# How can I calculate the values of the four resistors in a Wheatstone bridge?

I measured black-red = 400ohm, green-white = 350ohm in a 5kg loadcell.

How can I calculate the resistor values?

• Does the bridge have a datasheet? Does it have a fifth resistor (in series with one of the input wires)? Apr 19, 2022 at 11:27
• Measurements are meaningless because of connections. Apr 19, 2022 at 13:45

Many strain gauges have gain calibration resistors in the excitation leads such as here: -

Image from RS, Tedea Huntleigh Wire Lead Load Cell. And, this means that the excitation leads measure a higher resistance than the output leads. Given also that 350 Ω is pretty commonplace for a strain gauge, it's likely that the measurement of 400 Ω is because of the 350 Ω gauges plus 25 Ω in each excitation lead.

• Resistance between A and C should be 287.5 Ω nominal
• Resistance between A and D should be 287.5 Ω nominal
• Resistance between F and C should be 287.5 Ω nominal
• Resistance between F and D should be 287.5 Ω nominal

$$R_{AC} = 25 + 350 || 1050$$

• ok, i get it now, how did you calculate and got 287.5? I just got 290ohm AC, AD, FC, FD Apr 19, 2022 at 13:26
• guang ce yzc 1b 5kg loadcell Apr 19, 2022 at 13:26
• @TumenbayarMunkhjargal between B and C is a 350 ohm gauge in parallel with 3 series gauges of 350 ohm. That is a resistance of 262.5 ohms. Therefore between A and C is 262.5 ohm plus 25 ohm = 287.5 ohm. Apr 19, 2022 at 13:28
• could you please simplify that delta circuit into simple so I can understand it fully Apr 19, 2022 at 13:34
• I've added a picture @TumenbayarMunkhjargal Apr 19, 2022 at 13:41

This is a possible solution:

You could check by performing other measurements:

• between A and C = 312.5 $$\\Omega\$$

• between A and D = 312.5 $$\\Omega\$$

• between E and D = 262.5 $$\\Omega\$$

• between E and C = 262.5 $$\\Omega\$$

Just to be clear: if this is the topology you don't have access to node B.

guang ce yzc 1b 5kg loadcell

Update: different to this document, which resembles a datasheet and was mentioned yesterday in another question, it seems that the internal configuration uses 2 series resistors, as detailed in Andy's answer.

• it is physically loadcell 5kg, how to calculate it, not practically Apr 19, 2022 at 12:20
• A and C, AD, ED, EC are all 290ohm as I measured just now, I wonder why Apr 19, 2022 at 12:58
• @TumenbayarMunkhjargal looks like the answer you have been given is correct, with the tolerance of what you say you measured compared to the 287.5 ohms suggested. Apr 19, 2022 at 13:25