Holtgreven Truck Scale Resources - Manual (Page 41)

PT300 Users manual
Rev G, Feb 2011
Page 41 of 44
The 5 Ω thermistors in series with the Excitation leads are for temperature compensation. They
are balco thermistors that change resistance with temperature to maintain a constant signal
voltage. These thermistors are specially manufactured for Intercomp load cell material and strain
gage types.
Load cell leads color coding
Red : + Excitation (3.3 Volts)
Green : + Signal
White : - Signal
Black : - Excitation (ground)
Check load cell zero output value
The signal at the main circuit board with the signal leads in place should be approximately 0.1
millivolts with no load applied to the cell. Larger values could indicate a defective load cell, wiring,
or circuit board. The load cell must be tested separately to determine the source of the zero shift.
Check zero output from the load cell
You can use a power supply set to 3.3 Volts to provide the excitation voltage for this
measurement. Connect plus 3.3 volts to the red lead and ground to the black lead or use the scale
to provide the excitation by connecting the two excitation leads only (Black and Red). Turn the
scale on to apply excitation voltage.
Connect a meter capable of monitoring millivolt readings between the plus and minus signal leads
(Green and White) The zero signal output (With excitation voltage applied) should measure
approximately -1.00 to 1.00 millivolts). Larger values would indicate a defective load cell. This is
most likely the result of an overload of the cell during use.
Check load cell resistance values
Load cell resistance can vary depending on the model and capacity. The load cells will be one of
the standard resistance values given in the table below.
Nominal Cell
Resistance
350
Ω Ω Ω Ω
700
Ω Ω Ω Ω
1000
Ω Ω Ω Ω
Black – Red = 385
Ω
770
Ω
1082
Ω
Black – Green = 285
Ω
595
Ω
748
Ω
Black – White = 285
Ω
595
Ω
748
Ω
White - Green = 350
Ω
701
Ω
1000
Ω
With the load cells disconnected, the resistance readings should be as specified. It is normal for
all of the readings to be higher or lower than the values given by several percent. If there is
several Ohm difference between the Black-Green and Black-White measurements the cell is
defective. This test is most useful to detect a catastrophic failure of the cell or lead; you are
typically looking for an open or a short when using this check.