Working principle of DC potentiometer

A potentiometer is one of the most fundamental instruments of electrical instruments. The potentiometer measures voltage or potential difference by comparing an unknown voltage with a known voltage. Unlike the AC potentiometer, the DC potentiometer compares only the magnitude of unknown voltage with the known voltage.

A DC potentiometer can be used to measure the current, resistance, and power. If time can be measured then one can easily calculate the energy by multiplying the power with the time by the DC potentiometer. The potentiometer is also used for calibration of voltmeter, ammeter, and wattmeter.

Working principle of dc potentiometer

The working principle slide wire dc potentiometer is the same as the ac potentiometer, where the dc potentiometer compares the voltage of an unknown source with a source whose voltage is known. The circuit diagram is shown below:

circuit diagram of dc potentiometer.

The switch S is connected to calibrate position, which connects the standard cell to the circuit. This standard cell helps in standardizing the potentiometer.

Now the switch S is put in the operating position and the galvanometer key is kept open. The battery connected in series with the rheostat supplies the working current. The unknown voltage E1 is measured by finding a position for the sliding contact such that the galvanometer shows zero deflection.

The zero deflection represents that the unknown voltage E1 is equal to the voltage drop E2

Cromptons dc potentiometer

Cromptons potentiometer is a laboratory-type dc potentiometer. The working principle of dc slide wire potentiometer and Crompton’s dc potentiometer is the same. It is capable of measuring unknown voltage/emf to great precision.

Cromptons potentiometer consists of a dial switch with calibrated resistor that is divided into 15 equal steps. Resistance of each step is 10ohms. And it also consists of a small slide wire.

Advantages

  • A potentiometer is highly sensitive
  • It is a highly accurate instrument because it uses the comparing method for measurements, where the voltage of a reference source is known
  • It has a wide range of measurement

Disadvantages

  • Its operation is very time consuming

Applications of dc potentiometer

Calibration of voltmeter

Calibration of voltmeter requires a suitable stable DC supply of voltage. If there are little change occurs in the supply voltage, it can affect the calibration process of the voltmeter.

Arrangement for calibration of a voltmeter by potentiometer is shown below:

Circuit diagram for calibrating voltmeter by dc potentiometer.
1. voltage-ratio box is connected to a voltmeter, which is placed for calibration.
2. a stabilized dc supply is connected

The above network consists of a potential divider network consisting of 2 rheostats. One is for coarse adjustments and the other is for fine adjustments. Both the rheostats are connected to the stabilized supply voltage. With the help of these rheostats, it is possible to adjust the supply voltage so that the pointer coincides exactly with a major division of the voltmeter.

The voltage across the voltmeter is stepped down to a suitable value with the help of a voltage-ratio box. For accuracy of measurements, it is required to measure voltage near the maximum range of the potentiometer.

And if the potentiometer reading does not match with the reading of the voltmeter. A positive or a negative error is indicated.

Calibration of ammeter

A standard resistor S with a high current carrying capacity is connected in series with an ammeter for a test. The voltage drop across the standard resistor is measured by the potentiometer.

Circuit diagram for calibration of an ammeter by potentiometer is shown below:

Calibration of ammeter by the potentiometer. This is the application of dc potentiometer

Now the current through the resistor S can be computed

I = Vs/S

Where,

Vs = voltage drop across the resistor S         

S = resistance of the resistor

Now, by comparing the ammeter reading with the current found by calculation, a positive or negative error can be indicated if they do not match.

This method of calibration is very accurate because the resistance of the resistor S is exactly known and the current across the S is calculated.

Calibration of wattmeter

A standard resistor is connected in series with a current coil of wattmeter. The current coil is supplied with a low voltage current and the current through the current coil is measured by measuring the voltage drop across the standard resistor divided by the value of the standard resistor.

The potential coil of the wattmeter is supplied from normal supply through the potential divider. The voltage across the potential coil is measured directly by the potentiometer.

Then the power is calculated by

P = VI

Where,

V = voltage across the potential coil

I = current through the current coil of the wattmeter

Now, the wattmeter reading can be compared with the calculated value.

Measurement of current

The unknown current I,  whose value is to be measured is passed through a resistor R. The value of the resistor is such that voltage drop across it may not exceed the range of the potentiometer.

The circuit diagram of current measurement by potentiometer is shown below:

Measurement of current by the potentiometer. This is the application of dc potentiometer

So, the value of the unknown current is the voltage drop across the resistor divided by the value of the resistor.

I = V/R

Measurement of resistance

An unknown resistance is connected in series with a standard resistance S. A rheostat controls the current in the circuit. A two-pole double throw switch is also used in the circuit. The circuit is shown below:

Measurement of resistance by the potentiometer. This is the application of dc potentiometer

When the two poles double throw switch is put in position 1, the unknown resistance is connected to the potentiometer.

Let the reading of the potentiometer be VR

VR = IR                              (i)

Now the switch is put in position 2, this connects the standard resistor S to the potentiometer.

Let the reading of potentiometer be Vs

Vs = IS                             (ii)

From i and ii

VR/Vs = IR/IS

R = (VR/Vs)*S

The value of R is calculated accurately.

Measurement of power

In the measurement of power, 2 measurements are made. One is across the resistor S connected in series with the load and the other is across the output terminals of voltage—ratio box.

The load current is calculated from the voltage drop across the standard resistor

Measurement of power by the potentiometer. This is the application of dc potentiometer
Power measurement by potentiometer

The voltage drop across the load is calculated by the potentiometer reading across the output terminal of the voltage-ratio box

Load current I = VS/S

Where VS is the voltage drop across the standard resistor

The voltage drop across the load Vl = kVR

Where, k = multiplying factor of the voltage-ratio box.

So, the power consumed, P = VLI

P = K*VR*(VS/S)

Quick summary

  • A dc potentiometer works on the principle of comparing an unknown voltage with a known voltage for measurements. It measures only the magnitude of the unknown voltage.
  • DC potentiometer is used to measure the current, resistance, etc. and it comes very handy in calibrating the voltmeter, ammeter, etc.
  • The accuracy of the potentiometer makes it a very reliable instrument for measurements.

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