10
Scale Technical Description
LOAD CELL TRANSDUCERS
The function of the load cell transducer is to convert the weight applied to the weighing platform into an electrical
signal for further processing and subsequent display by the readout. WELCH ALLYN uses proprietarily designed load
cells in most of the scales to optimize performance and reliability. One of three types of load cell transducers may
be used in your WELCH ALLYN scale depending on the model number.
MODELS 5202, 6202D, 6202
The load cell employed in these devices is referred to as a “co-planar” beam. The design of the beams provides for
a thin, low profile construction in order to keep the height of the scale’s weighing platform to a minimum.
Each beam has strain gauges bonded to it that convert the weight-induced deflection of the beam into an output
voltage. When powered (“excited”) with a voltage the beam produces a linear voltage output that is directly
proportional and linear to the weight applied. Additional components mounted on the beam correct for
temperature effects and standardize the output voltage.
The weighing platforms of the models 5202 and 6202 contain four planar beams, one per corner. The output voltages
of the four separate beams are connected in parallel on a “junction” printed circuit board in order to provide the
total of the weight applied to the platform.
Dual platform systems, such as the model 6202D, incorporate an additional four planar beam load cells and junction
printed circuit board in the “satellite” platform. When the satellite platform is in use, all eight load cells are in parallel
and the resultant weight signals are passively summed.
LOAD CELL JUNCTION BOARD
A small printed circuit board is included in the weighing platform to sum the signals from the four co-planar beam
load cells. This board contains no active circuitry, but has five parallel wired connectors (4 in and 1 out) to take the
individual load cell signals and furnish their combined output to the instrument board.
MAIN & SATELLITE TERMINAL BOARD (6202D dual platform systems)
The On model 6202 dual platform scale two additional circuit boards are included for making connections between
the load cells of the two platforms.
The main scale has a terminal board that provides terminal blocks for connections of its platform load cells,
instrument board, and connector for the cross over cable to the satellite platform.
The satellite platform has a similar board, but it includes a relay for switching the satellite platform’s load cell
excitation voltage on or off depending on the position of the platform. When the platform is stowed in the “up”
position the excitation voltage to the satellite’s load cells is removed so that they do not contribute to the weight
signal.
When the satellite platform is lowered “down” into the weighing position, a micro switch in the satellite platform
mechanism closes, energizing a relay on the satellite terminal board, and excitation voltage is applied to the satellite
platform’s load cells. A signal representative of the weight applied to the satellite platform is then added to that of
the main platform so the combined weight may be processed and displayed.
