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May 20 2019

What is a DC Potentiometer? Working Principle and Applications

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

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Article Core

DC Potentiometer

Introduction

Potentiometer Basics

Operational Principle

1. Compensation Principle

2. Potentiometer Principle

   (1) Potentiometer Standardization

   (2) Measuring Unknown EMF Ex

3. Experiment

Classification

1. According to Different Methods of Obtaining Variable Standard Voltage

2. According to the High and Low Output Voltage of the Tested Terminal Voltage

3. According to the Measuring Range

4. According to the Applying Conditions

Advantages and Disadvantages

Application

1. Measuring EMF

2. Measuring Resistance

3. Measuring Current

4. Measuring High Voltage

5. Measuring Power

6. Calibrating Voltameter

7. Calibrating Ammeter

Applying Attention


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 supply to be measured, if there is a potential difference (in other words, a voltage), a current is generated in the coil, which interacts with the magnet causing rotation, and the value read by the pointer on the dial is the potential difference.

potentiometer

The comparative meter is a meter that compares the measured and known standard quantities and finally determines the measured values, and is divided into a compensation measuring instrument and a bridge measuring instrument. Known standard quantities used to compare with measured values have standard batteries, standard resistors, standard capacitors, and standard inductors. In addition,the comparator uses a galvanometer to achieve zero balance. Generally speaking, the measurement process of a comparative measuring instrument is a process of gradually reducing the difference obtained by comparison to zero by adjusting the alternative adjusting elements. In fact,this method is more accurate than direct reading measurements.

The compensation method is a basic method of electromagnetic measurement. And the compensation method measuring instrument has two kinds of compensation ways, full compensation method and difference compensation method. The full compensation method compares the measured value(voltage only) with the standard amount, and the detection is 0, and the measured value and standard value are equal. In addition, the measuring instrument theoretically does not draw energy from the measured object. The difference compensation is to use the standard quantity to be mostly compensated for the measured parts, and the differential detection is performed well to 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 to be measured, nor does it affect the state and parameters of the circuit under test when measuring the electrical quantity, so it is widely used in measuring work and high-precision measurement. The measured DC voltage error can be less than ±0.005%. It can be used for accurate measurement of electromotive force and voltage, and can also accurately measure current, resistance and power with standard resistors, also it can also be used to calibrate direct reading instruments such as precision meters and DC bridges. For some electrical instrument factories, it is used to determine the accuracy and calibration of the product, not only for DC circuits but also for AC circuits. Therefore, it is widely used in the automatic control system of the the industrial measurement.


Operational Principle

1. Compensation Principle

In a DC circuit, the power supply EMF is numerically equal to the terminal voltage of the two electrodes when the power supply is in open-circuit. Therefore, when measuring, it is required that no current flows through the power supply, and the terminal voltage of the power supply is measured, that is, the EMF of the power source. However, if the voltmeter is used directly to measure the terminal voltage of the power supply, when measuring, the voltmeter always has a current through the current reaction voltage, and the power supply has an internal resistance, the accurate EMF value cannot be obtained, and the measured potential difference is always less than the true value of the potential difference. In order to accurately measure the potential difference, the current shunted to the measuring branch must be equal to zero, and the DC potentiometer is designed accordingly.

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 pole pair (positive to positive), wherein E is an adjustable standard power source EMF, while EX is an unknown power source electromotive force, and a galvanometer G is connected in series in the middle 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 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 1. Compensation Circuit

2. Potentiometer Principle

The experimental device for measuring the potential difference according to the compensation method is called a potentiometer, and the measurement principle can be explained by using Figure 2 and Figure 3, respectively. Figure 2 is the principle diagram of the potentiometer calibration, in which ABCD is the auxiliary working circuit, the power supply E, the current limiting resistor R, the uniform resistance wire AB are connected in series to form a closed loop; MN is the compensation circuit, which consists of the power supply to be tested En and the galvanometer G. The resistance box R is used to adjust the working current I of the loop, in addition,by adjusting I, the magnitude of the potential difference V0 on the resistance wire per unit length can be adjusted. M and N are the two movable contacts on the resistance wire AB, which can be moved on the resistance wire, to take the appropriate potential difference from AB to compensate for the potential difference on the measuring branch, which is equivalent to En in the compensation circuit (Figure 1), in other words, providing a variable power supply. When the loop is turned on, 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, the spacing of the MN can be changed so that the galvanometer G reading is zero, and the voltage between the two points of the MN is equal to the electromotive force to be measured. To measure the EMF (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 2. 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   

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. Since , 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 the galvanometer G is zero. which is

Potentiometer Measurement Schematic 

Figure 3. Potentiometer Measurement Schematic

Connecting with equation , it flows that 9.png

Therefore, Ex to be measured can get.

Experiment

purpose

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

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


RA

0.1

RB

0.01

RC

0.001


Es

10

1

8.6

Es=1.0186 V

Ex

14

9

9

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

1500

150

75

30

20

15

8

U / V








I / A








r / W









87-1 type potentiometer block diagram

87-1 type potentiometer block diagram


Classification

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+…+InUs=I0R, adjusting R causes the galvanometer P to be zero, so Ux=Us=I0R is measured. However, The accuracy of ammeter limits the accuracy of potentiometer.

Constant Resistance Variable Current Potentiometer Principle Circuit 

Figure 4. 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 5. 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.


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.Relying on standard resistance, galvanometer, standard power supply, if the resistance wire RAB is very uniform and accurate, the EMF of the standard power supply is accurate and stable, and the galvanometer is very sensitive, which can be used as a standard instrument to verify the meter.

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

3. High "internal resistance". But it does not affect the circuit to be tested. 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 to be tested is not affected during the measurement. 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 each time the compensation is reached, fine adjustments must be made, therefore, the operation is cumbersome and time consuming.


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 is measured by the potentiometer.

Potentiometer Measures Resistance 

Figure 6. 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 7. 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 8. 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:

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

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

Potentiometer Calibrates Ammeter 

Figure 9. Potentiometer Calibrates Ammeter


Applying Attention

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

(1) Power Supply. The working battery is required to have sufficient capacity, generally more than one thousand times of the working current; or a regulated current with high stability. And it is necessary to ensure that the measurement error caused by the relative change of the operating 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 lines sensitivity can be satisfied, try not to use a galvanometer with too high 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 multidiverter 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 noted also.

(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 dry battery; when it is greater than 10 mA, it should be powered by 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 continuously charging or discharging the standard battery, resulting in gradual damage to the standard batteries.


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5 comments

    • 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.

    • 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.

    • 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?

    • 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.

    • 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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