Calculating a Bridge-Circuit with load
In a loaded bridge circuit, a current between points A and B must be considered. One way to calculate this circuit is to consider the resistor R5 as a load resistor, which is connected to a voltage source with internal resistance Ri:
Specific example with the following values:
U = U0 = 12 V
R1 = 400Ω, R2 = 800Ω, R3 = 600Ω, R4 = 300Ω, R5 = 500Ω
Step 1: Calculating UAB with no load, that means no current between the points A and B
U2 = R2 / (R1+R2) = 800Ω / 1200Ω * 12V = 8V
U4 = R4 / (R3+R4) = 300Ω / 900Ω * 12V = 4V
=> UAB = 8 V - 4 V = 4 V Δ U0, if no current flows, i.e. R5 is not connected!
Step 2: Calculate Ri
If we consider the voltage source from figure a to be ideal, it has no internal resistance and can be short-circuited. We get the equivalent circuit as shown and can calculate the internal resistance:
Rtotal = Ri = R1 || R2 + R3 || R4 = 466.7Ω
Step 3: Calculate the terminal voltage
Therefore we consider pic b.
UAB / R5 = U0 / (R5 + Ri)
=> UAB = 2,07 V