Testing The Load Cell Assembly; Ramp Test; Resistance Test; Excitation And Output Voltage Test - OHAUS FD SERIES Service Manual

CHAPTER 3 SCALE TESTING AND CALIBRATION

3.3 TESTING THE LOAD CELL ASSEMBLY

This section contains three methods of testing the Load cell.

3.3.1 Ramp Test

Refer to Appendix A and perform the Ramp Test. If the Ramp Test passes, continue with the rest of this
procedure.

3.3.2 Resistance Test

1. Refer to Section 4.1 and remove the Top Housing.
2. Remove the Load Cell cable connector from J1 on the Main PC Board. Using an Ohmmeter,
measure in accordance with Table 3-1 Load Cell Resistance Readings on the cable. If the
resistance readings vary more than the table readings allow, replace the Load Cell in accordance
with the procedure in Section 4.4. This resistance test is useful in determining if the Load Cell has
been severely damaged by a short or open circuit which has occurred on the cell or wiring. If the
Load Cell resistance readings are good, continue with this procedure.
NOTE: Pins 1, 2, 3, and 4 must be isolated from chassis ground and shield. Resistance should be
greater than 5 megohms. A lower reading indicates a short in either in the Load Cell cable or the Load
Cell.
3. Reconnect Load cell cable to J1.

3.3.3 Excitation and Output Voltage Test

NOTES: 1. The following steps involve power applied to the scale.
2. Solder contacts located above connector J1 can be used as measuring points. See Figure 3-2.
1. Insert the 4 Pan Support Caps into the Load Cell Frame and place the Pan on top.
2. Turn on power to the scale by momentarily connecting pin 1 and 5 on J2.
3. With no load on the Pan, first measure the excitation voltage across pins 1 and 4 of J1 on the PCB.
This voltage must be 5.0 Volts dc with the Load cell connected. If the voltage is lower, disconnect
the Load Cell cable from the PC Board and measure again. If the voltage is 5 Volts dc, the Load
Cell is defective and must be replaced. If the voltage remains low, the regulated 5 volt power
supply on the PC Board is defective and the Main PC Board must be replaced. For Main PC Board
replacement, see Section 4.3.
TABLE 3-1 LOAD CELL RESISTANCE READINGS
MEASUREMENT POINTS
Pins 1 & 4
Pins 2 & 3
3-2
READING
408 + 10 Ohms
350 + 4 Ohms