Schering Bridge Construction and Theory

Schering Bridge is very important capacitance measuring bridge circuit. It is also capable of measuring the dielectric loss of the capacitor in addition to its capacitance value. It compares the capacitance of an imperfect capacitor in term of the capacitance of a standard capacitor. We can represent an imperfect capacitor with its loss-free capacitance (here it is C2) in series with a resistance (here it is r) as shown in the diagram of Schering Bridge.

We normally use this type of bridge circuit for measuring capacitance at a very high voltage level.

Construction of Schering Bridge

Before going to the actual theory of Schering Bridge, let us draw its basic circuit diagram.

Schering Bridge
Schering Bridge

This is essentially an ac bridge with four arms, AB, BC, CD, and DA. According to our Schering Bridge diagram, AB consists of a standard capacitor of capacitance C1. The arm BC consists of the capacitor under measurement of capacitance C2 with resistance r in series and the arm CD consists of a resistor of known resistance R3. The arm AD consists of the parallel of a resistance R4 and a capacitance C4. Now, we will apply the alternating voltage between node B and D, as shown in the circuit diagram. Lastly, we will make the node D grounded for better safety of the operator since it is a high voltage apparatus.

Theory of Schering Bridge

Since this bridge is an ac bridge, we can easily compare it with standard ac bridge. The figure below shows the general arrangement of a standard ac bridge.

AC Bridge
AC Bridge

At balanced condition, there would not be any voltage difference across the detector of the bridge. And at that condition, we can write,

Now by comparing, we can find out the impedance of the ac bridge in terms of impedances of the Schering Bridge.

So, after balancing the bridge circuit, we can write

By equating real and imaginary part separately with zero, we get,

The Schering bridge is able to determine the quality of the capacitor in terms of its dissipation factor. Pure capacitor leads the current by 90°. But an imperfect capacitor leads the current with a phase angle less than 90°. The difference of the angle from 90° is referred to as defect angle of the capacitor. The tangent of the defect angle is called the dissipation factor of the capacitor. In other words, the ratio of the resistive component and the reactive component of a capacitor is the dissipation factor. Hence, for this capacitor under measurement, we can write

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