How to Connect Third Party Load Cells to the Phidget Bridge

Phidgets load cellsThere’s a vast selection of load cells available from sensor companies around the world. While all these load cells do basically the same job, there are some subtle differences that are worth clarifying when connecting them to the 1046 Phidget Bridge. Lots of resources exist online for load cells, but likely contain more information than necessary. This guide will boil down the information and give you what you need to know for hooking up practically any load cell to the Bridge, including what load cells can’t be connected, what to do with extra wires, and how to figure out which leads are which when no documentation is provided.

Choosing a Third Party Load Cell

The Phidget Bridge supports most strain gauge load cells, including compression load cells, s-type load cells, shear beam and more. All of these load cells have excitation lines and output line(s) that output a very small voltage, which is amplified by the bridge. The Phidget Bridge provides 5Vdc of power. As such, load cells that require a greater excitation voltage will not work on the 1046 Phidget Bridge.

Connections of a Load Cell

Most load cells will come with documentation that will name each wire, making it straightforward to hook up.

The Phidget Bridge has four terminals: 5V, +, -, and G. When looking at the specifications for your load cell, most will refer to “excitation” lines or “input”. The positive (+) excitation line connects to the 5V terminal and the negative (-) line connects to the ground (G) terminal. The next two lines will be called “output” or “signal”. There will be one positive and one negative output line and these connect to the + and – terminals on the Bridge.

Phidget Bridge Connection

Phidget Bridge Connection for a 5-wire load cell

Some load cells will have additional wires. In a load cell with five wires, the extra wire is going to be the shield wire. It’s usually yellow and connects to the ground terminal with the other wire.

In a load cell with six wires, the extra wires are “sense” lines, these are lines drawn off the excitation lines. There will be a positive and a negative sense line, which correspond to the polarity of the excitation lines. To connect these lines to the load cells, connect the positive (+) sense line to the 5V terminal and the negative (-) sense line to the ground terminal with the excitation lines.

Determining Connections Without Documentation

If  no documentation is provided, it is possible to use a multimeter to determine which wire is which, provided there are no electronics in the sensor.

  1. First, measure resistance between all the wires.
    • In a 4-wire load cell, there are six combinations. There will be two pairs with unique resistances that are larger than any other resistances, two pairs with matching lowest resistances and two pairs with matching mid-resistances.
    • In a 5-wire load cell, there are ten combinations. The shield wire will very likely be thicker than the other wires, but if it’s not, it won’t generate any resistance when paired with the other wires, so it will be apparent. The other four wires will respond exactly as a 4-wire load cell, explained above.
    • In a 6-wire load cell, there are fifteen combinations. There will be four pairs that share the highest resistance, one pair that has a unique mid-resistance, two pairs that have practically no resistance, four pairs that have matching lowest resistances (but much greater than zero) and four pairs that have matching mid-resistances
  2. A pair of wires with the highest resistance mark the excitation lines. In a 6-wire load cell, the pairs of wires with no resistance should be kept together and treated as a single excitation line. Wire these lines to 5V and G on the Phidget Bridge.
  3. Connect the other pair with a unique resistance to the + and – terminals.
  4. Apply a load. If the mV/V responds in the opposite way to your expectations, flip the + and – wires.

Calibrating the Load Cell

When you get your load cell connected, you’ll want to calibrate it. You can use this simple formula to convert the measured mv/V output from the load cell to the measured force:

Expected Force or Weight = K ⋅ (Measured mV/V - Offset)

Where K is gain value that will change depending on what unit of force or weight you want to measure. Since the offset varies between individual load cells, it’s necessary to measure it for each sensor. Record the output of the load cell at rest on a flat surface with no force on it. The mv/V output measured by the Phidget Bridge is the offset.

Once you’ve found the offset, measure something with a known weight and solve the equation for K. You can also calibrate the load cell at multiple known weights and use these points to model a linear function. For more information on calibration, you can read our past blog post.

We’ve just covered the basics of connecting load cells to the Phidget Bridge, but if you’re interested to learn more about load cells then read the load cell primer or 1046 user guide.

 

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