Schering Bridge (Measurement of Capacitance)

Schering Bridge (Measurement of Capacitance)

This is the most common bridge used for measurement of unknown capacitance, dielectric loss, relative permittivity and power factor. Figure 2.9 shows the basic circuit arrangement of the bridge and its phasor diagram under balance conditions.

 Two branches consist of non-inductive resistance R₃ and a standard capacitor C₂. The standard capacitor is usually a high-quality mica capacitor (low-loss) for general measurement or an air capacitor (having very stable value and a very small elastic field) for insulation measurement. One of the arms consists of a variable capacitor connected in parallel with a variable non-inductive resistance R₄. The remaining arm consists of unknown capacitor C_X whose capacitance is to be determined. Connected in series with a resistance R_X to represent loss in the capacitance C_X, the impedance of four arms are we obtain that the dissipation factor is the reciprocal of the quality factor Q and therefore 

If the frequency and resistor R₄ in Schering bridge is fixed, the capacitor C₄ can be calibrated to read the dissipation factor directly.