Chapter 10: Troubleshooting
Check Load Cells
(2/01) 10-3
Check Load Cells
1.
Remove power from the system. Fully disconnect each load cell and check for
proper input/output resistances (see Table 10-2).
Measuring Points Resistance
Any lead to shield or ground Infinity
+Exc (Green) to -Exc (Black) 385 ohms minimum
+Sig (White) to –Sig (Red) 348-352 ohms
Table 10-2: Load Cell Measuring Points
2.
If resistance is within specification, perform a shorted-signal symmetry test.
• 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.
• Remove the lead from the +Excitation wire and place it on the –Excitation wire.
The two resistance values should be approximately equal.
3.
If the load cells pass the shorted-signal test, reconnect them and reapply power to
the scale. Confirm that the proper excitation voltage is reaching the load cells by
placing multimeter leads on the excitation positions of each load cell terminal.
4.
If proper excitation voltage is reaching the load cells, check the output signal from
each cell by disconnecting the signal leads and measuring voltage output. 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.
5.
If any load cell has an unusual signal, remove all load from that cell.
• With the power 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 would be 30 mV and
the no-load zero output should be within ± 0.45 mV.
6.
If a load cell fails any of the above tests, replace it.
Note: Remove signal leads from
terminals to measure output.
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