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 Rv is replaced by a fixed resistance R1 of the same value as the nominal value of the unknown resistance Ru . As the resistance Ru 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 Rv is replaced by R1. Thus, from equation (7.1), we have:
V0= Vi * ( Ru / Ru + R3)- ( R1 / R1+ R2)
When Ru is at its nominal value, i.e. for Ru 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 Ru.