In this tutorial, we are going to learn about Potentiometer.
Slide wire= platinum – silver alloy.
Slide wire contact= copper- gold silver alloy.
Resistors =magnetic material high stability
The principle of an ac potentiometer and dc Potentiometer are identical. The main difference between ac and DC potentiometers is that in ac, the magnitude of the unknown EMF is compared with the standard cell’s EMF. In ac, however, the magnitude as also the phase angle of the unknown voltage is compared in order to attain balance. The potentiometer must be modified to allow for this condition. These are the points that must be taken into consideration to ensure ac potentiometer smooth operation.
- Non-inductive wires and resistance coils of an AC potentiometer are best to avoid reading errors.
- Stay or external magnetic fields can affect the reading. They must be removed or measured at the time of measurement and a correction factor should then be applied.
- Harmonics should not be present in the sources of ac supplies. Otherwise, it may be impossible to achieve a balance.
- As sinusoidal as possible should the ac source be.
- The voltage or current to which the potentiometer circuit is connected should come from the same source.
What is DC Potentiometer?
The switch S must be in the “operate” position. With the galvanometer key open, the battery supplies the working power through the rheostat. You can adjust the setting of the rheostat to alter the working current through the slide wire. Method E1 is measured by finding a position for the slide contact that allows the galvanometer to show zero deflection. This indicates null condition. Zero galvanometer deviation means which is unknown voltage like E1 is equal to the voltage fall E2 across position a-c of the slide wire. It is now possible to determine the unknown voltage values by evaluating the voltage drop along the section a-c.
CROMPTON’ DC POTENTIOMETER
A Laboratory- type Crompton’s DC potentiometer. It is composed of a dial switch with fifteen steps (or more). Each steep has a 10 O resistance. The dial switch therefore has a total of 150 O resistances. This potentiometer has a 10 MA working current, so each dial switch step corresponds to 0.1 volt. The range of the dial switch’s operation is therefore 1.5 volt. The dial switch is connected in series to a circular slide wire. The circular slide wire has a 10 o resistance. The range of the slide wire there is therefore 0.1 volt. Each division of the slide wire corresponds to a voltage drop equal to 0.1 volt.
MEASUREMENT HIGH VOLTAGE BY A POTENTIOMETER
Measurement of high voltage using a potentiometer It is necessary to make special arrangements in order to measure extremely high voltage by the potentiometer (e.g.100 volts), as this voltage is beyond what is possible with a normal potentiometer. A volt – ratio box can be used in conjunction with the potentiometer to measure voltages above the range of the potentiometer, which is generally 1.8 volt. The volt- ratio consists of a simple resistance potentiometer with tapping on the input side. Each terminal is marked with the maximum voltage that can be applied as well as the multiplier factor for the potential scale. The appropriate input terminal of the volt-ratio boxes is used to measure high EMF. The leads to the potentiometer can be taken from the two tap points designated for this purpose.
Measurement Resistance by A Potentiometer
This is the connection diagram to measure unknown resistance using a potentiometer. The unknown resistance, R, is connected in series to the standard resistor S. The circuit’s rheostat controls the current flow. The circuit also has an ammeter to show whether the working current is within the limits of the potentiometer. The working current value is not known unless it is measured. The known resistance is connected with the potentiometer when the double-pole double throw switch in position 1 is turned on. The reading of the potentiometer at that position should be VR.
CALIBRATION OF VOLTMETER BY POTENTIOMETER
The main requirement for calibration of voltmeters is that the voltage supply must be stable. Any change in voltage will affect the calibration process. Two rheostats make up the potential divider network. The coarse control and the fine control of the calibrating voltage are handled by one rheostat. These controls allow you to adjust the supply voltage such that the pointer matches exactly with a major division on the voltmeter. With the help of a volt-ratio box, the voltage across the voltmeter can be lowered to a level suitable for the potentiometer. To get precise measurements, you need to measure voltages within the maximum range of your potentiometer as much as possible. A positive or negative error will be indicated if the reading of your potentiometer is not in line with the reading on the voltmeter. With the help of the voltmeter reading and the potentiometer, a calibration curve can be drawn.
CLIBRATION DURING WATTMETER BY PTENTIOMETER
The current coil of the wattmeter is calibrated by using a low voltage supply. The potential coil is the supplied from the normal supply via potential divider. The potentiometer measures the voltage V across the potential of the wattmeter. The voltage drop across the current coil is standard resistor by the current through the coil. VI is the true power, where V is voltage across the current coil. This value can be used to compare the wattmeter reading, and a calibration curve could be drawn.
Advantages of Potentiometer
- Highly accurate
- No power loss in balance cond.
- Voltage measurement is independent of source resistance.
Basic Potentiometer Principle
- Measurement of unknown EMF.
- Its working current is never change.