Adam Equipment CBW Series Service Manual page 13

This will cause the bridge to become unbalanced. When the bridge has a voltage
across it from +E to -E then the signal output at +S and -S will show a voltage of zero
volts with no load and a small voltage proportional to the load as the load is
increased. Typical load cells show 20-30mv of signal if the excitation voltage is
10volts and the load cell is fully loaded.
Most load cells used in scales are not as simple as this example.
additional resistance elements added to compensate for temperature variations and
to set the outputs to correct voltages. The circuit of a typical load cell is shown
below-
A method to check the basic function and integrity of a load cell can be done using a
ohmmeter and voltmeter with up to a 10volt power supply.
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 by
which it increases will be dependent 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.
@Adam Equipment Company 2005
410 ohms ±30 ohms
350 ohms ± 2 ohms
13
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