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Schering Bridge Measurement of Capacitance Using Schering Bridge _ Electrical4u
Schering Bridge Measurement of Capacitance Using Schering Bridge _ Electrical4u
April 3, 2018 | Author: riddler_007 | Category:
Capacitor
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Electrical Impedance
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Permittivity
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Series And Parallel Circuits
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Electronic Engineering
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7/24/2015Schering Bridge Measurement of Capacitance using Schering Bridge | Electrical4u electrical4u.com Online Electrical Engineering SHOW MENU SHOW RELATED TOPICS Schering Bridge Measurement of Capacitance using Schering Bridge Under Measurement Search Schering Bridge Theory This bridge is used to measure to the capacitance of the capacitor, dissipation factor and measurement of relative permittivity. Let us consider the circuit of Schering bridge as shown below: http://www.electrical4u.com/scheringbridgemeasurementofcapacitanceusingscheringbridge/ 1/8 http://www. r3 is a pure resistor (i. Now the supply is given to the bridge between the points a and c. The detector is connected between b and d.com/scheringbridgemeasurementofcapacitanceusingscheringbridge/ 2/8 . c4 is a variable capacitor. c2 is a standard capacitor. non inductive in nature). c1 is the unknown capacitance whose value is to be determined with series electrical resistance r1. From the theory of ac bridges we have at balance condition.7/24/2015 Schering Bridge Measurement of Capacitance using Schering Bridge | Electrical4u Here.e.electrical4u. And r4 is a variable non inductive resistor connected in parallel with variable capacitor c4. z2. z3 and z4 in the above equation.com/scheringbridgemeasurementofcapacitanceusingscheringbridge/ 3/8 . we get Equating the real and imaginary parts and the separating we get.electrical4u. http://www.7/24/2015 Schering Bridge Measurement of Capacitance using Schering Bridge | Electrical4u Substituting the values of z1. e4 and e2 respectively. From the above Schering bridge phasor diagram.7/24/2015 Schering Bridge Measurement of Capacitance using Schering Bridge | Electrical4u Let us consider the phasor diagram of the above Shering bridge circuit and mark the voltage drops across ab. we can calculate the value of tanδ which is also called the dissipation factor.com/scheringbridgemeasurementofcapacitanceusingscheringbridge/ 4/8 . for the measurement of small capacitance we need to apply high voltage and high frequency as compare to low voltage which suffers many disadvantages. The equation that we have derived above is quite simple and the dissipation factor can be calculated easily. What is the need of high voltage schering bridge? The answer to this question is very simple. e3. Let us discuss more features of this high voltage Schering bridge: http://www.electrical4u. As we have discussed that simple schering bridge (which uses low voltages) is used for measuring dissipation factor.bc. capacitance and measurement of other properties of insulating materials like insulating oil etc.cd and ad as e1. Now we are going to discuss high voltage Schering bride in detail. Earthed screens are placed http://www. The point c is earthed.com/scheringbridgemeasurementofcapacitanceusingscheringbridge/ 5/8 . (d) The fixed standard capacitor c2 has compressed gas which works as dielectric therefore dissipation factor can be taken as zero for compressed air. so that the voltage across bc and dc are few volts above the point c.7/24/2015 Schering Bridge Measurement of Capacitance using Schering Bridge | Electrical4u (a) The bridge arms ab and ad consists of only capacitors as shown the bridge given below and impedances of these two arms are quite large as compared to the impedances of bc and cd. The arms bc and cd contains resistor r3 and parallel combination of capacitor c4 and resistor r4 respectively. (b) The high voltage supply is obtained from a transformer 50 Hz and the detector in this bridge is a vibration galvanometer. As impedances of bc and cd are quite small therefore drop across bc and cd is small.electrical4u. (c) The impedances of arms ab and ad very are large therefore this circuit draws low current hence power loss is low but due to this low current we need a very sensitive detector to detect this low current. And from this measured value of capacitance relative permittivity can calculated easily by using the very simple relation: Where r is relative permeability. Let us study how Schering bridge measures relative permittivity: In order measure the relative permittivity. c is the capacitance with specimen as dielectric.7/24/2015 Schering Bridge Measurement of Capacitance using Schering Bridge | Electrical4u between high and low arms of the bridge to prevent errors caused due to inter capacitance. http://www. There is another way to calculate relative permittivity of the specimen by changing electrode spacing. and ε is permittivity of free space. Let us consider diagram shown below Here A is the area of electrode.com/scheringbridgemeasurementofcapacitanceusingscheringbridge/ 6/8 . d is the spacing between the electrodes. A is the net area of electrodes. we need to first measure capacitance of a small capacitor with specimen as dielectric.electrical4u. Enter your suggestion/comment here Name : Enter your name here Email : Enter your email here Location : Enter your location here Suggestion / Comment : http://www.com/scheringbridgemeasurementofcapacitanceusingscheringbridge/ 7/8 . c is the effective combination of cs and co. cs is the capacitance of specimen. we will get the final expression for of εr as: Like Share 27. t is the gap between the electrode and specimen (here this gap is filled by compressed gas or air).7/24/2015 Schering Bridge Measurement of Capacitance using Schering Bridge | Electrical4u d is the thickness of the specimen. as two capacitors are connected in series. the expression for capacitance reduces to On equating (1) and (2). when we remove specimen and the spacing readjusted to have same value of capacitance. co is capacitance due to spacing between electrode and specimen.electrical4u. εo is permittivity of free space. From figure above.435 people like this. εr is relative permittivity. 7/24/2015 Schering Bridge Measurement of Capacitance using Schering Bridge | Electrical4u Enter your comments/suggestion here Enter the above code here Regenerate Submit Reset © 20112015 electrical4u.The content is copyrighted to electrical4u and may not be reproduced on other websites. http://www.com/scheringbridgemeasurementofcapacitanceusingscheringbridge/ 8/8 .electrical4u.
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