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2-1
CHAPTER 2 THEORY OF OPERATION
2.2
LOADCELL THEORY
The Loadcell assembly used in Trooper balances is part of a Spider Assembly. The Spider Assembly
consists of two spider shaped brackets in which the Loadcell is mounted in between in a sandwich
fashion. The Spider Assembly holds the Loadcell in proper alignment with preadjusted down-stop
screws which are locktited. The bottom half of the Spider Assembly is mounted to the base of the scale.
The upper half of the Spider Assembly forms the sub-platform for the scale. The platform assembly
is mounted directly to this. Loadcells may not be ordered as a replacement part since they are part
of the Spider Assembly.
For electrical components, refer to schematic diagram Figure 5-2. For mechanical components, refer
to Figure 5-1. One basic type of Loadcell is used in the Trooper balances. The Loadcell contains four
Strain Gauges in a full bridge configuration. The full bridge has two Strain Gauges mounted on top
of the Transducer and two mounted on the bottom. A temperature compensation sensor is also
mounted on the top of the Loadcell.
The Loadcell consists of an aluminum beam with 4 Strain Gauges bonded at the hinged areas
(thinnest part of the beam). Two of the Strain Gauges are located on top of the beam and two on the
bottom. These 4 electrical strain gauges are wired to form a Wheatstone bridge. When a load is placed
on the beam, it bends at the hinged areas. The bending changes the resistance of the Strain Gauges
and the resulting resistance output from the Wheatstone bridge is proportional to the load. Six Down
Stop screws which are part of the upper Spider Assembly engages and limits the external load at
slightly over full load capacity and limits the bending of the beam, preventing damage due to
overloading. The Spider Assembly does not contain up-stop screws which protects the Transducer
assembly from exceeding maximum upward movement.
2.3
PRINTED CIRCUIT BOARD THEORY (See figure 2-1)
2.3.1 Power Supply Circuit U12
Power to the scale is supplied by a 9 V dc, 500mA Power Adapter to connector J7 or by 6 "C" size
Alkaline batteries. Power is regulated through U12 (3-terminal positive voltage regulator) to output
a reg5 V dc which supplies the board.
2.3.2 Power Monitor Circuit U9
Battery voltage is monitored by a power monitor circuit U9. When the battery voltage goes low to a
preset point, a reset signal (Low Bat) is sent from the power monitor circuit to the microprocessor U7.
The microprocessor activates the display which indicates a low battery voltage condition.
2.1 TROOPER THEORY OF OPERATION
This chapter contains information on the basic theory of operation of the Loadcell and scale circuit
descriptions of the printed circuit board. An exploded view drawing is included in Chapter 5 which
identifies all servicable components of the scale. The descriptions in this manual refer to the name of
the components identified in the exploded view drawing.
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