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Mar 13 2020

Variable Resistor: Basics, Application Circuits&Common Malfunctions

Catalog

I Introduction

II Definition, Symbol and Labeling of Variable Resistor

2.1 Definition

2.1.1 What is Variable Resistance?

2.1.2 What is Variable Resistor?

2.2 Symbol

2.3 Labeling Method of Variable Resistor

III How The Variable Resistor Works

IV Features of Variable Resistor Shape

V Structure and Function of Variable Resistor

5.1 Basic Structure

5.2 Schematic Diagram of Two Variable Resistors

5.3 The Role of The Variable Resistor

VI Types of Variable Resistors

6.1 Resistance box

6.2 Sliding Rheostat

6.3 Potentiometer

6.4 Specific Classification of Variable Resistors

6.4.1 Film Variable Resistor

6.4.2 Wire Wound Variable Resistor

VII Typical Application Circuits of Variable Resistor

7.1 Variable Resistor Circuit in Transistor Bias Circuit

7.2 Stereo Balance Control Variable Resistor Circuit

VIII Causes and Solutions of Variable Resistor Malfunctions

8.1 Causes of Variable Resistor Malfunctions8.2 Characteristics of Variable Resistor Malfunctions

8.3 Methods For Repairing Variable Resistor

8.4 Testing a Variable Resistor with a Multimeter

8.4.1 Method

8.4.2 Precautions

IX Active Variable Resistors with Wide Range of Load Impedance

X One Question Related to Variable Resistors

  10.1 Question

  10.2 Answer

 I Introduction

A resistor is a current-limiting element. After the resistor is connected to the circuit, the resistance of the resistor is fixed. It generally has two pins, which can limit the current flowing through the branch connected to it. Those whose resistance cannot be changed are called fixed resistors, and those with variable resistance are called potentiometers or variable resistors.

Setting Up A Variable Resistor, Rheostat or Fixed Resistory

II Definition, Symbol and Labeling of Variable Resistor

2.1 Definition

2.1.1 What is Variable Resistance?

Variable resistance is a kind of resistance, which can play the role of resistance in electronic circuits. The difference from ordinary resistance is its resistance can be continuously changed within a certain range. In some cases where the resistance value is required to change but does not change frequently, a variable resistor can be used.

2.1.2 What is Variable Resistor?

A variable resistor is an electronic component with adjustable resistance. It consists of a resistor and a rotating or sliding system. It is usually used in the circuit that needs to adjust the resistance frequently and plays the role of adjusting the voltage, adjusting the current, or controlling the signal. Its main parameters are basically the same as those of the fixed resistor.

2.2 Symbol

The symbol of the variable resistor is R and the unit is Ω.

2.3 Labeling Method of Variable Resistor

(1) The variable resistor uses the direct standard method to indicate the nominal resistance value, that is, the nominal resistance value is directly marked on the variable resistor. In the case of high current applications, the variable resistor is also marked with the rated power parameter. In addition, the resistance value of small variable resistors is expressed in three digits, which is the same as that of resistors.

(2) For variable resistors used in small-signal circuits, we generally only care about its nominal resistance and have no power requirements.

III How The Variable Resistor Works

When a voltage is applied between two fixed electric shocks of the resistor body, the position of the contact on the resistor body is changed by rotating or sliding the system, and a position is formed between the movable contact and the fixed contact. Certainly related voltage.

In other words, the resistor body of the variable resistor has two fixed ends. By manually adjusting the rotating shaft or sliding handle to change the position of the moving contact on the resistor body, the relationship between the moving contact and any fixed end is changed. The resistance value changes the magnitude of voltage and current.

IV Features of Variable Resistor Shape

(1) The volume of the variable resistor is larger than that of the general resistor, and at the same time, the variable resistor in the circuit is less, and it can be easily found in the circuit board.

(2) There are three pins in the variable resistor, and they are different from each other. One is a moving pin and the other two are fixed. Generally, the two fixed pins can be used interchangeably, but the fixed and moving pin cannot be used interchangeably.

(3) There is an adjustment port on the variable resistor. Use a flat-blade screwdriver to protrude into this adjustment port. Turn the screwdriver to change the position of the moving plate and adjust the resistance value.

(4) The nominal resistance value can be seen on the variable resistor. This nominal resistance value refers to the resistance value between two fixed chip pins, and is also a fixed chip pin and a moving chip pin. The maximum resistance value between.

(5) The vertical variable resistor is mainly used in small-signal circuits. Its three pins are vertically downward and mounted vertically on the circuit board. The resistance adjustment port is in the horizontal direction.

(6) Horizontal variable resistors are also used in small-signal circuits. Its three pins are at 90 ° to the resistance plane, and are mounted vertically on the circuit board with the resistance adjustment port facing upward.

(7) The variable resistance of the small plastic case is smaller and has a circular structure. Its three pins are down and the resistance adjustment port is up.

(8) Variable resistors (wire-wound structure) for large power applications. The volume is large, and the moving blade can slide left and right to adjust the resistance.

V Structure and Function of Variable Resistor

5.1 Basic Structure

The variable resistor is chiefly composed of a moving piece, a carbon film body, and three pins.The three pins are two fixed pins (also called fixed pieces) and one moving piece pin. The moving piece of the variable resistor can be rotated left and right. When using a flat-blade screwdriver to reach into the adjustment port and rotate, the contacts on the moving piece can slide on the resistance piece.

According to diverse uses, the resistance material of the variable resistor includes metal wire, metal sheet, carbon film or conductive liquid. For currents of general magnitude, metal type variable resistors are frequently used. When the current is slight, it is better to use a carbon film type. When the current is large, the electrolytic type is most suitable.

5.2 Schematic Diagram of Two Variable Resistors

Schematic Diagram of Two Variable Resistors 

Figure3. Schematic Diagram of Two Variable Resistors

5.3 The Role of The Variable Resistor

(1) A variable resistor is an adjustable electronic component, which is composed of a resistor body and a sliding system. The variable resistor resistance is a resistor that can be adjusted for the current or change of the circuit In the case of circuit resistance, the light can be dimmed, and the motor can be controlled to start its speed.

(2) The variable resistor mainly controls the current in the series circuit by changing its own resistance, thereby protecting some electrical components with requirements for the current. The variable resistor is generally used in circuits that do not require frequent adjustment, mainly To fix the same value for the resistor.

VI Types of Variable Resistors

6.1 Resistance Box

Variable resistors are divided into three types: resistance box, sliding rheostat, and potentiometer. The resistance box is a variable resistance device that uses a conversion device to change its resistance value. This conversion device usually adopts a decimal disc type (knob type) structure, and can also adopt a plug type and an end button type structure as required. The circuit of the resistance box can be divided into series lines and series-parallel lines.

Compared with the sliding rheostat, the resistance box can continuously change the resistance in the connected circuit, while the sliding rheostat cannot display the resistance value of the connected circuit.

Resistance Box

Figure4. Resistance Box

6.2 Sliding Rheostat

A sliding varistor is one of the commonly used devices in electricity. Its working principle is to change the resistance by changing the length of the resistance line in the circuit, thereby gradually changing the current in the circuit. The resistance wire of a sliding rheostat is generally a nickel-chromium alloy with a high melting point and a large resistance, and a metal rod is generally a metal with a low resistance. As a result, when the cross-sectional area of the resistor is constant, the longer the resistance wire, the greater the resistance; the shorter the resistance wire, the smaller the resistance.

Sliding Rheostat

Figure5. Sliding Rheostat

6.3 Potentiometer

A potentiometer is a resistance element with three lead-out terminals whose resistance can be adjusted according to a certain change law. A potentiometer usually consists of a resistor and a movable brush. When the brush moves along the resistor body, a resistance value or voltage having a certain relationship with the amount of displacement is obtained at the output end.

The potentiometer can be used as a three-terminal element or a two-terminal element. The latter can be regarded as a variable resistor. Because its role in the circuit is to obtain an output voltage that has a certain relationship with the input voltage (external voltage), it is called a potentiometer.

Potentiometer

Figure6. Potentiometer

6.4 Specific Classification of Variable Resistors

The variable resistor can be divided into film-type variable resistor and wire-wound variable resistor according to the material.

6.4.1 Film Variable Resistor

Membrane variable resistors are usually composed of a resistor body (synthetic carbon film), a movable contact (a movable metal reed or a carbon contact), an adjustment part, and three pins (or solder pads). Two of the fixed pins are connected to both ends of the resistor body, and the other pin (center tap) is connected to the movable contact piece. You can change the resistance between the center tap and the two fixed pins by turning the adjustment part with a small flat-blade screwdriver and changing the contact position of the movable contact with the resistor.

Membrane variable resistors are available in hermetic, semi-hermetic, and non-hermetic configurations.

(1) Fully sealed film variable resistors are also called solid variable resistors. The resistor is made of carbon black, quartz powder, organic binder and other materials, and then pressed into plastic or epoxy resin. The matrix of the material is polymerized by heating. The movable contacts use carbon contacts and the adjustment parts are made of plastic. The resistor body and the movable contact are sealed by a metal casing (there is an adjustment hole above the metal casing). Its advantage is that it has good dustproof performance and rarely has bad contact failure.

(2) The manufacturing process of the resistor body of the semi-sealed film variable resistor and the resistor body of the fully sealed variable resistor is basically the same. The movable contact piece adopts a metal reed, and the outer plastic cover is sealed. When the plastic cover is rotated, the movable contact piece also rotates with it. This variable resistor is easy to adjust, but its dust resistance is not as good as a fully sealed film type variable resistor.

(3) Unsealed film variable resistors are also called chip tunable resistors. The resistor body is made of carbon black, graphite, quartz powder, organic binder, etc. to form a suspension, which is coated on glass fiber board or glue. Made from wooden boards. The movable contact piece uses a metal reed, and the reed has an adjustment hole, and no separate adjustment component is provided. Its disadvantages are poor dust-proof performance, the contacts are susceptible to oxidation, and prone to failure due to poor contact with the synthetic carbon film.

6.4.2 Wire Wound Variable Resistor

(1) High-power wire-wound varistor is also called sliding wire varistor, which is divided into axial ceramic tube-type wire-wound variable resistor and porcelain disc-type wire-wound variable resistor. It adopts an unsealed structure.

(2) Low-power wire-wound variable resistors include round vertical wire-wound variable resistors, round horizontal wire-wound variable resistors, and square wire-wound variable resistors, all of which are fully sealed. Package structure.

In addition, the variable resistor can be divided into a vertical variable resistor and a horizontal variable resistor according to the structure.

Wire Wound Variable Resistor

Figure7. Wire Wound Variable Resistor

VII Typical Application Circuits of Variable Resistor

7.1 Variable Resistor Circuit in Transistor Bias Circuit

The figure below shows a variable-resistor voltage-dividing bias circuit. In the circuit, the transistor VT1 constitutes a high-frequency amplifier, and RP1, R1, and R2 constitute a voltage-dividing bias circuit. The output voltage of the voltage dividing circuit is determined by the resistance of three resistors, RP1, Rl, and R2. R1 and R2 are fixed resistors. The variable resistor RP1 is adjusted, and then the VT1 static operating current is adjusted. The amount of current determines whether VT1 can work in the best state.

Variable Resistance Voltage Divider Bias Circuit 

Figure8. Variable Resistance Voltage Divider Bias Circuit

7.2 Stereo Balance Control Variable Resistor Circuit

The following figure shows the left and right channel gain balance adjustment circuits in the audio amplifier. RP1 in the circuit is a variable resistor in series with R1.

Left and Right Channel Gain Balance Adjustment Circuit in Audio Amplifier 

Figure9. Left and Right Channel Gain Balance Adjustment Circuit in Audio Amplifier

In the audio circuit, for a two-channel amplifier, we need to strictly require that the left and right channel amplifiers have equal gain (balanced), but the discreteness of the circuit components makes this impossible. In order to ensure that the gains of the left and right channel amplifiers are equal, a left and right channel gain balance adjustment circuit needs to be provided, which is referred to as a stereo balance circuit.

In the right channel circuit, the resistance of R2 is determined, so that the gain of the right channel amplifier is fixed. Taking the gain of the right channel amplifier as a reference, changing the resistance of RP1 so that the gain of the left channel amplifier is equal to the gain of the right channel amplifier can achieve the same gain of the left and right channel amplifiers.

VIII Causes and Solutions of Variable Resistor Malfunctions

8.1 Causes of Variable Resistor Malfunctions

(1) The use time is long, causing oxidation.

(2) The failure of the circuit caused the variable resistor to overcurrent and burned the carbon film. At this time, the burned trace of the variable resistor can also be seen from the appearance.

8.2 Characteristics of Variable Resistor Malfunctions

(1) Damage to the carbon film of the variable resistor

The carbon film of the variable resistor is worn or burned. At this time, the contact between the moving piece and the carbon film is poor or cannot be contacted.

(2) Poor contact between the moving piece of the variable resistor and the carbon film causes the contact resistance between the moving piece and the carbon film to increase.

(3) The variable resistor pin is broken.

8.3 Methods For Repairing Variable Resistor

(1) When the track of the contact of the moving blade on the carbon film is worn, the contact on the moving blade can be bent inward to change the original track of the contact of the moving blade.

(2) The contacts of the moving blade are dirty. You can clean the contacts with pure alcohol.

(3) There is a disconnection between one stator and the carbon film. At this time, if it is used as a variable resistor (not used as a potentiometer), this stator that is not disconnected can be used instead. Resistance value.

(4) A pin is broken due to twisting. A lead can be welded with a hard wire as a pin.

Test a Variable Resistor

Figure10. Test a Variable Resistor

8.4 Testing a Variable Resistor with a Multimeter

8.4.1 Method

The detection method of the variable resistor is basically the same as that of the resistor. The resistance between the primers is measured with an ohmic block. The measurement can be performed directly on the circuit board, or the variable resistor can be disconnected from the circuit. .

(1) Measure the nominal resistance of the variable resistor. The multimeter is placed in the proper range of the ohmic block. The two meter bars are connected to the two fixed pin pins of the variable resistor. At this time, the measured resistance value should be equal to the nominal resistance value of the variable electrical accessory, otherwise the variable resistance is explained. Device is damaged.

(2) Measure the resistance between the moving resistor and the stator of the variable resistor. The multimeter is placed in the proper range of the ohmic block. One meter rod is connected to the fixed piece, and the other one is connected to the moving piece. In this measurement state, when the variable resistor moving piece is rotated, the needle is deflected and the resistance value increases from zero To the nominal value, or decrease from the nominal value to zero.

8.4.2 Precautions

Due to the particularity of the variable resistor, the following issues should be noted during the detection process:

(1) If the resistance between the moving piece and a fixed piece is 0Ω, at this time, you should see whether the moving piece has turned to the end of the fixed piece. To exclude the effects of external circuits).

(2) If the resistance value between the moving piece and any certain piece is greater than the nominal resistance value, it means that the variable resistor has an open circuit fault.

(3) In the measurement, if the measured resistance between a moving piece and a certain piece is less than the nominal resistance value, it does not mean that it is damaged, but you should look at the position of the moving piece, which is different from ordinary resistors.

(4) When taking off the measurement, you can use the appropriate range of the multimeter's ohmic stop.-One rod is connected to the pin of the pad, and the other rod is connected to a foot. Then use a flat screwdriver to slowly rotate the pad in a clockwise or counterclockwise direction. At this time, the hands should continuously change from 0Ω to the nominal resistance.

The same method is used to measure the change of wake value between another fixed film and moving film. The measurement method and test result should be the same. In this way, the variable resistor is good, otherwise the variable resistor is damaged.

Digital Multimeter

Figure11. Digital Multimeter

IX Active Variable Resistors with Wide Range of Load Impedance

Power resistors, variable resistors, and other electronic loads are often used to test power supplies and voltage regulators, as shown in the following figure:

Active Variable Resistors with Several Orders of Magnitude Constant Resistance 

Figure12. Active variable resistors with several orders of magnitude constant resistance

Although the function is the same as a mechanical potentiometer, it is based on an active device, which can provide a wide range of load resistance, high resistance adjustment resolution, and less heat than a mechanical potentiometer. Analyzing the circuit shown in the figure above, the voltage expressions of the non-inverting and inverting ends of the operational amplifier are:

Formula 

Figure13. Formula

These two voltages are equal, so

Formula 

Figure14. Formula

The whole circuit can be regarded as the resistance of the non-inverting terminal IN + and the inverting terminal IN-. The non-inverting and inverting equivalent resistances are constant and independent of the test voltage (VIN). RSENSE includes several series resistors that provide multiple orders of magnitude in impedance selection. For example, if 10Ω is required, the terminal is IN + and "B" near IN-1 (points A, C, and D are not connected). For high power loads, pay attention to the rated power of the sense resistor and nFET.

 

The power supply of the operational amplifier can be a battery or any other DC power supply. Its maximum working current is only 20 μA. It is powered by a 9V battery. Under normal circumstances, the active load can be used for 1-2 years.

10.1 Question

Volume control regulator in a CD receiver, radio and amplifier also use

A.transistor

B.variable resistor

C.thermistor

D.fixed resistor

10.2 Answer

B


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