Potentiometer

DC Potentiometer

The potentiometer measures voltage or potential difference by comparing an unknown voltage with a 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 the calibration of the 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:

Construction 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 the 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 ammeter
• Calibration of wattmeter

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 in very handy in calibrating the voltmeter, ammeter, etc.
• The accuracy of the potentiometer makes it a very reliable instrument for measurements.

AC Potentiometer

A potentiometer is an instrument which measures an unknown voltage by balancing it with a known voltage. The known source may be DC or AC. The working phenomenon of the DC potentiometer and AC potentiometer is the same. But there is one major difference between their measurements, the 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. 

There are two types of AC potentiometers:

1. Polar-type potentiometer.
2. Coordinate type potentiometer.

Polar type Potentiometer

In such types of instruments, two separate scales are used to measure magnitude and phase angle on some reference of the unknown e.m.f. There is a provision on the scale that it could read phase angles up to 3600. It has an electrodynamometer-type ammeter along with a DC potentiometer and phase-shifting transformer which is operated by a single-phase supply.


In a phase-shifting transformer, there is a combination of two ring-shaped laminated steel stators connected perpendicularly to each other as shown in the figure. One is directly connected to the power supply and the other one is connected in series with variable resistance and capacitor. The function of the series components is to maintain a constant AC supply in the potentiometer by doing small adjustments in it.

Between the stators, there is a laminated rotor having slots and winding which supplies voltage to the slide-wire circuit of the potentiometer. When current start flowing from stators, the rotating field is developed around the rotor which induces an e.m.f. in the rotor winding.

The phase displacement of the rotor emf is equal to the rotor movement angle from its original position and it is related to the stator supply voltage. The whole arrangement of the winding is done in such a way that the magnitude of the induced emf in the rotor may change but it does not affect the phase angle and it can be read on the scale fixed on the top of the instrument.


The induced emf in rotor winding by the stator winding 1 can be expressed as


The induced emf in the rotor winding by the stator winding 2,


From equations (1) and (2), we get


Therefore, the resultant induced emf in the rotor winding due to two stator winding



Where, Ø gives the phase angle.

Coordinate type Potentiometer

In the coordinate AC potentiometer, two separate potentiometers are caged in one circuit as shown in the figure. The first one is named as the in-phase potentiometer which is used to measure the in-phase factor of an unknown e.m.f. and the other one is named as quadrature potentiometer which measures the quadrature part of the unknown e.m.f. the sliding contact AA’ in the in-phase potentiometer and BB’ in the quadrature potentiometer are used for obtaining the desired current in the circuit. By adjusting rheostat R and R’ and sliding contacts, the current in the quadrature potentiometer becomes equal to the current in the in-phase potentiometer and a variable galvanometer shows the null value. S1 and S2 are signs-changing switches which are used to change the polarity of the test voltage if it is required for balancing the potentiometer. There are two step-down transformers T1 and T2 which isolate the potentiometer from the line and give earthed screens protection between the winding. It also supplies 6 volts to potentiometers.

Now to measure unknown e.m.f. its terminals are connected across sliding contacts AA’ using selector switch S3. By doing some adjustments in sliding contacts and rheostat, the whole circuit gets balanced and the galvanometer reads zero at the balanced condition. Now the in-phase component VA of the unknown e.m.f. is obtained from the in-phase potentiometer and quadrature component VB is obtained from the quadrature potentiometer.

Thus, the resultant voltage of the coordinate AC potentiometer is



And the phase angle is given by

Applications of AC Potentiometer

• Measurement of self-inductance.
• Calibration of voltmeter.
• Calibration of Ammeter.
• Calibration of watt meter.