Adam Equipment CBC Series Service Manual page 13

Before the load cell is connected to the power supply use the ohmmeter to measure the
resistance between the wires. Typical resistance values are:
+E to -E
+S to -S
+E to +S or any other combination similar approximately 270 ohms.
If the load cell has Sense connections in addition they are connected to the excitation internal
to the load cell.
Connect the load cell +E and -E to a power supply, typically 10 volts. Never use more than
12 volts as it may damage the load cell. Measure the voltage between +S and -S.
With no load the voltage should be approximate 0mv ±3mv.
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
CBC/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 2005
410 ohms ±30 ohms
350 ohms ± 2 ohms
= 2mv/V * 5V * 20Kg = 6.7mv
13
30kg