Measurement of Capacitance using Schering Bridge:

A very important bridge used for the precision measurement of capacitors and their insulating properties is the Schering Bridge Experiment. Schering Bridge basic circuit arrangement is given in Fig. 11.25. The standard capacitor C₃ is a high quality mica capacitor (low-loss) for general measurements, or an air capacitor (having a very stable value and a very small electric field) for insulation measurement.

For balance, the general equation is

where

as

Equating the real and imaginary terms, we get

and

The dial of capacitor C₁ can be calibrated directly to give the dissipation factor at a particular frequency.

The dissipation factor D of a series RC circuit is defined as the contangent of the phase angle.

Also, D is the reciprocal of the quality factor Q, i.e. D = 1/Q. D indi­cates the quality of the capacitor.

Commercial units measure from 100 pf – 1 μf with ± 2% accuracy. The dial of C₃ is graduated in terms of direct readings for C_x, if the resistance ratio is maintained at a fixed value.

This Schering Bridge is widely used for testing small capacitors at low voltages with very high precision.

The lower junction of the bridge is grounded. At the frequency normally used on this bridge, the reactances of capacitor C₃ and C_x are much higher than the resistances of R₁ and R₂. Hence, most of the voltage drops across C₃ and C_x and very little across R₁ and R₂. Hence if the junction of R₁ and R₂ is grounded, the detector is effectively at ground potential. This reduces any stray-capacitance effect, and makes the bridge more stable.