Load cell basics

 

Load cell

What is a load cell

Its usually asked in a way that what is meant by a load cell, answer is quite simple, Load cell is an instrument used to measure force. This force may act on a load cell in any form like tension or compression. The load cell is a transducer that converts force applied to it into an electrical signal usually millivolts.

So by using electronics circuitry, we convert this electrical signal of millivolts can be shown on display converted into load in kg or tonn, and further this can be transmitted into a standard signal of 4 to 20 mA or 0 -5 v.

Load cell working principle

what s the working principle of a load cell ??
As discussed above load cell converts force into an electrical signal, most of the time load cell is designed by using a strain gauge, when the force applied to strain gauge its resistance changes, and using it in a wheat stone bridge we convert this change in resistance to measure force, however pneumatic and hydraulic type load cell is also used for force measurement.

Load cell checking procedure

If we have a load cell and we want to check and test a load cell, whether it is working ok or not,
we can check it by connecting to a display meter or transmitter and by putting a load on it we can check on the display of the meter that it's showing accurate weight.




In industrial instrumentation load cells are used to measure the force and load of bins hoppers weighbridges and belt weigh feeders, suppose we don't have a meter or transmitter to test a load cell working, then we will check it with a  digital multimeter.
For that test of a specific load cell, we will give excitation volt to the load cell and at the output wires of the load cell, we will measure millivolt.
Each load cell has specifications printed on the load cell or a tag of paper, which shows the maximum capacity and output millivolt of the load cell.
Suppose a load cell of 50 kg capacity and 2mv/V is its output, its mean when a load of 50 kg is applied to this load cell and excitation volt is 10 volt then for each volt it will output 2 mv, in total output will be 20 mv, if the load at load cell is 25 kg then the output will be 10mv. so with the change in load, our output varies and by measuring the output with a multimeter we can check that our load cell ok or not.


Load cell Connection





To answer a question, that how do I connect a load cell??
 In four-wire load cell sensors,  two wires are used to five excitation volts, as in the fig. above shown the load cell circuit diagram or wiring diagram, red and black wires are for the excitation volt, and at two wires green and yellow an output is acquired in millivolt.
 
In six wires load cell, connections are the same as the four-wire but two extra wires are used for reference. Reference wires are used to take a reference that voltages are reaching properly on the load cell and no voltage drop in cabling, so the controller can measure the exact weight.

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