Technical solutions

For each problem we encounter in the automation projects we participate in, we publish on this page the solutions we have found. You will find both programming and electrical solutions.

Mai 30, 2020
published by AZTEC Automation
in Sensors

How do we connect a weight cell?

Recently we went to a concrete station where we were asked to help them with measuring the quantities of ingredients. No scale indicates values ??and they could not produce concrete. The measuring cells had been connected, but the electricians were not sure of the correctness of the connections. Because they knew everything was connected but not functional, they suspected automation problems.

When a problem arises, we first suspect electrical connections. For this reason I checked how the cells were electrically connected and I found mistakes. By modifying the electrical connections of the cells, the measurement was possible.

Because we had this experience, we chose to publish how a weight cell can connect electrically correctly. One solution would be to find the cell documentation and read the connection order from there. Experience tells us that it is a laborious job of searching the net, and the electricians on site certainly do not do such a thing. What is certain is that any electrician has a measuring device and can easily check if a cell is good and that the correct order of the conductors.

As you can see in the picture above, weight cells can have 4-wire or 6-wire cable. Those with 6 conductors have 2 conductors called direction conductors, or feedback conductors, which help to correct the received signal when the cable is longer than the one provided. For this it is necessary that the receiver can connect the 6 conductors. If the receiver can only connect 4 conductors, it is enough to connect the sense conductors to the excitation conductors. However, they are connected internally in the cell. You can rely on the color code presented above, but if you forget the order you are still not lost.

Whatever cell you work with, even if other colors of the conductors may appear, the black conductor will always be present. This is the "-" (minus) conductor for powering the cell. It's on the excitation circuit.
Step 1: finding the other conductor for excitation, the "+" (plus) one. There are resistors between the conductors, which you can measure with the ohmmeter. If you had cell documentation you would see what the right values ??are for these resistors. If you do not have it, these values ??could be: 1000 ohms, 350 ohms, 750 ohms, 450 ohms. When measured, these values ??can be + -. Measuring between the black conductor and the other conductors you will have different values. Write them down and then compare, finding the value that approximates the above. Find with which conductor you obtained the value and that will be the + conductor for excitation. if you have a cable with 6 conductors, between black and the sense conductors you will have: 0 ohm if it is the sense one with -, and the value found above, if it is the sense one with +.
Step 2: Find the signal conductors. After the measurement from step 1, you will be left with 2 free conductors. These will be the signal ones, but measure the resistance to know that the cell is good. It should be one of the above values.
Step 3: measuring the output signal. You have found the conductors, and you want to check if the cell is outputting the right signal. Power the cell on the excitation conductors, but do not connect the signal conductors. The cells accept a supply voltage between 5VDC and 12VDC. Do not exceed 12VDC. At the output it will output a voltage proportional to the input voltage. Depending on the cell, it will output 2mV / V, 3mV / V etc. Thus the signal range will be between 0 mV and resolution x input voltage, ie 10mV if it has 2mV / V and is powered at 5VDC. If the cell has a load on it, you will need to measure a value between the above. If you measure values of the order of volts, either you have not found the right conductors, or the cell is no longer good. In this case, resume the resistance measurements or change the cell.
Step 4: Connect the signal conductors and check the automation.

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