Deflection-type D.C. bridge

Deflection-type d.c. bridge

A deflection-type bridge with d.c. excitation is shown in Figure . This differs from the Wheatstone bridge mainly in that the variable resistance R_v is replaced by a fixed resistance R₁ of the same value as the nominal value of the unknown resistance R_u . As the resistance R_u changes, so the output voltage V₀ varies, and this relationship between V₀ and Ru must be calculated.

This relationship is simplified if we again assume that a high impedance voltage measuring instrument is used and the current drawn by it, Im , can be approximated to zero. (The case when this assumption does not hold is covered later in this section.) The analysis is then exactly the same as for the preceding example of the Wheatstone bridge, except that R_v is replaced by R₁. Thus, from equation (7.1), we have:

V₀= V_i * ( R_u / R_u + R₃)- (  R₁ / R₁+ R₂)

When R_u is at its nominal value, i.e. for R_u D R1, it is clear that V₀ D₀ (since R₂ D R₃). For other values of Ru, V₀ has negative and positive values that vary in a non-linear way with R_u.