Wheatstone bridge

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This model can be used to determine the null conditions of a Wheatstone bridge or to determine the out-of-null output voltage as a function of arm resistance. Resistors R1 ... R4 are the four arms of the bridge and RL represents the input resistance of the detector or null voltage read-out device. This model requires an iterative solution; the "guess" variable can be the voltage or current in any one of the arms. In the example variable sheet shown below, an off-null sensitivity of 24.8 mV per ohm is demonstrated for a nearly balanced 100 ohm bridge with an excitation voltage of 10 volts and RL=100K.

Variable Sheet
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St Input     Name    Output    Unit      Comment
             E1      4.9751369 volts     Voltage across R1 (volts, mV, �V)
             I1      .04975137 Amps      Current through R1 (Amps, mA, �A)
   100       R1                Ohms      Resistance of arm 1 (Ohms, Kohms, MOhms)
             E2      4.9999876 volts     Voltage across R2 (volts, mV, �V)
             I2      .04999988 Amps      Current through R2 (Amps, mA, �A)
   100       R2                Ohms      Resistance of arm 2 (Ohms, Kohms, MOhms)
             E3      5.0248631 volts     Voltage across R3 (volts, mV, �V)
             I3      .04975112 Amps      Current through R3 (Amps, mA, �A)
   101       R3                Ohms      Resistance of arm 3 (Ohms, Kohms, MOhms)
             E4      5.0000124 volts     Voltage across R4 (volts, mV, �V)
             I4      .05000012 Amps      Current through R4 (Amps, mA, �A)
   100       R4                Ohms      Resistance of arm 4 (Ohms, Kohms, MOhms)
             EL      24.85071  mV        Voltage across RL (volts, mV, �V)
             IL      2.4851E-7 Amps      Current through RL (Amps, mA, �A)
   100000    RL                Ohms      Resistance of detector (Ohms, Kohms, MOh
   10        Vb                volts     Excitation voltage (volts, mV, �V)


Rule Sheet
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S Rule
* E1=I1*R1  "Ohms Law for R1
* E2=I2*R2  "Ohms Law for R2
* E3=I3*R3  "Ohms Law for R3
* E4=I4*R4  "Ohms Law for R4
* EL=IL*RL  "Ohms Law for RL
* E1+E3=Vb  "Kirchoffs Law for voltage
* E2+E4=Vb  "Kirchoffs Law for voltage
* I1=I3+IL  "Kirchoffs Law for current
* I2=I4-IL  "Kirchoffs Law for current
* EL=E3-E4  "Kirchoffs Law for voltage