terminals comprising a relatively wide and thin metal sheet, a portion of which contacts said driving electrode and extending with a region which does not contact said driving electrode or the support for said element; a vibration absorbing resilient layer applied to at least one side of said region and connecting means separate from said region for urging said terminals against said driving electrodes into firm electrical contact at the node of mechanical vibration of the fundamental vibration mode of said vibrating element.

2. The piezoelectric transformer according to claim 1 wherein said connecting means comprises a rectangular resilient insulator frame having a pair of transverse legs and a pair of vertical legs arranged to define an opening utilized to receive said vibrating element and said thin metal sheets so as to urge said thin metal sheets against said driving electrodes.

3. The piezoelectric transformer according to claim 1 wherein said connecting means is also used as a holder for said piezoelectric transformer.

4. The piezoelectric transformer according to claim 1 wherein one end of each of said thin metal sheets is bent to form grooves for receiving that portion of said connecting means urging said terminals against said driving electrodes at said node.

5. The piezoelectric transformer according to claim 2 wherein the inner surfaces of said transverse legs in contact with said thin metal sheets are serrated.

6. A piezoelectric transformer comprising a ferroelectric piezoelectric vibrating element including a driving region provided with driving electrodes on the opposite surfaces thereof and an output region provided with an output electrode; terminals for said driving electrodes, each of said terminals comprising a relatively side and thin metal sheet having a vibration absorbing resilient pad applied to at least one side thereof in a region not contacting with its driving electrode; and connecting means separate from said region for urging said terminals against said driving electrodes into firm electrical contact at the node of the mechanical vibration of the fundamental vibration node of said vibrating element.