Bridge Circuits: Voltage Divider & AC Bridge Circuits | Sensor & Industrial Instrumentation - Electronics and Communication Engineering (ECE) PDF Download

Bridge Circuits

Resistor temperature compensation can be achieved in voltage divider circuits or bridge circuits. However, when trying to measure small changes in resistance, as required in strain gauges, the signal resolution in bridge circuits is much higher than with voltage dividers. Bridge circuits are used to convert small changes in impedance into voltages. These voltages are referenced to zero, so that the signals can be amplified to give high sensitivity to impedance changes.

➢ Voltage Dividers
The two resistive elements R₁ and R₂ in a strain gauge sensor can be connected in series to form a voltage divider. The elements are driven from a supply, Vs. Since the temperature coefficient of resistance is the same in both elements, the voltage at the junction of the elements, V_R, is independent of temperature, and is given by:

➢ Ac Bridge Circuits
The basic concept of dc bridges can also be extended to ac bridges, the resistive elements are replaced with impedances, as shown in Figure 3.13(a). The bridge supply is now an ac voltage. This type of set up can be used to measure small changes in capacitance as required in the capacitive pressure sensor.
The differential voltage δV across S is then given by:
where E is the ac supply electromotive force (EMF).
When the bridge is balanced, V = 0, and (3.20) reduces to:
Z₂Z₃ = Z₁Z₄.........(3.21)
Or
R₂(R₃ + j/ωC₁) C₁) = R₁(R₄ + J/ωC₂)
Because the real and imaginary parts must be independently equal
R₃ x R₂ = R₁ x R₄.......(3.22)
and
R₂ x C₂ = R₁ x C₁.......(3.23)

Figure 3.13 (a) An ac bridge using block impedances, and (b) a bridge with R and C components