Measurement of capacitance using bridges.pptx

Measurement of capacitance using bridges

1. Measurement of capacitance By Dr. Yogesh Y.Pundlik

2. Schering Bridge The  Schering Bridge  is an electrical circuit used for measuring the insulating properties of electrical cables and equipment. It is an AC  bridge  circuit, developed by Harald   Schering . It has the advantage that the balance equation is independent of frequency.

3. Low Voltage Schering Bridge

4. Phasor Diagram

5. High Voltage Schering Bridge

6. De Sauty’s Bridge De Sauty Bridge  measures an unknown capacitance in term of a standard capacitance i.e. comparing two capacitance's Two ratio arm of this  bridge  consist pure resistor and two other arms consist capacitors where one is of unknown value and another is standard capacitor.

7. De Sauty’s Bridge

8. Balance Condition

9. Modified De Sauty’s Bridge

10. Phasor Diagram

11. Wein’s Bridge The Wien’s bridge use in AC circuits for determining the value of unknown frequency. The bridge measures the frequencies from 100Hz to 100kHz. The accuracy of the bridges lies between 0.1 to 0.5 percent. The bridge is used for various other applications like  capacitance  measurement, harmonic distortion analyser and in the HF frequency oscillator.

12. Wein’s Bridge

13. Wein’s Bridge The Wien’s bridge is frequency sensitive. Thereby, it is difficult to obtain the balance point in it . The input supply voltage is not purely sinusoidal, and they have some harmonics . The harmonics of the supply voltage disturbs the balance condition of the bridge. To overcome this problem the filter is used in the bridge. The filter is connected in series with the null detector.

14. Wein’s Bridge When the bridge is in the balanced condition, the potential of the node B and C are equal, i.e., the E 1  = E 2  and E 3  = E 4   The phase and the magnitude of E 3  = I 1 R 3  and E 4  = I 2 R 4  are equal, and they are overlapping each other. The current I 1  flowing through the arm BD and the current I 2  flowing through R 4  is also in phase along with the I 1 R 3  and I 2 R 4 .

15. Wein’s Bridge The total voltage drop across the arm AC is equal to the sum of the voltage drop I 2 R 2  across the resistance R 2  and the capacitive drop I 2 /wC 2  across the capacitance C 2 . When the bridge is in a balanced condition, the voltage E 1  and E 2  both are equals in magnitude and phase . The phase of the voltage E 1  and the voltage drop I R R 1  across the arms R 1  is also same. The resistance R 1  is in the same phase as that of the voltage E 1 . The phasor sum of E 1  and E 3  or  E 2  and E 4  will give the resultant supply voltage.

16. Wein’s Bridge At balance condition,  On equating the real part,

17. Wein’s Bridge On comparing the imaginary part, By substituting the value of ω = 2πf, Note: The slider of the  resistance  R 1  and R 2  mechanically connect to each other. So that, the R 1  = R 2  obtained.

18. Applications of Wein’s Bridge The bridge is used for measuring frequency in the audio range. Resistances R1 and R3 can be ganged together to have identical values. Capacitors C1 and C3 are normally of fixed values The audio range is normally divided into 20 - 200 - 2 k - 20 kHz range In this case, the resistances can be used for range changing and capacitors, and C3 for fine frequency control within the range.  The bridge can also be use for measuring capacitance. In that case, the frequency of operation must be known. The bridge is also used in a harmonic distortion analyzer, as a Notch filter, an in audio frequency and radio frequency oscillators as a frequency determine element. An accuracy of 0.5% - 1% can be readily obtained using this bridge. Because it is frequency sensitive, it is difficult to balance unless the waveform of the applied voltage is purely sinusoidal.