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What is a DC Potentiometer? Working Principle and Applications

Author: Apogeeweb
Date: 20 May 2019
DC Potentionmeter

The DC potentiometer is a DC resistance instrument made by the principle of compensation. The applied disciplines are mechanical engineering, electrical measuring instruments, inspection instruments and standards.

Principle of D.C Potentiometer, D.C Potentiometer Theory and Principle | How It Works | Principle of Slide Wire Potentiometer


Ⅰ Introduction

Ⅱ Operational Principle

2.1 Compensation Principle

2.2 Potentiometer Principle

2.3 Experiment

Ⅲ Potentiometer Types

Ⅳ Advantages and Disadvantages

Ⅴ Potentiometer Application

Ⅵ Applying Attention

Ⅶ Frequently Asked Questions about DC Potentiometer

Ⅰ Introduction

The DC potentiometer mainly consists of two electrodes, a coil with a certain resistance, a permanent magnet, a shaft, a pointer, and a dial. When the two poles are connected to the power source under test, if there is a potential difference (in other words, a voltage), a current will be generated in the coil. The interaction of the current with the magnet causes the pointer to rotate. The pointer reading on the dial is the potential difference.


Figure 1. Potentiometer Structure and Symbol

A comparator is an instrument that compares the measured value with a known standard quantity and finally determines the measured value. It is divided into compensation measuring instruments and bridge measuring instruments. The known standard quantities used by the comparator to compare with the measured value are standard batteries, standard resistances, standard capacitors, and standard inductances. In addition, the comparator uses a galvanometer to achieve zero balance.


Generally speaking, the measurement process of a comparative measuring instrument is a process in which the comparative difference is gradually reduced to zero by adjusting the optional adjustment components. In fact, this method is more accurate than direct reading measurement.


The compensation method is a basic method of electromagnetic measurement. The compensation method measuring instrument has two compensation ways, full compensation method and different compensation methods. The full compensation method compares the measured value (only voltage) with the standard value. If the detection value is 0, the measured value is equal to the standard value. In addition, the measuring instrument theoretically does not extract energy from the measured object. The difference compensation is to use the standard quantity to make most of the compensation for the tested parts, and the difference detection effect is better, which can reduce the measurement error.


A potentiometer is a precision instrument that uses the compensation principle to directly measure the electromotive force (EMF) or potential difference in electromagnetic measurement. Its outstanding advantage is that it does not absorb any energy from the circuit under test when measuring electricity, and does not affect the state and parameters of the circuit under test, so it is widely used in measurement work and high-precision measurement.


The DC voltage measurement error can be less than ±0.005%. It can be used for accurate measurement of electromotive force and voltage, it can also be used to accurately measure current, resistance and power with standard resistance, and it can also be used to calibrate direct-reading instruments such as precision instruments and DC bridges. For some electrical factories, it is not only used for DC circuits and AC circuits, but also for determining product accuracy and calibration. Therefore, it has been widely used in industrial measurement automatic control systems.


Ⅱ Operational Principle

2.1 Compensation Principle

In a DC circuit, the electromotive force(EMF) of the power supply is numerically equal to the terminal voltage of the two electrodes when the power supply is open. Therefore, when measuring, it is required that no current flows through the power source, and the terminal voltage of the measured power source is the electromotive force of the power source.


However, if you directly use a voltmeter to measure the terminal voltage of the power supply during measurement, the voltmeter will always have current flowing through it and generate a reaction voltage. The power supply has an internal resistance, so it is impossible to obtain an accurate electromotive force value, so the measured potential difference is always smaller than the true value of the potential difference. In order to accurately measure the potential difference, the current shunted to the measurement branch must be equal to zero, so a DC potentiometer is designed.


The principle of compensation is to use an EMF to offset another one. The principle can be illustrated in Figure 1. Two power sources E and a positive pair (positive to positive), wherein E is the adjustable standard power source EMF, while EX is an unknown power supply electromotive force, and a galvanometer G is connected in series to form a closed loop. If you want to measure the EMF of EX, you can adjust the power supply E to make the galvanometer reading zero, there is no current in the circuit. At this time, it indicates that EX=E, the potential difference between EX two ends and the potential difference between E's two ends compensate each other, meanwhile, the circuit is in compensation status. If the value of E in the compensation state is known, it can be determined the value EX the method applying this compensation principle under such situation is called a compensation method, and the circuit is called a compensation circuit.

 Compensation Circuit

Figure 2. Compensation Circuit

2.2 Potentiometer Principle

The experimental device that measures the potential difference according to the compensation method is called a potentiometer, and its measurement principle can be explained in Figure 2 and Figure 3 respectively.


Figure 2 is a schematic diagram of potentiometer calibration, where ABCD is an auxiliary working circuit. The power supply E, the current limiting resistor R, and the uniform resistance line AB are connected in series to form a closed-loop; MN is the compensation circuit, which is composed of the power supply En to be tested and the galvanometer G.


The resistance box R is used to adjust the working current I of the loop. By adjusting I, the potential difference V0 on the resistance wire per unit length can be adjusted. M and N are two movable contact resistance wires AB, the movable resistance wires, take appropriate potential difference AB to compensate the potential difference on the measurement branch, which is equivalent to the compensation circuit (Figure 1), in other words, the power supply provides variable.


When the loop is connected, according to Ohm's law, the voltage between any two points on the resistance wire AB is proportional to the distance between the two points. Therefore, by changing the spacing of MN, the reading of the ammeter G is zero, and the voltage between the two points of MN is equal to the measured electromotive force. To measure the electromotive force (potential difference) EX, it must be done in two steps:


(1) Potentiometer Standardization

Use the high accuracy of the standard power supply En, making the current I in the working circuit can accurately reach a certain calibration value I0. This adjustment process is called standardization of the potentiometer, also called the calibration of the potentiometer.

Principle Diagram of the Potentiometer Calibration 

Figure 3. Principle Diagram of the Potentiometer Calibration


According to the calibration principle and wires connection of Figure 2, move the sliding contacts M and N, fix the distance length between M and N on the Lmn, and adjust the resistor R in the working circuit to make the calibration circuit in the compensation loop reach balance, that is, the current flowing through the galvanometer G is zero, at this time:

During operation, the working current in the ABCD remains the same, because the RAB is a uniform resistance wire, having   ①, bring   ②


It is obvious that the voltage drop per unit length on the resistance wire is called the normalization coefficient of the operating current, and the unit is V/m. In actual operation, as long as V0 is determined, the calibration process is completed.


It can be seen from Equation ② that when the V0 is a constant current (ie, the operating current in ABCD remains unchanged), the length Lmn (mechanical quantity) between the two points of the resistance wire MN can be used to reflect the EMF Ex (electrical quantity) to be measured. For this, the V0 value must be determined. For the convenience of reading, take V0 = 1.0V/m. SinceEquation, and the resistance of the resistance wire is stable, thus the required value can be obtained only by adjusting the operating current in the ABCD.


(2) Measuring Unknown EMF

The process of measuring the EMF to be measured is exactly the opposite of the process of calibration.

When the above calibration process is completed, according to the connection of Figure 3, adjust the distance  between M' and N', so that the potential difference between the two points M' and N' is equal to the EMF to be measured, and the compensation is done. At this time, The current flows of the galvanometer G are zero, which is

Potentiometer Measurement Schematic 

Figure 4. Potentiometer Measurement Schematic

Connecting with equation ②, it flows that 9.png

Therefore, Ex to be measured can get.

2.3 Experiment

  • purpose

1. Acquire the basic working principle and structural characteristics of the DC potentiometer.

2. Know the principle and method of measuring electromotive force and voltage by learning the compensation method.

3. Measure the electromotive force and internal resistance of a battery with a DC potentiometer.

  • Experiment instruments

87-1 student potentiometer, standard battery, battery to be tested, galvanometer, DC stabilized power supply, resistor box

  • Experiment principle

1. compensation method

compensation method

The current is zero and the electromotive force is adjustable.

schematic diagramadjust the E0, having Ig=0, E0=Ex


2. Working principle of box potentiometer

schematic diagramt=20 ℃, Es=1.0186 V

1) Standardize the instrument first and determine the working current.

2) Measure the electromotive force by the compensation method.


3. components connection

schematic diagram

  • Experimental content

1) Measure EMF 5 times of dry battery

Ex=1.5 V    dry battery  

E=4.0 V  working voltage  

Es=1.0186 V     standard battery

Rb: resistor box, act as a current limit

Setting parameters













Es=1.0186 V





Ex=1.499 V

2) Measure the internal resistance of the dry battery for the r-I curve

schematic diagram

According to the full circuit Ohm's law:


The internal resistance r of the power supply has the following relationship with the electromotive force Ex and the terminal voltage U of the power source:

The internal resistance of the dry battery changes with the output current and the battery power consumption, making the r-I curve

Record measured data in this table

R / W








U / V


I / A


r / W


87-1 type potentiometer block diagram

87-1 type potentiometer block diagram


Ⅲ Potentiometer Types

1. According to different methods of obtaining variable standard voltage, it is divided into fixed-resistance variable flow type (Figure 4) and constant current variable resistance type (Figure 5).

(1) Constant Resistance Variable Flow Measurement Process

When the switch S is closed, the current I0=I1+I2+…+In,Us=I0R, adjusting R causes the galvanometer P to be zero, so Ux=Us=I0R is measured. However, the accuracy of the ammeter limits the accuracy of the potentiometer.

Constant Resistance Variable Current Potentiometer Principle Circuit 

Figure 5. Constant Resistance Variable Current Potentiometer Principle Circuit


(2) Constant Flow Resistance Measurement Process

S is set at 1, and R0 is adjusted so that the galvanometer P is zero, I0=En/Rn; S is set to 2, and R is adjusted so that P is zero again so that Ux=I0R’=R’/Rn×En is measured. The accuracy is determined by the standard battery and resistance accuracy, which is better performed.

Principle Circuit of Constant Rheological Resistance Potentiometer 

Figure 6. Principle Circuit of Constant Rheological Resistance Potentiometer


2. According to the high and low output voltage of the tested terminal voltage, it is divided into high-resistance potentiometer (output resistance is greater than 10KΩ /V, used to measure the voltage of large resistance and the EMF of high internal resistance power supply, the working current is small, no large capacity is needed operating power supply.) and low resistance potentiometer (output resistance is less than 100Ω /V, used to measure the voltage of small resistor and the EMF of low internal resistance power supply, the working current is large, which should be powered by large-capacity power supply).


3. According to the measuring range, it is divided into high-voltage potentiometer (measurement upper limit is 2V, output resistance up to 2×204Ω, working current I0=0.1mA) and low-voltage potentiometer (measurement upper limit is 20mV, output resistance is 20Ω, and working current I0=1mA).


4. According to the applying conditions, it is divided into laboratory type and portability type.


Ⅳ Advantages and Disadvantages

The accuracy of the DC potentiometer measurement is mainly dependent on the following factors:

(1) Accuracy and thickness uniformity of each length of the resistance wire.

(2) Standard power supply accuracy.

(3) Galvanometer sensitivity.

(4) Operating current stability.


• Advantages

Measuring the potential difference with a potentiometer has the following advantages:

1. High accuracy. Rely on standard resistance, galvanometer, and standard power supply. If the resistance wire RAB is very uniform and accurate, the electromotive force of the standard power supply is accurate and stable. The galvanometer is very sensitive and can be used as a standard instrument to verify the meter.


2. Wide measurement range, high sensitivity. Measuring small changes in voltage can be done easily.


3. High "internal resistance", but does not affect the circuit under test. It avoids the disadvantage that the voltmeter always shunts from the circuit under test when measuring the potential difference. Due to the principle of potential compensation, the original state of the circuit under test is not affected during the measurement process. When measuring the voltage with a voltmeter, a part of the current is always separated from the circuit under test, thereby changing the original state of the circuit to be tested. The lower the internal resistance of the voltmeter, the greater the effect. When measurement with a potentiometer, the current in the compensation loop is zero (of course it is not absolute, the higher the sensitivity of the galvanometer, the closer to zero), thus the impact of the circuit to be tested is negligible.


• Disadvantages

In the measurement process, the potentiometer is easy to change (such as the auxiliary loop power supply E is unstable, the variable resistor R is changed, etc.), in order to ensure the normalization of the working current during measurement, each measurement must have two basic steps, calibrated and measured. And every time compensation is achieved, fine adjustments must be made, so the operation is cumbersome and time-consuming.


Ⅴ Potentiometer Application

The advantages of the potentiometer make it widely used in measuring voltage with high precision.


1. Measuring various electromotive forces, especially small ones

For example, the thermoelectric potential of a thermocouple, the electromotive forces of various electrolytes and electrodes, the Hall electromotive force of a Hall element, and so forth.


2. Measuring Resistance

The standard resistor Rs is connected in series with the unknown resistor Rx, and the Us and Ux are measured by the potentiometer.

Potentiometer Measures Resistance 

Figure 7. Potentiometer Measures Resistance


3. Measuring Current

String the known standard resistors to make the sampling resistor and measure the voltage across the terminals, bring 11.png

 Potentiometer Measures Current

Figure 8. Potentiometer Measures Current


4. Measuring High Voltage

R1 and R2 form a voltage dividing circuit that adapted to the potential difference measuring limit, and the divider circuit resistance should not be too small. In addition, at practical measurement, DC voltage dividers with different accuracy and voltage division ratio should be available for selection.

Potentiometer Measures High Voltage

Figure 9. Potentiometer Measures High Voltage


5. Measuring Power

When measuring, voltage and current should be measured respectively, the power 13.png


6. Calibrating Voltameter

Another type of electromotive force with adjustable output is adjusted. Connect the voltmeter to both ends of the electromotive force to be tested, adjust the output voltage of the electromotive force to be measured, and record the readings of the DC potentiometer and the voltmeter Ex and V, then ΔV=Ex-V, ΔV-V is the calibration curve of the voltmeter.


7. Calibrating Ammeter

The ammeter to be calibrated is connected in series with a standard resistor. When the ammeter is read as I, the voltage Us on the Rs is measured by the potentiometer, and the current flowing through Rx is 14.png. There is no shunting action on the circuit, so Is is the current flowing through the ammeter, 15.png is the measurement error value of the ammeter.


In practical measurement, it must be noted that:

① the rated current of the standard resistor should be greater than the measured current;

② the voltage drop across the standard resistor should not exceed the upper limit of the potentiometer.

Potentiometer Calibrates Ammeter 

Figure 10. Potentiometer Calibrates Ammeter


Ⅵ Applying Attention

1. Selection of matching instruments for DC potentiometer
They include an operational power supply, galvanometer, standard battery, and so on.


(1) Power Supply. The working battery requires sufficient capacity, which is generally more than one thousand times the working current; or a regulated current with high stability. And it must be ensured that the measurement error caused by the relative change of the working current does not exceed one-tenth of the allowable error of the potentiometer.

(2) Galvanometer. It is the balance indicator of the potentiometer in the standard circuit and measuring circuit. In the measurement process, it plays an important role. Whether it is applied correctly will directly affect the measurement results. It can be reasonably applied according to the following principles:


① Select the sensitivity that is matching the measurement accuracy. Generally, when the line's sensitivity can be satisfied, try not to use a galvanometer with too high a sensitivity.


② Choose to work in a slightly under-damped state. In a typical measuring device, the resistance of the galvanometer circuit is changed as the measured value changes. Therefore, if necessary, a current multi-inverter should be connected, and its resistance should be properly selected so that the galvanometer always works close to the critical damping state.


In short, under the premise of meeting the sensitivity requirements, the galvanometer should work under a slightly under-damped state, and try to use a galvanometer with a shorter period.


(3) Standard Battery. The standard battery means the standard operating current of the potentiometer, requiring a small internal resistance. The allowable error of the accuracy level should be less than 1/5 to 1/10 of the error of the instrument to be inspected, and the annual change value should meet the technical requirements of the corresponding level.


2. In addition to paying attention to the rational selection of the matching instrument of the potentiometer, the following points must be also noted.


(1) The power supply of the potentiometer should be selected according to the working current. When the working current is less than 10mA, it can be powered by a dry battery; when it is greater than 10 mA, it should be powered by a storage battery.


(2) Pay attention to the standard battery, auxiliary power supply, polarity of the measured potential, and the measured value, to avoid connect the wrong polarity, and select the appropriate range.


(3) When using, the working current will change slightly due to various reasons, therefore, during operation, the working current should be checked at any time, and the changes should be calibrated in time.


(4) Do not connect the standard battery to the circuit for a long time. Otherwise, there will be a possibility of continuous charging or discharging the standard battery, resulting in gradual damage to the standard batteries.


Ⅶ Frequently Asked Questions about DC Potentiometer

1. What is a DC potentiometer?

A DC potentiometer is a potentiometer in which the supply is a battery and the balance is under direct current conditions. ... A DC potentiometer can measure voltage directly and measure resistance, current, power, and temperature indirectly.


2. What are the applications of DC potentiometer?

The uses of DC potentiometer are described below:
Measurement of Electric Current
Measurement of Voltage
Measurement of Resistance
Measurement of Electric Power
Calibration of Ammeter
Calibration of Voltmeter
Calibration of Wattmeter


3. What is the working principle of DC potentiometer?

The principle of a potentiometer is that the potential dropped across a segment of a wire of uniform cross-section carrying a constant current is directly proportional to its length. The potentiometer is a simple device used to measure the electrical potentials(or compare the e.m.f of a cell).


4. What is the difference between AC and DC potentiometer?

But there is one major difference between their measurements, DC potentiometer only measures the magnitude of the unknown voltage. Whereas an AC potentiometer measures both the magnitude and phase of unknown voltage by comparing it with a known reference.


5. How is DC potentiometer made direct reading?

The calibration of a potentiometer (i.e., making it direct reading) is called standardisation ofpotentiometer. This potential difference is equal to the e.m.f. of the standard cell. Thus potentiometer wire can be marked in terms of voltages and the instrument becomes direct reading.


You May Also Like

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

Operating Principle of Potentiometer and Method of Trouble Repair

The Function and Connection Method of Potentiometer

The Differences Between Potentiometer and Adjustable Resistor

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Related Articles

pinglun 5 comments

    • pingluntus
    • Joey Chan on 2019/7/26 17:54:04

    Very nice notes.. Easy to understand. You have done a good research work, continue, thank you.

    • pingluntus
    • Lily Chou on 2019/7/26 18:00:17

    Good research article, you should make all of tutorial with pdf files, because we can learn easy with pdf. Thank you.

    • pingluntus
    • Brain Willett on 2019/7/26 18:04:07

    Can a potentiometer be used for 4- 20 mA output? If input voltage is 12v then what will be the value of potentiometer resistance?

    • pingluntus
    • Robert K. Hampton on 2019/7/27 15:52:38

    Potentiometers are variable voltage dividers with a shaft or slide control for setting the division ratio. They are manufactured in panel-mount as well as breadboard (printed-circuit board) mount versions. Any style of potentiometer will suffice for this experiment. I think your article will help a lot for the beginners.

    • pingluntus
    • John Elinoff on 2019/7/27 16:00:57

    Because this is a useful article, I recommend making it into a pdf file as a more convenient referrence. Operating a potentiometer network is more complex than using a single potentiometer.

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