Solution: Wheatstone bridge
Exercise 1: Pressure sensor within a Measuring bridge
Block diagram Sensor
Sensitivity of conversion stage 1:
p1 = 1000 mbar => x1 = 8 mm
p2 = 2000 mbar => x2 = 18 mm
E1 = Δx = 18 mm – 8 mm = 0,01 mm/mbar
. Δp 1000 mbar
Sensitivity of conversion stage 2:
R´ = x1 x R0 = 8 mm x 2 kΩ = 640 Ω
. l 25 mm
R`` = x2 x R0 = 18 mm x 2 kΩ = 1440 Ω
. l 25 mm
E2 = ΔR = 1440 Ω – 640 Ω = 80 Ω
. Δx 18 mm – 8 mm mm
Total conversion factor of the sensor:
Etotal = E1 x E2 = 0,01 mm/mbar x 80 Ω/mm = 0,8 Ω/mbar .
Measuring bridge circuit
circuit measuring bridge
When p rises => R2´ rises => VBridge rises
Dimensioning of the resistance values R3 and R4:
Cond.: At p = 1000 mbar the bridge is balanced.
According to the text: At 1000 mbar the wiper of the potentiometer is 8 mm away from pin 3.
=> R1´ = 1360 Ω => R2´ = 640 Ω
Cond.: R1 = R3
. R2 R4
Choose e.g. R3 = 1360 Ω und R4 = 640 Ω
Measuring voltage at 1100 mbar
VBridge = V2 - V4
V2 = R2´ x V = 720 Ω x 10 V = 3,6 V
. R1´+ R2´ 2000 Ω
R2´(p = 1100 mbar) = 640 Ω + 100 Ω x 0,8 Ω/mbar = 720 Ω
R1´= 2000 kΩ - 720 Ω = 1280 Ω
V4 = R4 x V = 640 Ω x 10 V = 3,2 V
. R3 + R4 2000 Ω
=> VBridge = 3,6 V - 3,2V = 0,4V