Variable attenuator
BACKGROUND OF THE INVENTION 
     It has recently been required that variable attenuators used in audio products have a large maximum attenuation characteristic. Many variable attenuators have a comparatively low total resistance of, for example, lower than 10 kΩ, and it is now required that these attenuators be able to produce a maximum attenuation in the order of -100 dB to -120 dB. 
     A variable attenuator generally consists of a resistance element having conductive members typically made of a silver paste provided at the respective end portions of the resistance element. Terminals are secured to the conductive members, and a sliding member connected electrically to a tap terminal is slidingly movable along the resistance element. In such a variable attenuator, a sliding member is moved along the resistance element while a voltage is applied between the two terminals, so as to extract an attenuation voltage in accordance with the amount of movement of the sliding member. However, even when the sliding member is moved to the end portion of its path, a slight amount of voltage usually occurs across the sliding member due to a resistance existing in those portions of the conductive members that are between the end portion of the sliding members and the terminals secured to the conductive members. This offers an obstruction to the improvement of maximum attenuation amount in a variable attenuator having a comparatively low total resistance. For example, when the total resistance is 10 kΩ, with the resistance in the conductive members being 1Ω, the maximum attenuation amount is normally about -80 dB (20 log 1/10000), which is far lower than the currently demanded value of -100 dB to -120 dB. 
     Therefore, an object of the present invention is to provide a variable attenuator which will be able to obtain a large maximum attenuation amount even when the attenuator has a low total resistance. 
     Another object of the present invention is to provide a variable attenuator having a simple construction and a large maximum attenuation amount. 
     Still another object of the present invention is to provide a variable attenuator, which can be easily manufactured at a low cost and which has a large attenuation amount. 
     A further object of the present invention is to provide a variable attenuator which is especially suitable for controlling the sound volume of audio devices. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a first embodiment of the present invention, from which the patterns of the resistance element and conductive members and the arrangement of the sliding member may be understood; 
     FIG. 2 is a diagram of the equivalent circuit of the embodiment as shown in FIG. 1; 
     FIG. 3 is a plan view of a second embodiment of the present invention, from which the patterns of the resistance element and conductive members and the arrangement of the sliding member may be understood; and 
     FIG. 4 is a plan view of a third embodiment of the present invention, from which the patterns of the resistance element and conductive members and the arrangement of the sliding member may be understood. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, terminals 1b and 3b are provided on respective conductive members 1 and 3, each consisting of a coated or printed layer of silver paste, for example. The conductive members 1 and 3 are each connected to respective end portions of a substantially horseshoe-shaped main resistance element 4. A sliding member 2 has a sliding portion 2a freely movable over the main resistance element 4 from the conductive member 3 to the conductive member 1. The conductive member 1 has a branching conductive portion 1a made of a conductive material, and the branching conductive portion 1a consists of a leg portion extending from the conductive member 1 and along the outer edge portion of the resistance element 4 so as to be outside the path of the sliding member 2, and an arm portion extending from the leg portion across the path of the sliding member 2. The branching conductive portion 1a may be made from the same material as the conductive members 1 and 3, such as a coated or printed layer of, for example, silver paste. The sliding member 2 is movable between a point A on the conductive member 3 and point B on the conductive member 1. The inner end of the sliding member 2 is connected electrically to a tap terminal, as is well known in the art. 
     Assuming that C represents a point where the path of the sliding member 2 over the resistance element 4 crosses the branching conductive portion 1a; R the resistance between the terminal 3b and the point C; R 1  the resistance between the point C and the point B in a current path 5 starting at the point C, passing the point B where the path of the sliding member 2 terminates, and ending at the terminal 1b; r the resistance between the point B and the terminal 1b; and r 1  the resistance in a bypass, i.e., the current path 6 starting at the point C, passing through the branching conductive portion 1a, and ending at the terminal 1b, the equivalent circuit of the variable attenuator of the present invention may be as shown in FIG. 2. 
     Referring the FIG. 2, the same reference numerals and symbols as in FIG. 1 represent the same portions of the device as shown therein. As can be noted clearly from FIG. 2, the total resistance of the variable attenuator as shown in FIG. 1 is equal to a series resistance obtained by putting together R and a combined parallel resistance of (R 1  +r) and r 1 . 
     When the resistance in the bypass 6, i.e., the branching conductive portion 1a, is sufficiently smaller than that in the path 5 including the point B, the greater part of the electric current flows through the bypass 6. Consequently, the amount of electric current flowing through the path 5 can be reduced and the voltage caused by the resistance r can be lowered considerably. 
     When an electric current of 1 mA is applied from the terminal 3b to the terminal 1b, which is used as ground with R, R 1 , r and r 1  set to 10 kΩ, 98Ω, 1Ω, and 1Ω, respectively, and electric current of only 0.01 mA flows through the current path including the point B, i.e., the current path 5. Therefore, the voltage caused by the resistance r between the point B and terminal 1b is 10 -5  V (0.01 mA×1Ω=0.01 mV). Since the total resistance in the variable attenuator is approximately 10 kΩ, the voltage between the terminals 3b and 1b is approximately 10 V. Then, the voltage caused by the resistance r converted into a maximum attenuation amount is -120 dB (20 log 10 -5  /10). 
     FIG. 3 shows a second embodiment of the present invention. In this embodiment, the branching conductive portion 1a forming the bypass 6 is supplied by a combination of a conductive strip 7, resistance portion 8 and a conductive part 9, and the equivalent circuit of this is the same as that shown in FIG. 2. Therefore, the second embodiment produces the same effect as the first embodiment shown in FIG. 1. 
     FIG. 4 shows a third embodiment of the present invention, in which the arm portion of the branching conductive portion of the embodiment as shown in FIG. 1 is omitted. In the third embodiment provided with a branching conductive portion 1a&#39;, the flow of electric current is somewhat different from that in the embodiment as shown in FIG. 1, but in practice a maximum attenuation amount which poses no problems can be obtained. 
     Thus, the third embodiment which has a simple construction is also useful.