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Dec 6 2018

Potentiometer Circuit: Potentiometer Problems and Repair

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

Introduction

Potentiometers are widely used and are electronic components that are often used in circuit design. They are adjustable and are considered as one of variable resistors. In general, it is constructed by a resistor carrying box rotation/sliding system. Generally speaking, a moving contact is displaced on the resistor body to obtain a part of the voltage output.

Catalog

Introduction

Ⅰ How Does a Potentiometer Work

Ⅱ Potentiometer Wiring

2.1 How to Wire a Potentiometer

2.2 Potentiometer Structure

2.3 How to Use a Potentiometer

Ⅲ Potentiometer Repair

3.1 Potentiometer Problems

3.2 Potentiometer Repair Method


Ⅰ How Does a Potentiometer Work

Viewed from the outside, the pulse potentiometer is the same as the ordinary potentiometer. It is connected to pins 1, 2 and is connected to two metal mats of different lengths. It is connected to pin 3 for one week. Metal slides with 12 or 24 teeth. When the pulse potentiometer rotates, four states can appear: pin 3 is connected to pin 1;pin 3 is connected to pin 2 and pin 1; pin 3 is connected to pin 2; pin 3 is disconnected to Pin 2 and pin 1.

Potentiometer Principle

In practical use, pin 3 is generally grounded as the data input terminal. The pins 1 and 2 are connected with the IO port of the single chip computer as data output terminals. As shown in the figure, pin 1 is connected with the single chip microcomputer's P1.0, pin 2 is connected with the single chip computer's P1.1. When the pulse potentiometer is left-handed or right-handed, P1.0 and P1.1 periodically produce the waveforms shown. If the 12-point pulse potentiometer rotates once, 12 sets of such waveforms will be generated, 24 sets of such waveforms will be generated by the 24-point pulse potentiometer; a set of waveforms (or a cycle) contains four working states. So as long as the waveforms of P1.0 and P1.1 are detected, whether the pulse potentiometer rotates left-handed or right-handed can be identified. 

Potentiometer Principle

When a voltage is applied between the two fixed electric shocks of the resistor body, the position of the contact on the resistor body is changed by the rotation or sliding system, and a position of the movable contact is obtained between the movable contact and the fixed contact. A certain relationship between the voltage. It is mostly used as a voltage divider, which is a potentiometer that is a four-terminal component. Potentiometers are basically sliding varistor, there are several styles, generally used in speaker volume switch and laser head power adjustment potentiometer is a kind of adjustable electronic components. It consists of a resistor body and a rotating or sliding system. When a voltage is applied between the two fixed electric shocks of the resistor body, the position of the contact on the resistor body is changed by the rotation or sliding system, and a position of the movable contact is obtained between the movable contact and the fixed contact. A certain relationship between the voltage.

How Potentiometer Assembles and Works

When the rocker potentiometer is used as a current adjustment in the DC circuit, there will be a current passing through the sliding arm of the rocker potentiometer, and the resistance value will increase abnormally due to the anodization. In this case, it is recommended to connect the terminal to which the resistor is connected to the negative pole and the sliding arm to the positive pole. If the direct current directly passes through the rocker potentiometer, the anode of the rocker potentiometer will be oxidized and damaged, so that the impedance of the rocker potentiometer becomes larger. Therefore, it is better to connect the negative pole of the current to the terminal that is in contact with the carbon diaphragm. Upper, the positive pole is connected to the terminal of the brush (rocker potentiometer contact piece). When the rocker potentiometer is used as a variable resistor, it is recommended to use a voltage divider for adjusting the voltage. At the same time, the load resistance RL of the rocker potentiometer should be no less than 10 times the nominal resistance of the rocker potentiometer.

Potentiometer Principle

 

Regarding the volume rotation adjustment knob on the radio, a potentiometer is generally required. The potentiometer is mainly composed of three lead terminals, wherein there are fixed resistors between the two fixed terminals, and the other end is Slide on this resistor through the contacts. However, since the potentiometer is an electromechanical component, it relies on the sliding of the brush on the resistor body to obtain an output voltage in a certain relationship with the brush displacement, and then the resistance value from the sliding end and the fixed end of the potentiometer will The position changes with the position of the sliding end. That is to say, the radio potentiometer obtains different signal voltages by changing the position of the sliding end on the resistor body, thereby realizing the adjustment of the operation control volume.

 

Ⅱ Potentiometer Wiring

2.1 How to Wire a Potentiometer

Potentiometers are resistive elements with three leading ends whose resistance values can be adjusted according to a certain law of variation. Potentiometers usually consist of resistors and movable brushes. When the brush moves along the resistor the resistance value or voltage which is related to the displacement is obtained at the output.

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, and since it functions in the circuit to obtain an output voltage which is related to the input voltage (applied voltage), it is called a potentiometer.

The potentiometer is a continuously adjustable resistor. When the knob or slider of the potentiometer is adjusted, the movable contact slides on the resistor body. At this time, an output voltage which is related to the potentiometer applied voltage and the movable arm angle or stroke can be obtained at the output end of the potentiometer.

When the potentiometer is used as a varistor, it should be connected to the two ends of the device, so that a smooth and continuously changing resistance value can be obtained within the stroke range of the potentiometer. When the potentiometer is used as a current controller, one of the selected current outputs must be the sliding contact terminal.

2.2 Potentiometer Structure

(1) Carbon Film Potentiometer Structure

Figure 1. is a schematic diagram of the structure of a carbon film potentiometer.

Carbon Film Potentiometer Structure

Figure 1. Carbon Film Potentiometer Structure

 

(2) Multi-turn Potentiometer Structure

Figure 2. is a schematic diagram of the structure of a multi-turn potentiometer.

Muti-ture Potentiometer Structure

Figure 2. Muti-ture Potentiometer Structure

2.3 How to Use a Potentiometer

When the handle of the potentiometer is turned, the rotor slides on the resistor body, and the resistance between the GK105 rotor and the two stators changes. When the resistance of the moving piece to a fixed piece is increased, the resistance of the moving piece to the other fixed piece is reduced, as shown in Figure 3.When the resistance of the rotor to a stator is reduced, the resistance of the rotor to the other stator increases.

Potentiometer Adjustment Resistor

Figure 3. Potentiometer Adjustment Resistor

The potentiometer is also equivalent to a series circuit composed of two resistors in the circuit. The rotor divides the resistor body of the potentiometer into two resistors R1 and R2, as shown in Figure 4.

Potentiometer Equivalent Series Circuit

Figure 4. Potentiometer Equivalent Series Circuit

When the moving piece slides toward the fixed end 1 , the resistance of R1 decreases, and the resistance of R2 increases. When the moving piece slides toward the fixed end 2, the resistance of R1 increases, and the resistance of R2 decreases. The sum of the resistances of Rl and R2 is always equal to the nominal resistance of the potentiometer.

The main requirements for the potentiometer are:

(1) Resistance meets the requirements

(2) The contact between the center sliding end and the resistor body is good, and the rotation is smooth.

For potentiometers with switches, the switch parts should be accurate, reliable and flexible. Therefore, the performance of the potentiometer must be checked before use.

(1) Measurement of resistance value: Firstly, according to the resistance value of the potentiometer to be tested, select the appropriate resistance gear of the multimeter, measure the resistance value, that is, the resistance value between the two ends of the AC, and compare with the nominal resistance value. See if the two are consistent. At the same time, the sliding contact is rotated and its value should be fixed. If the resistance is infinite, the potentiometer is damaged.

(2) Then measure the contact between the center end and the resistor, that is, the resistance between the two ends of the BC. The method is that the multimeter ohmic file is in the appropriate range, during the measurement process, slowly rotate the rotating shaft, pay attention to observe the reading of the multimeter, the normal situation, the reading changes smoothly in one direction, if there is a phenomenon of jumping, falling or nowhere, the active contact has Poor contact failure.

3) When the center end slides to the head end or the end, the resistance value of the center end and the coincident end is 0 in the ideal state. In the actual measurement, there will be a certain residual value (generally depending on the nominal value, generally less than 5 Ω). normal phenomenon. Potentiometer (or trimming resistor, etc.) conventional pin (only for example, a potentiometer with 3 pins), the resistance value of the two ends is fixed, and the resistance value of the pin at either end of the intermediate pin is variable; It is equivalent to dividing the potentiometer into two series resistors from the middle pin, and the total resistance of the series is fixed; therefore, if used as a variable voltage divider resistor, one end is connected to the input voltage, and the middle end is connected to the output, and the rest is left. Terminal grounding; if used as a variable resistor, one end is connected to the input voltage, the middle end is connected to the output, and the remaining end can be suspended or connected to the intermediate end.

 

Ⅲ Potentiometer Repair

3.1 Potentiometer Problems

Common potentiometer problems generally manifest themselves in the internal open circuit of the pin, the resistor body is burned out, the switch is damaged, the rotating noise is too large, and the resistor body is worn out, etc.

1. Potentiometers are often poorly contacted due to damage to the carbon film.

When it is judged from the appearance that the potentiometer has a poor contact failure, the outer casing can be disassembled to check the degree of wear. If it is only a bad contact caused by mild wear, the carbon film can be wiped clean with anhydrous alcohol or carbon tetrachloride, and then the pressure of the sliding arm on the carbon film can be appropriately adjusted.

2. There are three cases of potentiometer switch structure damage

First, the customs are constantly open or open;

Second, the contact is poor, and the connection is not working;

The third is that the switch part is off.

These three types of faults can be detected by the multimeter or seen by the eyes. For repairs, new components must be replaced for the first and third cases. In the second case, the switch can be repaired according to the problem. If the contact is bad, the contact is oxidized, and the fault can be removed. If the spring is weak, the contact is poor, and the spring can be replaced.

Potentiometer Principle

3.2 Potentiometer Repair Method

At present, there is two main types of potentiometers: a potentiometer with no switch and another potentiometer with a switch. For the use of these two potentiometers, it is easy to cause some problems, such as switch-off or switch-on failure, or short circuit caused by the poor line contact, and the internal components of the potentiometer fall off. So how to fix these three types of problems? First of all, it is generally recommended to replace the potentiometer. It is difficult to restore the components if they fall off. Even if they can be glued back, they are dangerous in use. For some faults such as poor contact, it is generally due to the oxidation of the contacts. There are some methods. After the contacts of the potentiometer are wiped clean, the fault is eliminated. For another situation, that is, the spring force of the potentiometer decreases and causes poor contact, etc, replacing the new potentiometer spring.

Generally, the use of potentiometers for a long time will often lead to poor contact between lines due to carbon film damage and other phenomena. When the potentiometer fails to contact properly, the shell of the potentiometer can be dismantled to check the wear degree of the internal components. For example, if the reason of bad contact is only caused by slight wear of potentiometer, it can be cleaned by some industrial special purpose, or carbon tetrachloride can be used to clean the carbon film material of potentiometer. After cleaning, the pressure on the carbon film of the slide arm of potentiometer can be adjusted according to the pressure of the slide arm, and then it can be loaded back. 

In the non-linear potentiometer, the leads on the two stators cannot be connected to each other wrongly, otherwise it will affect the control effect of the potentiometer in the circuit. For example, after the two fixed pin pieces of the volume potentiometer are connected reversely, the volume is already very loud when the handle is turned a little bit, and the volume hardly increases when the volume knob is turned again, losing the linear control characteristics of the volume controller. There are many types of power amplifier volume potentiometers, but the most common one is a dual potentiometer, which is used to control the volume of the left and right channels. It is usually single-axis, and double-layer dual-axis volume potentiometers also have. The general problems with the volume potentiometer include noise when the volume is controlled by the rotation, or the volume of the two channels is different, then the volume potentiometer should be replaced. After the potentiometer has been used for a long time, the carbon film aging appears slightly cracks are the main reason that the quality of the volume potentiometer is degraded. This is because the potentiometer needs to be completely replaced.

How To Fix a Scratchy Volume Potentiometer?

 

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