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Potentiometer Circuit: Potentiometer Problems and Repair

Author: Apogeeweb
Date: 6 Dec 2018
potentiometer voltage divider

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


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



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

Ⅳ Frequently Asked Questions about Potentiometer Problems

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 in a circle. 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; pins 3 is disconnected to Pin 2 and pin 1.


Potentiometer Principle


In practical use, pin 3 is generally grounded as the data input terminal. 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 two fixed electric shocks of the resistor body, the position of the contact on the resistor body is changed through the rotation or sliding system, and the position of the movable contact is obtained between the movable contact and the fixed contact. There is a certain relationship between voltages. The potentiometer is a four-terminal component, which is mainly used as a voltage divider. The potentiometer is a kind of adjustable electronic component, which is basically a sliding rheostat. There are several styles of potentiometers, which are generally used for speaker volume switch and laser head power adjustment. 


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


For the volume rotation adjustment knob on the radio, a potentiometer is generally required. The potentiometer is mainly composed of three lead terminals, of which there is a fixed resistance between the two fixed terminals, and the other end slides on the resistance through the contact. However, because the potentiometer is an electromechanical component. It relies on the sliding of the brush on the resistor body to obtain an output voltage that has a certain relationship with the displacement of the brush, and then the resistance value of the potentiometer from the sliding end and the fixed end will change with the change of the position of the sliding end. In other words, 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

The potentiometer is a resistance element with three front ends, and its resistance value can be adjusted according to a certain change rule. The potentiometer is usually composed of a resistor and a movable brush. When the brush moves along the resistor, the resistance value or voltage related to the displacement is obtained at the output terminal.


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 that is related to the input voltage (applied voltage), it is called a potentiometer.


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


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-turn Potentiometer Structure

2.3 How to Use a Potentiometer

When the potentiometer handle rotates, 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 part to one fixed part increases, the resistance of the moving part to the other fixed part decreases, as shown in Figure 3. When the resistance of the rotor to one stator decreases, 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 part slides toward the fixed end 1, the resistance of R1 decreases, and the resistance of R2 increases. When the moving part 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: First, according to the resistance value of the potentiometer to be tested, select the appropriate resistance gear of the multimeter to measure the resistance value, that is, the resistance value between the two ends of the AC, and compare it with the nominal resistance value. See if the two are consistent. Rotate the sliding contact at the same time, and its value should be fixed. If the resistance is infinite, the potentiometer will be damaged.


(2) Then measure the contact between the center end and the resistance, that is, the resistance at both ends of BC. The method is that the ohm file of the multimeter is within the proper range, and during the measurement process, slowly rotate the shaft. Pay attention to the reading of the multimeter. Under normal circumstances, the reading changes smoothly in one direction. If there is a jump or no drop, the active contact is poorly contacted or malfunctioning.


(3) When the center end slides to the head end, in an ideal state, the resistance value between the center end and the coincident end is 0. In the actual measurement, there will be a certain residual value (generally depending on the nominal value, generally less than 5 Ω). This is a 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 the damage of the carbon film.

When judging from the appearance that the potentiometer has bad contact and failure, the shell can be removed to check the degree of wear. If it is only due to poor contact caused by mild abrasion, the carbon film can be wiped clean with absolute alcohol or carbon tetrachloride, and then the pressure of the sliding arm on the carbon film can be adjusted appropriately.


2. There are three cases of potentiometer switch structure damage.

First, the customs are constantly 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 with a multimeter or can be observed with the naked eye. When repairing, the first and third boxes must be replaced with new parts. 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 eliminated. If the spring is weak and the contact is poor, the spring can be replaced.

Potentiometer Principle

3.2 Potentiometer Repair Method

Currently, there are two main types of potentiometers: one is a potentiometer without a switch, and the other is a potentiometer with a switch. For the use of these two potentiometers, it is easy to cause some problems, such as turn-off or on-off failure, or short circuit caused by poor line contact, and the internal components of the potentiometer fall off. So how to solve these three types of problems?


First of all, it is generally recommended to replace the potentiometer. If the part falls off, it is difficult to restore the original state. Even if they are glued back, they are dangerous in use. Some failures, such as poor contact, are generally caused by contact oxidation. Some methods can solve this problem, for example, after the potentiometer contacts are wiped clean, the fault is eliminated. Another situation is that the spring force of the potentiometer is reduced, causing poor contact and so on. In this case, a new potentiometer spring can be replaced.


Under normal circumstances, long-term use of the potentiometer will often lead to poor contact between the lines due to damage to the carbon film. When the potentiometer can not be folded, you can remove the potentiometer shell to check the degree of wear of the internal components. For example, if the reason for poor contact is only the slight abrasion of the potentiometer, some industrial-specific methods can be used to clean it, or carbon tetrachloride can be used to clean the carbon film material of the potentiometer. After cleaning, the pressure on the carbon film of the potentiometer slide arm can be adjusted according to the pressure of the slide arm, and then load it 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, the volume potentiometers of two fixed pins are reversely connected. When you only turn the volume knob a little bit, the sound is already loud, but when you turn the knob again, the volume hardly increases, which is the loss of the linear control characteristics of the volume controller.


There are many types of volume potentiometers for power amplifiers, but the most common one is a double potentiometer, which is used to control the volume of the left and right channels. It is usually single-axis, but there are also double-layer dual-axis volumetric potentiometers. The general problems of the volume potentiometer include: when the volume is controlled by rotation, noise is generated, or the volume of the two channels is different, then the volume potentiometer should be replaced. After the potentiometer is used for a long time, the aging of the carbon film and the appearance of micro-cracks are the main reason for the decrease in the quality of the volume potentiometer. At this time, the volume potentiometer needs to be completely replaced.

How To Fix a Scratchy Volume Potentiometer?


Ⅳ Frequently Asked Questions about Potentiometer Problems

1. What is a potentiometer in a circuit?

A potentiometer is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. ... Potentiometers are commonly used to control electrical devices such as volume controls on audio equipment.


2. How is a potentiometer connected in a circuit?

Connect the voltmeter's terminals to the input and output terminals on the pot. Turn the voltmeter on and turn the dial to feed a signal. Turn the knob on top of your pot to adjust the signal. If the signal reading on the voltmeter goes up and down when you turn the knob, your potentiometer works.


3. How do I know if my potentiometer is bad?

First, connect all connection like in this image.
Now, rotate the potentiometer knob using your finger it will change the Lamp light source High to Low or Low to High. if Lamp light source is change, the potentiometer is good condition or is not change Lamp light source, the potentiometer is bad condition.


4. What happens when a potentiometer fails?

Potentiometer valve position indicator failures are normally electrical in nature. An electrical short or open will cause the indication to fail at one extreme or the other. If an increase or decrease in the potentiometer resistance occurs, erratic indicated valve position occurs.


5. How do you test if a potentiometer is working?

Put the probes of your ohmmeter on the two ends. It should read within a few ohms of the rated resistance of your potentiometer. If you get a different reading, one of your probes is on the wiper. Try different combinations of two probes until you get one that gives you the right reading.


6. Can a potentiometer be repaired?

Those controls are not available, they have to be repaired. The only way to repair it is to heat up the tone control shaft with high power hot iron ( I use 350 Watts solder gun) and special wooden knob I made for this purpose- original plastic knob would melt.


7. Why copper wire is not used in potentiometer?

Copper has low resistivity and a high temperature coefficient of resistance and hence we do not use it in potentiometer.


8. How does a 3 wire potentiometer work?

A potentiometer has 3 pins. Two terminals (the blue and green) are connected to a resistive element and the third terminal (the black one) is connected to an adjustable wiper. The potentiometer can work as a rheostat (variable resistor) or as a voltage divider.


9. How to Use Potentiometers to Adjust Current and Voltage

Connect one end of the limiting resistor to the positive terminal of the power supply. Attach the other end if the limiting resistor to one of the fixed terminals of the potentiometer. Link the positive probe of the current meter to the wiper of the potentiometer.


10. How do you reduce the value of a potentiometer?

We could replace the potentiometer with one of another value, or we could simulate a lower-value potentiometer by placing a resistor in parallel with it, diminishing its maximum obtainable resistance. This will decrease the span of the circuit from 10kΩ to something less.


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