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I am trying to understand the specifications of a load cell and there are the input and output resistance as you can see here, it says 1090±10Ω for input resistance and 1000±10Ω of output resistance.

This document says the following:

OUTPUT BRIDGE RESISTANCE - The output resistance of the cell. It is measured by placing an ohmmeter between the signal or output leads

INPUT BRIDGE RESISTANCE - The input resistance of the load cell. It is measured by placing an ohmmeter across the input or excitation leads. It is usually higher than the output bridge resistance because of the presence of compensating resistors in the excitation circuit.

Then I understand the output resistance is the measured resistence between the S+ and S- (see representation of the bridge below), that is 2R/2 = R (two resistor and series = 2R, and then 2R and 2R in parallel). But in my first read of input bridge resistance definition I understood that it is the value that I measure between E+ and E- (that AFAIK is R too). The part that I don't understand is about the input bridge resistance being "usually higher than the output bridge resistance because of the presence of compensating resistors in the excitation circuit."

I would appreciate if somebody could explain me this? I would say the contrary, if I have other resistances in parallel with the nodes E+ and E- the measured resistance between this nodes would be lower and not higher than the resistance measured between S- and S+, but I am wrong, why?

Thanks a lot.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ The compensating resistances are in series with the upper two resistors in your diagram. \$\endgroup\$ – Dave Tweed Apr 11 '14 at 19:56
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Try this: -

schematic

simulate this circuit – Schematic created using CircuitLab

In my schematic, R5 and R6 would be the compensation resistors (usually compensating temperature effects). I've inserted values of R5 and R6 that give 1090 ohms from the perspective of the excitation wires but still gives 1000 ohms from the perspective of the sense wires.

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  • \$\begingroup\$ Hi, thanks for the answer. I don't understand how R5 and R6 can compensate temperature effects, or why they exists. \$\endgroup\$ – koike Apr 12 '14 at 8:49
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    \$\begingroup\$ That's a different question dude. It's basically to do with the gauge factors of the gauges changing with temperature - basically this corresponds to an alteration of Vout per microstrain and R5 and R6 lower or increase excitation to fight this change. \$\endgroup\$ – Andy aka Apr 12 '14 at 8:52

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