Chapter 10: Troubleshooting
Check Load Cells
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Check Load Cells
Check each load cell for proper bridge resistances.
Measuring Points Resistance
Any lead to shield or ground Infinity
+Exc (Green) to -Exc (Black) 350 Ohm minimum
+Sig (White) to -Sig (Red) 348 to 352 Ohms
If bridge resistances are within specification, perform a “shorted signal”
symmetry check. Short the signal leads together and place one multimeter
lead on the shorted signals and one lead on the +excitation wire. Note the
resistance value indicated. Next, remove the lead from the +excitation
wire and place it on the -excitation wire. Both resistance values should be
equal within 1 Ohm.
If the cells pass the above test, reapply power to the scale platform.
Confirm that proper excitation voltage is reaching the load cells by
placing multimeter leads on the excitation positions of each load cell
terminal (excitation voltage can vary from 5 VDC to 15 VDC depending
upon the application and digital indicator used).
If proper excitation voltage is reaching the load cells, check the output
signal from each cell. If one cell has a particularly high or low dead-load
output it is suspect (the maximum output possible from any cell is 30
mV at 15 VDC excitation and loaded to gross capacity).
If any cell has an unusual signal, remove all load from that cell by raising
the platform. With the power still on, measure the output from the
suspect load cell. The “no load” zero output should be + 1.5% of the full
scale output. For example, if the excitation voltage is 15 VDC then the
full scale output is 30 mV. Therefore, the no load zero output should be
within + 0.45 mV. If the load cell is out of specification, replace it.
If a load cell fails any of the above test, replace it.
Note: Remove signal leads from
terminals to measure output.
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