Alternating current bridges are of outstanding importance for measurement of electrical quantities. Measurement of inductance, capacitance etc may be made conveniently and accurately by employing AC bridge networks. An AC bridge is an improved version of the Wheatstone bridge and consists of a source of excitation and a detector sensitive to small alternating potential differences. The Schering Bridge is one such type of AC bridge used for the measurement of capacitances.
The Schering bridge unlike the other bridges consists of four arms. One arm has a range selection is provided so as to select the correct point of balance of the bridge. This arm provides the point to select the range between which the bridge can be balanced. The other arm consists of a fixed capacitor connected in parallel to a variable resistance which is used as one of the variable arm of the bridge. The third arm has a standard variable resistance which is varied along with the other variable resistance to obtain exact balanced point. The circuit diagram and the phasor diagram of the bridge are as shown below,
The balance condition for the bridge is as shown below
Let C1 = Capacitor whose capacitance is to be measured
r1 = a series resistance representing the loss in the capacitor C1
C2 = a standard capacitor
R3 = a non - inductive resistance
C4 = a variable capacitor
R4 = a variable non-inductive resistance in parallel with variable capacitor C4
Therefore now at balanced condition,
Two independent balance equations are obtained if C4 and R4 are chosen as the variable elements. Dissipation factor
D =tan1* =T C r11
Therefore values of capacitance C1 and its dissipation factor are obtained from the values of bridge elements at balance permanently setup Schering bridges are sometimes arranged so that balancing is done by adjustment of R2 and C4 with C2 and R4 remaining fixed. Since R3 appears in both the balance equations and therefore there is some difficulty in obtaining balance but that will have same advantages as explained below.
The equation of the capacitance
and since R & C are fixed the dial resistor R3 may be calibrated to read the capacitance directly.42Dissipation factor D =1TC R and in case the frequency is fixed, the dial capacitor C4 can be44calibrated to read the dissipation factor directly. It should however be understood that the calibration for dissipation factor holds good for one particular frequency, but may be used at another frequency if correction is made by multiplying by the ratio of frequencies
The unit trainer has an inbuilt 1 KHz oscillator circuit for giving input to the bridge
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