A & A Scales PT300, Service Manual, Page: 27 / 52

PT300, Service Manual
Rev B, October, 2000
26
Load Cells:
Each PT300 scale has two load cells. These load cells are the transducers that
convert mechanical energy (weight) to electrical energy. Each load cell has 4
strain gages (1000 ohms each) and a temperature compensation resistor. The
four strain gages are wired together to form a Wheatstone bridge.
Color coding of the wires.
Red - + Excitation (5 Volts in this case)
Green - + Signal
White - - Signal
Black - - Excitation (ground)
1K 1K
1K 1K
EX+
5+
5-
EX-
Figure 12 Load Cell Schematic
The 200 ohm resistor in series with the Excitation lead is for temperature
compensation. It is a balco resistor that change resistance with temperature to
maintain a constant signal voltage. These resistors are specially manufactured for our
load cell material and strain gage types.
With the load cells disconnected from the summing board the resistances should be as
follows:
Blk - Red = 1200 Ohms
Blk - Grn = 751 Ohms
Blk - Wht = 751 Ohms
Wht - Grn = 1000 Ohms
With the two excitation leads soldered in place and the power on you should measure
approximately -1.00 to 1.00 millivolts between the plus and minus signal leads. The
signal at the main circuit board with everything soldered in place and the tare set
should be approximately 0.16 millivolts.