Adam Equipment CBD Series Service Manual page 16

With a mass on the load cell the output voltage should increase. The amount of
increase will depend upon the sensitivity of the load cell, capacity of the load
cell, the excitation voltage and the amount of mass placed on the load cell.
Most load cells have a sensitivity of either 2mv/V or 3mv/V.
The expected change to the output is:
Vout = (2mv/V) * Vext * Mass
capacity of load cell
For example a 2mv/V load cell of 30Kg capacity with 20Kg placed on it. Vext is
5 volts would give a output of approximately:
Vout = (2mv/V) * Vext * Mass
capacity of load cell
If the load cell has been damaged the no load voltage will likely be greater than
3mv or the loaded voltage will be grossly different from the expected value.
Remember the loaded voltage will be offset by the amount of zero load voltage
measured.
The load cell can be tested while it is connected to the A/D converter circuit
board, using the scale power supply for excitation voltage.
TESTING LOAD CELLS IN THE SCALE
The signal from the load cells is amplified by the circuits on the A/D circuit
board.
The signal needs to be amplified to make it acceptable for the A/D converter
used. The CBD/CBW scales amplify the input by 200, and change the no load
voltage to about +2.5V.
The amplified and offset voltage is measured at pin 6 of IC7, the INA118P
amplifier IC. This voltage is sent to the filter and then to the A/D converter.
@Adam Equipment Company 2007
= 2mv/V * 5V * 20Kg = 6.7mv
30kg
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