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Dec 4 2018

What Is a Potentiometer, and How to Choose the Correct Potentiometer?

Warm hints: This article contains about 4000 words and reading time is about 18 mins.

Introduction

Potentiometer is actually a variable resistor. It is called potentiometer because its function in the circuit is to obtain the output voltage which is related to the input voltage (applied voltage). It is usually manufactured to maintain its original characteristics no matter how long it is used. If the position sensor is used, the potentiometer can be a linear or rotary position sensor. The potentiometer outputs a voltage value proportional to the position of the slider along the variable resistor. Because temperature change, abrasion and fouling between slider and variable resistor will cause resistance change and affect the accuracy of potentiometer, so the accuracy of potentiometer is too low. Due to the development of materials, especially conductive plastics, potentiometers can maintain their original characteristics after a long time of use, and improve their performance at the same time. 

Article Core

Potentiometer

Purpose

Introduce what a potentiometer is, and how to choose the correct potentiometer.

Application

Two-terminal Component

Keywords

Potentiometer

 Catalog

Introduction


Definition


Attributes


Type of Potentiometer


Function of Potentiometer


 

 

 

How to Choose the Correct Potentiometer

The requirements of the structure.

The resistance change characteristics.

Select the type of potentiometer according to the requirements of use.

Select the parameters of the potentiometer according to the requirements of the circuit.



1. Definition

Potentiometers are one of the most commonly used tunable electronic components. The potentiometer is derived from the development of a rheostat. It consists of a resistor body and a rotating or sliding system. The contact brush of the boom slides on the resistor body to continuously change the resistance between the boom and the two ends. value. The potentiometer is a resistance element having three terminals and the resistance value can be adjusted according to a certain variation rule. Potentiometers usually consist of a resistor and a movable brush. When the brush moves along the resistor body, a resistance value or voltage which is in a certain relationship with the displacement amount is obtained at the output end. The potentiometer can be used as a three-terminal component or as a two-terminal component. The latter can be regarded as a variable resistor, because its role in the circuit is to obtain with the input voltage (applied voltage) into a certain relationship with the output voltage, so called the potentiometer.

There are many kinds of potentiometers, such as ordinary rotary potentiometer, switched potentiometer, miniature switched potentiometer, direct-slip potentiometer, multi-loop potentiometer, fine-tuning potentiometer, double-connected potentiometer and so on. The character symbol of potentiometer is "Rp". 

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A potentiometer is a type of rheostat. Usually consists of a six-leg potentiometer resistor body and a rotating or sliding system, that is, a moving contact moves on the resistor body to obtain a partial voltage output.

The structure characteristic of potentiometer is that the resistor of potentiometer has two fixed ends. By manually adjusting the rotating shaft or sliding handle to change the position of the moving contact on the resistor, the resistance value between the moving contact and any fixed terminal is changed, thus the magnitude of voltage and current is changed. 

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Potentiometer is an adjustable electronic element. It consists of a resistor and a rotating or sliding system. When a voltage is applied between the two fixed contacts of the resistor, the position of the contacts on the resistor is changed by rotating or sliding system, and a voltage related to the position of the moving contacts can be obtained between the moving contacts and the fixed contacts. It is mostly used as a voltage divider, when the potentiometer is a four-terminal element. The potentiometer is basically a sliding rheostat. There are several types of potentiometers, which are generally used to adjust the power of loudspeaker volume switch and laser head. The potentiometer is an adjustable electronic component. 

Rheostat is used for voltage division. On the exposed resistor body, one or two movable metal contacts are pressed. The contact position determines the resistance between either end of the resistor body and the contact. According to the material branch winding, carbon film, solid core potentiometer; according to the relationship between the output and input voltage ratio and the rotation angle, the linear potentiometer (linear relationship), the function potentiometer (in a curve relationship). The main parameters are resistance, tolerance, and rated power. Widely used in electronic equipment, for volume control in audio and receiver.


2.Attributes

In addition to the nominal resistance and rated power, the main parameters of the potentiometer also have a resistance change characteristic, which refers to the relationship between the resistance value and the rotation angle or sliding stroke of the boom. Common potentiometer resistance changes are linear (X-type), exponential (Z-type), and logarithmic (D-type). The curve of the three types of potentiometer resistance as a function of the angle of rotation of the movable contact is shown in Figure (b). The ordinate in the figure represents the actual value of the resistance at a certain angle and the percentage of the total resistance of the potentiometer. The abscissa is the percentage of the rotation angle and the maximum rotation angle.

The resistance change of the X-type potentiometer is linear with the rotation angle. That is, the distribution of the conductive material on the resistor body is uniform, so the resistance per unit length is equal. It is suitable for some applications that require uniform adjustment, such as voltage dividers, bias current adjustment and other circuits. When the Z-type potentiometer starts to rotate, the resistance value changes little, and when the rotation angle approaches one end of the maximum rotation angle, the resistance value changes more obviously. Because the human ear has a slight increase in the tiny sound, it feels very sensitive, but after the sound reaches a certain value, even if the sound power is greatly increased, the feeling of the human ear does not change much. This potentiometer is suitable for The volume control circuit, because the potentiometer is used for volume control, the relationship between the volume and the potentiometer angle is approximately linear. The resistance change of the D-type potentiometer is opposite to that of the Z-type. The resistance value changes greatly when it starts to rotate, and the resistance value changes slowly when the rotation angle approaches the maximum value. The D-type potentiometer is suitable for circuits such as tone control.

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Resistance Change Characteristic Curve 

If the potentiometer has slight contact failure, the carbon film and the contact of the moving piece can be cleaned with pure alcohol; if the carbon film is seriously worn and the contact is poor in the potentiometer, the metal play contact can be gently bent inward or outward. , changing the movement trajectory of the metal brush on the carbon film; if the potentiometer has a certain piece of film and the carbon film open circuit (mostly open at the silver coating), and another fixed piece is not used or welded together with the moving piece, then The welding position of the two fixed pieces can be exchanged and can still be used normally. For example, the potentiometer A stator and the carbon film are broken, at this time, the potentiometer A can be soldered and the B-slice can be replaced. If the carbon film is worn, use a pencil to apply the carbon on it to maintain its use. If it is severely damaged or the conditions permit, it will be replaced as much as possible.


3. Type of Potentiometer

(1) Synthetic Carbon Film Potentiometer: It is the most used potentiometer at present. The resistor body is made of a mixture of carbon black, graphite, quartz powder, organic binder, etc., and is coated on a rubber board or a fiberglass board.

Advantages: high resolution and wide resistance range;

Disadvantages: large sliding noise, poor heat and humidity resistance.

Variety: Common synthetic carbon film potentiometer, small synthetic carbon film potentiometer with switch, single-band switch (no switch) potentiometer, double coaxial no switch (with switch) potentiometer, double-axis without switch ( With switch) potentiometer, small precision synthetic carbon film potentiometer, push-pull switch synthetic carbon film potentiometer, straight-slip synthetic carbon film potentiometer, precision multi-turn synthetic carbon film potentiometer.


(2) Wirewound Potentiometer: The resistor body is made of a wire wound around a metal or non-metal plate coated with an insulating material.

Advantages: high power, low noise, high precision and good stability;

Disadvantages: Poor high frequency characteristics.


(3) Metal Film Potentiometer: The resistor body is made by depositing a metal alloy film, a metal oxide film, a metal composite film, and a ruthenium oxide film material on a ceramic substrate by a vacuum technique.

Advantages: high resolution, sliding noise is smaller than synthetic carbon film potentiometer;

Disadvantages: small resistance range and poor wear resistance.


(4) Straight-slide Potentiometer: The resistor body is a rectangular strip, which changes the resistance value by linear movement of the slider connected to the slider.

Usage: Generally used for volume control or equalization control in TV sets and stereos.


(5) Single-turn Potentiometer and Multi-turn Potentiometer:

Single-turn potentiometer: its sliding arm can only rotate within less than 360 degrees, generally used for volume control;

Multi-turn potentiometer: For every revolution of its rotating shaft, the sliding arm contact changes only a small distance on the resistor body. When the sliding arm is from one extreme position to the other extreme position, the rotating shaft needs to rotate a plurality of turns. Generally used in precision adjustment circuits.


(6) Solid potentiometer: It is made by mixing materials heated with carbon black, graphite, quartz powder, organic binder, etc., and then pressed on a plastic substrate and then heated and polymerized.

Advantages: high resolution, good wear resistance, wide resistance range, high reliability and small volume;

Disadvantages: high noise and poor high temperature resistance. Variety: can be divided into small solid potentiometer, linear solid potentiometer, logarithmic solid potentiometer.


(7) Single Potentiometer and Double Potentiometer:

Single potentiometer: a set of potentiometers controlled by a separate shaft;

Double potentiometer: Usually two potentiometers with the same specifications are mounted on the same shaft. When the shaft is adjusted, the sliding contacts of the two potentiometers rotate synchronously, which is suitable for the volume adjustment of the two-channel stereo amplifier circuit. There are also some double-connected potentiometers that are asynchronous.


(8) Stepping Potentiometer: It consists of stepping motor, rotating shaft resistor, moving contact and so on. The moving contacts can be manually adjusted by a rotating shaft or driven by a stepping motor.           

Purpose: Used for volume control in audio power amplifier 


(9) With Switching Potentiometer: A switching device is attached to the potentiometer. The switch and potentiometer are coaxial. The movement and control mode of the switch can be divided into two types: rotary type and push-pull type.   

Purpose: It is mostly used for volume control and power switch in black and white TV sets. Small rotary switched potentiometer is mainly used for volume control (or current and voltage regulation) and power switch in semiconductor radio or other small electronic products.     

Types: Switching digits are single-pole single-throw, single-pole double-throw and double-pole single-throw. 


(10) Patch potentiometer: also known as sheet potentiometer, is a miniature linear potentiometer without manual rotating axis. Screw screwdriver and other tools are needed for adjustment.            

Category: It can be divided into single-loop potentiometer and multi-loop potentiometer, which belong to precision potentiometer and have vertical and horizontal structure. 


4. Function of Potentiometer 

The main functions of potentiometers in circuits are as follows: 

(1) As a divider potentiometer, it is a continuously adjustable resistor. When the rotating handle or sliding handle of the divider is adjusted, the moving contact slides on the resistor. At this time, the output voltage of the potentiometer, which is related to the applied voltage of the potentiometer and the angle or stroke of the movable arm, can be obtained at the output end of the potentiometer.            

(2) When a rheostat potentiometer is used as a rheostat potentiometer, it should be connected to two end devices, so that a smooth and continuous variation of resistance can be obtained within the range of the trip of the flower potentiometer.            

(3) When a potentiometer is used as a current controller, one of the selected current output terminals must be a sliding contact lead-out. 


5. How to Choose the Correct Potentiometer

Presumably everyone knows something about potentiometers. In fact, potentiometer is a resistor that can adjust resistance value according to its function. It has three feet and one foot at each end. The resistance value corresponding to these two feet is fixed, called nominal resistance value. The middle foot can slide on resistance, called slider, if resistance is used. If the resistance is made straight, the slider will slide straight on the resistance. This potentiometer is called push-pull potentiometer. In this way, the resistance of the slide and the pins at both ends can be adjusted. Its measurement method needs to use the resistance of the multimeter, first to measure whether the nominal value is accurate, then to measure the resistance of the slide and the pins at both ends, while sliding, while measuring, to see whether the resistance changes, can change is good, unchanged, or 0, or infinite is bad. 

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There are still many types of potentiometers. In the choice of potentiometers, it is necessary to select according to certain requirements and needs. Select equipment, and pay attention to the amount of a door-to-door pair. Here are some notes on the selection of potentiometers.


(1) The requirements of the structure.

When choosing potentiometer, we should pay attention to the size of potentiometer, the length of shaft shank and the style of shaft end, and whether the position of axis needs locking switch, single or multiple, single or multiple loops, etc. to make specific demands on the structure. For potentiometers that need to be adjusted regularly, a potentiometer milling the axle end into a plane should be selected to install the knob. For potentiometers that do not need constant adjustment, the potentiometer with grooves at the axle end can be selected so that the potentiometer can not rotate after adjustment with screwdriver, so as to maintain the relative stability of the working state. Potentiometers with locking devices should be selected for potentiometers requiring accuracy and no change after adjustment. 

Potentiometer with switch, the switch part is used for on/off control of the circuit power supply, and the potentiometer part is used to adjust the power. The switch forms with switch potentiometers are single-pole single-throw, single-pole double-throw and double-pole double-throw. The selection should be determined according to the needs. The potentiometer with switch is divided into push-pull switch potentiometer and rotary switch potentiometer. Push-pull switch potentiometers do not participate in the action when the switch is actuated. The advantage of this is that there is no wear on the resistor body, and the position of the installed potentiometer is not changed. When the switch of the rotary potentiometer is operated once, the movable contact will slide once on the resistor body, so the wear is large, which will affect the service life of the potentiometer.


(2) The resistance change characteristics. 

The resistance change characteristics of the potentiometer should be selected according to the application. For example, the potentiometer for volume control should be an exponential potentiometer. In the case of an exponential potentiometer, a linear potentiometer can be used instead, but a logarithmic potentiometer cannot be used. Otherwise, the volume vocabulary range will be reduced: The potentiometer for voltage division should use a linear potentiometer; the potentiometer for tone control should use a logarithmic potentiometer.

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(3) Select the type of potentiometer according to the requirements of use.

In circuits that are generally less demanding or where the environment is better, a synthetic carbon film potentiometer should be used first. The synthetic carbon film potentiometer has the characteristics of high resolution, wide resistance range, complete variety and low price, but has the disadvantages of moisture resistance and stable parts, and can be widely applied to indoor household appliances. For example, a volume potentiometer with a switch for a semiconductor radio, a synthetic carbon film potentiometer can be used; a power-regulating circuit in a television can use a straight-slip carbon film potentiometer; a high-load and trimmer potentiometer in other household appliances can also be used. A synthetic carbon film potentiometer is used. In addition, the synthetic carbon film potentiometer has a long mechanical life and can be used in circuits requiring a long wear life.

If the circuit needs to be finely tuned, and the power consumed is large. Wirewound potentiometers should be used. Due to the large distribution parameters, wirewound potentiometers are only suitable for low frequency circuits, so wirewound potentiometers should not be used in high frequency circuits. In addition, the wire-wound potentiometer has low noise, and such a potentiometer can be selected for circuits requiring low noise.


(4) Select the parameters of the potentiometer according to the requirements of the circuit.

The parameters of the potentiometer mainly include nominal resistance, rated power, maximum working voltage, linear accuracy and mechanical life. They are the basis for selecting potentiometers. When the type of the potentiometer is selected according to the requirements of use, the technical and performance parameters of the potentiometer should be selected according to the requirements of the circuit.

The mechanical life of different potentiometers is also different. Generally, the mechanical length of a synthetic carbon film potentiometer can be as high as 200,000 weeks, while the mechanical test of a glass glaze potentiometer is only 100-200 weeks. When using a potentiometer, the potentiometer with different mechanical life parameters should be selected according to the different requirements of the circuit for wear resistance.

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