MethodWhat is calculated?
Parallel full bridges deliver the electrical mean of their outputs but load one another in proportion to their output resistances. Only with equal sensitivities and output resistances is the summed signal also the mean of the forces. Following Keil, cells are matched to the one with the largest quotient R_A/C by inserting trim resistors in the output leads of the others so that all effective sensitivities are equal.
Equations
C*₁ = C₁ / (1 + R_A1·(1/R_A2 + 1/R_A3 + … + 1/R_An)) (Keil Gl. 5.10)
C_ges = ΣC*ᵢ (Keil Gl. 5.11)
R*_Ai = Cᵢ·Q_max, Q_max = max(R_Ai/Cᵢ) (Keil Gl. 5.12)
R_AZi = R*_Ai − R_Ai (Keil Gl. 5.13)
ProcedureStep by step
- Enter the certified sensitivities and measured output resistances of all cells.
- The cell with Q_max gets no resistor; the others get the computed R_AZ, half in each output lead.
- Use the group sensitivity C_ges to scale the amplifier.
Typical mistake
Placing the whole trim resistor in one output lead or using datasheet instead of measured output resistances.
PracticeApplication and limits
Platform scales, vessel weighing and multi-support force measurement with three or four load cells on one amplifier channel, especially with wider sensitivity tolerances.
Constant-voltage excitation; input resistances are not considered (matching is deliberately done in the output circuit). Corner-load errors from installation are not corrected.
SourceTechnical basis
Keil, Dehnungsmessstreifen, 2nd ed. 2017, sec. 5.6 parallel connection of full bridges, eqs. (5.10)–(5.13) after [5.10].
The source supports the equation structure and worked examples; this calculator does not replace calibration of the measuring chain.
FAQFrequently asked questions
Why trim the output rather than the supply circuit?
So as not to disturb existing supply-side adjustments; the output circuit also carries only tiny currents.
How accurately must R_A be known?
To about 0.01 Ω, otherwise the match is no better than the cell tolerance.
What happens without trimming?
The cell with the lowest output resistance dominates and eccentric loads are weighted wrongly.
In the same cluster
All strain-gauge calculators