GB2274355A - Variable resistor used in signal adding circuit including an inverting amplifying circuit - Google Patents

Abstract

The variable resistor comprises a base board; a resistor part 11, having a predetermined length, formed on the base board; a conductive part 2 formed on the base board a uniform distance apart from the resistor part; and a slider, made of an electrically conductive material, provided over and between the resistor part and the conductive part, the slider being slid in a state where an end thereof is in contact with the resistor part and the other end thereof is in contact with the conductive part; and an end 116 of the resistor part 11 is formed so that the slider is separated from the resistor part when the slider is positioned at the extreme end of a movable range thereof. Alternatively the end of the resistor part 11 is covered with insulating material or the end of the slider may be caused to be lifted up by a protruberance on the board. Use of the resistor enables elimination of an increase in noise level and a leakage of signals when a volume knob is turned to the extreme end to separate the signal line from the adding circuit. <IMAGE>

Description

VARIABLE RESISTOR The present invention generally relates to a variable resistor, and more particularly to a variable resistor used in a signal adding circuit in which audio signals, for example, are superimposed.

Fig.1 is a circuit diagram of a basic structure of an inverting amplifying circuit. In the figure, if the gain of an operational amplifier (opamp) G is sufficiently large, the gain Gio of the inverting amplifying circuit is represented, as a function of an input impedance Zs and a feedback resistance Rf, by the following equation.

Gio = Vo/Vi = -Rf/Zs (1) A circuit diagram shown in Fig.2 is a signal adding circuit in which the circuit structure of Fig.1 is provided. This signal adding circuit is used for mixing, for example, a plurality of audio signals input from other circuits. The signal adding circuit has n input terminals I1 to In. Signal lines L1 to Ln respective to input terminals I1 to In are combined into a single line, and then connected to the inverting input terminal of the op-amp G.

Each signal line Lk (k=1 to n) comprises: the input terminal Ik for an original audio signal; prestage circuit Zk which processes the audio signal for sound quality; input variable resistor VRk which adjusts the level of the audio signal supplied by the pre-stage circuit Zk; and fixed input resistor Rs which determines the input impedance of the op-amp G.

Corresponding input terminal Ik, pre-stage circuit Zk, input variable resistor VRk and fixed input resistor are connected in series.

The op-amp G is formed from the inverting amplifying circuit shown in Fig.1, and the noninverting input terminal thereof is grounded. The inverting input terminal and the output terminal of the op-amp G are connected to each other via a feedback resistor Rf.

Each of the original audio signals input to each signal line Lk is processed respectively by the corresponding pre-stage circuit Zk, and is output as an audio signal. A supplying level adjustment is then performed on each of the audio signals by the respective input variable resistor VRk, and thus mixed by an arbitrary mixing ratio. After that, the mixed audio signals are amplified by the inverting amplifying circuit, and then output to a speaker or an audio recording apparatus.

Fig.3A and 3B are views of an example of a conventional variable resistor used as an input variable resistor VRk (k=1 to n) as shown in Fig.2.

Fig.3A shows a stationary part of the conventional variable resistor, and Fig.3B shows an assembly of the conventional variable resistor in which a movable part is assembled on the stationary part shown in Fig.3A.

In the variable resistor shown in Fig.3A, a conductive part 2, a resistor part 3 and lead terminals 4-6 are provided on a surface of a circular board 1, and a slider hole 7 is formed in the center of the board 1. An end of the resistor part 3 is soldered to the lead terminal 4, and the other end thereof is soldered to the lead terminal 6.

The conductive part 2 is formed in an annular shape surrounding the slider hole 7. The resistor part 3 is made of a thin film deposited on the board 1 by, for example, vapor deposition and formed by a method, such as an etching method, by which method an arbitrary pattern can be formed.

The lead terminals 4-6 are arranged in a row on a portion of the outer fringe of the board 1. The center lead terminal 5 is connected to the conductive part 2.

As shown in Fig.3B, the above-mentioned stationary part is provided with a slider 10, and thus the assembly can function as a variable resistor. The slider 10, made of a conductive material, extends from a volume knob (not shown in the figures) rotatably mounted to the slider hole 7, connects the conductive part 2 with a point on the resistor part 3.

Since the slider 10 can slide on the resistor part 3 by rotating the volume knob, the lengths of the resistor part 3 between the lead terminals 4 and 5 and between the lead terminals 5 and 6 are varied, and accordingly the assembly can function as a variable resistor.

When the above-mentioned conventional variable resistor is used in the signal adding circuit of Fig.2, in order to prevent noise generated by the pre-stage circuit Zk of the signal line Lk to which no audio signal is supplied from being input to the op-amp G, the volume knob is normally turned to the extreme end so as to minimize the value of the input variable resistor VRk.

If the resistance value of each of the input variable resistors VRk in the signal lines Lk is set minimum, which condition is obtained by turning the volume knob of each of the variable resistors to the extreme end, the input impedance Zs for the op-amp G is represented by the following equation (2).

The gain Giomin of the inverting amplifying circuit is obtained by applying the equation (1) to the equation (2), and is thereby represented by the following equatidn (3).

That is, in the signal adding circuit in which the conventional variable resistor is used, if the resistance value of the input variable resistors VRk in the signal lines Lk, to which signal lines no audio signal is supplied, is set to minimum, the gain of the corresponding pre-stage circuit Zk is increased, which results in an increase in noise. This results in a problem that the output noise level of the op-amp G is increased in accordance with the number of signal lines Lk in which the resistance value of the input variable resistor VRk is set to minimum.

Additionally, even though the input variable resistor VRk is set to minimum, a slight signal leakage to the op-amp G occurs due to a residual resistance of the input variable resistor. Therefore, there is another problem in that the noise becomes significant when a very low level signal or no signal is supplied to the signal line in which the resistance value of the input variable resistor VRk is set to minimum, and accordingly audio signals supplied to the op-amp G has an undesired effect.

It is a general object of the present invention to provide an improved and useful variable resistor in which the above-mentioned problems are eliminated.

A more specific object of the present invention is to provide a variable resistor which eliminates an increasing the noise level and the leakage of signals when the variable resistor is used in an adding circuit including an inverting amplifying circuit and when a volume knob thereof is turned to the extreme end so as to separate the signal line from the adding circuit.

According to one aspect of the present invention, there is provided a variable resistor, the resistance of which is varied by changing the length of a resistor inside thereof by a movement of a slider, the variable resistor comprising: a base board; a resistor part, having a predetermined length, formed on the base board; a conductive part formed on the base board a uniform distance apart from the resistor part; a slider, made of an electrically conductive material, provided over and between the resistor part and the conductive part, the slider being slid in a state where an end thereof is in contact with the resistor part and the other end thereof is in contact with the conductive part; and interrupting means for interrupting the electrical continuity between the resistor part and the conductive part when the slider is positioned at the extreme end of a movable range thereof.

According to the above-mentioned construction, the slider connecting the conductive part to a point on the resistor part, when the slider is moved to the extreme end where the resistance value is set to minimum, the conductive part and the resistor part are brought into an electrically unconnected state by the interrupting means. Accordingly, the resistance value of the variable resistor, when the slider is moved to the extreme end where a resistance value is set to be minimum, becomes substantially infinity, which is an ideal condition.

Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.

Fig.l is a circuit diagram of a basic structure of an inverting amplifying circuit; Fig.2 is a circuit diagram of a signal adding circuit in which the inverting amplifying circuit of Fig.1 is used; Fig.3A is an illustration showing a stationary part of an example of a conventional variable resistor; and Fig.3B is an illustration showing a complete assembly of the variable resistor shown in Fig.3A; and Fig.4A is an illustration showing a stationary part of an embodiment of a variable resistor according to the present invention; and Fig.4B is an illustration showing a complete assembly of the variable resistor shown in Fig.4A.

A description will now be given, with reference to Figs.4A and 4B, of an embodiment of a variable resistor according to the present invention.

In Figs.4A and 4B, parts that are the same as the parts shown in Figs.3A and 3B are given the same reference numerals, and descriptions thereof will be omitted.

A resistor part 11 shown in Figs.4A and 4B, corresponding to the resistor part 3 shown in Figs.3A and 3B, is made of a thin film and formed by a thin film pattern forming method, such as a thin film etching method. The pattern of the resistor part 11 can be in an arbitrary form, and in this case the resistor part 11 is formed in an annular pattern concentrical to the annular conductive part 2 formed in the center portion of the board 1.

As shown in Fig.4A, the resistor part 11 comprises three portions, these being a sliding portion lla, a separating portion llb and a connecting portion llc. The sliding portion 11a has an arc-like pattern, and the inner fringe thereof, being parallel to the outer fringe of the conductive part 2, has a radius of Ra from the center of the slider hole 7. The separating part 11b is positioned beyond the slider 10 having a length Rb greater than the radius Ra.The connecting portion 11c extends in a radial direction so as to connect the resistor portion 11a with the separating portion llb. That is, as shown in Fig.4A, the resistor portion gila, the connecting portion 11c and the separating portion 11b form together a cranklike pattern.

When the volume knob (not shown in the figures) attached to the slide hole 7 is turned to the minimum position, the slider 10 rotates in counterclockwise and reaches the extreme end position as shown in Fig.4B. That is, when the slider 10 is at the extreme end position of the movable range thereof, the slider 10 is completely off the sliding portion Ila of the resistor 11. Accordingly, when the volume knob is turned to the minimum position, the conductive part 2 and the resistor part 11 are brought into a unconnected state. That is, the conductive part 2 and the resistor part 11 are insulated each other, and the resistance value set by the variable resistor becomes infinity.

When constructing a signal adding circuit shown in Fig.3 by using the variable resistor of this embodiment, since the resistance value of the variable resistor VRk becomes infinity when the volume knob of the variable resistor is turned to the minimum position, the gain Gio cut of the inverting amplifying circuit, which gain concerns the above-mentioned noise problem, is represented by the following equation (4); resulting in complete elimination of the noise.

Giocut = -Rf/oO= O (4) Additionally, since the pre-stage circuit Zk is completely cut off from the inverting amplifying circuit due to the separation of the slider from the resistor part 11, low level signal leakage to the inverting amplifying circuit due to a residual resistance as in the conventional variable resistor is eliminated. Thus, the audio signals supplied to the op-amp G can have a better attenuation characteristic than that using the conventional variable resistor.

Further, the signal adding circuit using the variable resistor according to the present invention can be manufactured at a lower cost than that using the conventional variable resistor, which may be provided with a mechanical switch used only for cutting off the signal line, because the pre-stage circuit Zk can be separated from the inverting amplifying circuit by merely forming the resistor part 11 in a slightly different pattern than that of the conventional one.

In the above-mentioned embodiment, although means for separating the slider from the resistor part is established by forming the pattern of the resistor part different from that of the conventional variable resistor, such means can be achieved by covering an end of the resistor part 11 with an insulating material.

In such a case, the slider 10 slides on the insulating material at the end of the resistor part 11, and thereby continuity between the slider 10 and the resistor part 11 can be interrupted.

Alternatively, a protrusion may be provided in a portion of the surface of the board 1 between the conductive part 2 and the resistor part near the extreme end of the movable range of the slider 10. In such a case, the slider 10 slides on the protrusion at the extreme end of the movable range thereof, and thereby the end of the slider 10 sliding on the resistor part 11 is slightly lifted up, resulting in interruption of continuity between the slider 10 and the resistor part 11.

It should be noted that although a rotational type variable resistor is described in the abovementioned embodiment, the present invention may be applied to a linear movement type variable resistor.

The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention as defined by the appended claims.

Claims (8)

1. A variable resistor the resistance of which is varied by changing a length of a resistor inside thereof by a movement of a slider, the variable resistor comprising: a base board; a resistor part, having a predetermined length, formed on said base board; a conductive part formed on said base board a uniform distance apart from said resistor part; a slider, made of an electrically conductive material, provided over and between said resistor part and said conductive part, said slider being slid in a state where an end thereof is in contact with said resistor part and the other end thereof is in contact with said conductive part; and interrupting means for interrupting the electrical continuity between said resistor part and the conductive part when said slider is positioned at the extreme end of a movable range thereof.

2. The variable resistor as claimed in claim 1, wherein said interrupting means comprises a separating means for separating said slider from said resistor part when said slider is at the extreme end of the movable range thereof.

3. The variable resistor as claimed in claim 2, wherein said separating means comprises an end of said resistor part, which end is formed so that when said slider is at the extreme end of the movable range thereof, said slider slides off, in a plane of a surface of said base board, from an end of said resistor part.

4. The variable resistor as claimed in claim 3, wherein: said conductive part is formed in an annular shape on said base board; said resistor part comprises an annular resistor member formed outside said conductive part and concentrical to said conductive part; said slider comprises an extending part radially extending from said conductive part, an end of said extending part is pivotally mounted on said conductive part, and the other end thereof is slid on said resistor part by turning said slider; said separating means comprises said end of said resistor part which end is formed so that a distance between the center of pivot and an inner fringe of said resistor part is greater than a distance between an end of said extending part of said slider and the center of pivot.

5. The variable resistor as claimed in claim 2, wherein said separating means comprises vertical separating means for separating said end of said slider from said resistor part in a direction generally perpendicular to the surface of said base board.

6. The variable resistor as claimed in claim 5, wherein said vertical separating means comprises a thin insulating material provided on said end of said resistor part.

7. The variable resistor as claimed in claim 5, wherein said vertical separating means comprises a protrusion provided on said base board between said end of said resistor part and said conductive part, said slider being lifted up from said resistor part when said slider is moved onto said protrusion.

8. The variable resistor substantially as hereinbefore described with reference to Figs.4A and 4B of the accompanying drawings.