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I have 4 strain gauge load cell to calaculate actual weight.I want to use summing method for 4 output signals. I searched at internet for summing box schematic I found this pic Now if V1 , V2 , V3 , V4 are the output signals for each load cell Why connecting them in parallel like that will make the resulting volt = (V1+V2+V3+V4)/4. ?

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  • \$\begingroup\$ Consider each bridge as a Thevenin source. Then look at what will happen to the output when one generates a signal with them all paralleled. \$\endgroup\$ – Phil G Mar 6 at 20:21
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because each gauge load cell will have 1/4 of the load

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  • \$\begingroup\$ please , can you clarify your answer with equation? \$\endgroup\$ – Islam Gamal Mar 6 at 20:49
  • \$\begingroup\$ If you load 1 cell with 1kg for example it will give you X mv/V . If you separate load to 4 cells every cell will give you 1/4 of that value. \$\endgroup\$ – Peter MP Mar 6 at 20:58
  • \$\begingroup\$ so do you mean i1 = v1/r , i2 = v2/r , v3/r , v4 = v4/r and it = i1+i2+i3+i4 and it = v/(r/4) so v = (v1+v2+v3+v4)/4 ? \$\endgroup\$ – Islam Gamal Mar 6 at 21:15
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I have also found many documents related to Junction Box or Summing Box for load cells that state that connecting the bridges in parallel is the way for averaging the signals, but I found no one that provided an explanation of the working principle for this.

Motivated by what Phil G pointed about using Thevenin equivalent circuit and as I found some authors that also suggest the same as this link, I decided to follow the suggested path for the following full-bridge circuit:

enter image description here

First find the Thevenin equivalent circuit as seen from Sig+ and Sig-

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Applying the following simplification: ∆R << R ⇒ RTH ≈ R as in this Application report

When connecting the equivalent circuits in parallel we can solve and find the equivalent output voltage

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As a final result we have that Vout=Vext*(ΔR1+ΔR2+ΔR3)/(3*R), which reflects the relationship between the output and the average of the ΔRs from all the bridges.

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