How to use it
- Choose Rx for balance to find the unknown resistor, or output voltage to analyse a bridge that is not balanced.
- Enter R1 and R2 (the left divider, R1 on top) and R3 (top of the right divider).
- In voltage mode enter Rx and the supply voltage.
- Read Rx, or the output voltage in mV, and the voltage at each midpoint.
How it works
The bridge is two voltage dividers across the same supply: R1 over R2 on the left, R3 over Rx on the right. The detector sits between the two midpoints.
- Balance: R1 / R2 = R3 / Rx, so Rx = R2 × R3 / R1.
- Midpoints: V_left = Vs × R2 / (R1 + R2), V_right = Vs × Rx / (R3 + Rx).
- Output (open circuit): Vout = V_right − V_left.
The output assumes a high-impedance meter or amplifier that draws no current.
Worked example
R1 = 100 Ω, R2 = 200 Ω, R3 = 150 Ω. Balance needs Rx = 200 × 150 ÷ 100 = 300 Ω; with 10 V both midpoints sit at 6.6667 V.
Unbalanced: 10 V, R1 = R2 = R3 = 1,000 Ω, Rx = 1,100 Ω. V_left = 5 V, V_right = 10 × 1,100 ÷ 2,100 = 5.2381 V, so Vout = 238.095 mV.
Tips
- Balance does not depend on the supply, so a slide-wire or decade-box bridge measures resistance accurately even with an unstable battery.
- The output does. In sensor bridges a 1 % supply change moves the output by 1 %; ratiometric ADCs use the same supply as reference to cancel it.
- Self-heating: high supply voltage heats the arms and shifts their resistance; keep the current low for precise work.