1. Field of the Invention
The present invention relates to a small-sized variable resistor used in a hearing aid, a measuring instrument, a communication device, a sensor or other industrial devices.
2. Description of Related Art
In recent years, small-size and light-weight fabrication of devices has become a notable trend. For instance, the hearing aid has progressed from a device which is carried in a user's pocket, to a device which hangs from the user's ear, to a device which is inserted in the user's ear canal (referred to a canal-type device). Especially small-sized parts are required for the circuit parts of typical canal-type hearing aids.
An ultra-small type of variable resistor having, for example, an outer diameter or a side dimension of about 2 mm may be used within a hearing aid. The hearing aid may include a stable spring part, providing a slider member, which is needed in this type of variable resistor. It is difficult to downsize this type of slider structure while still achieving excellent contact performance and slide characteristics.
A slider capable of ensuring sufficient elasticity, even when it is applied to a small-sized variable resistor, has been proposed in Japanese Unexamined Patent Publication No. JP-A-61-4162. The slider described in that document has a structure in which a base portion is provided at the central portion of the slider. An arm in a channel-like shape is provided on one side of the slider and an arm in a rectangular shape is provided on the other side of the slider. The arms are folded so as to bend in the same direction at both ends of the base portion, and the arm in the rectangular shape intersects with the arm in the channel-like shape via a gap of the channel.
In the case of the slider having the structure described above, the lengths of the arms can be extended even with the small size required, and therefore a variable resistor capable of ensuring elasticity and having high reliability can be obtained.
In the case of the variable resistor described above, the arm having the rectangular shape is brought into contact with a collector electrode formed at the central portion on the surface of a resistor substrate. The arm having the channel-like shape is slid on a resistor in a circular arc shape formed concentrically at the outer periphery of the collector electrode. Accordingly, when a rotor is rotated, the slider is rotated with a contact point portion formed at the front end of the arm having the rectangular shape as its center. Further, the base portion provided at the central portion of the slider is fixed at a position which is off-center with respect to the axis of rotation of the rotor.
In the above-described stricture, a moment in a direction inclined to the rotor is formed by a spring reaction of the slider. When there is play between the rotor and a case, contact pressure of the slider, and more particularly, contact pressure between the arm having the channel-like shape and the resistor, is lowered by some amount. Further, a drawback is caused when the sliding resistance between the rotor and the case is increased due to the inclination of the rotor.