100713-0726 EST
rcwilson:
You can view a loadcell as some equivalent voltage source with some internal resistance when just viewing the two output terminals.
The approximate equation, but very close for small unbalance, for the source voltage would be vout = Vin*K*f where Vin is the excitation voltage, K a scaling constant, and f the applied force. The equation for internal resistance is two sets of two parallel resistors and then these in series.
If you want to create the equivalent circuit of the bridge but referenced to one of the excitation terminals, then you have two voltage sources with approximations of vout1 = Vin*0.5 + Vin*K1*f, and vout2 = Vin*0.5 - Vin*K2*f and the difference of vout1 and vout2 is your desired signal, or vout = Vin*K1*f - Vin*K2( -f ) = Vin*(K1+K2)*f. The source resistance for each side is the parallel resistance of the two bridge leg resistors on the associated side.
Ideally a bridge would have identical resistance in each leg except for the delta resistance from the applied force.
So a nominal 350 ohm output bridge also has a 350 ohm input and is made up of four 350 ohm resistors. Two 350 ohms in parallel = 175 ohms, and then two 175 ohms in series = 350 ohms.
Real straingage bridges are not quite built this way so input and output internal resistance measurements are not the same. This is because other resistances are associated with the bridge to provide standardization, zero balance, and temperature compensation. Measured with an ohmmeter the output resistance will be slightly less than the input resistance.
A transducer I pulled from the cabinet has 1,020 ohms input and 1000 ohms output resistance.
You can find a discussion of the application of Thevevin's Theorem to bridge circuits on p67 -71 of "Basic Electrical Measurements", by Melville B. Stout, Prentice-Hqll, 1950.
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