3
\$\begingroup\$

The load cells here: http://www.cnloadcell.com/mlc700-postal-scale-load-cell and here: http://www.cnloadcell.com/mlc714n-personal-scale-weight-sensor have 4 wires (red, white, black, green).

The diagram is quite confusing, I think I can see it is somehow part of a wheatstone bridge.

sensor

But from reading up on Wheatstone bridges, each strain gauge should only have 2 wires and these gauges have 4? Is the diagram showing how to use just one gauge or many?

Two wires coming out the strain gauge:

enter image description here

\$\endgroup\$
4
\$\begingroup\$

Wheatstone Bridge style strain gauges have 4 wires, as shown in the circuit diagram you posted. Red and Black are for your excitation voltage, white and green is your differential output voltage that's scaled by that excitation voltage you provide.

\$\endgroup\$
  • \$\begingroup\$ thanks for that, sorry my knowledge of circuits is pretty poor. Do you have a link to an example? ( I added another circuit diagram to the question - it only shows two wires coming from the strain gauge [R4]). In the four wire circuit diagram, if all 4 wires come from the strain gauge, I don't understand where you "connect" to measure it? \$\endgroup\$ – Blundell Sep 13 '17 at 7:25
1
\$\begingroup\$

This is from an Analog Devices reference design for a weigh scale, showing how it is used in practice

enter image description here

\$\endgroup\$
  • \$\begingroup\$ it says Load Cell on that diagram, and I see a Wheatstone bridge next to it, but that still doesn't explain, how the 4 wires connect? (and see my comment on other answer) \$\endgroup\$ – Blundell Sep 13 '17 at 9:08
1
\$\begingroup\$

Found a bigger explanation: https://learn.sparkfun.com/tutorials/load-cell-amplifier-hx711-breakout-hookup-guide

These pins are labeled with colors; RED, BLK, WHT, GRN, and YLW. These colors correspond to the conventional color coding of load cells, where red, black, green and white wires come from the strain gauge on the load cell and yellow is an optional ground wire that is not hooked up to the strain gauge but is there to ground any small outside EMI (electromagnetic interference). Sometimes instead of a yellow wire there is a larger black wire, foil, or loose wires to shield the signal wires to lessen EMI.

enter image description here

In General, each load cell has four strain gauges that are hooked up in a wheatstone bridge formation as shown above.

The four wires coming out from the wheatstone bridge on the load cell are usually:

  • Excitation+ (E+) or VCC is red

  • Excitation- (E-) or ground is black.

  • Output+ (O+), Signal+ (S+)+ or Amplifier+ (A+) is white

  • O-, S-, or A- is green or blue

Some load cells might have slight variations in color coding such as blue instead of green or yellow instead of black or white if there are only four wires (meaning no wire used as an EMI buffer). You might have to infer a little from the colors that you have, but in general you will usually see these colors.

So basically the load sensor internally already has the wheatstone bridge and these 4 wires are the output from that (not that the four wires are expected to be used inside a wheatstone bridge.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.